ISRN Geophysics The latest articles from Hindawi Publishing Corporation © 2014 , Hindawi Publishing Corporation . All rights reserved. Heat Flow in the Campos Sedimentary Basin and Thermal History of the Continental Margin of Southeast Brazil Sun, 27 Apr 2014 14:27:08 +0000 Bottom-hole temperatures and physical properties derived from geophysical logs of deep oil wells have been employed in assessment of the geothermal field of the Campos basin, situated in the continental margin of southeast Brazil. The results indicate geothermal gradients in the range of 24 to 41°C/km and crustal heat flow in the range of 30 to 100 mW/m2 within the study area. Maps of the regional distributions of these parameters point to arc-shaped northeast-southwest trending belts of relatively high gradients and heat flow in the central part of the Campos basin. This anomalous geothermal belt is coincident with the areas of occurrences of oil deposits. The present study also reports progress obtained in reconstructing the subsidence history of sedimentary strata at six localities within the Campos basin. The results point to episodes of crustal extension with magnitudes of 1.3 to 2, while extensions of subcrustal layers are in the range of 2 to 3. Thermal models indicate high heat flow during the initial stages of basin evolution. Maturation indices point to depths of oil generation greater than 3 km. The age of peak oil generation, allowing for variable time scales for cooling of the extended lithosphere, is found to be less than 40 Ma. Roberta A. Cardoso and Valiya M. Hamza Copyright © 2014 Roberta A. Cardoso and Valiya M. Hamza. All rights reserved. Assessment of Economically Accessible Groundwater Reserve and Its Protective Capacity in Eastern Obolo Local Government Area of Akwa Ibom State, Nigeria, Using Electrical Resistivity Method Mon, 14 Apr 2014 06:49:13 +0000 The application of geophysical method employing vertical electrical sounding (VES) method in combination with laboratory analysis of aquifer sediments has been used to access the economically accessible groundwater reserve and its protective capacity in some parts of Eastern Obolo Local Government area, the eastern region of the Nigerian Niger Delta. Schlumberger electrode configuration was used to sound twelve VES to occupy the areas that have borehole locations and accessibility for the spread of current electrodes to at least 1000 m. Based on the results, the safe and economic aquifer potential has groundwater reserve of about  m3. The desired aquifer thickness and its depth of burial have average value of 52.02 m and 73.14 m, respectively. The area has a fair protective capacity. This is indicated by 58.33% weak, 16.67% moderate, and 25% good protective capacity for the area. This study was done in one of the oil cities, where contaminated Salt River water is used as the major source of water for domestic uses and it is believed that the settlers will appropriate this result and sue for safe groundwater at the indicated depths. N. J. George, E. U. Nathaniel, and S. E. Etuk Copyright © 2014 N. J. George et al. All rights reserved. Water Effects on the First-Order Transition in a Model of Earthquakes Sun, 16 Mar 2014 09:30:44 +0000 The study of 1D spring-block model of earthquake dynamics with consideration of water effects in preexisting fault deals with new forms of frictional force. An analytical study of the equation of motion enables us to establish that motion of geological fault is accelerated by water pressure. In the same setting the critical value of frictional velocity for which appears the discontinuous (first-order) transition from a stick-slip behavior to a creep motion strongly depends on water pressure. The investigation also displays the magnitude and probability of events as a function of water pressure; these two quantities decrease and increase, respectively, with the variation of water pressure. M. W. Dongmo, L. Y. Kagho, F. B. Pelap, G. B. Tanekou, Y. L. Makenne, and A. Fomethe Copyright © 2014 M. W. Dongmo et al. All rights reserved. Effect of Fracture Aperture on P-Wave Attenuation: A Seismic Physical Modelling Study Tue, 11 Feb 2014 07:15:22 +0000 We used the seismic physical modelling approach to study the effect of fracture thickness or aperture on P-wave attenuation, using a laboratory scale model of two horizontal layers. The first layer is isotropic while the second layer has six fractured blocks, each consisting of thin penny-shaped chips of 3 mm fixed diameter and same thickness to simulate a set of aligned vertical fractures. The thickness of the chips varies according to the blocks while the fracture density remains the same in each block. 