A new gravity data compilation for Venezuela was processed and homogenized. Gravity was measured in reference to the International Gravity Standardization Net 1971, and the complete Bouguer anomaly was calculated by using the Geodetic Reference System 1980 and 2.67 Mg/m3. A regional gravity map was computed by removing wavelengths higher than 200 km from the Bouguer anomaly. After the anomaly separation, regional and residual Bouguer gravity fields were then critically discussed in term of the regional tectonic features. Results were compared with the previous geological and tectonic information obtained from former studies. Gravity and topography data in the spectral domain were used to examine the elastic thickness and depths of the structures of the causative measured anomaly. According to the power spectrum analysis results of the gravity data, the averaged Moho depths for the massif, plains, and mountainous areas in Venezuela are 42, 35, and 40 km, respectively. The averaged admittance function computed from the topography and Free-Air anomaly profiles across Mérida Andes showed a good fit for a regional compensation model with an effective elastic thickness of 15 km.

Javier Sanchez-Rojas

New Bouguer Gravity Maps of Venezuela: Representation and Analysis of Free-Air and Bouguer Anomalies with Emphasis on Spectral Analyses and Elastic Thickness

In the present paper the problem of a polarized cylinder with a small cross-section, which is located at the contact of two horizontal layers with different resistivities, is studied. Such a polarization geometry simulates the self-potential (SP) field produced by a horizontal flow at the contact between the two layers. First, the expression of the self potential at the space domain is derived, applying the image technique. Then, the expression for the Fourier transform of the SP anomaly is found and the behavior of the amplitude spectrum is studied. Based on this study, a direct interpretation method at the spatial frequency domain is proposed, in order to calculate the depth of the flow and the reflection coefficient of the stratified medium. Experimentation with a synthetic model shows that the method works well (small deviations between true and calculated values). When the SP curve contains noise, deviations between calculated and true depths are smaller than those between calculated and true reflection coefficients. The proposed method, which is also applied on SP data from a geothermal system (Mauri et al., 2010), may be useful in detecting underground water or heat flows.

Georgios Aim. Skianis

The Self-Potential Anomaly Produced by a Subsurface Flow at the Contact of Two Horizontal Layers and Its Quantitative Interpretation

Observations from the NASA 10 cm polarimetric Doppler weather radar (NPOL) were used to examine structure, development, and oceanic transition of West African Mesoscale Convective Systems (MCSs) during the NASA African Monsoon Multidisciplinary Analysis (NAMMA) to determine possible indicators leading to downstream tropical cyclogenesis. Characteristics examined from the NPOL data include echo-top heights, maximum radar reflectivity, height of maximum radar reflectivity, and convective and stratiform coverage areas. Atmospheric radiosondes launched during NAMMA were used to investigate environmental stability characteristics that the MCSs encountered while over land and ocean, respectively. Strengths of African Easterly Waves (AEWs) were examined along with the MCSs in order to improve the analysis of MCS characteristics. Mean structural and environmental characteristics were calculated for systems that produced TCs and for those that did not in order to determine differences between the two types. Echo-top heights were similar between the two types, but maximum reflectivity and height and coverage of intense convection (&gt;50 dBZ) are all larger than for the TC producing cases. Striking differences in environmental conditions related to future TC formation include stronger African Easterly Jet, increased moisture especially at middle and upper levels, and increased stability as the MCSs coastally transition.

