Depth Slices Research Articles

View-synthesized ‘re-calibration’ of an array projector for 3D measurement from an arbitrary monocular view

We previously presented a method on how to calibrate a high-speed Multi-Aperture Array Projector (MAAP) in order to obtain three-dimensional (3D) surface measurements using only a single camera instead of a stereo-camera configuration. The MAAP calibration procedure however can be time-consuming as it requires the camera to encode each aperture's illumination frustum by imaging the pattern incident on a depth-wise scanned test plane. This is as if recording discretized depth slices of the illumination frustums. This can be detrimental as the calibration is coupled to a single specific camera view. It would be laborious and time-consuming to repeat the entire MAAP calibration process for an alternate camera view. This study demonstrates a so-called MAAP ‘re-calibration’ solution that artificially synthesizes MAAP calibration images from an arbitrary camera view without direct imaging. This presents the opportunity to decouple the MAAP calibration from a single camera view and obtain 3D measurements from alternate camera placement setups without having to perform the entire MAAP calibration process again. This study shows that the synthesized MAAP calibration images resemble well the ground truth images. As a result, the proposed method has similar 3D measurement performance compared to when calibrating the MAAP with true images.

Snapshot Coherence Tomographic Imaging

We demonstrate a high-throughput computation-efficient snapshot coherence tomographic imaging method by combining interferometric coding and compressive sampling. We first encode the depth distribution of a three-dimensional (3D) object into the spectrum of a light field, using the principle of optical coherence tomography (OCT), i.e., through a Michaelson interferometer, which generates an intermediate (x, y, λ) data-cube that encodes the raw (x, y, z) data of the object. We then sample the spectral data using a well-established compressive spectral imaging technique, called the coded aperture snapshot spectral imaging (CASSI), which yields a compressed 2D (x, y) measurement that captures the whole 3D tomographic information of the object. Finally, a developed iterative algorithm and end-to-end deep learning network are used for tomographic reconstruction from the single 2D measurement. Such integration of OCT and CASSI leads to a physically simple and computationally efficient system, allowing us to implement a large data size of more than 2000×2000 pixels in the transverse dimensions and up to 200 pixels (depth slices) in the axial dimension. Owning to the interferometry-based depth sensing mechanism, we achieve a high axial resolution of up to 13 μm within an axial field of view of 1.6 mm. Video-rate visualization of dynamic 3D objects at micrometer scale are shown through several examples.

GPR Prospecting at the Castle of Alceste in San Vito dei Normanni (Brindisi, Italy)

In this contribution we will show the results of a non-invasive GPR prospecting performed in the archaeological site of San Vito dei Normanni, in the outskirts of Brindisi, Apulia region, southern Italy, where the ancient population of the Messapians had an important settlement. The present work was performed in the framework of a school in the field financed by the Basilicata Region. The interpretation of the data has been based on the depth slices, on the processed Bscans and on the a-priori available information is provided. Future excavations will be driven also by the results achieved from this GPR prospecting.

Three-dimensional inversion of DCR data incorporating structural similarity constraint

The three-dimensional inversion of direct current resistivity data is an ill-posed problem, which should to be regularized to obtain a physically meaningful parameter distribution. This is usually realized by introducing additional stabilizing model objective functionals to the optimization problem or constraining the model parameters. The constraints may be defined relative to a reference model which prevents the divergence of parameters from the given reference model. A half-space of homogeneous resistivity, results of a previous survey (i.e. time lapse) or simply a vector of zeros may be assigned as the reference model. The reference model must be defined in accordance with the physical parameter, subject to the study (resistivity in our case). However, high-resolution geophysical images of subsurface, provided by a different method (e.g. ground penetrating radar) could also be used to define a reference model using appropriate tools. In this study, a method is proposed to carry out a three-dimensional constrained inversion of direct current resistivity data using a reference model derived from ground penetrating radar method. We have converted depth slices of radar data volume and the three-dimensional resistivity model into grayscale images to make them comparable on a common basis. An image quality assessment tool, namely the structural similarity index is used to evaluate the similarity/difference of the calculated model parameters with respect to the radar slices belonging to same depth. The method is tested on two different data sets acquired during archaeogeophysical investigations. The constrained inversion of direct current resistivity data revealed the buried anthropogenic structures more clearly, which verifies the use of the proposed methodology.

