Geoelectrical Cross-sections Research Articles

Fault structures and their geoelectric parameters in the epicentral zone of the 27 September 2003 Chuya earthquake (Gorny Altai) from resistivity data

This paper presents an integrated measurement technique based on DC methods (vertical electrical sounding, electrical resistivity tomography) which was used to identify faults and determine their geoelectric parameters in the western part of the Chuya basin. New information on the structure of the Chagan River valley located in the zone of the disastrous 27 September 2003 Chuya earthquake has been obtained from the results of these methods. Geoelectric cross-sections of the sedimentary sequence and the upper part of the basement were obtained from VES data, showing the block structure of the study area. Electrical resistivity tomography sections confirm the presence of a major fault between basement blocks of different heights and indicate the presence of faults bounding the valley on its right side and in the southwestern part.

Improved Structural Mapping and Conductive Targeting Delivered by a new 2.5D AEM Inversion Solver

The advantages of 2.5D (2D geology, 3D source) airborne electromagnetic inversion in 3D geological mapping applications and the identification of conductive drilling targets compared to the more commonly used CDI transforms or simple 1D inversions are demonstrated using examples from different geological settings.The 2.5D inversion application used in this work and described in Silic et al, 2015 is a substantially changed version of ArjunAir, Wilson et al., 2006, a product of CSIRO/AMIRA project P223F. The changes include a new forward model algorithm and a new inversion solver. The application enables the accurate simulation of 3D source excitation for full domain models inclusive of topography, non-conforming boundaries and very high resistivity contrasts. Solution is accurate for a geoelectrical cross-section which is relatively constant along a strike length that exceeds the AEM system footprint.The major innovation includes a new inversion solver with adaptive regularisation which allows the incorporation of a misfit to the reference model and the model smoothness function. The regularisation parameter is chosen automatically and changed adaptively at each iteration, as the model, the sensitivity and the roughness matrices are changing, Silic et al, 2015.Memory usage has been dramatically reduced and provides a usage estimate prior to execution. For speed the software has been parallelised using Intel MPI and can be used on standard computing hardware or computing clusters. Data from survey lines with lengths exceeding 30 kilometres can be inverted on high end laptop computers. The integrated software design allows the user to prepare a full survey inversion then execute this simply in a batch process. The user can visualise inversion progress at any time during process execution.We allow flexibility in the selection of components and in the estimation of noise. A non-specialist can obtain a high value result from our 2.5D AEM inversion in terms of it achieving a more realistic geological section.We show inversion examples from groundwater, minerals (VMS) and geological mapping AEM surveys projects and compare the results with known geology and drilling. We demonstrate the much improved mapping and target definition delivered by this inversion method when compared with the other more common transforms or inversion methods used on these projects.

Combined application of electrical resistivity and GIS for subsurface mapping and groundwater exploration at El-Themed, Southeast Sinai, Egypt

The study area is located at south central part of Sinai Peninsula at El-Thamed area. Geoelectrical resistivity method was used by measuring eighteen vertical electrical soundings using the Schlumberger configuration with total current electrode separation ranging between 3m and 4000m in order to investigate the deep groundwater aquifer and to delineate the subsurface structures of this area.The geoelectrical cross-sections show that the subsurface section consists of six geoelectrical units. The first unit is Quaternary deposits with thickness ranging from 0.6m to 30.9m and the resistivity values ranging from 177Ohm.m. to 39680Ohm.m. The second geoelectrical unit is composed of Egma Formation, with thickness ranging from 9.2m to 12.8m and the resistivity values ranging between 21Ohm.m. and 82Ohm.m. The third geo-electrical unit is composed of Paleocene Esna shale, also not detected at the study area with exception at profile 2, its thickness ranges from 15.6m to 166.5m and resistivity range from 8.9Ohm.m. to 20Ohm.m. This layer has not been detected at the extremely southern part of this profile, its depth ranges from 9.63m to 15.5m. The fourth geoelectrical unit is composed of Nubian Sandstone of Malha Formation (Lower Cretaceous) with depth ranging from 0.5m to 8.9m and its resistivity values ranging from 14Ohm.m. to 133Ohm.m. This layer represents the main aquifer in the study area with thickness ranging between 14.2m and 283m. The fifth geoelectrical unit is composed of claystone of Naqus Formation with depth ranging from 14.2 to 349m and its resistivity values ranging from 2.7Ohm.m. to 13Ohm.m., sometimes, it is extends to the maximum depth of penetration. The sixth geoelectrical unit is composed of Basement rocks. It is characterized by resistivity ranging from 970Ohm.m. to 25462Ohm.m. and depth ranging from 60.1m to 283m. This unit is detected at the eastern part of the study area.Structurally, the study area is affected by sets of faults trending mainly NE-SW (Aqaba Trend).

