Actualidad en Geofísica

PWG - Actualidad en Geofísica
HOME | Free E-book

Marzo de 2008 Detecting seismogenic stress evolution and constraining fault zone rheology in the San Andreas Fault following the 2004 Parkfield earthquake Authors: Taka'aki Taira, Paul G. Silver et al Link: Click Here Abstract We investigate temporal changes in seismic scatterer properties at seismogenic depth attributed to the 2004 M 6 Parkfield earthquake, making use of the San Andreas Fault Observatory at Depth repeating-earthquake target sequences, as well as nearby similar-earthquake aftershock clusters. We use a two-step process: (1) observing temporal variations in the decorrelation index, D(t), reflecting changes in the scattered wavefield of repeating-earthquake sequences and (2) estimating the spatial distribution of time-dependent scatterers by using a larger-aperture source array. We focus on three scatterers	exhibiting clear time dependence, using pairs of earthquakes that span or follow the 2004 Parkfield earthquake. They are found to be located on the fault at the northernmost extent of coseismic rupture, beneath Middle Mountain, with a depth range of 11 to 17 km. The shallowest and most prominent scatterer is located near a region of increased Coulomb stress, as well as significant postseismic slip following the 2004 Parkfield earthquake, and a large M = 5 aftershock. The other two deeper ones are also in regions of increased Coulomb stress. We show that D(t)1/2 is expected to be proportional to the level of stress in the fault zone, and then we constrain the form of fault zone rheology by comparing the time dependence of D(t)1/2 with geodetic or seismic measures of strain rate, assuming a power law rheology between stress and strain rate characterized by exponent n. Such a comparison yields n ranging from 1.6 through 3.3, a value that is more consistent with ductile behavior, rather than frictional sliding, at the base of the seismogenic zone.
Marzo de 2008 A vertical profile of PM10 dust concentrations measured during a regional dust event identified by MODIS Terra, western Queensland, Australia Authors: Hamish A. McGowan, Andrew Clark et al Link: Click Here Abstract Accurate determination of the spatiotemporal properties of dust plumes and their dust concentrations is essential for calibration of satellite products and the initialization and validation of numerical models that simulate the physical properties and affects of dust events. In this paper, we present a 500 m vertical profile of PM10 dust concentrations measured during a regional dust event in western Queensland, Australia. PM10 dust concentrations within the	haze were found to be >20 times background ambient values and decreased with height following an exponential function. We apply an over-land algorithm to MODIS Terra satellite images of the dust haze to enhance its visual appearance against the bright land surface and define its size. In conjunction with the measured attenuation of dust concentrations with height we calculate the PM10 dust load of the plume to be ~ 60% of that which would have been calculated assuming a constant dust concentration up to the dust ceiling height. Results extend previous findings from tower-based studies made close to the surface and confirm that atmospheric dust concentrations decrease rapidly with increasing height, thereby enabling more accurate calculation of atmospheric dust loads during synoptic-scale dust outbreaks.
Marzo de 2008 First discovery and formation process of authigenic siderite from gas hydrate–bearing mud volcanoes in fresh water: Lake Baikal, eastern Siberia Authors: Alexey Krylov, Oleg Khlystov et al Link: Click Here Abstract We report on the first authigenic siderite (FeCO3) concretions recovered from near-bottom sediments at gas hydrate-bearing mud volcanoes in fresh water (Lake Baikal, Eastern Siberia). The carbonates appear as firm ‘plate-type’ formations at the Malenky mud volcano (Southern Baikal Basin) and as soft nodules at the K-2 mud	volcano (Central Baikal Basin). Calcium is the main divalent component which substitutes iron in the carbonate lattice (7 to 20 mol%). The δ13C values of the carbonates (+3.3 to +6.8‰ at Malenky, and +16.5 to +21.9‰ PDB at K-2) indicate that their formation is due to methanogenesis. The latter was most likely caused by the microbial methyl-type (acetate) fermentation that is suggested from the isotopic composition of the accompanying methane hydrates and dissolved methane. General depletion of the siderites in 18O (−11.6 to −9.9 ‰ at Malenky, and −13.9 to −12.3‰ PDB at K-2) is mainly inherited from the isotope composition of pore water (−15.2 to −15.4‰ SMOW) at ambient temperature (3.5°C).
Marzo de 2008 Space geodetic investigation of the coseismic and postseismic deformation due to the 2003 Mw7.2 Altai earthquake: Implications for the local lithospheric rheology Authors: Sylvain Barbot, Yariv Hamiel et al Link: Click Here Abstract We use Envisat Advanced Synthetic Aperture Radar data and SPOT optical imagery to investigate the coseismic and postseismic deformation due to the 27 September 2003, Mw7.2 Altai earthquake, which occurred in the Chuya Basin near the Russia-China-Mongolia border. On the basis of the synthetic aperture radar (SAR) and SPOT data, we determined the rupture location and developed a coseismic slip model for the Altai earthquake. The inferred rupture location is in a good agreement with field observations, and the geodetic moment from our slip model is consistent with the seismic moment determined from the teleseismic data. While the epicentral area of the Altai earthquake is not optimal for radar interferometry (in particular, due to temporal decorrelation), we were able to detect	a transient signal over a time period of 3 years following the earthquake. The signal is robust in that it allows us to discriminate among several commonly assumed mechanisms of postseismic relaxation. We find that the postearthquake interferometric SAR data do not warrant poroelastic rebound in the upper crust. The observed deformation also disagrees with linear viscoelastic relaxation in the upper mantle or lower crust, giving rise to a lower bound on the dynamic viscosity of the lower crust of the order of 1019 Pa s. The data can be explained in terms of fault slip within the seismogenic zone, on the periphery of areas with high coseismic slip. Most of the postseismic deformation can be explained in terms of seismic moment release in aftershocks; some shallow slip may have also occurred aseismically. Therefore the observed postseismic deformation due to the Altai earthquake is qualitatively different from deformation due to other similarly sized earthquakes, in particular, the Landers and Hector Mine earthquakes in the Mojave desert, southern California. The observed variations in the deformation pattern may be indicative of different rheologic structure of the continental lithosphere in different tectonically active areas.
Marzo de 2008 A hybrid three-dimensional computational model of groundwater solute transport in heterogeneous media Authors: Yuan-Ching Kuo, Liang-Hsiung Huang et al Link: Click Here Abstract This study examines a new hybrid three-dimensional computational solution for groundwater solute transport in a layered aquifer. In this study, the horizontal length scale of the computational domain, referring to the nature geometry, is assumed much larger than the vertical length scale. Thus the computation of solute transportation in the horizontal direction is more complex than the computation in the vertical direction. Using the Dupuit-Forchheimer approximation of groundwater flow as a reference, this study employs the vertical/horizontal splitting (VHS) concept to divide the three-dimensional solute transport equations into depth averaged	(two-dimensional) and vertical variation (one-dimensional) equations. The two-dimensional equations are processed via the so-called finite analytic method, while the one-dimensional equations are solved analytically. The three-dimensional solute transport is effectively reduced to a hybrid three-dimensional model by combining the two-dimensional computations and the one-dimensional analytical solutions. The resultant hybrid three-dimensional method is capable of large computational time steps and large spatial grids and is thus suitable for simulating solute transport in large-scale sites of layered aquifers with negligible spatial variation of both thickness and hydraulic properties within each layer, where traditional numerical schemes fall short of satisfying the requirements of both accuracy and efficiency. Nevertheless, the use of this method is necessarily restricted when (1) the site is extremely heterogeneous and thus cannot be treated as layers or (2) the flows in the aquifer and in the aquitard are highly three-dimensional. This method is verified by analytical solutions as well as by the numerical model MT3D.
Marzo de 2008 Light elements in the core: Effects of impurities on the phase diagram of iron Authors: Alexander S. Côté, Lidunka Vočadlo et al Link: Click Here Abstract The stable phase of iron (Fe) in the Earth's core has been the subject of extensive studies, yet still remains controversial. Hexagonal close-packed structure (hcp) is the accepted stable form of pure Fe; however, it has been recently proposed that a	body-centered-cubic (bcc) structured alloy of Fe with lighter elements would be a strong candidate. Experiments and ab initio calculations have shown that small amounts of silicon can stabilize the bcc phase with respect to the hcp phase. In this study we examine the effect at core conditions by adding small concentrations of carbon (C), sulphur (S) and oxygen (O) to iron using electronic structure calculations. We analyze the effect of the light elements on the energy difference between the two structures, and we conclude that the bcc phase tends to be stabilized by the alloys, strengthening the case for a bcc-structured inner core of the Earth.
Marzo de 2008 Chemical equilibrium between ferropericlase and molten iron to 134 GPa and implications for iron content at the bottom of the mantle Authors: Haruka Ozawa, Kei Hirose et al Link: Click Here Abstract We have examined the solubility of oxygen in molten iron coexisting with ferropericlase up to 134 GPa and 3200 K by using laser-heated diamond-anvil cell (LHDAC) and analytical transmission electron microscope (TEM). The	results demonstrate that the oxygen solubility in liquid iron decreases with pressure to 38 GPa, whereas the pressure effect is small at higher pressures. If the molten outer core is in chemical equilibrium with the bottom thin layer of the mantle, ferropericlase could be significantly depleted in FeO at the core-mantle boundary (CMB). The liquid core containing 8 wt% oxygen, which is high enough to account for the core density deficit, coexists with ferropericlase with Mg#96 when the temperature is 4000 K. The very bottom of the mantle becomes depleted in iron by the consequences of chemical reaction with the core.
