High Resistivity Zones Research Articles

Three-Dimensional Audio-Magnetotelluric Imaging including Surface Topography of the Cimabanshuo Porphyry Copper Deposit, Tibet

Cimabanshuo deposit is a newly discovered porphyry copper (Cu) deposit with giant metallogenic potential, found in the western segment of the Gangdese metallogenic belt, Tibet. The average elevation of the deposit is greater than 5500 m and the terrain on which it is found is steep and complex. Therefore, it is untraversed, and the existing exploration works on it are weak. We used 59 AMT sites belonging to an array covering the main, proven mineralization zone and ore bodies of this deposit for an analysis of its underground electrical structure. Dimensionality and strike analysis revealed the apparent three-dimensional (3D) features near the Cu ore bodies. 3D inversion with topography was conducted for the AMT array data. A large range of high-resistivity anomaly (~500–2000 Ωm) appears beneath the proven Cu mineralization zone and ore bodies, which is interpreted as intrusive rocks with potassic alteration. Although containing chalcopyrite, it is characterized by middle–high resistivity due to a low sulfide content and poor connectivity. Moreover, a series of scattered conductors (~10–300 Ωm) around the Cu ore bodies are distributed in the shallow layer from near the surface to ~200 m, possibly indicating phyllic alteration containing pyritization and connected metal sulfides. The proven ore bodies of Cimabanshuo are mainly located at the junction regions between high-resistivity intrusive rocks and high-conductivity sericitization alteration zones. According to this research, the 3D inversion with topography of AMT data can visually display the 3D distribution of intrusive rocks and alteration zones beneath porphyry Cu deposits in high-elevation regions, and provides a reference for further exploration works.

Electrical Imaging of a Shallow Free-Phase Stray Gas Plume from an Abandoned Exploration Well

Geophysical imaging of free-phase gas (FPG) within aquifers is an emerging method for understanding the mechanisms controlling stray gas migration from oil and gas wells. Crystal Geyser is an unsealed and partially cased well that transports stray CO2 gas to the shallow subsurface. Accumulations of subsurface CO2 FPG near Crystal Geyser have been inferred, but the actual location and dimensions remained unclear. Here, the subsurface FPG distribution surrounding Crystal Geyser was characterized by interpreting 2D electrical resistivity images with previous drilling records and field mapping. An approximately 70-metre-wide FPG plume was located laterally between Crystal Geyser’s conduit and the Little Grand Wash Fault. The FPG plume spanned the vertical extent of approximately 20 to 55 metres below the ground surface, located within the Slick Rock Member sandstone with the relatively low permeability Earthy Member silty sandstone acting as a caprock. The FPG plume was identified from an anomalously high resistivity zone within the Slick Rock Member that was not caused by lateral lithofacies changes or fault displacement. The conceptual FPG migration pathways beneath Crystal Geyser are presented, based on the interpreted FPG distribution from the electrical resistivity images combined with previous site characterization and the principles of buoyant FPG migration. FPG accumulates within the Slick Rock Member by buoyant up-dip migration beneath siltstone capillary barriers of the Earthy Member. FPG leaks to the ground surface within high permeability preferential pathways along the Little Grand Wash Fault and the conduit of Crystal Geyser.

Exploring Potable Groundwater Sources Surrounding Manchar Lake

This study was carried out to explore the groundwater quality in Manchar Lake's surroundings, (one of Asia's largest lakes in Sindh Province of Pakistan), to discover sites where potable groundwater is available. To achieve this objective, Vertical Electrical Sounding (VES) was employed at 21 sites in shallow depths by adopting Schlumberger electrode array alignment. The maximum distance chosen between current electrodes (A & B) was 300 m, and 20 m was between potential electrodes (M & N). ABEM terrameter SAS 4000 and IX1D resistivity software was used for data collection and interpretation, respectively. The results revealed that except for two sites (13 and 19), all other sites do not have potable groundwater at any depth. At site 13, one out of three layers lies under a high resistivity zone, while at site 19, two out of four layers lie under a high resistivity zone, which indicates the presence of potable water. To verify VES findings, water samples from 5 trial bores made by hand percussion method were collected and analyzed for Electrical Conductivity (EC) and Total Dissolved Solids (TDS), which revealed saline water from all trial bores. To sum up, potable groundwater is not available in the vicinity of Manchar Lake at shallow depths.