Shallow groundwater investigation using time-domain electromagnetic (TEM) method at Itay El-Baroud, Nile Delta, Egypt

The Nile Delta is one of the oldest known ancient delta, largest and most important depositional complex in the Mediterranean sedimentary basin. Furthermore, it is a unique site in Egypt that is suitable for accumulation and preservation of the Quaternary sediments. In this work we applied time-domain electromagnetic (TEM) method to investigate the Quaternary sediments sequence as well as detecting the groundwater aquifer in the area of study.A suite of 232 TEM sounding at 43 stations were carried out using a “SIROTEM MK-3” time-domain electromagnetic system. A simple coincident loop configuration, in which the same loop transmits and receives signals, was employed with loop side length of 25m. The 1-D modeling technique was applied to estimate the depth and the apparent resistivity of the interpreted geoelectrical data.Based on the interpretation of the acquired geophysical data, four geoelectric cross-sections were constructed. These sections show that the Upper Quaternary sequence consists of three geoelectric layers. The Holocene Nile mud is separated into two layers: the agricultural root zone (Layer 1) and thick water saturated mud (Layer 2). The Upper Pleistocene sandy aquifer (Layer 3) is very complicated non-linear boundary. This aquifer is the most important unit since it is considered as the main water bearing unit in the study area.

The changes in the electric conductivity of the lithosphere in the source region of the strongest Olyutora earthquake in the Koryak highlands

The results of the magnetotelluric (MT) soundings before and after an earthquake are analyzed. The interpretation is based on the longitudinal and transverse MT curves (along and across the strike of the main tectonic elements, respectively). The MT curves are distorted by the ρ- and coast effect. The distortions due to the coast effect are estimated by the testing three-dimensional (3D) model. It is established that the coast effect distortion at the periods up to 1000 s is small and can be disregarded. The divergence of the longitudinal and transverse MT curves, which points to the presence of the deep faults, is thoroughly studied. The inversion of the MTsounding curves is carried out by the REBOCC program of the numerical two-dimensional modeling. This program implements the procedures of elimination of the ρ-effect and the joint inversion of the longitudinal and transverse MT sounding curves. The obtained geoelectrical cross sections provide an insight into the structure of electrical conductivity of the lithosphere before and after the earthquake. The more intense variations in the electric conductivity are observed in the zone of the deep faults. These variations are related to the changes in the porosity and saturation of the rocks by the highly mineralized fluids.