Marzo de 2008 Strength, porosity, and permeability development during hydrostatic and shear loading of synthetic quartz-clay fault gouge Authors: B. R. Crawford, D. R. Faulkner et al Link: Click Here Abstract The strength and permeability of fault zones must be quantified in order to accurately predict crustal strength and subsurface fluid migration. To this end, we performed experiments on mixtures of fine-grained quartz and kaolinite incremented at 10 wt% intervals between the two end-member components (analogues for natural fault gouge) in order to establish their strength and fluid flow properties during hydrostatic and shear loading. Hydrostatically compacted samples exhibited permeability reduction on increasing effective pressures from 5 MPa to 50 MPa, with the rate of reduction displaying strong dependency on the synthetic fault gouge composition. The permeability decreases continuously with	increasing kaolinite content. Porosity exhibits a distinct minimum that evolves with increasing effective pressure according to the relative compaction of the quartz and kaolinite end-members. Porosity evolution with increasing clay content is predicted satisfactorily by a simple ideal packing model. At the highest effective pressure (50 MPa), permeability reduced log-linearly over 4 orders of magnitude with increasing clay content. Mechanically, sheared gouge samples showed a continuous reduction in frictional strength with increasing clay fraction. Permeability decreased further on shear loading after initial hydrostatic compaction to 50 MPa. This was most evident for the pure quartz end-member, with two orders of magnitude additional reduction, whereas the clay-rich samples were reduced only tenfold, mostly before a shear strain of 5. Variation of permeability with both clay content and shear deformation may be adequately described by previously published empirical predictors for fault zone permeability. Clay content has the largest effect on permeability, and shear deformation affects permeability of quartz-rich gouges more than clay-rich gouges.
Marzo de 2008 Aplicación del Método de Monte Carlo para determinar los parámetros de recurrencia de terremotos a partir de catálogos paleosísmicos con pocos datos: Ejemplo de cálculos para California. Autor: Tom Parsons Link: Click Aquí Abstract Las observaciones de Paleoterremotos frecuentemente presentan escasez de datos como para poder establecer una confiable Función Densidad de Probabilidad (FDP) para el tiempo de recurrencia. Los sitios que presentan de 10 a 15 intervalos de recurrencia no proveen suficiente información confiable como para determinar la forma de la FDP en caso de utilizar técnicas estándar (vg Ellsworth et al, 1999). En este paper yo presento un método que pretende obtener una gran cantidad de parámetros de la FDP a partir de series paleosísmicas con pocos datos. A partir de iteraciones del tipo Monte Carlo es posible estimar cuantitativamente los parámetros de la FDP y además se puede obtener una distribución de éstos con miras del cálculo del riesgo. En los test que se realizaron con pocos datos el método obtuvo resultados que se agruparon alrededor del valor medio del input de la distribución (ver NIST/SEMATECH, 2006). Para series con pocos datos (menos de 10 intervalos), la media de la muestra tiende a reflejar la mediana de una FDP asimétrica, lo que posiblemente pueda llevar a una sobreestima-ción del riesgo. Por lo tanto, un enfoque del tipo Monte Carlo puede ser útil para la evaluación del riesgo a partir de pocos datos. Es posible establecer el grado de dependencia entre los parámetros y el coeficiente de variación. El método es descrito para ser utilizado con FDP tiempo-independientes y tiempo-dependientes, y además se presentan los resultados de 19 secuencias paleosísmicas en California.	Marzo de 2008 Monte Carlo method for determining earthquake recurrence parameters from short paleoseismic catalogs: Example calculations for California Author: Tom Parsons Link: Click here Abstract Paleoearthquake observations often lack enough events at a given site to directly define a probability density function (PDF) for earthquake recurrence. Sites with fewer than 10–15 intervals do not provide enough information to reliably determine the shape of the PDF using standard maximum-likelihood techniques (e.g., Ellsworth et al., 1999). In this paper I present a method that attempts to fit wide ranges of distribution parameters to short paleoseismic series. From repeated Monte Carlo draws, it becomes possible to quantitatively estimate most likely recurrence PDF parameters, and a ranked distribution of parameters is returned that can be used to assess uncertainties in hazard calculations. In tests on short synthetic earthquake series, the method gives results that cluster around the mean of the input distribution, whereas maximum likelihood methods return the sample means (e.g., NIST/SEMATECH, 2006). For short series (fewer than 10 intervals), sample means tend to reflect the median of an asymmetric recurrence distribution, possibly leading to an overestimate of the hazard should they be used in probability calculations. Therefore a Monte Carlo approach may be useful for assessing recurrence from limited paleoearthquake records. Further, the degree of functional dependence among parameters like mean recurrence interval and coefficient of variation can be established. The method is described for use with time-independent and time-dependent PDFs, and results from 19 paleoseismic sequences on strike-slip faults throughout the state of California are given.
Marzo de 2008 Measuring the potential predictability of ensemble climate predictions Authors: Youmin Tang, Hai Lin et al Link: Click here Abstract In this study, ensemble predictions of the El Niño Southern Oscillation (ENSO) and the Arctic Oscillation (AO) were conducted using two coupled models and two atmospheric circulation models, respectively, as well as various ensemble schemes. Several measures of potential predictability including ensemble mean square (EM2), ensemble spread and the ratio of signal-to-noise were explored in terms of their ability of estimating a priori the predictive skill of the ENSO and AO ensemble predictions. The emphasis was put on examining the relationship between the measures of predictability that do not use observations and the model prediction skill of correlation and mean square error (MSE) that	make use of observations. The relationship identified here offers a practical means of estimating the potential predictability and the confidence level of an individual prediction. It was found that the EM2 is a better indicator of the actual skill of ensemble ENSO and AO prediction than the ratio of signal-to-noise. When correlation-based metrics are used, the prediction skill is likely to be a linear function of EM2, i.e., the larger the EM2 the higher skill the prediction; whereas when MSE-based metrics are used, a “triangular relationship” is suggested between them, namely, that when EM2 is large, the prediction is likely to be reliable whereas when EM2 is small the prediction skill is highly variable. In contrast with ensemble weather prediction (NWP), the ensemble spread is not a good predictor in quantifying climate prediction skill in the models used in this study because the forced response may be much larger than the noise in the climate timescales compared to the NWP. A statistical framework was proposed to explain why EM2 is a good indicator of actual prediction skill in the ensemble climate predictions.
Marzo de 2008 A preliminary global map of the vector lunar crustal magnetic field based on Lunar Prospector magnetometer data Authors: N. C. Richmond and L. L. Hood Link: Click here Abstract Previous processing of the Lunar Prospector magnetometer (LP-MAG) data has yielded ~ 40% coverage of the Moon. Here, new mapping of the low-altitude LP-MAG data is reported with the goal of producing the first global vector map of the lunar crustal magnetic field. By considering all data regardless of the external plasma environment and using less restrictive editing criteria, 2360 partial and complete passes have been identified that can be used to investigate the lunar crustal magnetic anomalies. The cleanest global coverage is provided using 329 low-altitude nightside and terminator passes. An inverse	power method has been used to continue the final mapping data to constant altitude. Using the 329 optimal passes, global maps of the lunar crustal magnetic field are constructed at 30 and 40 km. Consistent with previous studies: (1) the largest concentrations of anomalies are mapped antipodal to the Crisium, Serenitatis, Imbrium, and Orientale basins and (2) isolated anomalies at Reiner Gamma, Rima Sirsalis, Descartes, and Airy are mapped. Anomalies previously unmapped by the LP-MAG experiment include (1) isolated anomalies near the craters Abel and Hartwig, (2) weak magnetization within the Nectarian-aged Crisium and Moscoviense basins, and (3) a relatively weak anomaly in an area dominated by crater chains associated with the formation of Nectaris. Future work with the new low-altitude data set is discussed and will include determining whether the lunar anomalies are capable of deflecting the solar wind and investigating directions of magnetization to evaluate a possible former core dynamo.
Febrero de 2008 Heat flow study at the Chinese Continental Scientific Drilling site: Borehole temperature, thermal conductivity, and radiogenic heat production Authors: Lijuan He, Shengbiao Hu et al Link: Click here Abstract The Chinese Continental Scientific Drilling (CCSD) Project offers a unique opportunity for studying the thermal regime of the Dabie-Sulu ultrahigh-pressure metamorphic belt. In this paper, we report measurements of borehole temperature, thermal conductivity, and radiogenic heat production from the 5158 m deep main hole (CCSD MH). We have obtained six continuous temperature profiles from this borehole so far. The temperature logs show a transient mean thermal gradient that has increased from 24.38 to 25.28 K km-1 over a period of about 1.5 years. We	measured thermal conductivities and radiogenic heat productions on more than 400 core samples from CCSD MH. The measured thermal conductivities range between 1.71 and 3.60 W m-1 K-1, and the radiogenic heat productions vary from 0.01 µW m-3 to over 5.0 µW m-3, with a mean value of 1.23 ± 0.82 µW m-3 for the upper 5-km layer of the crust. The heat productions in CCSD MH appear to be more rock-type than depth-dependent and, over the depth range of CCSD MH, do not fit the popular model of heat production decreasing exponentially with increasing depth. The measured heat flow decreases with depth from ~75 mW m-2 near the surface to ~66 mW m-2 at a depth of 4600 m. High heat flow anomalies occur at ~1000 and ~2300 m, and low anomalies occur at 3300-4000 m. A preliminary two-dimensional numerical model suggests that both radiogenic heat production and thermal refraction due to structural heterogeneity are at least partially responsible for the vertical variation of heat flow in CCSD MH.