Initial exploration results of the Collins epithermal Au‐base metal prospect, Aceh, Indonesia

AbstractInterpretation of various exploration data, in particular geochemical prospecting, offers a powerful and rapid assessment of grass‐root projects in a green‐field terrain. Here, we present an example of the Collins epithermal prospect in Aceh Province, Indonesia. In this area, the Au+ base‐metal‐bearing sheeted quartz veins (individually mostly 2–4 cm wide), which are controlled by a 250 m wide by 800 m long NNE‐trending structural corridor within Paleogene sandstone and volcanic rocks, are the product of two main stages of deposition. Stage I formed veins with a sliver of cryptocrystalline quartz wall zone followed by an inner zone of comb quartz with interstitial rhombic adularia that terminates in open space. Stage I or main‐stage sulfide mineralization consisting of early galena + sphalerite and later chalcopyrite occurs with the quartz + adularia. Small amounts of galena also occur in the wall zone. Stage II mineralization brecciated Stage I veins and overprinted them with silicification characterized by vuggy texture. Mineralization associated with this episode consists of earlier chalcopyrite + sphalerite + tennantite–tetrahedrite and later, vug‐filling Au–Ag alloy (Ag0.37–0.41Au0.62–0.59). The above mineralized veins are successively flanked by silicic selvages, an illite + chlorite + pyrite ± kaolinite zone and a chlorite + epidote + carbonate + pyrite zone. Local supergene alteration induced replacement of galena by plumbogummite and anglesite and chalcopyrite by covellite. Data from fluid inclusion microthermometry in quartz indicated that the inner zone of Stage I veins formed from fluids with a 2.3 wt% salinity (0.5–3.3 wt% NaCl equivalent), at 174°C (155–211°C). Combining these physico‐chemical parameters with the mineral assemblage, the mineralization occurred under a reduced environment. Rock and soil assays indicate that elevated Au concentrations (up to 16.5 ppm over 1 m) occur along northeast‐trending zones and show a strong correlation with Pb, while Cu (up to 2.58% over 1 m), Zn, As, Sb, and Mo anomalies lie mostly at the periphery. The high‐grade mineralized veins correlate with moderate to high resistivity and chargeability zones, and the pseudosections of such geophysical signals are interpreted as reflecting coalesced or enlarged veins at depth, or inclined veins in other localities. The intermediate sulfidation affinity for Collins points to potential mineralization at depth as well as preservation of Au‐rich and sulfide‐poor zones in the less eroded areas.

Crustal architecture and structural evolution of a Neoarchean sedimentary basin: geological and geophysical evidence from Metal Earth Chicobi transect in the Abitibi Subprovince, Superior Province, Quebec, Canada

Our understanding of crustal architecture and tectonic evolution has advanced due to integrated geological and geophysical research on Precambrian cratons. However, it is still a heated debate whether Archean crust structure developed by an allochthonous model similar to modern plate tectonics or autochthonous model emphasizing in-situ evolution of supracrustal belts. The < ca. 2696 Ma Chicobi belt is a sedimentary basin in the central Abitibi Subprovince of the Superior Province, and as such provides a unique opportunity to contribute to this debate. It comprises both Porcupine-type turbidite and Timiskaming-type polymictic conglomerate and cross-bedded sandstone, and unconformably overlies ca. 2728–2711 Ma metavolcanic rocks, including the Amos Group to the south. The metasedimentary rocks were deposited after an early (D1 > ca. 2696 Ma) deformation event, which is expressed by internally foliated clasts in conglomerate. The Chicobi belt was regionally folded and cut by shear zones during a second (D2) deformation event, and was re-folded by Z-shaped folds during a third (D3) dextral shearing event. The D2 event is constrained between ca. 2696 Ma, the pre-existing age of detrital zircon in the Scapa Group along strike of the Chicobi belt, and ca. 2681 Ma, the new age of the cross-cutting syenitic Gémini-Saint-Éloi Pluton.New high-resolution seismic reflection and magnetotelluric surveys were conducted along the same transect across the Chicobi belt. The seismic reflection survey imaged subhorizontal to shallowly north-dipping reflective bands at depths of 3–7 km. Regional-scale inversion of new and pre-existing gravity data indicates that these reflective bands are linked to the Beauchamp Fault south of the Amos Group. Magnetotelluric inversion model shows high resistivity (>1000 Ω·m) zones under metasedimentary rocks, as well as a synform defined by low resistivity (10–100 Ω·m) zones, which aligns with folded metavolcanic rocks south of the Chicobi belt.Integration of the geophysical data with surface mapping suggests that the upper-middle crustal architecture of the Chicobi belt is dominated by D2 upright folds overlying a fault zone. We suggest that deformation of the Chicobi belt is dominated by thrusting and folding, a deformation style compatible with allochthonous growth of the Abitibi Subprovince.

