Electrical Resistivity Tomography Sections Research Articles

Integrating geological, hydrogeological and geophysical data to identify groundwater resources in granitic basement areas (Guéra Massif, Chad)

In Chad, hard-rock aquifers are the main source of drinking water for the population located on basement areas. In these basement aquifers, and in particular those of the Guéra region, water drilling failure rates remain high despite research on one- and two-dimensional electrical resistivity techniques and lineaments as a means to improve access to the resource, mainly because these techniques are only used on an observational and structural basis to locate fractures. This study combines electrical resistivity tomography (ERT) with geology, hydrogeology and geomorphology, in order to characterise the structure and geometry of the aquifer system, assess borehole productivity and determine the factors controlling it. After validating the large dataset and its representativeness, 315 high- and low-yield wells, of which 41 have complete geophysical datasets, were selected. This large dataset allows a multi-parameter approach to (1) better characterise each facies according to its electrical resistivity and (2) clearly identify the main formations constituting the local conceptual hydrogeological model. The most suitable areas for productive boreholes are characterised by the presence of an overburden of <20 m depth, well-developed weathered and fractured horizons of granites and biotite granites (preferably) containing little or no clay, and a nearby drainage network. The most substantial flow rates are found in the first 30 m of the fissured horizon, below the base of the alterites. The experience gained from the present study will guide future analysis of ERT sections in order to reduce the probability of drilling dry wells.

Unveiling a hidden fortification system at “Faraglioni” Middle Bronze Age Village of Ustica Island (Palermo, Italy) through ERT and GPR prospections

We carried out a geophysical research project in the Middle Bronze Age village of Ustica (Palermo, Sicily, Italy), named “Faraglioni Village” after the stack formations which detach from the coast north of the archaeological site. The investigation, which comprised Electrical Resistivity Tomography (ERT) and Ground Penetrating Radar (GPR) techniques, allowed us to discover the buried foundations of an outwork fortification system never evidenced by previous archaeological studies, only hypothesised from the observation of aerial photography and partially outcropping boulders, which align roughly parallel to the main defensive wall of the Village.Our geophysical prospection involved the entire 250 m-long arc of the outward village defensive wall, with the acquisition of eleven ERT profiles and 27 GPR scans. The techniques were selected based on both favourable logistics and methods applicability: ERT sections allowed us to trace a series of high-resistivity anomalies arranged to form an arc-shaped structure along the perimeter of the defensive wall. GPR investigation was localised in the most accommodating patch of terrain of the site, with the effort of intercepting clear enough sections of the target, to determine more accurately its shape, depth, and overall dimensions. Our discovery paves the way for new investigations, mainly aimed at defining the timing of construction of the fortification system, as well as the function of the remains of other architectural structures identified close to the wall, which could represent the target of further geophysical investigations.

Spreading of Localized Information across an Entire 3D Electrical Resistivity Volume via Constrained EMI Inversion Based on a Realistic Prior Distribution

Frequency-domain electromagnetic induction (EMI) methods are commonly used to map vast areas quickly and with minimum logistical efforts. Unfortunately, they are often characterized by a very limited number of frequencies and severe ill-posedness. On the other hand, electrical resistivity tomography (ERT) approaches are usually considered more reliable; for example, they do not require specific calibration procedures and can be easily inverted in 2D/3D. However, ERT surveys are, by far, more demanding and time consuming, allowing for the deployment of a few acquisition lines per day. Ideally, the optimal would be to have the advantages of both approaches: ease of acquisition while keeping robustness and reliability. The present work raises from the necessity to cope with this issue and from the importance of enforcing realistic constraints to the data inversion without being limited to (over)simplistic spatial constraints (for example, characterizing the smooth and/or sharp regularization). Accordingly, the present research demonstrates, by means of synthetic and field data, how the EMI inversion—based on realistic prior models—can be further enhanced by incorporating additional pre-existing pieces of information. While the proposed scheme is quite general, in the specific examples discussed here, these additional pieces of information are, respectively, a reference model along a line across the survey area, and an ERT section. The field EMI results were verified against extensive ground penetrating radar (GPR) measurements and boreholes.