GEOELECTRICAL ANALYSIS ON THE GROUNDWATER OCCURRENCE FOR THE DEVELOPMENT OF EAST EL MINYA GOVERNORATE, EGYPT

The investigated area is a part of Nile valley which is surrounded by limestone plateaus of Egypt. It is located between Latitudes 28o 15′ - 28o 45’ N and Longitudes 30o 45- 31o 00′ E, and has a surface area of about 1362 Square kilometers. The geoelectrical resistivity measurements have been carried out, using a Schlumberger array to identify the resistivity and thicknesses of different subsurface layers and determine groundwater occurrences. For these purpose, 44 Schlumberger vertical electrical soundings (VES) were conducted with a maximum half current electrode spacing of 700 m. Results from the quantitative interpretation of sounding data indicate that, the area is generally underlain by five main geoelectrical units (A, B, C, D and E). The main characteristics of these geoelectrical layers as well as, the main features of the constructed geoelectrical cross sections have been interpreted in the case of determining the groundwater occurrences. Based on the results obtained, four water-bearing layers (aquifers) were detected in the investigated area. The Quaternary deposits represented by Pleistocene Neonile Sediments of sands and gravels with clay and Shale lenses (A3) act as the first aquifer which represents a limited distribution in the investigated area. The second water-bearing layer (C1) corresponds to water-bearing limestone of Maghagha Formation which lead to the second aquifer and represents also a limited distribution in the investigated area. The third one (D4) corresponds to water-bearing limestone of Samalut Formation which lead to the third aquifer and represents a wide distribution in the investigated area. The fourth water bearing zone (E) corresponds to water-bearing limestone of Minia Formation which lead to the fourth aquifer and represents a wide distribution in the investigated area. Two priority maps for groundwater exploitation in the investigated area have been presented for each of El Minia and Samalut aquifers. It can be concluded that, the priority is generally increasing toward the southern parts, especially at wadi El Saririia area, where El Minia Aquifer occupy the first category and at wadi El Sheikh Hasan where the two main aquifers of Samalut and El Minia facies are well represented

Preliminary hydrogeophysical investigation at the 10th of Ramadan City, Egypt, by 1D and 2D inversion of VES data

ABSTRACTElectrical resistivity survey in the form of vertical electrical sounding was conducted at the 10th of Ramadan City, Egypt, to analyse subsurface lithology distribution and the inter‐relation between the possible infiltrated wastewater from oxidation ponds and the surface seepage emerged at the city centre. Special interest was paid for tracing the low‐resistivity clayey layer, which can act as a barrier against surface water percolation and as a low permeable zone to form local perched water system. Vertical electrical sounding data by Schlumberger electrode configuration were taken at 17 stations along a line extended from the city centre to the oxidation ponds used for wastewater treatment. The data were inverted by using 1D and 2D inversion schemes to construct geoelectrical cross section along the surveyed line. The geoelectrical layers were interpreted principally in terms of lithology and water content by correlation with borehole information. From the obtained results, four layers were identified in the subsurface medium. The upper three layers are located above the regional water table, whereas the fourth one was interpreted to be fully water saturated. The second layer from the surface is of great importance as it is the low‐resistivity clayey layer. This layer is discontinuous along the survey line. Downward infiltration of wastewater from the oxidation ponds is possible especially at the pond No. 2 as the clayey layer was interpreted not to exist at the site. The infiltrated wastewater from the oxidation ponds was interpreted not to be the cause of the surface seepage emerged at the city centre because the clayey layer dips toward opposite direction. The probable sources of the seepage were considered the local surface infiltration from excess irrigation water in the nearby cultivated land and/or leakage from the underground water utilities in the city centre.

GEOHYDROLOGICAL INVESTIGATION USING VERTICAL ELECTRICAL SOUNDING AT BANARAS HINDU UNIVERSITY CAMPUS, VARANASI, U.P, INDIA

Geohydrological investigations are performed to assess the groundwater parameters for locating suitable sites for groundwater exploration and resource management in the main campus of Banaras Hindu University, Varanasi, U.P., India. Nine vertical electrical soundings (VES) using Schlumberger configurations were carried out at selected locations in the main campus of the university. The interpretation of sounding data has been accomplished using both curves matching as well as computer assisted automatic iterative resistivity sounding technique. On basis of interpreted sounding results, four geoelectrical cross sections have been generated along the profiles. The interpretation of data revealed four layers, generally one top thin layer overlying the other three thick layers. Interpreted results are corroborated with the borehole data. The results depict proper geohydrological conditions for existence of good aquifers suggesting continued supply of groundwater in the campus for extended period.