Geophysical Delineation of Barite-Galena Mineralization Using Coupled Electrical Resistivity (ER) and Induced Polarization (IP) Techniques. A Case Study of Iyamitet, Cross River State, South-South, Nigeria

A combined Electrical Resistivity (ER) and Induced Polarization (IP) techniques were carried out at Iyamitet, Cross-River State Nigeria with the aim of mapping the Barite-Galena mineralization zone within the area. Five traverses were established in orthogonal directions with length of 100 m. The traverses were established in grid format for better coverage of the study area and Dipole-Dipole electrode configuration was adopted for the data acquisition for both ER and IP. Res2Dinvx software was employed for the joint inversion of the data and the resulting 2D resistivity and chargeability images of the subsurface were interpreted qualitatively and semi-quantitatively to locate the mineralized zone. The result of the investigation revealed that the resistivity values of the suspected mineralized zones fall between 1023 ohm-m to 377599 ohm-m and the chargeability falls between 232 msec and 727 msec. The depth to the top of some of the mineralized zones is as shallow as 1.25 m and as deep as 19.8 m in other places. The results of the investigation have indicated the presence of the Barite-Galena ore within the area and this manifested as high resistivity and high chargeability zones along the traverses. The result of this investigation highlights the efficiency of combined geophysical techniques in locating mineralized zones in a basement area.

High-resolution electrical resistivity tomography and seismic refraction for groundwater exploration in fracture hard rocks: A case study in Kanthan, Perak, Malaysia

• High-resolution ERT and seismic results were validated by tube-well drillings. • Sharp vertical boundary between high and low resistivity zones indicates fault zone. • Depression zone in seismic refraction depth profile indicates highly fractured rock. • DLS method of Wenner-Schlumberger + dipole–dipole arrays are the most effective. • Groundwater source is discovered at the intersection of major discontinuities . This research article describes the integration of high-resolution electrical resistivity tomography (ERT) and seismic refraction to explore potential groundwater in fracture carbonate hard rocks. Data Levels Synthesization (DLS) method is developed where the data levels of two different optimized arrays were merged to produce one final subsurface resistivity image to achieve high-resolution ERT. Numerical modeling was initially conducted on three different geoelectrical arrays. The results of before and after applied DLS method were compared to evaluate the effectiveness of each geoelectrical array to delineate fracture and anomalies accurately. Lineament map is constructed beforehand to map the major discontinuities which could possibly be the potential target for water-containing and water-conducting zones in the subsurface. Two geophysical survey lines were conducted where each survey line covered both ERT and seismic refraction methods. Two tube wells were successfully drilled exactly on the fracture zone with a flow rate around 1000 L/h. The potential water-containing zones within the subsurface limestone bedrock were identified as zones of low to moderate geoelectrical resistivity values ranging from 10 to 400 Ωm and V p velocities exceeding 2000 ms −1 . The fault zones were determined by sharp vertical boundary between the high and low resistivities zones in ERT image and lateral decrease in the V p velocities of the limestone bedrock and depressions along the overburden-bedrock interface. The DLS method of Wenner-Schlumberger and dipole–dipole arrays had been proven effective in enhancing the horizontal and vertical resolutions to produce high-resolution ERT image validated with seismic refraction, lineament map, and tube well drillings.