2D reflection data were acquired with the physical model submerged in a water tank in a direction perpendicular to the fracture strikes using the pulse and transmission method. The induced attenuation was estimated from the preprocessed CMP gathers using the QVO method, which is an extension of the classical spectral ratio method of attenuation measurement from seismic data. The results of our analysis show a direct relationship between attenuation and the fracture thickness or aperture. The induced attenuation increases systematically with fracture thickness, implying more scattering of the wave energy in the direction of increasing aperture. This information may be useful to differentiate the effect caused by thin microcracks from that of large open fractures. Aniekan Martin Ekanem, Xiang Yang Li, Mark Chapman, Main Ian, and Jianxin Wei Copyright © 2014 Aniekan Martin Ekanem et al. All rights reserved. Subsurface and Petrophysical Studies of Shaly-Sand Reservoir Targets in Apete Field, Niger Delta Tue, 10 Dec 2013 19:31:25 +0000 Conventional departures from Archie conditions for petrophysical attribute delineation include shaliness, fresh formation waters, thin-bed reservoirs, and combinations of these cases. If these departures are unrecognized, water saturation can be overestimated, and this can result in loss of opportunity. Wireline logs of four (4) wells from Apete field were studied to delineate petrophysical attributes of shaly-sand reservoirs in the field. Shale volume and porosities were calculated, water saturations were determined by the dual water model, and net pay was estimated using field-specific pay criteria. Ten sand units within the Agbada formation penetrated by the wells were delineated and correlated and their continuity was observed across the studied wells. The reservoirs had high volume of shale (Vcl), high hydrocarbon saturation, low water saturation, and good effective porosity ranging 12.50–46.90%, 54.00–98.39%, 1.61–46.0%, and 10.40–26.80%, respectively. The pay zones are relatively inhomogeneous reservoirs as revealed from the buckle’s plot except in Apete 05. The direction of deposition of the sands was thus inferred to be east west. Empirical relationships apply with variable levels of accuracy with observation of the porosity-depth, water saturation-depth, and water saturation-porosity trends. Core data is recommended for better characterization of these reservoirs. P. A. Alao, A. I. Ata, and C. E. Nwoke Copyright © 2013 P. A. Alao et al. All rights reserved. Error Analysis in Measured Conductivity under Low Induction Number Approximation for Electromagnetic Methods Thu, 05 Dec 2013 13:12:24 +0000 We present an analysis of the error involved in the so-called low induction number approximation in the electromagnetic methods. In particular, we focus on the EM34 equipment settings and field configurations, widely used for geophysical prospecting of laterally electrical conductivity anomalies and shallow targets. We show the theoretical error for the conductivity in both vertical and horizontal dipole coil configurations within the low induction number regime and up to the maximum measuring limit of the equipment. A linear relationship may be adjusted until slightly beyond the point where the conductivity limit for low induction number () is reached. The equations for the linear fit of the relative error in the low induction number regime are also given. George Caminha-Maciel and Irineu Figueiredo Copyright © 2013 George Caminha-Maciel and Irineu Figueiredo. All rights reserved. Day-to-Day Variability of H and Z Components of the Geomagnetic Field at the African Longitudes Thu, 29 Aug 2013 15:58:18 +0000 The Day-to-day variability of the geomagnetic field elements at the African longitudes has been studied for the year 1987 using geomagnetic data obtained from four different African observatories. The analysis was carried out on solar quiet days using hourly values of the Horizontal, , and vertical, , geomagnetic field values. The results of this study confirm that Sq is a very changeable phenomenon, with a strong day-to-day variation. This day-to-day variation is seen to be superimposed on magnetic disturbances of a magnetospheric origin. T. N. Obiekezie, S. C. Obiadazie, and G. A. Agbo Copyright © 2013 T. N. Obiekezie et al. All rights reserved. Theoretical Study on the Flow Generated by the Strike-Slip Faulting Tue, 18 Jun 2013 11:54:21 +0000 The flow driven by the strike-slip faulting is theoretically analyzed in this paper. The surface of the strike-slip fault is generally near vertical, and the corresponding plates move in horizontal directions during the faulting. The focus of present paper is on the flow at the early stage when the