Geoelectrical investigation of seawater intrusion in the coastal urban area of Çanakkale, NW Turkey

Seawater intrusion (SI) constitutes one of the major concerns most commonly encountered in coastal areas. The city of Canakkale, which is situated in the northwestern part of Turkey and built on an alluvial coastal plain, has the second longest coastline in the country. Thus, SI poses a serious threat to shallow freshwater resources in the plain. Direct current resistivity method is the most widely used geoelectrical method for determining low-resistivity zones that usually correspond to the presence of SI. A total of 78 vertical electrical sounding (VES) measurements covering the entire coastal urban area were conducted in 2004 for groundwater exploration. In the present study, 61 soundings displaying an alignment along 13 lines in the direction of E–W were re-interpreted using a two-dimensional (2D) inversion technique based on a smoothness-constrained least squares method to delineate the extend of SI landward. The produced geoelectrical cross-sections successfully revealed an undulating seawater–freshwater interface. The reliability of the estimated solutions was demonstrated by a sensitivity analysis performed by calculating model resolution matrix. The results obtained via 2D inversion of VES data were also correlated well with those obtained from hydrogeological and borehole information within the study area. Consequently, the results of this study indicated that SI characterized by low resistivity (<5 Ωm) is mainly observed in the southern part of the Saricay stream which passes through the city center and flows into the Canakkale Strait (Dardanelles). Moreover, seawater input extends several hundred meters landward.

Geohydrological investigation using Schlumberger sounding in part of hard rock and alluvial area of Ahraura region, Mirzapur district, Uttar Pradesh, India

The study area is located in the part of hard rock and alluvial area of Ahraura region, Mirzapur district, Uttar Pradesh, India. Twenty-two vertical electrical soundings (VES) were conducted using Schlumberger configuration at suitable locations in study area for geohydrological or groundwater investigation. The VES data were interpreted using both techniques: curve matching technique and automatic iterative method. Geoelectrical cross section along the five profiles was conducted based on the results of vertical electrical soundings using Schlumberger configuration. A priori information, detailed survey, and appropriate locations of vertical electrical sounding were done using satellite imagery and integrated geophysical methods. The different types of hydro-resistivity maps are prepared based on the results of VES such as contour map of total transverse resistance and total longitudinal conductance, 3-D surface map of total transverse resistance and total longitudinal conductance, contour map of substratum, and one-grid vector resistant substratum map, giving the information about lithological changes and subsurface geology.

Contribution of Geophysics to the Management of Water Resources: Case of the Ariana Agricultural Sector (Eastern Mejerda Basin, Tunisia)

In Tunisia, the Mejerda basin is the most important agricultural sector. Identification of additional water sources and good management of current and new resources is required to maintain and increase its productivity. The present study concerns the Ariana region (north-east of Tunisia) covering the eastern part of Mejerda basin. Lithological columns, well logs, vertical electrical soundings, and gravity data were analyzed and interpreted to give more precise information for exploitation of the water resources of the Ariana region. Lithological columns, well logs, and geoelectrical models reveal that the permeable Quaternary alluvial deposits, the Upper Campanian–Lower Maestrichtian, Albian and Barremian limestones, and Valanginian–Hauterivian sandstones are the main aquifers in Ariana. Comparison of geoelectrical cross sections to the Bouguer anomaly map of the Ariana region reveals tectonic influence on the geometry of the aquifers. The folded structures attributed to the Tortonian compressive stress and the tectonic movements in N–S, E–W, NW–SE, and NE–SW directions have resulted in the compartmentalization of Ariana’s aquifers into subsided and raised blocks. This reconstituted geometry of the region influenced the depth of burial of permeable lithologies and inhibited groundwater circulation in some localities.

VYUŽITÍ ELEKTRICKÉ ODPOROVÉ TOMOGRAFIE K VIZUALIZACI VÝPLNÍ HISTORICKÝCH STEZEK

Electric resistivity tomography is suitaible method for geophysical survey of shallow subsurface subjects. We selected this method for identification of historical roads relicts. We choosed four localities for our study in the eastern part of Bohemia and north-western part of Moravia. Historical roads are detected by well defined vegetation features and the geoelectric cross sections were drawn across them. The relicts of historical roads are visible on cross resistivity sections. They manifest themselves as anomalous objects with diff erent resistivity. The resistivity is mostly higher than surroundings due to their higher compaction. They are located in depth about 1 m and create oval bodies.