Magnetotelluric data analysis using 2D inversion: A case study from Al-Mubazzarah Geothermal Area (AMGA), Al-Ain, United Arab Emirates

Geothermal manifestations (hot springs) emerge in the Al-Mubazzarah Geothermal Area (AMGA), Al-Ain city, Abu Dhabi Emirate, United Arab Emirates. This paper presents the application and results of a Magnetotelluric (MT) survey, which was carried out in 2017 at the AMGA geothermal field. The MT method was used to investigate the variations in the electrical conductivity beneath the AMGA. This study focuses on characterizing the patterns of subsurface electrical conductivity of the AMGA geothermal reservoir. Dimensionality analysis of the measured MT data indicate that 2D inversion is appropriate for the subsurface resistivity interpretation. The inversion results support a model consisting of three resistivity-defined layers; from top to bottom they are: (1) a shallow layer with resistivity ranging from 10 to 20 Ωm, representing recent alluvial and windblown deposits, (2) a second conductive layer with resistivities less than 10 Ωm, beneath the first layer. This layer is recognized as the Tertiary carbonate sequence in the region, (3) a deep, moderate to relatively high resistive zone, 10–30 Ωm beginning at 800 m depth and reaching 4 km depth in the northern part of the profile, representing Mesozoic basement rocks. The observed moderate to high resistivity zone (10–30 Ωm) in the 2D model may represent a region where the hot groundwaters originated (geothermal reservoir), with the hottest geothermal located at a depth greater than 4 km. The geothermal reservoir zone is also represented by a low to high density contrast and a low to moderate magnetic susceptibility, as indicated in the inverted potential field data models, and confirmed the existence of a north dipping major fault.

Electrical resistivity tomographic detection of the hidden Thakek fault, Northeast Thailand

ABSTRACTThe Khorat plateau covers most of northeast Thailand along with parts of southern Laos. Recent earthquake hazard assessment and other studies of the regional Thakek fault have rarely appeared in publications and reports. The Thakek fault zone in the Laos region includes three main principal fault planes, but it rapidly fades out when the fault passes through Thailand. A geophysical hidden fault study in the Khorat plateau is herein performed in Bung Kan and Nakhon Phanom provinces, Thailand, where the surface expression is difficult to document through conventional geological surveys. Some fault traces have been identified by satellite image interpretation and digital elevation model analysis. Geological field investigation and electrical resistivity tomography surveys were carried out in order to determine an optimal site for trenching. The electrical resistivity tomography results show zones of high and low resistivity interpreted as coarse sediment layers and mudstone, respectively. The maximum vertical fault offset occurring along the fault trace interpreted from the three‐dimensional electrical resistivity tomographic slices is 40 ± 5 m. Based on the electrical resistivity tomography results, a trench was excavated to directly investigate patterns of faulting and deformation. Based on structural geological observations at the trench wall, indications of faulting have been tentatively identified. In particular, faulting appears to have displaced mudstone over Quaternary sediments with vertical offsets &gt;3.5 m cutting the youngest unit. The result has implications for seismic hazard assessment in northeast Thailand.

Detecting fault zone characteristics and paleovalley incision using electrical resistivity: Loma Blanca Fault, New Mexico

We have combined electrical resistivity tomography (ERT), geologic information from boreholes and outcrops, and hydrogeologic data to investigate field-scale fault-zone cementation of the Loma Blanca Fault in the Rio Grande Rift. We have collected electrical resistivity data from 16 transects and geologic samples from 29 boreholes (completed as groundwater wells to 30 m depth) across and around the fault. The 2D ERT profiles, whose interpretations are constrained by geologic data, indicate (1) a high resistivity zone in cemented portions of the fault below the water table and (2) in the unsaturated zone, a low-resistivity feature along the cemented portions of the fault. The high-resistivity zone below the water table is consistent with a 10% reduction in porosity due to the fault zone cementation. With the same porosity in the unsaturated zone, the low-resistivity feature in the cemented fault zone is consistent with saturation &gt;0.7, in contrast to saturation 0.2–0.7 for sediment outside of the cemented fault zone. In addition, subsurface samples and ERT profiles delineate a buttress unconformity (i.e., steeply dipping erosional contact) corresponding to a paleovalley margin. This unconformity truncates the cemented fault zone and separates Pliocene axial-fluvial sand (deposited by an ancestral Rio Grande) from late Quaternary sand and gravel (deposited by the Rio Salado, a Rio Grande tributary). The cemented fault zone in the southern portion of the study area is a hydrogeologic barrier; north of the buttress unconformity, where the cemented fault zone has been removed by erosion, the fault is not a hydrogeologic barrier. The integration of geologic, geophysical, and hydrogeologic observations is key to developing our understanding of this complex system, and it allows us to demonstrate the utility of ERT in detecting subsurface fault-zone cementation.