Integrated subsurface investigation for magmatic sulfide mineralization in Betul Fold Belt, central India

Magnetotelluric (MT), Electrical Resistivity Tomography (ERT), Time-Domain Induced Polarization (TDIP), Geochemical and Geological studies followed by drilling and down-hole logging were undertaken with in the Betul Fold Belt (BFB) in Central India, to demarcate zones of magmatic Ni-Cu-PGE sulfide mineralization. The BFB is predominantly composed of circular to elongate gabbro bodies of the Padhar Mafic-Ultramafic Complex, intruded into a sequence of bimodal volcanic rocks and quartzites. Near-surface samples of ultramafic rocks were subjected to precise geochemical analysis and scanned by an electron microscopy with an energy dispersive spectrometer (SEM-EDS). This work indicated the presence of pyrite, pyrrhotite, chalcopyrite, pentlandite, and minor amounts of W-Cd bearing boweiite and palladenite assemblage. These minerals are regarded as favorable to the occurrence of Ni-Cu-PGE sulfide mineralization. MT data derived from two profiles were analyzed and modeled using 2D and 3D inversion algorithms. The robust conductivity anomalies obtained from the MT model have been interpreted coupled with electrical tomography, geology, and geochemistry data. The near-surface shallow depth conductors observed in the ERT sections are interpreted as the sulfide mineralized zones. They corroborate the MT results. These conductive zones reflect the occurrence of the magmatic Ni-Cu-PGE bearing sulfide mineralization associated with rocks of the mantle-derived Padhar Mafic Ultramafic Complex. This geophysical data, in conjunction with petrological and geochemical analysis of drill core samples have allowed the identification of the origin and paragenesis of sulphide mineralization in the study area. Geochemical studies suggest that the parental magma was generated from a subduction modified, metasomatized and an enriched mantle source that was subsequently emplaced in a magmatic continental arc setting. The interpreted conductors, observed at shallow depths (∼200-300 m), have been generated by secondary hydrothermal fluid circulation leading to vein formation in the host Padhar Mafic-Ultramafic Complex. The MT and electrical tomography models delineate the geological boundaries of the sulfide-bearing mineralized deposits in the BFB.

Geomorphometric and Geophysical Constraints on Outlining Drained Shallow Mountain Mires

Long-term draining of peatlands results in transformation of vegetation and obliteration of their morphological features. In many areas, efforts are made to restore the original ecosystems and increase their water retention potential. Using combined analyses of a LiDAR-based digital terrain model (DTM), colour-infrared (CIR) imagery data, ground-penetrating radar (GPR) data and electrical resistivity tomography (ERT) data, we tested the applicability of these methods in outlining the extent and subsurface structure of drained mires located in the Stolowe Mountains National Park area, Poland. The LiDAR-DTMs enabled parameterisation of physiographic features of the mires and determination of their extent, runoff directions and potential waterlogging areas. CIR analysis enabled classification of vegetation types. GPR prospecting revealed the bedrock morphology, thickness and internal structure of the peat deposits, showing that this technique can also provide data on variability in the decomposition of phytogenic deposits. The obtained ERT sections indicate both the thickness of peat deposits and variability in the bedrock internal structure. The results show that integrated analyses of data obtained with different methods can be an effective tool in outlining the original extent of peatlands, with potential application in the planning of peatland ecosystem restitution.

English

Vertical electrical sounding (VES) and 2D electrical resistivity tomography (ERT) employing the Schlumberger and Wenner configurations respectively were combined for groundwater exploration in parts of Enugu metropolis. The area is underlain by Enugu Shale and Mamu Formation. A total of twenty (20) VES and four (4) 2D ERT traverses were acquired in the study area. VES and 2D ERT data were processed and interpreted using INTERPEX 1X1D and RES2DINV respectively. Sounding curves shows dominant decrease in resistivity with depth while 2D ERT inverse model shows lateral variation in resistivity of the rock layers with lower resistivity at increasing depth. VES geoelectric layer sections show top sideritic clay - clay/shale sequence. Resistivity curve analysis also shows predominance of Q and K curve types. 2D map of aquifer layer resistivity, thickness and depth were constructed. Aquifer layer resistivity range from 2.00 to 190 ?m, while the thickness range from 10 to 75 m and a depth range of 23 to 150 m. The 2D ERT shows layer resistivity range of 0.1 to 250 ?m. Based on the combination of the two techniques for groundwater exploration in the study area, a water table depth of 18 m with a maximum drill depth of about 100 m was determined. However, it is recommended that the selection of point of drill should be on the low saturated regolith as observed on the 2D ERT sections.&nbsp;&nbsp; Key words: Aquifer, groundwater, VES, 2D ERT, resistivity.