Hydrogeophysical evaluation of aquifer hydraulic characteristics using surface geophysical data: a case study of Umuahia and environs, Southeastern Nigeria

The determination of aquifer hydraulic characteristics of Umuahia area from Dar-Zurrouk parameters was carried out using 30 vertical electrical sounding (VES) data and information from ten well-log data. The Schlumberger electrode configuration was employed for the field data acquisition with a maximum half current electrode separation (AB/2) of 500 m. The VES data were interpreted using the conventional partial curve matching technique to obtain initial model parameters which were used as input for computer iterative modeling using the OFFIX software. Aquifer apparent resistivity in the area varies from 109 to 850 Ωm with a mean value of 253.2 Ωm. The depth to water table varies from 0.7 m around Ajata Ibeku to 82.67 m at Mater Dei Umuahia. The aquifers of the study area are highly variable in thickness with Ajata Ibeku having the least thickness of 1.2 m. The hydraulic conductivity across the area ranges from 0.64 m/day to about 10.22 m/day, while the transmissivity have mean values of 308.16 and 6.257 m2/day for zones A and B, respectively. The mean storativity values of 1.92 × 10−4 and 1.74 × 10−5 for zones A and B, respectively, suggest that zone A has a fairly productive aquifer unlike zone B which revealed very poor hydraulic properties. Information extracted from iso-resistivity models, geo-electric cross-sections, litho logs, and electric logs revealed a sandy clay lithology at very shallow depths with a thick layer of shale/clay extending to over 300 m within zone B. These potential shallow aquiferous units are however perched, partially saturated and may not have sufficient yield.

A geoelectric model for the Uzon caldera, Kamchatka

The results of magnetotelluric and magnetovariational studies in the Uzon caldera are considered. An analysis of magnetotelluric parameters yielded the required method of interpretation. The MTS curves were interpreted in the framework of a 2D model using the REBOCC program. Geoelectric cross sections of the caldera were constructed along two orthogonal lines. Anomalies of high electrical conductivity were identified in the sediments and in the basement and were found to be confined to the locations of geothermal springs. The higher conductivity of these anomalies is here related to the presence of highly mineralized hydrothermal solutions. Electrical conductivity was used for an approximate estimation of porosity in the sediments and basement. A subvertical zone of higher porosity was identified at depths of 1.5–3.5 km in the caldera with a connection to the channelways of fluids rising into the sediments. It is hypothesized that highly mineralized solutions are diluted with vadose water in that zone and come through fissures onto the ground surface in the form of hot springs. The totality of these data suggested a conceptual model to characterize the main features in the generation of hydrothermal springs in the Uzon caldera.

Multi-geophysical Investigation of Geological Structures in a Pre-selected High-level Radioactive Waste Disposal Area in Northwestern China

A potential high-level radioactive waste (HLRW) disposal site in northwestern China was investigated to determine its suitability for such a use. The site is primarily covered with well-developed metamorphic granite rocks. The primary targets for geological repositories are three granite rock masses (I, II, and III). Only surface geological data were available from previous studies. The objective of this study was to evaluate the quality of the rock mass and identify any weak geological structures that could jeopardize this future underground repository. We used gravity and aeromagnetic data on a large scale to study the regional geological structures within and around the three rock masses. Subsequently, in 2009, a controlled source audio-frequency magnetotelluric (CSAMT) survey was conducted to study rock mass I in more detail. This paper introduces three-dimensional (3-D) tomography imaging of gravity and aeromagnetic data. 3-D tomography imaging was carried out on previously collected gravity and aeromagnetic data and, using the results of different depth slices, we evaluated the rock mass quality and interpreted the geology. The aeromagnetic depth slices show that at about 1-km deep in rock masses I and III there is a high magnetic susceptibility body, possibly caused by an early granite intrusion. The deep granite or metamorphic rocks appear as weak `magnetic anomalies in the magnetic slices. The results also indicate that rock mass II is more deeply buried and smaller than rock masses I and III. In the upper crust, inhomogeneous density variations have a strike direction consistent with that of the structure revealed in the aeromagnetic data. For rock mass I, we designed a CSAMT line to map small-scale outcropped faults to depth, and identify any hidden faults or weakened zones in the subsurface. Because of the highly resistive ground, we improved the electrode contact conditions by pouring salt water on both the transmitting and receiving electrodes. During data processing, we applied Hanning filtering to reduce static effects from lack of electrical homogeneity near the ground surface, and through experimentation found an optimum filter length to achieve maximum static effect reduction. Sounding data were inverted for geoelectrical cross sections using a two-dimensional smooth-model inversion method. Inversion artifacts were suppressed by imposing the model smoothness constraint. Several important results were revealed by the inversion: the inversion section correctly recognized two fractures at the surface, and also identified two new faults that were not observed during the previous geological survey. The inversion profile suggests that the narrow factures and deformation bands extend to a great depth. With further drilling confirmation, this interpretation will enhance understanding of the deep geological structures at the HLRW disposal site.