Febrero de 2008 A leaping, triggered sequence along a segmented fault: The 1951 ML 7.3 Hualien-Taitung earthquake sequence in eastern Taiwan Authors: Kate Huihsuan Chen, Shinji Toda et al Link: Click here Abstract As the most destructive seismic episode ever known in eastern Taiwan, the 1951 M L 7.3 Hualien - Taitung earthquake series consisted of sequential ruptures along four distinct fault segments. It provides a good opportunity to study earthquake triggering processes along an active fault at an oblique arc-continent collision boundary. This sequence initiated on 21October 1951 with the M L 7.3 Hualien main shock and a group of M6+ aftershocks nearby. The M L 6.0 Chihshang earthquake occurred 34 days later and 100 km away from the main shock. The M L 7.3 Yuli earthquake followed 3 m later and 5 km away from the Chihshang event. Two days later, the M L 6.0 Taitung earthquake shocked a region 40 km	away from the preceding M6 event and completed the sequence. The first triggered rupture outside the main shock area did not occur on the nearby Yuli fault segment but occurred 100 km away at the Chihshang fault. Calculations of static Coulomb stress change show that most of the major aftershocks were located in areas of enhanced static stress change. However, the stress transfer alone cannot explain triggering across 100 km. With the rate/state stress transfer model, we computed the temporal order of encouraged ruptures on different segments along the collision boundary. The results show that 34 days following the major shocks in Hualien, the Chihshang segment had a higher M6+ (M 6) earthquake probability due to its significantly higher (at least an order of magnitude) background seismicity rate than the other two segments. After the Chihshang event, the rate/state model predicted a higher M6+ earthquake probability in the Yuli segment, also matching the observation. In this case, the Yuli segment was triggered ahead of the Taitung segment because of its larger increase in Coulomb stress change.
Febrero de 2008 A new GPS-acoustic method for measuring ocean floor crustal deformation: Application to the Nankai Trough Authors: Ryoya Ikuta, Keiichi Tadokoro et al Link: Click here Abstract We developed a new geodetic method of monitoring crustal deformation on the ocean floor. The measurements were conducted at two sites beneath the Pacific Ocean, near the Nankai Trough, where the Philippine Sea plate subducts into the Pacific plate. The first site was at a depth of 700 m in Suruga Bay, and the second one was at a depth of 2000 m in the Kumano Basin. At each survey site, three seafloor transponders, whose positions were repetitively measured, were deployed to define a benchmark unit. An ultrasonic signal was generated from a surface vessel drifting over the benchmark unit that was transmitting and receiving the signal and the	replies from the transponders. Both sound speed structure and the benchmark unit positions were determined simultaneously using a tomographic technique. We repeatedly carried out measurements over the two sites. For the Kumano Basin we made six measurements from July 2004 to August 2005. The benchmark unit location's horizontal precision was 5 cm, and its vertical precision was 10 cm. For Suruga Bay we made five measurements, and the positions were less accurate, with horizontal variation of 7 cm and vertical variation of 16 cm. We investigated the reason for the decreased precision in Suruga Bay with a numerical experiment, and the results revealed that the geometry of the measurement, that is, the vessel's path, is one of the most important variables in our method. At the Kumano Basin a 21.5-cm southward displacement of the benchmark unit was detected just below the site before and after a large earthquake (Mw 7.5). Our observation system therefore proved itself capable of detecting seafloor crustal deformation associated with crustal activities in offshore areas.
Febrero de 2008 Zeta potential estimation of volcanic rocks on 11 island arc-type volcanoes in Japan: Implication for the generation of local self-potential anomalies Authors: Koki Aizawa, Makoto Uyeshima et al Link: Click here Abstract From streaming potential measurements, we deduced the zeta potential of 73 volcanic rock samples collected in 11 volcanoes where self-potential (SP) surveys had also been conducted. Experiments with crushed rock samples and 0.001 mol/L NaCl solution showed a large variation in streaming potential coefficient, which ranged from -2860 to 2280 mV/MPa (deduced zeta potential ranged from -45.1 to 37.2 mV). Among the 73 samples, 9 samples showed positive values, and 11 samples showed small absolute values less than 10 mV. Even in a volcano, the	zeta potential values of rocks were sometimes highly variable. To investigate the reason for the variation in zeta potential, we performed X-ray fluorescence (XRF) measurements on the rock samples. However, no significant correlations between zeta potential (or isoelectric point) and weight percentage of major elements were found, implying that the zeta potential of the volcanic rocks is not controlled by chemical composition but by mineralogical composition or crystalline structure. In volcanic areas, rocks with positive or small absolute values of zeta potential, which are considered to be unusual, frequently exist in areas along with local positive SP anomalies. Although positive SP anomalies are usually interpreted as an indication of hydrothermal upflow, this correlation suggests that in addition to the groundwater flow pattern, the polarity of zeta potential should also be considered for interpretation of SP data. Therefore, in cases where detailed zeta potential measurements have not been conducted, local positive SP anomalies should be interpreted with caution.
Febrero de 2008 Characteristics of ambient seismic noise as a source for surface wave tomography Authors: Yingjie Yang and Michael H. Ritzwoller Link: Click here Abstract Interstation cross correlations of ambient seismic noise from 1 year of continuous data at periods between 6 and 50 s are used to study the origin of the ambient noise using stations located in Europe, southern Africa, Asia, and three regions within North America. The signal-to-noise ratios (SNR) of Rayleigh waves for positive and negative correlation time lags at periods of 8, 14, 25 and 50 s are used to determine the azimuthal distribution of strong ambient noise sources. Ambient noise in both the primary (10-20 s) and secondary microseism bands (5-10 s) comes dominantly from the directions of relatively nearby coastlines	with stronger noise occurring in the Northern Hemisphere in northern winter and in the Southern Hemisphere in southern winter, consistent with the hypothesis that oceanic microseisms are generating this noise. The observed differences in the directivity of noise in the primary and secondary microseism bands are the consequence of propagation and attenuation, rather than the location of generation. At intermediate and long periods (>20 s), there is much less seasonal variation in both signal strength and directivity. We argue that our results are explained most simply by near-coastal sources rather than deep ocean sources at all periods. Although the dominant ambient noise sources are distributed inhomogeneously in azimuth, strong ambient noise emerges from most directions when using recordings that are 1 year in duration. Simulations illustrate that this is what ensures the accuracy of the empirical Green's functions and ambient noise tomography.
Febrero de 2008 Three-dimensional radar observation of a submillimeter meteoroid fragmentation Authors: J. Kero, C. Szasz et al Link: Click here Abstract We present and discuss two examples of pulsating meteor events observed with the tristatic 930 MHz EISCAT UHF radar system. One of them provides the first strong observational evidence of a submillimeter-sized meteoroid breaking apart into two distinct fragments. The received power fluctuates regularly in the time profiles of all three	receivers, but the fluctuation frequencies are different. The pulsations are interpreted as being due to interference from two distinct scattering centers and the three detected pulsation rates are utilized to calculate the differential velocity of the fragments. The result is consistent with interference from two fragments of unequal cross-sectional area over mass ratio, separating from each other due to different deceleration along the trajectory of their parent meteoroid. The other meteor event is an example of a meteoroid undergoing quasi-continuous disintegration. This manifests itself as simultaneous pulsations at all three receivers. Both observations indicate a head echo target upper size limit of the order of half the 32 cm radar wavelength.
Febrero de 2008 Mesoscopic fluid flow simulation in double-porosity rocks Authors: Jing Ba, Jian-Xin Nie et al Link: Click here Abstract A discrete Fourier transform algorithm is designed to simulate mesoscopic fluid flow(MFF) in double-porosity rocks. Double-porosity equations with MFF are derived. Results from pseudo-spectral simulation show MFF transfers	fast P waves and the first kind of slow P waves' energy to the second kind of slow P waves, and the Biot diffusive mode significantly attenuates the second kind of slow P waves' energy. We use a novel approach with a numerical grid method to solve the double-porosity wave equations. We conclude that the wavefield's attenuation should be attributed to a two-step mechanism: (1) MFF and (2) macroscopic Biot diffusion. Numerical estimations have shown that MFF in the double-porosity model can produce a high attenuation (0.32DB/10 m, 1/Q = 0.1083) in the seismic band (50 Hz).