Seismicity depth distribution analysis in southern Pakistan

This study is focused on analytical analysis of spatial dispersion of seismicity depth in southern Pakistan. The southern Pakistan spreads over Sindh, Balochistan, and frontal offshore areas of the Arabian Sea. Thereby, earthquake-prone sources are diversified in their seismotectonic characteristics such as active Makran subduction zone, transform boundary along Murray ridge and Chaman-Ornch-Nal fault system. The drifting mosaic of lithospheric platform is adding elastic fuel at a rate of 1.5–4 mm/year to the potential faults at shallow, intermediate, and deep levels. Unlike conventional approach, we have fragmented the composite catalog of earthquakes (during 325 BC to 25 February 2020) into different depth slices to reckon comparative seismicity changes at various depth intervals. Seismicity depth patterns are found significant to delineate potential seismotectonic plate boundaries and associated fault network in their intraplate regions. The changes in seismicity characteristics are comprehended with b-value variations for segregated depth windows. The interplate seismicity is observed in various clusters of shallow to deep focal depths, reflecting the seismically active zones of respective plate margins. Critical analysis of focal depths unveiled some anomalous deep focal depths for several intraplate earthquakes located in southern Sindh region. It is suggested to re-estimate focal depth anomalies by identifying uncertainties in data recording procedures or investigate physical mechanisms which synchronized with seismotectonic environments of the region.

Application of geoelectrical resistivity method for the assessment of shallow aquifer quality in landfill areas.

In this article, a valuable approach utilizing the relationship between select physical water and soil characteristics and geoelectrical resistivity data was used to recognize and trace groundwater contamination by using the geoelectrical resistivity data of a landfill area. It can reduce uncertainty in geoelectrical resistivity interpretation. By interpreting and calibrating the resistivity model with the lithology and physical characters of water samples, it was possible to identify the unique paths of landfill leachate that occurred throughout a shallow aquifer. The water physical property analysis showed that the landfill area was contaminated by a relatively high amount of total of dissolved solids (TDSs). A scatter plot of TDS values and directly measured resistivity showed that resistivity decreased with increasing TDSs. The movement direction of the landfill leachate in the aquifer system was clearly observed in a depth slice of the resistivity distribution. The aquifer is considered to be contaminated starting from the landfill zone and extending to the northeastern part of the study area.

Object-Based Image Analysis of Ground-Penetrating Radar Data for Archaic Hearths

Object-based image analysis (OBIA) has been increasingly used to identify terrain features of archaeological sites, but only recently to extract subsurface archaeological features from geophysical data. In this study, we use a semi-automated OBIA to identify Archaic (8000–1000 BC) hearths from Ground-Penetrating Radar (GPR) data collected at David Crockett Birthplace State Park in eastern Tennessee in the southeastern United States. The data were preprocessed using GPR-SLICE, Surfer, and Archaeofusion software, and amplitude depth slices were selected that contained anomalies ranging from 0.80 to 1.20 m below surface (BS). Next, the data were segmented within ESRI ArcMap GIS software using a global threshold and, after vectorization, classified using four attributes: area, perimeter, length-to-width ratio, and Circularity Index. The user-defined parameters were based on an excavated Archaic circular hearth found at a depth greater than one meter, which consisted of fire-cracked rock and had a diameter greater than one meter. These observations were in agreement with previous excavations of hearths at the site. Features that had a high probability of being Archaic hearths were further delineated by human interpretation from radargrams and then ground-truthed by auger testing. The semi-automated OBIA successfully predicted 15 probable Archaic hearths at depths ranging from 0.85 to 1.20 m BS. Observable spatial clustering of hearths may indicate episodes of seasonal occupation by small mobile groups during the Archaic Period.