Estimation of subsurface hydrological parameters around Akwuke, Enugu, Nigeria using surface resistivity measurements

As few boreholes may be available and carrying out pumping tests can be expensive and time consuming, relationships between aquifer characteristics and the electrical parameters of different geoelectric layers exist. Data from 19 vertical electrical soundings (VESs; 13 of these selected for evaluation) was recorded with a Schlumberger electrode configuration in the area around Akwuke, Enugu, Nigeria. The data was interpreted by computer iterative modelling with curve matching for calibration purposes. Geoelectric cross-sections along a number of lines were prepared to ascertain the overall distribution of the resistivity responses of the subsurface lithology. Identified probable shallow aquifer resistivity, thickness and depth values are in the range of 28–527 Ωm, 2.1–22.5 m and 3.1–28.3 m respectively. As our aquifer system consists of fine-grained, clay–silty sand materials, a modification of the Archie equations (Waxman–Smits model) was adopted to determine the true formation factor using the relationship between the apparent formation factor and the pore water resistivity. This representation of the effects of a separate conducting path due to the presence of clay particles in the aquifer materials was used in making reliable estimations of aquifer properties. The average hydraulic conductivity of 8.96 × 10-4 m s-1 transmissivity ranging between 1.88 × 10-3 and 2.02 × 10-3 m2 s-1 estimated from surface resistivity measurements correlated well with the available field data. Results of the study also showed a direct relationship between aquifer transmissivity and modified transverse resistance (R2 = 0.85).

Geophysical Evidence through a CSAMT Survey of the Deep Geological Structure at a Potential Radioactive Waste Site at Beishan, Gansu, China

This paper describes a controlled-source audio-frequency magnetotellurics (CSAMT) survey conducted in August 2009 over a potential high-level radioactive waste (HLRW) disposal site in northwestern China. The site is primarily covered with well-developed metamorphic rocks. The purpose of the CSAMT survey was to map the outcropped faults to depth and identify any hidden faults or weakened zone in the subsurface. The site is located in the arid Beishan area, Gansu Province. Substantial challenges were encountered in acquiring quality electrical field data because of the highly resistive ground. Satisfactory electrode contact conditions were generally maintained by applying salt-saturated water to both the transmitting electrodes and the receiving electrodes. The excitation frequency ranges from 9,600 Hz to 1 Hz with a target depth of investigation of 1,000 m. The CSAMT data were processed in several steps. Low-pass filtering was applied to remove the static effect caused by the local electrical inhomogeneities near the ground surface. An optimum filter length was found through experiment to yield the maximum static effect reduction. The pre-processed data are inverted for geoelectrical cross sections using a 2-D inversion method. Inversion artifacts were suppressed by imposing a model smoothness constraint. The inversion reveals several important results. First, the inversion cross sections correctly recognized the fractures and deformation bands mapped at the surface. The cross sections also identified four new faults that were not observed in the geological survey. The inversion profiles suggested that the narrow factures and deformation bands observed along survey line 1 extend to a great depth. The profile helped identify a possible weak mineralization zone along survey line 2. Considering the regional tectonic stress direction, the lower resistivity zone suggests that the faults parallel to survey line 2 are subject to an extensional or transtensional force that produces a broad and broken alteration zone. Although subject to further drilling confirmation, these interpretation results greatly enhance the understanding of the deep geological hazards at the Beishan site.