Febrero de 2008 Full waveform inversion of reflection seismic data for ocean temperature profiles Authors: Warren T. Wood, W. Steven Holbrook et al Link: Click here Abstract We show that ocean temperature profiles can be accurately recovered using only acoustic methods employed at the sea surface. Using a towed air gun array and a hydrophone streamer, thermohaline boundaries are ensonified at a suite of frequencies and angles, yielding travel time trajectories and reflectivities. These data are	inverted via full waveform inversion to estimate sound speed and, subsequently, a temperature profile. The high lateral data density of the seismic technique offers the potential of acoustically derived temperature profiles to be used to constrain models of ocean mixing and internal waves. Results on realistic synthetic data show that sound speed can be recovered with arbitrary accuracy when using broadband data, with known source function and recording geometry. Application to field seismic data (corroborated by expendable bathythermograph) shows that even with a seismic acquisition system not specifically calibrated for seismic oceanography, temperature contrasts within the ocean can be recovered to within one degree Celsius.
Febrero de 2008 Calibration of hydrological model GR2M using Bayesian uncertainty analysis Authors: David Huard and Alain Mailhot Link: Click here Abstract An outstanding issue of hydrological modeling is the adequate treatment of uncertainties in model calibration and prediction. The current paradigm is that the major sources of uncertainties, namely input, output and model uncertainty should be accounted for directly, instead of assuming they	can be safely lumped into the output uncertainties. In this paper, Bayesian analysis is used to calibrate the conceptual hydrologic monthly model GR2M taking into account input, output, structural and initial state uncertainty through error models and priors. Calibration is performed under different error assumptions to study the influence of the initial state uncertainty, the consequences of large input errors, the impact of error assumptions on calibrated parameter posterior distributions and the definition of error models. It is shown how such an analysis can be used to separate, a posteriori, the different sources of errors, and in particular, to identify structural errors from data errors.
Febrero de 2008 Derived operating rules for a reservoir operation system: Comparison of decision trees, neural decision trees and fuzzy decision trees Authors: Chih-Chiang Wei and Nien-Sheng Hsu Link: Click here Abstract This article compares the decision-tree algorithm (C5.0), neural decision-tree algorithm (NDT) and fuzzy decision-tree algorithm (FIDs) for addressing reservoir operations regarding water supply during normal periods. The conventional decision-tree algorithm, such as ID3 and C5.0, executes rapidly and can easily be translated into if-then-else rules. However, the C5.0 algorithm cannot discover dependencies among attributes and cannot treat the non-axis-parallel class	boundaries of data. The basic concepts of the two algorithms presented are: (1) NDT algorithm combines the neural network technologies and conventional decision-tree algorithm capabilities, and (2) FIDs algorithm extends to apply fuzzy sets for all attributes with membership function grades and generates a fuzzy decision tree. In order to obtain higher classification rates in FIDs, the flexible trapezoid fuzzy sets are employed to define membership functions. Furthermore, an intelligent genetic algorithm is utilized to optimize the large number of variables in fuzzy decision-tree design. The applicability of the presented algorithms is demonstrated through a case study of the Shihmen Reservoir system. A network flow optimization model for analyzing long-term supply demand is employed to generate the input-output patterns. Findings show superior performance of the FIDs model in contrast with C5.0, NDT and current reservoir operating rules.

Febrero de 2008 Simulation assessment of the direct-push permeameter for characterizing vertical variations in hydraulic conductivity Authors: Gaisheng Liu, Geoffrey C. Bohling et al Link: Click here Abstract The direct-push permeameter (DPP) is a tool for the in situ characterization of hydraulic conductivity (K) in shallow, unconsolidated formations. This device, which consists of a short screened section with a pair of pressure transducers near the screen, is advanced into the subsurface with direct-push technology. K is determined through a series of injection tests conducted between advancements. Recent field work by Butler et al. (2007) has shown that the DPP holds great potential for describing vertical variations in K at	an unprecedented level of detail, accuracy and speed. In this paper, the fundamental efficacy of the DPP is evaluated through a series of numerical simulations. These simulations demonstrate that the DPP can provide accurate K information under conditions commonly faced in the field. A single DPP test provides an effective K for the domain immediately surrounding the interval between the injection screen and the most distant pressure transducer. Features that are thinner than that interval can be quantified by reducing the vertical distance between successive tests and analyzing the data from all tests simultaneously. A particular advantage of the DPP is that, unlike most other single borehole techniques, a low-K skin or a clogged screen has a minimal impact on the K estimate. In addition, the requirement that only steady-shape conditions be attained allows for a dramatic reduction in the time required for each injection test.
Febrero de 2008 A new stochastic model for fracture transmissivity assessment Authors: Tomasz J. Kozubowski, Mark M. Meerschaert et al Link: Click here Abstract A new stochastic model is proposed for fracture counts and transmissivities in a borehole interval.	This new model incorporates several empirical observations, including: (i) Clustering of fractures; (ii) Exponential fracture spacings; (iii) Transmissivities extending over several orders of magnitude; (iv) Power law probability tail for transmissivities at finer scales; (v) Log-normal transmissivities at larger scales; and (vi) Dependence between fracture counts and transmissivities. Several example applications are provided using borehole data from Sweden.
Febrero de 2008 Coseismic and early postseismic slip for the 2003 Tokachi-oki earthquake sequence inferred from GPS data Authors: Shin'ichi Miyazaki and Kristine M. Larson Link: Click here Abstract Early afterslip following the 2003 Tokachi-oki earthquake is investigated using subdaily GPS time series. Afterslip results are compared with	the coseismic slip for the M8 mainshock and the M7.4 aftershock. Afterslip between those two earthquakes is inferred at the southwestern adjacent region of the mainshock, between two epicentral regions, which possibly triggered the aftershock in the southwest. Subsequently, deeper slip occurs. The afterslip loci are distinct from the rupture regions. The non-uniform propagation of afterslip may reflect the depth-dependence of the effective normal stress and the distance between the closest unstable slip patches.
Febrero de 2008 PP and PS seismic response from fractured tight gas reservoirs: a case study Authors: Tang Jianming, Zhang Shaonan et al Link: Click here Abstract In this paper, we present an example of using PP and PS converted-wave data recorded by digital micro-eletro-mechanical-systems (MEMS) to evaluate a fractured tight gas reservoir from the Xinchang gas field in Sichuan, China. For this, we analyse the variations in converted shear-wave splitting, Vp/Vs ratio and PP and PS impedance, as well as other attributes based on absorption and velocity dispersion. The reservoir formation is tight sandstone, buried at a depth of about 5000 m, and the converted-wave data reveal significant shear-wave splitting over the reservoir formation.	We utilize a rotation technique to extract the shear-wave polarization and time delay from the data, and a small-window correlation method to build time-delay spectra that allow the generation of a time-delay section. At the reservoir formation, the shear-wave time delay is measured at 20 ms, about 15% shear-wave anisotropy, correlating with the known gas reservoirs. Furthermore, the splitting anomalies are consistent with the characteristics of other attributes such as Vp/Vs ratio and P- and S-wave acoustic and elastic impedance. The P-wave shows consistent low impedance over the reservoir formation, whilst the S-wave impedance shows relatively high impedance. The calculated gas indicator based on absorption and velocity dispersion yields a high correlation with the gas bearing formations. This confirms the benefit of multicomponent seismic data from digital MEMS sensors.
Febrero de 2008 Intelligent approaches for the synthesis of petrophysical logs Authors: M Reza Rezaee, Ali Kadkhodaie-Ilkhchi et al Link: Click here Abstract Log data are of prime importance in acquiring petrophysical data from hydrocarbon reservoirs. Reliable log analysis in a hydrocarbon reservoir requires a complete set of logs. For many reasons, such as incomplete logging in old wells, destruction of logs due to inappropriate data storage and measurement errors due to problems with logging apparatus or hole conditions, log suites are either incomplete or	unreliable. In this study, fuzzy logic and artificial neural networks were used as intelligent tools to synthesize petrophysical logs including neutron, density, sonic and deep resistivity. The petrophysical data from two wells were used for constructing intelligent models in the Fahlian limestone reservoir, Southern Iran. A third well from the field was used to evaluate the reliability of the models. The results showed that fuzzy logic and artificial neural networks were successful in synthesizing wireline logs. The combination of the results obtained from fuzzy logic and neural networks in a simple averaging committee machine (CM) showed a significant improvement in the accuracy of the estimations. This committee machine performed better than fuzzy logic or the neural network model in the problem of estimating petrophysical properties from well logs.
Febrero de 2008 1D and 3D resistivity inversions for geotechnical investigation Authors: Sultan Awad Sultan et al Link: Click here Abstract The resistivity method is frequently used in the investigation of the shallow parts of the earth. Interpretation of such data is usually done assuming a layered earth. However, a more complete imaging can be obtained if 3D models are used. Thirty-five vertical electrical soundings (VES) were carried out in a regular mesh at the northwestern part of Greater Cairo in order to characterize different geological units and to study	their quality for building foundations. Models obtained from 1D inversion of each VES, together with borehole information, were used for construction of eight geoelectrical sections which exhibit the main geoelectrical characteristics of the geological units present in the area. The 3D inversion of the data indicated a complex subsurface electrical resistivity distribution conditioned by lithology, water content and tectonic structures. The results indicate that the subsurface consists of different geologic units such as gravel and sand, sand, clay and limestone. The main results are related to the characterization of the clay formations in the north of the survey area, which is revealed by low-resistivity values (<100 O m) and sand layers associated with high-resistivity values (>600 O m) depicted in the central part of the study zone.