An Integrated Geophysical and Geochemical Investigation of an Open Dumpsite in a Sedimentary Formation

The extents of migration of leachate plumes and the probabilistic assessment of human health risks were carried out on Oke-Diya dumpsite, Sagamu, Southwestern Nigeria. Geophysical methods involving Very Low Frequency-Electromagnetic (VLF-EM) and Electrical Resistivity Topography (ERT) techniques, and Atomic Absorption Spectrometric (AAS) geochemical method were used to map and detect the presence and extent of leachate migration. The VLFEM pseudo-sections revealed conductive zones of responses 25 - 50 to a depth of about 15 m and approximated distance 50 m from the study area. The 2D and 3D ERT models delineated high conductive zones with very low resistivity values ranging from 0.551-6.54 and 2.4respectively. These conductive regions were observed to depths above 13.4 m and approximated distance 60 m from the study area in the 2D models. The 3D sections comprising of the horizontal depth slices, vertical planes and volumetric displays revealed regions with higher conductivity to a depth of about 21.9 m and approximated distance 50 m from the dumpsite. The mean concentrations of heavy metals Pb, Ni, Cd, Zn, Fe and Cu in the soil samples were higher than the concentrations in the control samples. The human health risk assessment indicated unacceptable values for non-carcinogenic adverse effects for both children and adults. The probabilistic cancer risks were within the acceptable range for Pb and unacceptable for Cd for both the children and adults. Generally, this study had identified areas of concentrated plumes accumulation and migration pathways, which were in agreement with earlier studies in sedimentary terrain.

Imaging contamination surrounding a wastewater dumpsite using 2 terrain conductivity meters

Terrain conductivity meters are effective tools for imaging shallow groundwater and soil contamination; however, the capability of different meters as well as the comparability of their results is a controversial issue. We used the EMP-400 Profiler and EM-31 terrain conductivity meters to investigate the soil and groundwater contamination around a wastewater lake in Wadi Uranah, western Saudi Arabia. Both devices are used simultaneously to collect apparent conductivity data along 19 profiles with a total length of 40 kilometers and the collected data are subjected to similar processing routines and plotting parameters. The results show that despite differences in the absolute conductivity values acquired by both meters, they produce significantly comparable subsurface images of apparent conductivity distribution. The results also show that each meter has a set of advantages and disadvantages. The EMP-400 Profiler is lighter, smaller, and easier to handle and operate in field and has the capability to operate at 3 different frequencies, yielding images at variable depth slices. On the other hand, subsurface apparent conductivity images provided by the EM-31 show apparent conductivity images with more details those of the EMP-400 Profiler. The apparent conductivity maps, collected by both meters, confirm that the flow and accumulation of contaminants in the area are structurally controlled and trending the northeast-southwest direction.

Subsurface electrical resistivity modelling over a suspected fault zone at Ojirami, Southwestern Nigeria

The subsurface of a suspected fault zone at Ojirami Southwestern Nigeria was investigated using the four-electrode array model of the electrical resistivity method of geophysical prospecting. Measurements were taken along four traverses using the 1-D and 2-D electrical resistivity field techniques. The Schlumberger and dipole–dipole electrode configurations were respectively utilized for each of the techniques. The 1-D data were interpreted by partial curve matching and computer iteration using the IPI2Win® software. The data obtained from the 2-D ERI were processed and migrated using the Dipro for Windows® 4.0 inversion software. Inversion was done using the finite element modelling (FEM) method. The second-order smoothness constraint was used and five iterations were carried out on each data set. Three 2-D resistivity models and one geoelectric section were generated from the inverted resistivity data. A resistivity depth slice map of the depth range of 2.5–5 m was also generated for the assessment of the spatial configuration of the suspected fault zone. Geoelectric units which include the topsoil/outcrop, weathered/faulted bedrock and the fresh bedrock were delineated by the 2-D resistivity models and the geoelectric section. The suspected fault zone manifested as zone of comparatively low resistivity (8–81 Ω-m) within two resistive (800 to > 5000 Ω m) basement bedrocks on the resistivity images and geoelectric section. The resistivity depth slice map also revealed the fault zone to be a low resistivity discontinuity within two flanks of fresh bedrocks. The study concluded that the study area is indeed a fault zone.