Geo-Electric and 3D-Imaging of Groundwater Distribution along Flood Plain Deposits of River Niger at Jebba, North-Central Nigeria

Twenty five profiles of Vertical Electrical Sounding were obtained at Jebba area, North-central Nigeria with the aim of ascertain the nature of groundwater distribution in the area. The area is characterized by arid climate of low rainfall and high evaporation. The old alluvial plains surface is covered with a silty-sand and gravel which are subject to new reclamation. The first sedimentary cover unit < 0.5 m is draped by silty-clay causing some of the area under analysis to be water-logged. It is underlain by a sandy layer more than one meter thick beneath the surface. The geo-electric cross-sections produced from the interpreted twenty five VES data using iterative computer program revealed three to four geo-electric layers. The top most layer ranged between 5 ?m-1722 ?m and is suggestive of lateritic clay with a thickness of 0 m-1.5 m. Layer two has thickness of 1.5 m-28 m with an average apparent resistivity of 30 ?m-122 ?m and is suggestive of weathered basement layer. Layer three has a geo-electric layer of thickness between 28 m-45 m with an average apparent resistivity of between 50 ?m-350 ?m which is suggestive of partly weathered/fractured basement and fourth layer suggestive of the fresh basement. The geo-electric sections suggested two system aquifer, weathered layer aquifer system and the partly weathered/fractured basement aquifer system. These aquifer units possess largely good hydraulic characteristics and groundwater potential. This study enables the delineation of shallow aquifers, their subsurface disposition and identified promising areas for elaborate groundwater development in the area. An integration of such hydro-geophysical study with lithologic logs/drilling data would enhance accurate delineation of aquifers of the study area. Another high point of this study is the application of the leapfrog hydro software to prepare 3D Imaging of the sub-surface and to delineate areas high groundwater potentials.

Delineation of Shallow Aquifers of Umuahia and Environs, Imo River Basin, Nigeria, Using Geo-Sounding Data

This work was undertaken to study the hydrogeophysical characteristics of Umuahia and environs,ImoRiver Basinusing geophysical data. Thirty (30) vertical electrical sounding data with a maximum electrode spacing of (AB/2) of 500 meters were acquired using the Schlumberger electrode configuration. The VES data were interpreted using the conventional partial curve matching technique to obtain initial model parameters, which were used as input for computer iterative modelling using the OFFIX software. The study revealed three (3) to six (6) geo-electric layers with underlying shale beds. Aquifer apparent resistivity in the study area ranges from about 50 to 850 Ωm with the aquifer depth varying between 0.53 - 82.67 m. Similarly, the aquifer thickness in the study area ranges from 0.8 - 110 m, with very low thicknesses around Ajata Ibeku. These shallow aquifers with very small thicknesses in the western areas around Ajata Ibeku are believed to be perched or fractured shale units with very low or no yield. Finally, transverse resistances in the study area range from 200 - 3400 Ωm2 with low values in the western part. Information extracted from iso-resistivity models, geoelectric cross sections, litho logs and electric logs revealed a sandy clay lithology at very shallow depths with a thick layer of shale/clay extending to over 300 m within the western part. These shallow aquiferous units are however perched, partially saturated and may not have sufficient yield. Finally, this study has helped to delineate the aquifers of the study into two distinct zones, thus revealing a groundwater divide.

Using Petrophysical Properties of Volcanic Rocks in the Interpretation of Geophysical Data (Volcano Ebeko, Kuril Islands)

Petrophysical properties of volcanic rocks were investigated on the North-Eastern fumarolic field of the volcano Ebeko. The attempt is made to use this data in order to interpret the geo-electrical cross sections of the fumarolic field subsurface space.