Febrero de 2008 Pseudo-3D imaging of a low-slip-rate, active normal fault using shallow geophysical methods: The Geleen fault in the Belgian Maas River valley Authors: Kris Vanneste,Koen Verbeeck et al Link: Click here Abstract In a detailed site survey for paleoseismic trenching, we applied shallow geophysical prospecting techniques, including ground-penetrating radar (GPR), electric resistivity tomography (ERT), and resistivity mapping to identify, locate, and visualize in 3D the Geleen fault, an active normal fault bordering the Roer Valley graben in northeast Belgium. Because of a low slip rate, the geomorphic expression of this fault is very faint in the relatively young deposits of the Maas River valley. ERT profiles show the fault as a broad, near-vertical anomaly characterized by sharp lateral resistivity contrasts, with an associated vertical offset of sediment layers. We observed offsets of ~15 m and ~5 m for the base and top, respectively, of a middle-late Pleistocene	fluvial gravel layer. Shallow ERT and GPR profiles indicate that younger sediments are also affected by faulting, but the amount and sense of offset appear to be conflicting: ERT profiles show a near-surface, high-resistivity layer with an apparent reverse offset, and GPR profiles portray the fault as a sharp and laterally consistent disruption of reflectors, often without a clearly identifiable offset. Resistivity maps at different depths map the fault as a narrow, linear, lateral resistivity gradient matching the anomalies observed with other techniques. This method proved to be efficient in determining the precise position and orientation of dip-slip faults, and could potentially be very useful for the identification of lateral changes in fault geometry, such as splays and step-overs. Subsequent trenching confirmed the presence of a normal fault at the location predicted by the geophysical survey. Correlation with the sediments exposed on the trench walls demonstrated that, close to the surface, resistivity and dielectric permittivity contrasts mostly occur in a postdepositional soil, which developed differently on either side of the fault. This explains why shallow geophysical variations do not reflect the true fault offset.
Febrero de 2008 Analysis of the induced seismicity of the Lacq gas field (Southwestern France) and model of deformation Authors: · T. Bardainne, · N. Dubos-Sallée et al Link: Click here Abstract The goal of this paper is to propose a model of deformation pattern for the Lacq gas field (southwest of France), considering the temporal and spatial evolution of the observed induced seismicity. This model of deformation has been determined from an updating of the earthquake locations and considering theoretical and analogue models usually accepted for hydrocarbon field deformation. The Lacq seismicity is clearly not linked to the natural seismicity of the Pyrenean range recorded 30 km farther to the south since the first event was felt in 1969, after the beginning of the hydrocarbon recovery. From 1974 to 1997, more than 2000 local events (ML < 4.2) have been recorded by two permanent local seismic networks. Unlike previously published results focusing on limited time lapse studies, our analysis relies on the data from 1974 to 1997. Greater accuracy of the absolute locations have been obtained using a well adapted algorithm of 3-D location, after improvement of the 3-D P-wave velocity model and determination of specific station corrections for different clusters of events. This updated	catalogue of seismicity has been interpreted taking into account the structural context of the gas field. The Lacq gas field is an anticlinal reservoir where 3-D seismic and borehole data reveal a pattern of high density of fracturing, mainly oriented WNW-ESE. Seismicity map and vertical cross-sections show that majority of the seismic events (70 per cent) occurred above the gas reservoir. Correlation is also observed between the orientation of the pre-existent faults and the location of the seismic activity. Strong and organized seismicity occurred where fault orientation is consistent with the poroelastic stress perturbation due to the gas recovery. On the contrary, the seismicity is quiescient where isobaths of the reservoir roof are closed to be perpendicular to the faults. These quiescient areas as well as the central seismic part are characterized by a surface subsidence determined by repeated levelling profiles. Moreover, the temporal evolution of the distribution of the seismicity clearly exhibits a spatial migration from the centre to the boundaries of the reservoir. We conclude that the entire field is strained but this deformation is seismically expressed only where faults are parallel to the isobaths of the reservoir roof and where these faults plunge towards outside the field according to one of the two theoretical deformation models considered in our study. Then we propose a temporal scenario of deformation along the principal axis of seismic deformation. SEE MORE
Febrero de 2008 Electrical imaging of deep crustal features of Kutch, India Authors: · R. S. Sastry, · Nandini Nagarajan et al Link: Click here Abstract A regional Magnetotelluric (MT) study, was carried out with 55 MT soundings, distributed along five traverses, across the Kutch Mainland Unit (KMU), on the west coast of India, a region characterized by a series of successive uplifts and intervening depressions in the form of half graben, bounded by master faults. We obtain the deeper electrical structure of the crust beneath Kutch, from 2-D modelling of MT data along the 5 traverses, in order to evaluate the geo-electrical signatures, if any, of the known primary tectonic structures in this region. The results show that the deeper electrical structure in the Kutch region presents a mosaic of high resistive crustal blocks separated	by deep-rooted conductive features. Two such crustal conductive features spatially correlate with the known tectonic features, viz., the Kutch Mainland Fault (KMF), and the Katrol Hill Fault (KHF). An impressive feature of the geo-electrical sections is an additional, well-defined conductive feature, running between Jakhau and Mundra, located at the southern end of each of the five MT traverses and interpreted to be the electrical signature of yet another hidden fault at the southern margin of the KMU. This new feature is named as Jakhau-Mundra Fault (JMF). It is inferred that the presence of JMF together with the Kathiawar Fault (NKF), further south, located at the northern boundary of the Saurashtra Horst, would enhance the possibility of occurrence of a thick sedimentary column in the Gulf of Kutch. The region between the newly delineated fault (JMF) and the Kathiawar fault (NKF) could thus be significant for Hydrocarbon Exploration. SEE MORE
Febrero de 2008 Finite cube elements method for calculating gravity anomaly and structural index of solid and fractal bodies with defined boundaries Author: Mostafa E. Mostafa Link: Click here Abstract In this paper, we present the finite cube elements method (FCEM); a novel numerical tool for calculating the gravity anomaly g and structural index SI of solid models with defined boundaries and variable density distributions, tilted or in normal position (e.g. blocks, faulted blocks, cylinders, spheres, hemispheres, triaxial ellipsoids). Extending the calculation to fractal objects, such as Menger sponges of different orders and bodies defined by polyhedrons, demonstrates the robustness of FCEM. In addition, approximating the cube element by a sphere of equal volume makes the calculation of gravitation and related derivatives much simpler.	In gravity modelling of a sphere, cubes with edges of 100 m and 200 m achieve a good compromise between running time and overall error. Displaying the distribution of SI of the studied models on contour maps and profiles will have a strong impact on the forward and inverse modelling of potential field data, especially for Euler deconvolution. For Menger sponges, plots of gravity elements g and its derivatives show similar patterns independent of fractal order. Moreover, both the pattern and magnitude of SI are independent of fractal order, allowing the use of SI as a new invariant measure for fractal objects. However, SI pattern and magnitude strongly depend on the depth to the buried bodies as do other elements In this study, we also present a new type of plot; the structural index against distance variation diagrams from which we extract the three critical SI (CSI) values, one per axis. The inversion of gravity anomaly data at CSI values gives the optimal mean location of the buried body. SEE MORE
Febrero de 2008 NASA Scientist Finds a New Way to the Center of the Earth Link: Click here Abstract Humans have yet to see Earth's center, as did the characters in Jules Verne's science fiction classic, "Journey to the Center of the Earth." But a new NASA study proposes a novel technique to pinpoint more precisely the location of Earth's center of mass and how it moves through space. Knowing the location of the center of mass, determined using measurements from sites on Earth's surface, is important because it provides the reference frame through which scientists determine the relative motions of positions on Earth's surface, in its atmosphere and in space. This information is vital to the study of global sea level change, earthquakes, volcanoes and Earth's response to the retreat of ice sheets after the last ice age. The accuracy of estimates of the motion of Earth's center of mass is uncertain, but likely ranges from	2 to 5 millimeters (.08 to .20 inches) a year. Donald Argus of NASA's Jet Propulsion Laboratory, Pasadena, Calif., developed the new technique, which estimates Earth's center of mass to within 1 millimeter (.04 inches) a year by precisely positioning sites on Earth's surface using a combination of four space-based techniques. The four techniques were developed and/or operated by NASA in partnership with other national and international agencies. Results of the new study appear in the June issue of Geophysical Journal International. Scientists currently define Earth's center in two ways: as the mass center of solid Earth or as the mass center of Earth's entire system, which combines solid Earth, ice sheets, oceans and atmosphere. Argus says there is room for improvement in these estimates. "The past two international estimates of the motion of the Earth system's mass center, made in 2000 and 2005, differ by 1.8 millimeters (.07 inches) a year," he said. "This discrepancy suggests the motion of Earth's mass center is not as well known as we'd like." SEE MORE
Febrero de 2008 Natural gas back in favour with US power companies Author: Jeff Tollefson Link: Click here Abstract Natural gas produces fewer emissions than coal, but supplies are shrinking and its cost is rising. K. BURKE/CORBIS. Electric utilities in the United States are quietly shifting their sights from coal to natural gas as the lower-risk fuel while they wait for the nation's carbon policy to be decided. Recent proposals to build coal-fired power plants have been sunk after hitting a wall of public and political opposition in Texas, Kansas and Florida. Investors and lenders - including some of Wall Street's leading investment banks - are increasingly worried about the imposition of greenhouse-gas regulations in the coming years.	Add to this the rising prices of materials such as steel and concrete, which are driving up construction costs, and coal no longer looks like the safe investment it did just a few years ago. This new calculus hit home in 2007, when plans for more than 50 coal-fired power plants were cancelled, according to consulting firm Global Energy Decisions, based in Boulder, Colorado. In their place, experts say, utilities are turning to natural gas as a quicker, cheaper and lower-emission alternative to meet growing demand while lawmakers sort out the nation's carbon policy. "Over the past year to two years there's been this 180-degree turn from concerns about dependence on natural gas to concerns about climate change," says Larry Makovich, an energy analyst and senior power adviser at Cambridge Energy Research Associates in Massachusetts. "The US power sector has started to move back towards natural gas in a very substantial way." SEE MORE
Febrero de 2008 Nonmonotonic traveling wave solutions of infiltration into porous media Authors: David A. DiCarlo, Ruben Juanes et al Link: Click here Abstract In uniform soils that are susceptible to unstable preferential flow, the water saturation may exhibit a nonmonotonic profile upon continuous infiltration. As this nonmonotonicity (also known as saturation overshoot) cannot be described by the conventional Richards equation, there have	been proposed possible extensions to the unsaturated flow equations, including a nonmonotonic capillary pressure-saturation curve and a second-order hyperbolic term. Here, we present analytic traveling wave solutions to the extended Richards equation. These new solutions indeed display a nonmonotonic saturation profile, similar to previous simulation results. We show that these extensions need a regularization term to produce a unique solution. We develop complete analytic solutions using a relaxation regularization term, and we discuss the results in terms of recent measurements of saturation overshoot.