Hydro-geologic, Corrosivity and Geotechnical Implications of Geoelectric Sounding Survey at FUTA Cooperative Housing Estate, Ilaramokin near Akure Southwestern Nigeria


 Aim: Groundwater potential evaluation, corrosivity and geotechnical competence investigations of the subsurface layers were carried at FUTA Cooperative Housing Estate, Ilaramokin near Akure Southwestern Nigeria in order to facilitate proper positioning of facilities within the estate.
 Methodology: A total of 14 vertical electrical sounding (VES) data was acquired across the estate using Schlumberger array.
 Results: The VES survey results delineated 3 - 5 geoelectric layers across the estate which corresponds to the topsoil, weathered layer, partially weathered basement, partially fractured basement and the presumed fresh bedrock. The groundwater potential map of the estate revealed that the groundwater potential is moderate (GWP of 0.5 - 0.75) to high (GWP of 0.75 - 1.0) in the western and southeastern parts of the estate. The depth slice corrosivity maps (1.0, 2.0 and 3.0 m) of the estate indicated that the corrosivity of the subsurface layers within the estate reduces with depth. The 0.5 m depth slice isoresistivity map of the estate indicates that the central and a segment of the southwestern parts of the estate are characterized with low resistivity (50 - 100 Ωm) indicating incompetence, while the estate flanks (west and east) are considered to be moderately competent (100 - 350 Ωm). Likewise, the 1.0 m depth slice isoresistivity map of the estate reveals that the central and western parts of the estate are characterized with low resistivity (50 - 100 Ωm) suggesting incompetent layer, while the northern, southern and eastern parts of the estate are moderately competent (100 - 350 Ωm).
 Conclusion: These results suggested that structural failures can be expected on any engineering structures sited at the central, northern, southern and eastern parts of the estate. Groundwater efforts should be concentrated at the western and southeastern parts of the estate.

Investigate the layout and age of a large‐scale mausoleum in Hangzhou, China using combined geophysical technologies and archaeological documents

AbstractGeophysical techniques are used to detect mounds and burial chambers, but detecting the shape and layout of large mausoleums containing main tombs and ancillary facilities is a developing field. The shape and layout of mausoleums are closely related to cultural practices. In this study, integrated geophysical technologies were used to survey a large‐scale mausoleum in Hangzhou, China. The layout of the main tomb was deduced from magnetic gradient measurements, and the depth and scale were inferred from electrical resistivity tomography (ERT). The location and burial depth of ancillary facilities, including the mausoleum path, were characterized using the depth (time) slice of ground‐penetrating radar (GPR) three‐dimensional (3D) attribute analysis. An integrated interpretation of geophysical results combined with archaeological documents provides the plan and section layouts of the mausoleum. Based on integrated analyses and archaeological documents, the age of the mausoleum was inferred (middle to late Southern Song Dynasty). Subsequent test excavations partly confirmed the results and showed that the combination of geophysical technologies and archaeological documents are effective and valuable for detecting large‐scale mausoleums.

3-D P-wave velocity structure of oceanic core complexes at 13°N on the Mid-Atlantic Ridge