Subsurface geoelectrical structure from 3-d inversion of magnetotelluric data of Gisenyi geothermal field, western part of Rwanda

In this paper, a set of Magnetotelluric (MT) data collected from 69 stations at the Gisenyi geothermal field in the western part of Rwanda were used to derive a 3D electrical resistivity distribution model to investigate deep geologic structures related to the geothermal resources. The MT method is a geophysical technique commonly used in geothermal exploration projects. The aim of this study is to investigate subsurface electrical resistivity distribution in the Gisenyi geothermal area to gain an understanding of the process related to geothermal activities and to model deep crustal structures. MT data analysis using phase tensors revealed the nature of the subsurface structures of the geothermal area prior to performing a 3D MT inversion. The dimensionality analysis results revealed a 3D conductivity structure in the deeper portion. The ModEM 3D MT inversion code was used to infer the subsurface electrical distribution from the MT responses. The recovered 3D resistivity distribution model showed a conductive region (<10 Ω.m) at deeper depths beneath a thick high-resistivity zone with a resistivity more than 100 Ω.m. The 3D model indicates patches of high and low resistivity at shallow depths related the presence of recent volcanic lavas and basement rocks from the surface to depths of a few hundred meters. The MT model revels important subsurface structures that can be related to the geothermal resource in the area. The deep-seated conductive body is presumably associated to the heat source of the Gisenyi geothermal system.

GPR geophysical method as a remediation tool to determine zones of high penetration resistance of soil

In this study, we demonstrated application of ground penetrating radar towards assessing zones of high penetration resistance (PR) within soil horizons with a view to characterize the subsurface media. Soil attributes changes both in time and space can be difficult to understand. This may be largely due to concealing nature of subsurface media and convectional methods of soil investigations that are usually restricted to observation at the surface and discrete points. Thus making interpolations, inferences and conclusions from such methods inadequate and less reliable. However, ground penetrating radar (GPR) – a geophysical method of investigation was used in this study in lieu of the traditional methods. Soil variability measured in its PR (strength) is related to its compactness-function of bulk density. GPR utilizes electromagnetic (EM) energy in the range of 10 MHz to 3GHz that propagates through the investigative materials. Analysis of the two-way travel time of EM wave gives information about the velocity of the travelling wave. Data were acquired on a loamy silty soil at a farmland in Krakow as a test site simulated with compaction by tractor passes to induce densification of the subsurface layers. Results of the test depicted distinguishing feature of the compacted zones which is reduction in (EM) waveform amplitude and rapid decay of the EM frequency spectra. The research outcome shows swift, cheap and less cumbersome method of soil investigation.

An optimal sample selection-based logistic regression model of slope physical resistance against rainfall-induced landslide