Febrero de 2008 Model for high-frequency Strombolian tremor inferred by wavefield decomposition and reconstruction of asymptotic dynamics Authors: E. De Lauro, S. De Martino et al Link: Click here Abstract We study the volcanic tremor time series recorded by a broadband three-component seismic network installed at Stromboli volcano during 1997. By using decomposition methods in both frequency and time domains, we prove that Strombolian tremor can be described as a linear combination of nonlinear signals in time domain. These "components" are similar to those obtained for explosion quakes, with the only difference being the amplitude enhancement. We characterize each of these nonlinear signals both in terms of their wavefield properties as well as dynamic systems. Moreover, we take into account	the complex processes of magma flow and turbulent degassing, looking at time and amplitude modulation of tremor on a suitable scale. The distribution of tremor amplitudes is Gaussian while the intertimes between the maxima in a suitable scale are described by a Poisson clustered process. Starting from these analyses, a first approximate model for volcanic tremor field can be deduced. The recorded signals, i.e., the elastic vibrations at a point, can be described by a nonlinear equation which gives limit cycles (different observed "nonlinear modes"). This equation is governed by a time-dependent threshold which represents the variability of bubble flux. We take into account some inelasticity in the medium perturbing the elastic potential with a Gaussian function on a suitable scale. It acts as a radiance function modulating the frequency of the limit cycle. This proposed model is able to reproduce waveform, Fourier spectrum, and phase space dimension of one of the extracted nonlinear wave packets.
Febrero de 2008 Dynamic modeling of the 2004 Mw 6.0 Parkfield, California, earthquake Authors: Shuo Ma, Susana Custódio et al Link: Click here Abstract We present two spontaneous rupture models of the 2004 Mw 6.0 Parkfield earthquake constrained by near-source ground motions. We start with a stress drop distribution calculated from a kinematic slip distribution. Using a linear slip-weakening friction law, we utilize trial and error to obtain both the stress conditions and frictional parameters on the fault that produce synthetics consistent with records. The material contrast across the San Andreas Fault is incorporated using different one-dimensional velocity structures on each side of the fault. An	approximately constant S parameter of 0.3 and a uniform slip-weakening distance of 0.15 m are used in the dynamic models. In our preferred dynamic model, consistent with the ground motion and GPS, the slip is bounded by seismicity streaks at 5 and 10 km depths, confirming a locked zone at depth. The stress drop is approximately 10 MPa in the hypocentral region and about 2 MPa elsewhere. The material contrast across the fault causes significant normal stress variations (~1 MPa), leading to a larger strength drop to the southeast than to the northwest. The main rupture front propagates at nearly a constant subshear rupture velocity ~3 km/s in both directions. The total radiated energy determined from the preferred dynamic model is 1.1 × 1013 J, seismic moment is 1.0 × 1018 Nm, and fracture energy is 3.0 × 1013 J. The limited number of aftershocks in the slipped area suggests the important role of stress on the distribution of seismicity in the locked zone.
Enero de 2008 Time reverse modeling of low-frequency microtremors: Application to hydrocarbon reservoir localization Authors: Brian Steiner, Erik H. Saenger et al Link: Click here Abstract Time reverse modeling is applied to synthetic and real low-frequency microtremors measured at the Earth surface with synchronized seismometers. In contrast to previous time-reverse applications, no single event or first arrival time identification is applied for microtremor localization. Synthetic low-frequency microtremors are numerically generated within a small underground area and	modeled with a finite-difference algorithm simulating two-dimensional elastic wave propagation. Time reverse modeling using synthetic microtremors shows that small underground source areas can be accurately located. Different source characteristics which emit mainly P-waves or S-waves vertically influence the localization accuracy. Time reverse modeling is applied to two real microtremor data sets acquired 16 months apart above known oil reservoirs nearby Voitsdorf, Austria, to investigate whether spectral anomalies observed above the reservoirs originate from the reservoirs. Time reverse modeling indicates that low-frequency microtremor signals originate from the reservoir locations and provides a possible method for reservoir localization.
Enero de 2008 Determination of magnetic properties of rocks by analysis of demagnetization curves: Hematite-ilmenite bearing rocks from SW Sweden Authors: Erik Eneroth, Leif Johansson et al Link: Click here Abstract Granulites from the high-grade Sveconorwegian (corresponding to Grenvillian) gneiss terrane of SW Sweden were investigated in order to relate magnetic properties to the granulite facies mineralogy and the amphibolite facies retrograde mineralogy. We used standard paleomagnetic techniques, susceptibility measurements, electron and optical microscopy, and a new way to analyze demagnetization curves based on vector difference sums. The new technique allowed quantitation of the relative contributions of partial natural remanent magnetization (NRM) components as well as numerical extraction of thermal and coercive force (alternating field, AF) unblocking data. Two major components were identified, one stable which is carried by hematite-ilmenite, and one less stable carried by multidomain (MD) magnetite. The age of the	hematite-ilmenite component is circa 930 Ma, and it has steep negative inclination, coercivities in excess of 150 mT, and unblocking temperatures between 540°C and 640°C. Two corresponding poles are reported (1) paleolatitude/paleolongitude (plat/plong) (218°/- 24.3°) with Dp/Dm (9.4°/9.9°) (paleopole) and (2) plat/plong (203°/-51°) with Dp/Dm (4.4°/5.6°) (VGP). The MD magnetite component is a recent or Cretaceous-Paleogene, steeply positive inclined NRM component associated with strong induced magnetization (plat/plong 2°/-74° and Dp/Dm 11°/14°) (VGP). The MD magnetite was unblocked at conditions of 6-40 mT and 60-400°C. Our new analysis method was successful in showing that retrograde amphibole formation strongly decreases the importance of the hematite-ilmenite component on behalf of the MD magnetite. It has a diminishing effect on a negative aeromagnetic anomaly in the area. The 930 Ma components constitute 80-0% of the vector difference sums depending on rock composition. However, as this fraction goes down to 60%, the induced magnetization seems to outweigh the hematite-ilmenite influence on aeromagnetic anomalies due to its opposite direction. These rocks still have a strong and stable hematite-ilmenite dominated NRM.
Enero de 2008 Detection and localization of hydromechanical disturbances in a sandbox using the self-potential method Authors: A. Crespy, A. Revil et al Link: Click here Abstract Four sandbox experiments were performed to understand the self-potential response to hydro-mechanical disturbances in a water-infiltrated controlled sandbox. In the first two experiments, ~0.5 mL of water was abruptly injected through a small capillary at a depth of 15 cm using a syringe impacted by a hammer stroke. In the second series of experiments, ~0.5 mL of pore water was quickly pumped out of the tank, at the same depth, using a syringe. In both type of experiments, the resulting self-potential signals were measured using 32 sintered Ag/AgCl medical electrodes. In two experiments, these electrodes were located 3 cm below the top surface of the tank. In two other experiments, they were placed along a vertical section crossing the position of the capillary. These electrodes were connected to a voltmeter with a sensitivity of 0.1 µV and an acquisition	frequency of 1.024 kHz. The injected/pumped volumes of water produced hydro-mechanical disturbances in the sandbox. In turn, these disturbances generated dipolar electrical anomalies of electrokinetic nature with an amplitude of few microvolts. The source function is the product of the dipolar Green's function by a source intensity function that depends solely on the product of the streaming potential coupling coefficient of the sand to the pore fluid overpressure with respect to the hydrostatic pressure. Numerical modeling using a finite element code was performed to solve the coupled hydro-mechanical problem and to determine the distribution of the resulting self-potential during the course of these experiments. We use 2D and 3D algorithms based on the cross-correlation method and wavelet analysis of potential fields to show that the source was a vertical dipole. These methods were also used to localize the position of the source of the hydromechanical disturbance from the self-potential signals recorded at the top surface of the tank. The position of the source agrees with the position of the inlet/outlet of the capillary showing the usefulness of these methods for application to active volcanoes.