Bradley W. Klotz

Paul Kucera

Observations of Coastally Transitioning West African Mesoscale Convective Systems during NAMMA

Direct current Resistivity (DCR) method was carried out to characterize the hydrogeological connection between the Tabular Middle Atlas (TMA) and the Saïs Basin. The TMA is one of the most important aquifers in northern Morocco that supplies the deep aquifer of the Saïs Basin. Electrical resistivity tomography (ERT) survey was focused on the Bittit area that is one of the most important outlet discharges, and it is representative of the relations between the TMA and the Saïs Basin. The high resolution capabilities of the electrical tomography were used to define the geological draining features in the framework of water paths from the TMA to the karstic springs. The synthetic data were calculated for the similar model expected in field data inversion and inversion result of these synthetic data used as a guide for the interpretation of the inverse data resistivity sections. Joint interpretation of geophysical, geological, structural, and synthetic simulation data allowed identifying a conductive horizontal shallow layer overlying two subvertical families of fractures of NE-SW and NW-SE directions. This result leads to propose hydrological behaviour of water from the Tabular Middle Atlas and the Saïs Basin at the Bittit Spring, which takes into account for both horizontal flows through stratification joints or karst and through subvertical fractures.

Kh. Qarqori

M. Rouai

F. Moreau

G. Saracco

O. Dauteuil

D. Hermitte

M. Boualoul

C. Le Carlier de Veslud

Geoelectrical Tomography Investigating and Modeling of Fractures Network around Bittit Spring (Middle Atlas, Morocco)

In mature oil fields undergoing enhanced oil recovery methods, such as CO2 injection, monitoring the reservoir changes becomes important. To understand how reservoir changes influence compressional wave (P) and shear wave (S) velocities, we conducted laboratory core experiments on five core samples taken from the Morrow A sandstone at Postle Field, Oklahoma. The laboratory experiments measured P- and S-wave velocities as a function of confining pressure, pore pressure, and fluid type (which included CO2 in the gas and supercritical phase). P-wave velocity shows a response that is sensitive to both pore pressure and fluid saturation. However, S-wave velocity is primarily sensitive to changes in pore pressure. We use the fluid and pore pressure response measured from the core samples to modify velocity well logs through a log facies model correlation. The modified well logs simulate the brine- and CO2-saturated cases at minimum and maximum reservoir pressure and are inputs for full waveform seismic modeling. Modeling shows how P- and S-waves have a different time-lapse amplitude response with offset. The results from the laboratory experiments and modeling show the advantages of combining P- and S-wave attributes in recognizing the mechanism responsible for time-lapse changes due to CO2 injection.

Aaron V. Wandler

Thomas L. Davis

Paritosh K. Singh

An Experimental and Modeling Study on the Response to Varying Pore Pressure and Reservoir Fluids in the Morrow A Sandstone

Using a regional climate model asynchronously coupled to a dynamic vegetation model, this study examines future climate predictions for the West Africa region and how dynamic vegetation feedback may influence such predictions. Without accounting for the impact of vegetation dynamics, the model predicts a future decrease of annual rainfall over Sahel. Dynamic vegetation feedback reverses this trend, leading to a substantial increase of annual rainfall. Regardless of how vegetation is treated, the predicted future trend of precipitation in the Sahel region follows a specific seasonal pattern, with a decrease during the pre- and early-monsoon season (May-June and early July) due to the warming-induced enhancement of spring convective barrier and an increase after the monsoon is fully established (typically in July-August-September) due to enhanced moisture import from a warmer ocean. Dynamic vegetation feedback reduces the magnitude of the predicted rainfall reduction in the early season and increases the magnitude of the predicted rainfall increase later in the rainy season.  The future decrease of early-season rainfall has significant agronomic implications.

Guiling Wang

Clement A. Alo

Changes in Precipitation Seasonality in West Africa Predicted by RegCM3 and the Impact of Dynamic Vegetation Feedback

Constructing a statistical model to characterize the physical conditions associated with large earthquake occurrence is crucial for disaster mitigation. With the aim to formulate such a model, we previously developed a statistical evaluation system which assesses the correlation of the spatiotemporal relationship between different kinds of physical quantities with the occurrence times of large inland earthquakes. In this study, we focused on assessing the relationship between geodetic and seismic quantities and attempted to find the pair of related quantities that most likely indicates preparatory processes of large earthquakes in Japan. We assessed the quantities prior to M &#x2265; 6.0 inland earthquakes for the period of 2001&#8211;2007 in terms of probability gains and error diagram. Our system revealed that the pair of absolute value of dilatation rate and seismic energy showed the highest statistical performance. Further validation of this result is required by updating the database of physical quantities.