Due to the insufficient ability of the slope to resist deformation and frequent rainfall, geological disasters often occur on the mountain slopes, especially rainfall-induced landslide. However, the method of assessing the ability of mountain slopes to resist rainfall-induced landslide is still not available. Therefore, using a typical example, this study mainly focuses on developing a mapping model for disaster resistance of rainfall-induced landslide in mountain slopes taking data mining of historical slope damage records and conditioning factors into account. To be specific, specifically, 18 assessment factors, elevation, slope gradient, slope aspect, slope position, plane curvature, section curvature, comprehensive curvature, terrain roughness, CRDS, terrain wetness index, sediment transport index, stream power index, micro-landform, lithology, average annual rainfall, NDVI, distance from faults, and distance from streams, are selected as the evaluation factors for the mountain slope disaster resistance. Afterward, a geospatial database is built to assess the disaster resistance of mountain slopes by combining 477 historical rainfall-induced landslides in the study area. Then, a sample dataset is selected with the negative and positive sample ratio of 1:1. An optimal training sample is selected by tenfold cross-validation, while the testing sample is randomly selected from the sample dataset comprising 30% of the sample dataset. A logistic regression (LR) model is subsequently obtained by the optimal training sample, which contributes to the simulation analysis of the disaster resistance of mountain slopes throughout the study area. Finally, the simulation results are classified into five disaster resistance classes: low, relatively low, medium, relatively high, and high. It should be noted that the area of low and relatively low resistance zones accounts for only 25.06% of the total area, but 83.07% of the historical rainfall-induced landslides are located in this region. While the area of relatively high and high resistance zones accounts for 59.26% of the total area, only 7.77% of the historical rainfall-induced landslides are located there. In addition, an examination based on the receiver operating characteristic (ROC) curve shows that the area under curve values of the testing sample, training sample, sample dataset, and geospatial cells are 0.8552, 0.8924, 0.8605, and 0.8527, respectively. It is found that over the last 1.5 years, the location of 10 rainfall-induced landslides was mainly located in the relatively low and low resistance zones. Thus, it can be concluded that the LR assessment model for disaster resistance of mountain slopes, which is based on a data mining analysis of historical data has high stability and reliability in the assessment of mountain slope disaster resistance against rainfall-induced landslides.

Analysis of the potential application of geophysical survey (induced polarization and DC resistivity) to a long-term mine planning in a sulfide deposit

Medium- and long-term mine planning depends on a large series of probes to determine the continuity and content of mineralized zones and the development direction of the exploration. In addition to being logistically complex and onerous, this process is financially wasteful. In this way, Geophysics is an alternative and indirect investigative tool traditionally used in various phases of mineral exploration. This project proposes to assess the possible continuity of a gold and sulfide quartz lode deposit related to an adjacent abandoned open-pit mine by applying DC resistivity and induced polarization methods through electrical tomography. The most promising results indicated a zone of high chargeability (> 20 mV/V) with high potential of mineralization but not coincident to the high resistivity zones. This specific zone was characterized by a vertical tubular stockwork related to vertical fracture joints generated by the stress inversion of the 3rd (NE) and 4th (NW) phases of regional deformation. The integrated analysis resulted in a potential target for future exploration, in addition to the optimization of application of financial resources in probing procedures.

Joint seismological–magnetotelluric investigation of shallow and implosive non-DC and DC earthquakes beneath the gravitationally unstable Heisei-Shinzan Lava Dome, Unzen Volcano, Japan

Shallow, implosive non-double-couple (NDC) earthquakes have been detected beneath Unzen Volcano, Japan, by a routinely operated seismometer network. Here, we analyze the waveforms from 14 temporary seismic stations around Mt. Fugen (Fugendake) and Heisei-Shinzan Lava Dome, which formed during the 1990–1995 eruptions, to further investigate this shallow seismicity. We also estimate the subsurface electrical resistivity structure using broadband magnetotelluric observations and investigate its relationship to the shallow seismicity. We find that (1) implosive NDC earthquakes occur at the craters that were active during the 1990–1991 eruptions, which are currently buried by the approximately 200-m-thick Heisei-Shinzan Lava Dome; (2) the detected NDC earthquakes generally possess downward P-wave arrivals, although upward P-wave arrivals are observed at a few seismic stations; (3) the tensile axes of the NDC earthquakes show various slip directions; (4) repeated NDC earthquakes are in some cases detected within 20 s of the initial event; and (5) a high-resistivity zone exists beneath the NDC earthquakes. We conclude that the implosive NDC earthquakes were generated by the collapse of small, randomly oriented, vapor-filled voids at the two buried craters, with active volcanic conduits still present beneath these craters. The shallow DC earthquakes consist of two clusters that are located 2–3 km below the NDC earthquakes. The focal mechanisms of the shallower cluster (approximately 2 km depth) indicate various fault mechanism, whereas those of the deeper cluster (approximately 3 km depth) indicate strike-slip faulting. The top of the shallower cluster corresponds to the base of the conductive zone, which suggests that the DC earthquakes in the shallower cluster are generated in a zone of high pore pressure that is capped by a hydrothermally altered clay layer.