Enero de 2008 Secular variation of the geomagnetic dipole during the past 2000 years Authors: Jean-Pierre Valet, Emilio Herrero-Bervera et al Link: Click here Abstract We have constructed a very simple model of a time-varying geocentric dipole based on the archeomagnetic records obtained at four widely separated sites on the globe for the past 2 ka. The predictions of the model in terms of directional variations have been tested against actual archeomagnetic data from 12 sites distributed over the globe. They were also compared with the Hongre et al. (1998) time-varying spherical harmonic model and with the CALS7K-2 model by Korte et al. (2005). We find that the misfits between the predictions of our simple dipole model and the data are equivalent to those of the spherical harmonic models for the European sites and not strikingly larger for the rest of the world. Many discrepancies can be accounted for by uncertainties inherent to the archeomagnetic records, which, along with the small number and	poor geographical distribution of sites, leads us to conclude that the present state of the database does not allow the extraction of secular variations described by terms going beyond degree 2. It appears also that dipole tilt could be responsible for the main part of the secular variation associated with time constants exceeding 102 years. In a second step, we used the paleointensity records contained in the same database to construct the curve depicting the variations of the true dipole moment. The present decrease of the dipole did not begin prior to 1000 years ago, and the dipole was actually increasing from 0 until A.D. 500. The dipole moment of CALS7K is lower than the present estimate, probably due to large repartition of energy to higher harmonics to minimize the misfit between the inversion and the data. The tilt and strength of the dipole can predict the dipole field at any site and were used to derive the contribution of the nondipole field to values of paleointensity at Paris during the past 2 ka. The results show that the "archeomagnetic jerks" are associated with various configurations depending on the phase relationship between the nondipolar and dipolar parts of the field.
Enero de 2008 Discrete visco-elastic lattice methods for seismic wave propagation Author: G. S. O'Brien Link: Click here Abstract 2D and 3D discrete particle or lattice methods for the simulation of seismic waves are presented for different visco-elastic media. It is demonstrated that a numerical implementation of the method is capable of modelling visco-elastic seismic wave propagation. Lattice methods represent the medium under investigation as particles or nodes	interacting through local force rules. Three schemes are developed, a Maxwell body, a Kelvin-Voigt body and a Zener Body (or standard linear solid). The force acting between particles is chosen to represent each of these different rheologies. Each of these schemes was tested against analytical solutions for wave propagation in 2D and 3D unbounded visco-elastic media. The seismograms generated for each different rheology fit well with the expected theoretical seismograms with the maximum misfit error energy being less than 3% for each different rheology. As such, lattice methods offer an alternative approach to seismic wave modelling in elastic and visco-elastic media.
Enero de 2008 Monitoring mountain permafrost evolution using electrical resistivity tomography: A 7-year study of seasonal, annual, and long-term variations at Schilthorn, Swiss Alps Authors: C. Hilbich, C. Hauck et al Link: Click here Abstract A combined geophysical and thermal monitoring approach for improved observation of mountain permafrost degradation is presented. Time-lapse inversion of repeated electrical resistivity tomography (ERT) measurements allows both active layer dynamics and interannual permafrost conditions to be delineated. Analysis of a comprehensive ERT monitoring data set from a 7- year study at Schilthorn, Swiss Alps, confirmed the applicability of ERT monitoring to observations of	freezing and thawing processes on short-term, seasonal, and long-term scales. Long-term resistivity changes were evaluated on the basis of seasonal resistivity variations showing an annual cycle with high resistivities in frozen and low resistivities in unfrozen state. One important result is the detection of a sustained impact of the extraordinarily hot European summer of 2003 on the permafrost regime, which is more severe than previously assumed from borehole temperatures. Combined analyses of ERT monitoring and borehole temperature data revealed substantial ground ice degradation as a consequence of the 2003 summer, which did not recover in the following years despite suitable subsurface temperature conditions. Resistivity changes that are nonconforming to long-term temperature evolution are attributed to the limited availability of liquid water and/or changes in material characteristics (e.g., pore volume changes as a result of subsidence).

Enero de 2008 Paleomagnetic reorientation of San Andreas Fault Observatory at Depth (SAFOD) core Authors: Josep M. Parés, Anja M. Schleicher et al Link: Click here Abstract We present a protocol for using paleomagnetic analysis to determine the absolute orientation of core recovered from the SAFOD borehole. Our approach is based on determining the direction of the primary remanent magnetization of a spot core recovered from the Great Valley Sequence during	SAFOD Phase 2 and comparing its direction to the expected reference field direction for the Late Cretaceous in North America. Both thermal and alternating field demagnetization provide equally resolved magnetization, possibly residing in magnetite, that allow reorientation. Because compositionally similar siltstones and fine-grained sandstones were encountered in the San Andreas Fault Zone during Stage 2 rotary drilling, we expect that paleomagnetic reorientation will yield reliable core orientations for continuous core acquired from directly within and adjacent to the San Andreas Fault during SAFOD Phase 3, which will be key to interpretation of spatial properties of these rocks.
Enero de 2008 Spatiospectral localization of global geopotential fields from the Gravity Recovery and Climate Experiment (GRACE) reveals the coseismic gravity change owing to the 2004 Sumatra-Andaman earthquake Authors: Shin-Chan Han and Frederik J. Simons et al Link: Click here Abstract Regional mass fluxes owing to transport and adjustment within the Earth system that are implicitly contained in the monthly Gravity Recovery and Climate Experiment (GRACE) global geopotential coefficients are revealed by localizing global spectra using spatiospectrally concentrated window functions. We have analyzed 45 monthly global GRACE harmonic coefficient	series in order to find the coseismic signature associated with the 2004 great Sumatra-Andaman earthquake. A significant gravity change after the earthquake is found in the time series of the GRACE coefficients after localization with a single band-limited window centered near the north of the island of Sumatra. This change is undetectable from the original global coefficients or from coefficients localized elsewhere on the globe. A step function with its discontinuity at 26 December 2004 usefully models the coseismic gravity change. The localized GRACE coefficients contain the jumps (associated with the earthquake) up to degree and order 55, although not all of them within this band produce changes that are statistically significant. The gravity change calculated from the localized GRACE coefficients displays 30 µGal peak-to-peak variations that are very well correlated with an independently derived seismic model based on elastic dislocation theory.
Enero de 2008 Self-potential signals induced by periodic pumping tests Authors: Alexis Maineult, Elmar Strobach et al Link: Click here Abstract We measured the variations of the self-potential (SP) during periodic pumping tests performed at a test site located near a freshwater reservoir (Kemnader See, Bochum, Germany). Successions of injection and production intervals were applied in a borehole penetrating a jointed sandstone aquifer. We report the SP observations for tests with periods ranging between 10 and 60 min and flow rates between 10 and 25 L min−1. The SP responses at the surface exhibit the imposed period but are not truly harmonic contrary	to the hydraulic pressure and SP measured in monitoring wells. In the grassy zone around the injection well, the amplitude of the SP signals decreases with distance from the injection well (around one order of magnitude at 10 m) in rough agreement with predictions for radial flow in a homogeneous medium around an infinite source. The shape of the SP responses also evolves with distance. Fourier spectral analysis reveals that the surface signals generally contain two main components at the main period and at half the period with the relative weight of the subperiodic components increasing with distance. Furthermore, the characteristics of the SP responses depend on whether the boreholes are left open or closed by packers. The comparison between surface and borehole measurements suggests that nonlinear phenomena are acting, probably related to the saturation and desaturation processes occurring in the vadose zone.
Enero de 2008 Automatic reconstruction of fault networks from seismicity catalogs: Three-dimensional optimal anisotropic dynamic clustering Authors: G. Ouillon, C. Ducorbier et al Link: Click here Abstract We propose a new pattern recognition method that is able to reconstruct the three-dimensional structure of the active part of a fault network using the spatial location of earthquakes. The method is a generalization of the so-called dynamic clustering (or k means) method, that partitions a set of data points into clusters, using a global minimization criterion of the variance of the hypocenters locations about their center of mass. The new method improves on the original k means method by taking into account the full spatial covariance tensor of each cluster in order to partition the data set into fault-like, anisotropic	clusters. Given a catalog of seismic events, the output is the optimal set of plane segments that fits the spatial structure of the data. Each plane segment is fully characterized by its location, size, and orientation. The main tunable parameter is the accuracy of the earthquake locations, which fixes the resolution, i.e., the residual variance of the fit. The resolution determines the number of fault segments needed to describe the earthquake catalog: the better the resolution, the finer the structure of the reconstructed fault segments. The algorithm successfully reconstructs the fault segments of synthetic earthquake catalogs. Applied to the real catalog constituted of a subset of the aftershock sequence of the 28 June 1992 Landers earthquake in southern California, the reconstructed plane segments fully agree with faults already known on geological maps or with blind faults that appear quite obvious in longer-term catalogs. Future improvements of the method are discussed, as well as its potential use in the multiscale study of the inner structure of fault zones.