faulting is activated. Standard procedures for deriving the exact solution of the induced flow are first demonstrated. Based on the derived solution, flows generated by three kinds of faulting are examined to observe and compare the evolution of velocity profiles and the corresponding kinetic energy. The results show that the flow energy rapidly decays as the speed of the moving plates begins to slow down. Moreover, mathematical methods proposed in this study provide a useful basis for related studies on not only geophysics, but also fluid mechanics, industry manufacturing, heat-conduction problems, and other possible applications. Chi-Min Liu, Ray-Yeng Yang, and Hwung-Hweng Hwung Copyright © 2013 Chi-Min Liu et al. All rights reserved. Streaming Potential and Electroosmosis Measurements to Characterize Porous Materials Tue, 11 Jun 2013 11:58:23 +0000 Characterizing the streaming potential and electroosmosis properties of porous media is essential in applying seismoelectric and electroseismic phenomena for oil exploration. Some parameters such as porosity, permeability, formation factor, pore size, the number of pores, and the zeta potential of the samples can be obtained from elementary measurements. We performed streaming potential and electro-osmosis measurements for 6 unconsolidated samples made of spherical polymer particles. To check the validity of the measurements, we also used alternative analysis to determine the average pore size of the samples and, moreover, used a sample made of sand particles to determine the zeta potential. D. T. Luong and R. Sprik Copyright © 2013 D. T. Luong and R. Sprik. All rights reserved. Estimating the Thickness of Sedimentation within Lower Benue Basin and Upper Anambra Basin, Nigeria, Using Both Spectral Depth Determination and Source Parameter Imaging Tue, 11 Jun 2013 11:50:26 +0000 The Total Aeromagnetic Data covering the study area was subjected to First Vertical Derivative, Spectral Depth Analysis, and Source Parameter Imaging (SPI). The result from the First Vertical Derivative shows that the Northern part of the area is covered by the young biotite granite of Precambrian origin, and the western edge is covered by the old granite, gneisses, and migmatite of Western Nigeria, while the remaining area is covered by the cretaceous sedimentary deposits. The entire area was divided into forty-eight sections. Spectral Depth Analysis was run for each of these forty-eight sections; the result shows that a maximum depth above 7 km was obtained within the cretaceous sediments of Idah, Ankpa, and below Udegi at the middle of the study area. Minimum depth estimates between 188.0 and 452 meters were observed around the basement regions. Results from Source Parameter Imaging show a minimum depth of 76.983 meters and a maximum thickness of sedimentation of 9.847 km, which also occur within Idah, Ankpa, and Udegi axis. The disparity observed in depth obtained by each method is discussed based on the merit and demerit of each method, and the depths obtained were compared with results from previous researchers. Geophysical implication of the result to oil and gas exploration in the area is briefly discussed. Adetona A. Abbass and Abu Mallam Copyright © 2013 Adetona A. Abbass and Abu Mallam. All rights reserved. Thermomagnetic Features of Crust in Southern Parts of the Structural Provinces of Tocantins and São Francisco, Brazil Wed, 27 Mar 2013 16:02:10 +0000 In the present work we report results of a regional scale investigation of the thermal and magnetic characteristics of the crust in the southern sector of the geologic provinces of Tocantins and São Francisco, Brazil. Updated compilations of aeromagnetic and geothermal data sets were employed for this purpose. Use of such techniques as vertical derivative, analytic signal, and Euler deconvolution in analysis of aeromagnetic data has allowed precise locations of the sources of magnetic anomalies and determination of their respective depths. The anomalies in the Tocantins province are considered as arising from variations in the magnetic susceptibilities and remnant magnetizations of alkaline magmatic intrusions of the Tertiary period. The lateral dimensions of the bodies are less than 10 km, and these are found to occur at shallow depths of less than 20 km. On the other hand, the anomalies in the cratonic areas are related to contrasts in magnetic properties of bodies situated at depths greater than 20 km and have spatial dimensions of more than 50 km. Analysis of geothermal data reveals that the cratonic area is characterized by geothermal gradients and heat flow values lower when compared with those of the Tocantins province. Suze Nei P. Guimaraes and Valiya M. Hamza