SUMMARY The Mid-Atlantic Ridge at 13°N is regarded as a type locality for oceanic core complexes (OCCs), as it contains, within ∼70 km along the spreading axis, four that are at different stages of their life cycle. The wealth of existing seabed observations and sampling makes this an ideal target to resolve contradictions between the existing models of OCC development. Here we describe the results of P-wave seismic tomographic modelling within a 60 × 60 km footprint, containing several OCCs, the ridge axis and both flanks, which determines OCC crustal structure, detachment geometry and OCC interconnectivity along axis. A grid of wide-angle seismic refraction data was acquired along a series of 17 transects within which a network of 46 ocean-bottom seismographs was deployed. Approximately 130 000 first arrival traveltimes, together with sparse Moho reflections, have been modelled, constraining the crust and uppermost mantle to a depth of ∼10 km below sea level. Depth slices through this 3-D model reveal several independent structures each with a higher P-wave velocity (Vp) than its surrounds. At the seafloor, these features correspond to the OCCs adjacent to the axial valley walls at 13°20′N and 13°30′N, and off axis at 13°25′N. These high-Vp features display dipping trends into the deeper crust, consistent with the surface expression of each OCC's detachment, implying that rocks of the mid-to-lower crust and uppermost mantle within the footwall are juxtaposed against lower Vp material in the hangingwall. The neovolcanic zone of the ridge axis has systematically lower Vp than the surrounding crust at all depths, and is wider between OCCs. On average, throughout the 13°N region, the crust is ∼6 km-thick. However, beneath a deep lava-floored basin between axial OCCs the crust is thinner and is more characteristically oceanic in layering and velocity–depth structure. Thicker crust at the ridge axis suggests a more magmatic phase of current crustal formation, while modelling of the sparse Moho reflections suggests the crust–mantle boundary is a transition zone throughout most of the 13°N segment. Our results support a model in which OCCs are bounded by independent detachment faults whose dip increases with depth and is variable with azimuth around each OCC, suggesting a geometry and mechanism of faulting that is more complicated than previously thought. The steepness of the northern flank of the 13°20′N detachment suggests that it represents a transfer zone between different faulting regimes to the south and north. We propose that individual detachments may not be linked along-axis, and that OCCs act as transfer zones linking areas of normal spreading and detachment faulting. Along ridge variation in magma supply influences the nature of this detachment faulting. Consequently, not only does magma supply control how detachments rotate and migrate off axis before finally becoming inactive, but also how, when and where new OCCs are created.

Multiscale phase inversion for 3D ocean‐bottom cable data

ABSTRACTWe invert three‐dimensional seismic data by a multiscale phase inversion scheme, a modified version of full waveform inversion, which applies higher order integrations to the input signal to produce low‐boost signals. These low‐boost signals are used as the input data for the early iterations, and lower order integrations are computed at the later iterations. The advantages of multiscale phase inversion are that it (1) is less dependent on the initial model compared to full waveform inversion, (2) is less sensitive to incorrectly modelled magnitudes and (3) employs a simple and natural frequency shaping filtering. For a layered model with a three‐dimensional velocity anomaly, results with synthetic data show that multiscale phase inversion can sometimes provide a noticeably more accurate velocity profile than full waveform inversion. Results with the Society of Exploration Geophysicists/European Association of Geoscientists and Engineers overthrust model shows that multiscale phase inversion more clearly resolves meandering channels in the depth slices. However, the data and model misfit functions achieve about the same values after 50 iterations. The results with three‐dimensional ocean‐bottom cable data show that, compared to the full waveform inversion tomogram, the three‐dimensional multiscale phase inversion tomogram provides a better match to the well log, and better flattens angle‐domain common image gathers. The problem is that the tomograms at the well log provide an incomplete low‐wavenumber estimate of the log's velocity profile. Therefore, a good low‐wavenumber estimate of the velocity model is still needed for an accurate multiscale phase inversion tomogram.

GPR and ERT explorations at the archaeological site of Kılıç village (Isparta, SW Turkey)

An archaeo-geophysical survey was conducted through ground-penetrating radar (GPR) and electrical resistivity tomography (ERT) techniques at the archaeological site of Kılıç village located 45 km from the cities of Isparta and Burdur in the Lakes Region of SW Turkey. In the exploration, a GPR reflection profiling survey was primarily conducted over 72 profiles 0.5 m apart using a 500 MHz shielded antenna in the S-N direction. The presence of three-aisled basilica displaying a central nave with an aisle on each side and an apse at the end was successfully revealed. Based on the GPR results, the width of the inner and outer walls of the basilica are about 3 and 1.7 m, respectively. An ERT survey was then carried out via Wenner-Schlumberger array on 37 parallel profiles 1 m apart in NW–SE direction. Depth slices obtained from both two- and three-dimensional inversions of apparent resistivity data were considered in the evaluation. The ERT findings displaying the main structure of the basilica support the results of the GPR survey. Since Kılıç village may be a part of the ancient Baris town according to some views, these findings together with the previous one obtained from about 100 m from the present area may indicate new assessments on the subject. Therefore, we strongly suggest a more detailed archaeological investigation in the village by considering the risk of damage due to illegal excavations.