Alluvium Assessment of Gold-Bearing Layer in Buried Old River Channel by using Electrical Resistivity Method at Batu Melintang, Jeli, Kelantan

The electrical resistivity imaging (ERI) survey was conducted to delineate the distribution of sediment in the gold-bearing layer and the nature of bedrock within the study area at Batu Melintang. The study area is known as a highly potential zone of gold mineralization due to its location within the Bentong-Raub suture zone. Miners are having difficulties to have a proper mining activities plan as the alluvium distribution is not well understood. The objectives of this study are to identify the sediment in the subsurface of the mining area and to locate the high potential deposit containing gold. ERI survey concerns with resistivity (ohm.m) value of rocks in the subsurface. This research was conducted by using the Schlumberger array with the total number of survey lines are four. All the survey line were done in 200m with 2.5m electrode spacing except for survey line 3 which was done at 100m with 1.5m electrode spacing. All the data obtained is processed in RES2DINV software. The results show variable resistivity ranging from 1-4000ohm.m with a depth of investigation is approximately 35m for 200m length of survey line and depth of 16m for 100m length of survey line. Low resistivity zones of 10-800ohm.m show the values for alluvium present in the area while high resistivity zones of 2000-4000ohm.m indicate values for bedrock. Alluvium layer is identified located around 15m depth, whereas the depth of bedrock is about 20m. This survey gives the general idea of the alluvium distribution, old river channel and the depth of bedrock that can be used for volume estimation and facilitates to create a proper sustainable mining plan in the future

Characterization of the unconventional Tuscaloosa marine shale reservoir in southwestern Mississippi, USA: Insights from optical and SEM petrography

This study presents new optical petrography and electron microscopy data, interpreted in the context of previously published petrophysical, geochemical, and mineralogical data, to further characterize the Tuscaloosa marine shale (TMS) as an unconventional reservoir in southwestern Mississippi. The basal high resistivity zone has a higher proportion of Type II sedimentary organic matter than the overlying TMS, indicating it is more prone to oil generation. Optical petrography and electron microscopy reveal a heterogeneous clay matrix with ubiquitous pyrite grains, quartz, feldspar, glaucony, foraminifera, shell fragments, and rarer occurrences of apatite and crinoid fragments as well as liptinite, alginite, inertinite, and vitrinite. Our petrographic observations suggest that higher abundances of detrital quartz grains coupled with minimal authigenic cements result in higher porosity and permeability. However, the TMS is also more clay-rich than other unconventional shale oil and gas plays, which can impair the effectiveness of hydraulic fracture stimulation. Thin section observations reveal alternating clay and calcium carbonate laminae that are interpreted to reflect changes in sediment flux. Planktonic foraminifera indicate an overlying oxygenated water column while benthic inoceramid fragments and pervasive authigenic pyrite suggest anoxic or dysoxic bottom water conditions. Apatite fragments in thin section suggest mixing events and an influx of nutrient-rich sediments. Overall, these observations suggest that a variety of paleodepositional environments occurred in the TMS and the lithofacies diversity resulting from these small-scale depositional cycles makes it difficult to determinatively identify areas conducive to enhanced economic hydrocarbon recovery.