Copyright © 2013 Suze Nei P. Guimaraes and Valiya M. Hamza. All rights reserved. A Simulation Study of the Formation of Large-Scale Cyclonic and Anticyclonic Vortices in the Vicinity of the Intertropical Convergence Zone Wed, 20 Mar 2013 18:30:23 +0000 A regional nonhydrostatic mathematical model of the wind system of the lower atmosphere, developed recently in the Polar Geophysical Institute, is utilized to investigate the initial stage of the origin of large-scale vortices at tropical latitudes. The model produces three-dimensional distributions of the atmospheric parameters in the height range from 0 to 15 km over a limited region of the Earth’s surface. Time-dependent modeling is performed for the cases when, at the initial moment, the simulation domain is intersected by the intertropical convergence zone (ITCZ). Calculations are made for various cases in which the initial forms of the intertropical convergence zone are different and contained convexities with distinct shapes, which are consistent with the results of satellite microwave monitoring of the Earth’s atmosphere. The results of modeling indicate that the origin of convexities in the form of the intertropical convergence zone, having distinct configurations, can lead to the formation of different large-scale vortices, in particular, a cyclonic vortex, a pair of cyclonic-anticyclonic vortices, and a pair of cyclonic vortices, during a period not longer than three days. The radii of these large-scale vortices are about 400–600 km. The horizontal wind velocity in these vortices can achieve values of 15–20 m/s in the course of time. Igor V. Mingalev, Natalia M. Astafieva, Konstantin G. Orlov, Victor S. Mingalev, Oleg V. Mingalev, and Valery M. Chechetkin Copyright © 2013 Igor V. Mingalev et al. All rights reserved. Using Microseismicity to Estimate Formation Permeability for Geological Storage of CO2 Tue, 26 Feb 2013 09:05:28 +0000 We investigate two approaches for estimating formation permeability based on microseismic data. The two approaches differ in terms of the mechanism that triggers the seismicity: pore-pressure triggering mechanism and the so-called seepage-force (or effective stress) triggering mechanism. Based on microseismic data from a hydraulic fracture experiment using water and supercritical CO2 injection, we estimate permeability using the two different approaches. The microseismic data comes from two hydraulic stimulation treatments that were performed on two formation intervals having similar geological, geomechanical, and in situ stress conditions, yet different injection fluid was used. Both approaches (pore-pressure triggering, and the seepage-force triggering) provide estimates of permeability within the same order of magnitude. However, the seepage-force mechanism (i.e., effective stress perturbation) provides more consistent estimates of permeability between the two different injection fluids. The results show that permeability estimates using microseismic monitoring have strong potential to constrain formation permeability limitations for large-scale CO2 injection. D. A. Angus and J. P. Verdon Copyright © 2013 D. A. Angus and J. P. Verdon. All rights reserved. Propagation and Attenuation of Elastic Waves in a Double Porosity Medium Wed, 13 Feb 2013 15:17:09 +0000 This study solves the mathematical model for the propagation of harmonic plane waves in a dissipative double porosity solid saturated by a viscous fluid. The existence of three dilatational waves is explained through three scalar potentials satisfying wave equations. Velocities of these waves are obtained from the roots of a cubic equation. Lone shear wave is identified through a vector potential satisfying a wave equation. The displacements of solid particles are expressed through these four potentials. The displacements of fluid particles in pores and fractures can also be expressed in terms of these potentials. A numerical example is solved to calculate the complex velocities of four waves in a dissipative double porosity solid. Each of the complex velocities is resolved to define the phase velocity and quality factor of attenuation for the corresponding wave. Effects of medium properties and wave frequency are analyzed numerically on the propagation characteristics of four attenuated waves. It seems that and S waves are not very sensitive to the pore/fluids characteristics, except the fracture porosity. Hence, the recovery and analysis of slower (, ) waves become more desired to understand the fluid-rock dynamism in crustal rocks. J. S. Nandal and T. N. Saini Copyright © 2013 J. S. Nandal and T. N. Saini. All rights reserved. Propagation of Rayleigh Wave in a Two-Temperature Generalized Thermoelastic