Enero de 2008 A mathematical model for the formation and dissociation of methane hydrates in the marine environment Authors: Sabodh K. Garg, John W. Pritchett et al Link: Click here Abstract To elucidate the geological processes associated with hydrate formation and dissociation in the marine environment under a wide range of conditions, we have developed a one-dimensional numerical computer model (simulator). The numerical model can be used to simulate the following aspects of hydrate formation, decomposition, reformation, and distribution: (1) burial history of deep marine sediments and associated phenomena (e.g., sediment compaction and consequent reduction in sediment porosity and permeability, fluid	expulsion, time evolution of temperature and pressure, heat flux), (2) in situ generation of biogenic methane from buried organic carbon and methane solubility in formation brine, (3) methane hydrate formation, decomposition, reformation, and distribution in response to changes in gas concentration, pressure, temperature and fluid salinity (the hydrate formation and decomposition are treated as equilibrium processes), (4) influence of sulfate reduction zone under the seafloor on hydrate formation, (5) possible presence of a free gas zone beneath the gas hydrate stability zone, and (6) multiphase (i.e., liquid brine with dissolved gas, free gas, gas hydrate) flow through a deformable porous matrix. The model provides for a reduction/increase in permeability due to the formation/decomposition of the gas hydrate. Initial applications of the model to study hydrate distribution at the Blake Ridge (site 997) and Hydrate Ridge (site 1249) are described. Model results are compared with chlorinity, sulfate, and hydrate distribution data.
Enero de 2008 Nickel isotopic anomalies in troilite from iron meteorites Authors: David L. Cook, Robert N. Clayton et al Link: Click here Abstract We measured nickel isotopes, via multicollector inductively coupled plasma mass spectrometry (MC-ICPMS), in troilite (FeS) from ten iron meteorites from both non-magmatic (IAB) and magmatic (IIAB, IIIAB, IVA) groups. These are the	first reported measurements of 64Ni (the least abundant Ni isotope) in meteoritic sulfides. No excesses of 60Ni from the decay of the short-lived radionuclide 60Fe (t1/2 = 1.49 My) were found in any of the samples. Resolvable deficits in 60Ni were observed in five samples. These deficits are inconsistent with the effects from 60Fe decay, given the Fe/Ni ratios in these troilites. Also, variations were found in 61Ni and 64Ni in several samples. The variations in Ni isotopic compositions of these samples may be due to the preservation of a component within the troilite that is characterized by the nucleosynthetic signature expected for Ni produced either by a type II supernova or by an AGB star.
Enero de 2008 Earthquake-generated tsunamis in the Mediterranean Sea: Scenarios of potential threats to Southern Italy Authors: Stefano Lorito, Mara Monica Tiberti et al Link: Click here Abstract We calculated the impact on Southern Italy of a large set of tsunamis resulting from earthquakes generated by major fault zones of the Mediterranean Sea. Our approach merges updated knowledge on the regional tectonic setting and scenario-like calculations of expected tsunami impact. We selected three potential source zones located at short, intermediate and large distance from our target coastlines: the Southern Tyrrhenian thrust belt; the Tell-Atlas thrust belt; and the western Hellenic Arc. For each zone we determined a Maximum Credible	Earthquake and described the geometry, kinematics and size of its associated Typical Fault. We then let the Typical Fault float along strike of its parent source zone and simulated all tsunamis it could trigger. Simulations are based on the solution of the nonlinear shallow water equations through a finite difference technique. For each run we calculated the wavefields at desired simulation times and the maximum water elevation field, then produced traveltime maps and maximum wave-height profiles along the target coastlines. The results show a highly variable impact for tsunamis generated by the different source zones. For example, a large Hellenic Arc earthquake will produce a much higher tsunami wave (up to 5 m) than those of the other two source zones (up to 1.5 m). This implies that tsunami scenarios for Mediterranean Sea countries must necessarily be computed at the scale of the entire basin. Our work represents a pilot study for constructing a basin-wide tsunami scenario database to be used for tsunami hazard assessment and early warning.
Enero de 2008 Did Tsunami-Launched Gravity Waves Trigger Ionospheric Turbulence over Arecibo? Authors: Andrew N. Hock, Nathalie A. Cabrol et al Link: Click here Abstract We report on measurements of ionospheric plasma dynamics conducted at the Arecibo Observatory between 20:00 and 24:00 local time (LT) on December 25 and 26, 2004 using the 430 MHz incoherent scatter radar (ISR). For interpretive purposes these measurements are supported by data from two nearby ionosondes and Global Positioning System (GPS) satellites. The ISR detected different ionospheric behaviors during the vertical-transmission periods on the consecutive, magnetically quiet nights. On the night of December 25 the ionosphere descended smoothly and spread F signatures faded. For about two hours on the following evening the bottomside ionosphere rose by ~ 50 km, inducing plasma irregularities and intense spread F.	Alternating cycles of bottom-side plasma rising and falling persisted through the remainder of the experiments. We postulate that this sinusoidal behavior is a response to gravity waves propagating above Puerto Rico. Nearly simultaneous data from two nearby stations show that GPS signals were modified by variations in total electron content (TEC) indicating the presence of traveling ionospheric disturbances (TIDs). The December 26 experiments were conducted about a day after an MW= 9.2 earthquake launched tsunami waves first across the Indian, then into the Atlantic and Pacific Oceans. We suggest that coupling at the tsunami sea-air interface launched gravity waves that propagated for great distances beneath the mesopause. GPS data recorded TEC variation in Asia, Europe, and the Caribbean, suggesting that TIDs were induced on a global scale at the wake of tsunami-launched gravity waves. Energy from imperfectly ducted gravity waves leaked into the ionosphere, partially over Puerto Rico. The wind-velocity field of these gravity waves caused local ionospheric plasma to rise, seeding bottomside irregularities via the generalized Rayleigh-Taylor instability.
Enero de 2008 Daisyworld: A review Authors: Andrew J. Wood, Graeme J. Ackland et al Link: Click here Abstract Daisyworld is a simple planetary model designed to show the long-term effects of coupling between life and its environment. Its original form was introduced by James Lovelock as a defense against criticism that his Gaia theory of the Earth as a self-regulating homeostatic system requires teleological control rather than being an emergent property. The central premise, that living organisms can have major effects on the climate	system, is no longer controversial. The Daisyworld model has attracted considerable interest from the scientific community and has now established itself as a model independent of, but still related to, the Gaia theory. Used widely as both a teaching tool and as a basis for more complex studies of feedback systems, it has also become an important paradigm for the understanding of the role of biotic components when modeling the Earth system. This paper collects the accumulated knowledge from the study of Daisyworld and provides the reader with a concise account of its important properties. We emphasize the increasing amount of exact analytic work on Daisyworld and are able to bring together and summarize these results from different systems for the first time. We conclude by suggesting what a more general model of life-environment interaction should be based on.
Enero de 2008 Glacial cycles and carbon dioxide: A conceptual model Author: Andrew McC. Hogg Link: Click here Abstract The correlation between Antarctic temperature and atmospheric carbon dioxide concentration is a key feature of Quaternary climate cycles. The cycle is characterised by pronounced temporal asymmetry; with rapid increase in both	temperature and CO2 at the glacial termination. Here I compare observed climate cycles with results from a simple model which predicts the evolution of global temperature and carbon dioxide over the glacial-interglacial cycle. The model includes a term which parameterises deep ocean release of CO2 in response to warming, and thereby amplifies the glacial cycle. In this model, temperature rises lead CO2 increases at the glacial termination, but it is the feedback between these two quantities that drives the abrupt warming during the transition from glacial to interglacial periods.
Diciembre de 2007 Life in the Atacama: A scoring system for habitability and the robotic exploration for life Authors: Andrew N. Hock, Nathalie A. Cabrol et al Link: Click here Abstract The science goals of the Life in the Atacama (LITA) robotic field experiment are to understand habitat and seek out life in the Atacama Desert, Chile, as an analog to future missions to Mars. To those ends, we present a new data analysis tool, the LITA Data Scoring System (DSS), which (1) integrates rover and orbital data relevant to environmental habitability and life detection, and (2) provides a standard metric, or "score" to	evaluate (a) the potential habitability, and (b) the strength of evidence for life at all locales along the rover's traverse. Designed and tested during the 2005 field campaign, first results from the DSS indicate that the three selected sites in the Atacama Desert are generally inhospitable. The strength of evidence for life is positively correlated with potential habitability at two of the three sites. Using factor analysis, we find three factors explain 79.9% of the variance in biological observations and five factors explain 96.2% of the variance in potential habitability across all sites. These factors are used to focus a discussion of scoring variable definitions for future robotic missions in the Atacama and of instrument selection and strategy development for future robotic missions on Earth and Mars.

<< Anterior | Home: www.geofisica.cl | Siguiente >>

geophysics job course geophysics training education geophysics geophysics monitoring geology geophysics borehole geophysics geophysics mining exploration geophysics environmental geophysics geophysics introduction engineering geophysics