3D Characterization of a Coastal Freshwater Aquifer in SE Malta (Mediterranean Sea) by Time-Domain Electromagnetics

Electromagnetic (EM) geophysical methods are well equipped to distinguish electrical resistivity contrasts between freshwater-saturated and seawater-saturated formations. Beneath the semi-arid, rapidly urbanizing island of Malta, offshore groundwater is an important potential resource but it is not known whether the regional mean sea-level aquifer (MSLA) extends offshore. To address this uncertainty, land-based alongshore and across-shore time-domain electromagnetic (TDEM) responses were acquired with the G-TEM instrument (Geonics Ltd., Mississauga, ON, Canada) and used to map the onshore structure of the aquifer. 1-D inversion results suggest that the onshore freshwater aquifer resides at 4–24 m depth, underlain by seawater-saturated formations. The freshwater aquifer thickens with distance from the coastline. We present 2D and 3D electromagnetic forward modeling based on finite-element (FE) analysis to further constrain the subsurface geometry of the onshore freshwater body. We interpret the high resistivity zones that as brackish water-saturated bodies are associated with the mean sea-level aquifer. Generally, time-domain electromagnetic (TDEM) results provide valuable onshore hydrogeological information, which can be augmented with marine and coastal transition-zone measurements to assess potential hydraulic continuity of terrestrial aquifers extending offshore.

Three-dimensional resistivity structure in MCT zone around Chamoli region, Garhwal Himalaya and its seismogenic implication

Broadband Magnetotelluric (MT) data were recorded at 28 sites in and around Chamoli region of Uttarakhand, India. This dataset was processed and inverted to image the subsurface electrical resistivity of the region and to study its correlation with the seismic velocity model. The dimensionality analysis of the MT data reveals a complex strike direction hinting at the general 3D nature of the impedance tensor. Hence, 3D inversion of MT data was performed using the code ‘AP3DMT’. The final 3D resistivity model was obtained after several 3D inversion experiments. The inverted resistivity model shows broad intra-crustal low resistivity (<10 Ωm) feature distributed in the depth range 4–20 km. Width of this low resistivity feature varies with depth and it is maximum in the depth 8–16 km. Further, there are few high resistivity blocks intervening this broad low resistivity feature. The low resistivity features are interpreted to be a result of the fluids generated by metamorphic dehydration in deeper depths and pushed upward through the fractured zones. The deeper low resistivity zone is associated with the source area of the main shock of Chamoli earthquake of March 29, 1999 (Mb 6.3). The low resistivity features at the shallow depth (2–11 km) are interpreted as the fluid saturated fracture zones associated with swarms of aftershocks of the Chamoli 1999 earthquake. At shallow depths, hypocenters of the swarms and aftershocks are mainly concentrated in the high resistivity zone adjacent to these low resistivity features. However, at the deeper depths hypocenters of the main shock and its aftershocks are in the low resistivity zone, coinciding with the seismogenic source area of Chamoli Earthquake. Out of 423-recorded aftershocks, 336 are located in and around low resistivity zone while remaining 87 aftershocks are located in resistive zone, mainly at shallow depths (<4 km). The low resistivity fluid-filled zone is consistent with the low velocity zone deciphered through earthquake tomography study of the region.

Vertical Electric Sounding of Leachate Contaminant Plumes at a Dumpsite in Obigbo, Rivers State, Nigeria

The quality of underground water at a dumpsite in Obigbo, Rivers State, Nigeria, was investigated by collecting 7 vertical electrical soundings (VES) using Wenner electrode configuration with current electrode spacing (AB/2) ranging from 1.0 m to 300 m. Field data were acquired using the SAS 300C resistivity meter and accessories, coordinates and elevation were measured using the Global Positioning System (GPS) at each sounding station. The VES data showed that the area is composed of clay, sandy clay and sand. The contaminated zones have low resistivity and corresponding high conductivity. Two zones were identified, zones of low resistivity and high resistivity with values of 11.3 Ωm to 21.4 Ωm, and 357.0 Ωm to795.0 Ωm respectively. There is thus, both lateral and downward movement of the contaminant leachate plumes. The results also revealed that the surrounding soil and groundwater around the landfill has not been contaminated to depths exceeding 15.6 m, which is shallower than the productive aquifer depth greater than 37.0 m. Judging from the age of the dumpsite, this site has a good protecting capacity, probably as a result of the presence of sufficient confined layers of clay that have impeded or slowed down the percolation of leachate into the aquifer. The aquifer is the underlying rocks of sand and gravel that are porous which constitute the water body. Keywords : Dumpsite, Resistivity, Sounding, Leachate, Aquifer.