Masashi Kawamura

Takeshi Kudo

Koshun Yamaoka

Spatiotemporal Relationship between Geodetic and Seismic Quantities: A Possible Clue to Preparatory Processes of M &#x2265; 6.0 Inland Earthquakes in Japan

Three measuring lines were arranged on one of free planes of magnetite cuboid samples. Apparent resistivity data were acquired by MIR-2007 resistivity meter when samples were under uniaxial compression of servocontrol YAW-5000F loadingmachine in laboratory. Then we constructed the residual resistivity images using electrical resistivity tomography (ERT) and plotted the diagrams of apparent resistivity anisotropy coefficient (ARAC) <svg style="vertical-align:-0.1638pt;width:17.112499px;" id="M1" height="13.95" version="1.1" viewBox="0 0 17.112499 13.95" width="17.112499" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns="http://www.w3.org/2000/svg"><g transform="matrix(.017,0,0,-.017,.062,13.688)"><path id="x1D706" d="M529 97q-70 -109 -136 -109q-41 0 -56 94q-23 144 -37 284q-38 -88 -99 -202.5t-93 -156.5q-26 -8 -76 -19l-9 21q71 78 145.5 193t124.5 232q-5 84 -15 128q-12 55 -29.5 75.5t-42.5 20.5q-21 0 -45 -13l-8 24q16 17 46 30t55 13q43 0 70 -46.5t40 -169.5&#xD;&#xA;q27 -249 51 -392q7 -46 23 -46q24 0 70 60z" /></g><g transform="matrix(.012,0,0,-.012,9.45,5.525)"><path id="x2217" d="M471 153q-22 -15 -61 -13q-25 31 -45.5 49.5t-56.5 41.5q4 -71 28 -134q-17 -33 -42 -46q-24 12 -42 46q24 63 28 134q-36 -23 -56.5 -41.5t-45.5 -49.5q-36 -2 -61 13q0 28 19 59q65 10 130 43q-65 33 -130 43q-19 31 -19 59q22 15 61 13q25 -31 45.5 -49.5t56.5 -41.5&#xD;&#xA;q-4 71 -28 134q17 33 42 46q24 -12 42 -46q-24 -63 -28 -134q36 23 56.5 41.5t45.5 49.5q36 2 61 -13q0 -28 -19 -59q-65 -10 -130 -43q65 -33 130 -43q19 -31 19 -59z" /></g></svg> and the included angle α between the major axis of apparent resistivity anisotropy ellipse and the axis of load with pressure and effective depth. Our results show that with increasing pressure, resistivity and the decreased (D region) and increased (I region) resistivity regions have complex behaviors, but when pressure is higher than a certain value, the average resistivity decrease and the area of D region expand gradually in all time with the increase of pressure, which may be significant to the monitoring and prediction of earthquake, volcanic activities, and large-scale geologic motions. The effects of pressure on <svg style="vertical-align:-0.1638pt;width:17.112499px;" id="M2" height="13.95" version="1.1" viewBox="0 0 17.112499 13.95" width="17.112499" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns="http://www.w3.org/2000/svg"><g transform="matrix(.017,0,0,-.017,.062,13.688)"><use xlink:href="#x1D706" /></g><g transform="matrix(.012,0,0,-.012,9.45,5.525)"><use xlink:href="#x2217" /></g></svg> and α are not very outstanding for dry magnetite samples.

Tao Zhu

Jian-Guo Zhou

Jin-Qi Hao

Experimental Studies on the Changes in Resistivity and Its Anisotropy Using Electrical Resistivity Tomography

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International Journal of Geophysics - Table of contents 2012

International Journal of Geophysics | Hindawi

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International Journal of Geophysics publishes research focused on all areas of theoretical, observational, applied and computational geophysics.

International Journal of Geophysics