Working with Gaussian Random Noise for Multi-Sensor Archaeological Prospection: Fusion of Ground Penetrating Radar Depth Slices and Ground Spectral Signatures from 0.00 m to 0.60 m below Ground Surface

The integration of different remote sensing datasets acquired from optical and radar sensors can improve the overall performance and detection rate for mapping sub-surface archaeological remains. However, data fusion remains a challenge for archaeological prospection studies, since remotely sensed sensors have different instrument principles, operating in different wavelengths. Recent studies have demonstrated that some fusion modelling can be achieved under ideal measurement conditions (e.g., simultaneously measurements in no hazy days) using advance regression models, like those of the nonlinear Bayesian Neural Networks. This paper aims to go a step further and investigate the impact of noise in regression models, between datasets obtained from ground-penetrating radar (GPR) and portable field spectroradiometers. Initially, the GPR measurements provided three depth slices of 20 cm thickness, starting from 0.00 m up to 0.60 m below the ground surface while ground spectral signatures acquired from the spectroradiometer were processed to calculate 13 multispectral and 53 hyperspectral indices. Then, various levels of Gaussian random noise ranging from 0.1 to 0.5 of a normal distribution, with mean 0 and variance 1, were added at both GPR and spectral signatures datasets. Afterward, Bayesian Neural Network regression fitting was applied between the radar (GPR) versus the optical (spectral signatures) datasets. Different regression model strategies were implemented and presented in the paper. The overall results show that fusion with a noise level of up to 0.2 of the normal distribution does not dramatically drop the regression model between the radar and optical datasets (compared to the non-noisy data). Finally, anomalies appearing as strong reflectors in the GPR measurements, continue to provide an obvious contrast even with noisy regression modelling.

Mapping of subsurface contamination zone using 3D electrical resistivity imaging in Hilla city, Iraq

Electrical resistivity imaging (ERI) method is very useful in mapping subsurface contamination zones. In this study, 3D ERI is used to identify the distribution and depth of subsurface soil contamination zone near Al_Furat state company for chemical and pesticides industries in Hilla city, Iraq. A 3D pure image of electrical resistivity distribution is obtained from a perpendicular square grid (80 × 80 m) which consists of three parallel and three perpendicular lines. The electrical resistivity values range from < 1 to 21 O.m, and the total root mean square (RMS) after four iterations is about 7%. The image gives eight different depth slices for the estimated model with a depth interval of about 1 m. The maximum depth of investigation is 13.7 m. The results mapped the contamination zone, where high electrical resistivity values (about 21 O.m) are observed possibly due to accumulation of alkaline waste disposed from the company. The ERI results show that the subsurface layers up to a depth of 4.99 m are of moderate-to-high electrical resistivity values. The study reveals that 3D ERI is an effective tool for subsurface contamination zone mapping in various depths, which extends in this study from the near-surface to a depth of about 5 m below the ground level.

Petrographical and geophysical investigation of the Ecca Group between Fort Beaufort and Grahamstown, in the Eastern Cape Province, South Africa

Abstract The outcrop of the Ecca Group in the Eastern Cape Province was investigated in order to reveal petrographic and geophysical characteristics of the formations that make up the group which are vital information when considering fracking of the Karoo for shale gas. The petrographic study reveals that the rocks of the Ecca Group are both argillaceous and arenaceous rock with quartz, feldspar, micas and lithics as the framework minerals. The sandstones are graywackes, immature and poorly sorted, thus giving an indication that the source area is near. The observed heavy minerals aswell as the lithic grains signify that the minerals are of granitic, volcanic and metamorphic origin. The porosity result shows that of all the formations that make up the Ecca Group, the Whitehill Formation is the most porous with an average porosity of about 2.1% and also least dense with an average dry density of about 2.5 g/cm3. The least porous unit is the Ripon Formation with porosity of about 0.8% but has the highest dry density of approximately 2.8 g/cm3. The magnetic map shows some ring-like structures which coincide with dolerites that were mapped in the field. As revealed by the depth slices result, dolerite intrusions are pervasive in the northern part of the study area, extending to a depth of about 6000 m below the ground surface. The appearance of dolerite intrusions at the targeted depth (3000 - 5000 m) for gas exploration could pose a serious threat to fracking of the Karoo for shale gas.