Solid Half-Space Tue, 12 Feb 2013 14:25:11 +0000 The Rayleigh surface wave is studied at a stress-free thermally insulated surface of an isotropic, linear, and homogeneous two-temperature thermoelastic solid half-space in the context of Lord and Shulman theory of generalized thermoelasticity. The governing equations of a two-temperature generalized thermoelastic medium are solved for surface wave solutions. The appropriate particular solutions are applied to the required boundary conditions to obtain the frequency equation of the Rayleigh wave. Some special cases are also derived. The speed of Rayleigh wave is computed numerically and shown graphically to show the dependence on the frequency and two-temperature parameter. Baljeet Singh Copyright © 2013 Baljeet Singh. All rights reserved. Merge-Optimization Method of Combined Tomography of Seismic Refraction and Resistivity Data Mon, 31 Dec 2012 17:32:10 +0000 This paper discussed a novel application called merge-optimization method that combines resistivity and seismic refraction data to provide a detailed knowledge of the studied site. This method is interesting because it is able to show strong accuracy of two geophysical imaging methods based on many of data points collected from the conducted geophysical surveys of disparate data sets based strictly on geophysical models as an aid for model integration for two-dimensional environments. The geophysical methods used are high resolution methods. The resistivity imaging used in this survey is able to resolve the subsurface condition of the studied site with low RMS error (less than 2.0%) and 0.5 metre electrodes interval. For seismic refraction method, high resolution of seismic is used for correlation with resistivity results. Geophones spacing is 1.0 metre and the total number of shot-points is 15, which provides very dense data point. The algorithms of merge-optimization have been applied to two data sets collected at the studied site. The resulting images have been proven to be successful because they satisfy the data and are geometrically similar. The regression coefficient found for conductivity-resistivity correlation is 95.2%. Andy A. Bery Copyright © 2012 Andy A. Bery. All rights reserved. The Dependence of Electrical Resistivity-Saturation Relationships on Multiphase Flow Instability Wed, 17 Oct 2012 11:19:33 +0000 We investigate the relationship between apparent electrical resistivity and water saturation during unstable multiphase flow. We conducted experiments in a thin, two-dimensional tank packed with glass beads, where Nigrosine dyed water was injected uniformly along one edge to displace mineral oil. The resulting patterns of fluid saturation in the tank were captured on video using the light transmission method, while the apparent resistivity of the tank was continuously measured. Different experiments were performed by varying the water application rate and orientation of the tank to control the generalized Bond number, which describes the balance between viscous, capillary, and gravity forces that affect flow instability. We observed the resistivity index to gradually decrease as water saturation increases in the tank, but sharp drops occurred as individual fingers bridged the tank. The magnitude of this effect decreased as the displacement became increasingly unstable until a smooth transition occurred for highly unstable flows. By analyzing the dynamic data using Archie’s law, we found that the apparent saturation exponent increases linearly between approximately 1 and 2 as a function of generalized Bond number, after which it remained constant for unstable flows with a generalized Bond number less than −0.106. Zoulin Liu and Stephen M. J. Moysey Copyright © 2012 Zoulin Liu and Stephen M. J. Moysey. All rights reserved. Study of Rainfall from TRMM Microwave Imager Observation over India Wed, 17 Oct 2012 09:10:15 +0000 This paper presents a technique to estimate precipitation over Indian land (6–36°N, 65–99°E) at spatial grid using tropical rainfall measuring mission (TRMM) microwave imager (TMI) observations. It adopts the methodology recently developed by Mishra (2012) to monitor the rainfall over the land portion. Regional scattering index (SI) developed for Indian region and polarization corrected temperature (PCT) have been utilized in this study. These proxy rain variables (i.e., PCT and SI) are matched with rainfall from precipitation radar (PR) to relate rain rate with PCT, SI, and their combination. Retrieval techniques have been developed using nonlinear relationship between rain and proxy variables. The results have been compared with the observations (independent of training data set) from PR. Results have