Derivation of empirical relationships between geotechnical parameters and resistivity using electrical resistivity tomography (ERT) and borehole data at Sohag University site, upper Egypt

The new site is located in desert region of rugged topography. Quaternary boulders, gravels, sands and clays intercalation are the main rock types in the area. Geophysical evaluation of the subsurface lithology and structures using 2D and 3D electrical resistivity tomography (ERT) of the proposed site for construction was the main target of the present study, in addition finding empirical relationships between rock's electrical resistivity and some geotechnical parameters are also another target. Four geoelectrical resistivity layers were discriminated after the 2D inversion. Inspection of all 2D inverted profiles and 3D electrical resistivity depth slices showed rapid lateral and vertical variations in electrical resistivity values indicating heterogeneity in soil section. Clayey gravels or clayey sands and clays facies are predominant and hence swelling actions of the foundation beds is highly expected. The results of electrical resistivity tomography (ERT) were correlated with that inferred from the boreholes within the investigated area. The geotechnical parameters: liquid limit L.L%, plastic limit P.L%, free swelling F·S% and density Qu were calculated for representative samples collected from drilled boreholes. All samples showed considerably high L.L%, low to moderate P.L% with high F·S% which characterizes clayey soil. A trial for finding empirical relationships between the geotechnical properties and the rock electrical resistivity (L.L%, P.L%, F·S% and Qu) were carried out. These relations revealed that there is a considerable correlation between them when the rock has a resistivity more than 400 Ω m and poor correlation when the electrical resistivity is less than 400 Ω m. Accordingly, inferring applicable empirical equations between rock electrical resistivity and geotechnical parameters can be strong in the dry successions with a limited clays content, but in the wet or clayey soil materials the correlation will be weak and risky in application. From all obtained results and relationships, highly engineering awareness should be taken into account during establishing any construction in this site.

Audio-magnetotelluric investigation of Bakreswar Geothermal Province, Eastern India

Bakreswar Geothermal Province (BGP) lies over the Archaean gneisses and schists formations. The heat flow and geothermal gradient of BGP are \(\sim \)230 mW/m\(^{2}\) and \(95{^{\circ }}\hbox {C}/\hbox {km}\), respectively. The Bakreswar hot spring (BHS) (\(23{^{\circ }}52^\prime 30^{\prime \prime }\hbox {N}\); \(87{^{\circ }} 22^{\prime }30^{\prime \prime }\hbox {E}\)) is located over NNW–SSE trending fault. It lies in the eastern flank of BGP and is connected by Sone–Narmada–Tapti (SONATA) and deep-seated ONGC fault systems. Earlier studies indicated that the geothermal fluid flowed towards the same path as that of deep ONGC fault. In view of this, 24 audio-magnetotelluric soundings in the frequency range 10 kHz–10 Hz were conducted at one end of the deep ONGC fault, which is about 20 km northwest of BHS. The data analysis indicated that the data are internally consistent. Analysis of dimensionality and strike are indicative of complex conductivity structures. The 3D inversion of full impedance was performed to obtain a conductivity model of the study area and resulted in root mean square (RMS) for sites close to unity. The results have been discussed by extracting cross-section along 3 to 7 profiles (four of the profiles are perpendicular to strike, whereas three profiles were oblique to the strike direction and are perpendicular to east–west trending ONGC faults). Several low conducting zones are observed in the profiles drawn parallel and perpendicular to the strike. These common conductors show consistent model features but modified resistivity and dimension. Depth slices from 200 to 1400 m suggest that the conductors in the northern part of the study area are deeper than the conductors in the south. The study area is dominated by several shallow conductors which coincide with several fractures present in the study area. The conductors are surrounded by a resistive feature that can act as an impermeable bed and probably restricts the flow of water within the zone. The four major conductors along the oblique profiles (profiles perpendicular to deep ONGC fault) coincide with deep ONGC fault, through which probably the geothermal fluid flows.