also been validated with the observations from automatic weather station (AWS) rain gauges. It is observed from the validation results that nonlinear algorithm using single variable SI underestimates the low rainfall rates (below 20 mm/h) but overestimates the high rain rates (above 20 mm/h). On the other hand, algorithm using PCT overestimates the high rain rates (above 25 mm/h). Validation results with rain gauges show a CC of 0.68 and RMSE of 4.76 mm when both SI and PCT are used. Anoop Kumar Mishra and Rajesh Kumar Copyright © 2012 Anoop Kumar Mishra and Rajesh Kumar. All rights reserved. Free Field Surface Motion at Different Site Types due to Near-Fault Ground Motions Sun, 29 Jul 2012 13:08:02 +0000 Seismic hazards during many disastrous earthquakes are observed to be aggravating at the sites with the soft soil deposits due to amplification of ground motion. The characteristics of strong ground motion, the site category, depth of the soil column, type of rock strata, and the dynamic soil properties at a particular site significantly influence the free field motion during an earthquake. In this paper, free field surface motion is evaluated via seismic site response analysis that involves the propagation of earthquake ground motions from the bedrock through the overlying soil layers to the ground surface. These analyses are carried out for multiple near-fault seismic ground motions at 142 locations in Mumbai city categorized into different site classes. The free field surface motion is quantified in terms of amplification ratio, spectral relative velocity, and spectral acceleration. Seismic site coefficients at different time periods are also evaluated for each site category due to near-fault ground motions from the acceleration response spectra of free field surface motion at each site and the corresponding acceleration response spectra at a reference rock outcrop site. Jagabandhu Dixit, D. M. Dewaikar, and R. S. Jangid Copyright © 2012 Jagabandhu Dixit et al. All rights reserved. The Effects of Marine Cloud Brightening on Seasonal Polar Temperatures and the Meridional Heat Flux Tue, 08 May 2012 08:25:17 +0000 Marine cloud brightening (MCB) is one of several proposed solar radiation management (SRM) geoengineering schemes designed to ameliorate some of the undesirable effects of climate change, for example polar ice loss and associated increased sea levels. Satellite measurements over the last 40 years show a general reduction in polar sea ice area and thickness which is attributed to climate change. In our studies, HadGEM1, a fully coupled climate model, is used to predict changes in surface temperatures and ice cover as a result of implementing MCB in a double carbon dioxide concentration atmosphere. The meridional heat flux (MHF) is the mechanism within the earth system for the transport of energy from tropical to polar regions. This poleward transport of heat in a double carbon dioxide atmosphere amplifies the effects in polar regions, where it has a significant impact on both temperatures and ice cover. The results from this work show that MCB is capable of roughly restoring control temperatures and ice cover (where control is defined as 440 ppm carbon dioxide, a predicted 2020 level) in a double carbon dioxide atmosphere scenario. This work presents the first results on the impact of MCB on the MHF and the ability of the MCB scheme to restore the MHF to a control level. Ben Parkes, Alan Gadian, and John Latham Copyright © 2012 Ben Parkes et al. All rights reserved. Structural Interpretation of the Mamfe Sedimentary Basin of Southwestern Cameroon along the Manyu River Using Audiomagnetotellurics Survey Wed, 14 Mar 2012 13:24:25 +0000 Five audiofrequency magnetotelluric AMT soundings were collected northwest-southeast along the Manyu river in the Mamfe sedimentary basin of southwestern Cameroon. The soundings were performed with frequencies in the range 3 to 2500 Hz and covered a distance of approximately 28 km. Sounding curves and geoelectric and geological sections were processed, and the results were compared with rocks' resistivity to characterize the lithostratigraphy of the eastern part of the basin. The results show above 1000 m depth, sedimentary layers with resistivities in the range of 1 to 100 Ohm-m, which decrease with depth. We identified three types of sedimentary rocks: laterite-clay mixture, shale, and sandstones. Various faults were also identified, illustrating the structural complexity of the Mamfe basin, along the Manyu River. Jean Jacques Nguimbous-Kouoh, Eric M. Takam Takougang, Robert Nouayou, Charles Tabod Tabod, and Eliezer Manguelle-Dicoum Copyright © 2012 Jean Jacques Nguimbous-Kouoh et al. All rights reserved.