Subsurface characterization of a construction site in Nanjing, China using ERT and CPTU methods

In geotechnical engineering, the need for efficient and economical site characterization is the main motivation for the development of new techniques. This study combines electrical resistivity tomography (ERT) with piezocone penetration test (CPTU) to develop a rapid site characterization tool. The feasibility and efficiency of delineating subsurface structures, mapping the lithology, and estimating the hydraulic conductivity of subsoils are verified at a construction site. The lithologic information obtained via this rapid tool is validated using borehole data. According to CPTU results, the subsurface discontinuity exhibited in the ERT section is related to variation in the water content of silty sands. Accordingly, rapid and economical subsurface characterization can be achieved by using ERT survey to optimize the number and locations of point-based geotechnical tests, especially CPTU. We outline the applicability of using CPTU and ERT-derived resistivity data to predict the hydraulic conductivity of cohesive and sandy soils, respectively. A non-simplified Archie's law is used to relate the resistivity of sandy soils to their hydraulic conductivity, and appropriate values of this law's model parameters are proposed.

A data mining approach for improved interpretation of ERT inverted sections using the DBSCAN clustering algorithm

SUMMARY Geophysical imaging using the inversion procedure is a powerful tool for the exploration of the Earth's subsurface. However, the interpretation of inverted images can sometimes be difficult, due to the inherent limitations of existing inversion algorithms, which produce smoothed sections. In order to improve and automate the processing and interpretation of inverted geophysical models, we propose an approach inspired from data mining. We selected an algorithm known as DBSCAN (Density-Based Spatial Clustering of Applications with Noise) to perform clustering of inverted geophysical sections. The methodology relies on the automatic sorting and clustering of data. DBSCAN detects clusters in the inverted electrical resistivity values, with no prior knowledge of the number of clusters. This algorithm has the advantage of being defined by only two parameters: the neighbourhood of a point in the data space, and the minimum number of data points in this neighbourhood. We propose an objective procedure for the determination of these two parameters. The proof of concept described here is applied to simulated ERT (electrical resistivity tomography) sections, for the following three cases: two layers with a step, two layers with a rebound, and two layers with an anomaly embedded in the upper layer. To validate this approach, sensitivity studies were carried out on both of the above parameters, as well as to assess the influence of noise on the algorithm's performance. Finally, this methodology was tested on real field data. DBSCAN detects clusters in the inverted electrical resistivity models, and the former are then associated with various types of earth materials, thus allowing the structure of the prospected area to be determined. The proposed data-mining algorithm is shown to be effective, and to improve the interpretation of the inverted ERT sections. This new approach has considerable potential, as it can be applied to any geophysical data represented in the form of sections or maps.

Mapping of Gold Mineralization in Ichadih, North Singhbhum Mobile Belt, India Using Electrical Resistivity Tomography and Self-Potential Methods

The North Singhbhum Mobile Belt (NSMB) lies between two crustal-scale shear zones, the South Purulia Shear Zone (SPSZ) at the north, and the Singhbhum Shear Zone (SSZ) at the south. The Babaikundi-Birgaon Axis (BBA) is an east-west trending secondary shear zone in the NSMB lying south of SPSZ, which is known for the occurrence of gold along with other sulfide minerals within the quartz reefs. The present study has been carried out at north of Ichadih village, which lies on the BBA. The present study attempts combined electrical resistivity tomography (ERT) and self-potential methods along a profile of 470-m length in the Ichadih area to map the potential mineralized zones. Initially, ERT sections generated by Wenner, Wenner-Schlumberger, dipole-dipole, and joint inversion of the combined array have been analyzed, and results are validated through the analysis of model resolution. A prominent low resistivity (10–100 Ωm) associated with quartz reef/limonitic quartz reef at a depth of approximately 30–85 m, encircled by a high resistive (more than 10,000 Ωm) formation associated with metasedimentary rock, has been delineated using joint inversion of the combined array. Further, analysis of self-potential (SP) anomaly using particle swarm optimization inversion also confirms the prominent low SP anomaly at a depth of 77.3 m, which is analogous to the low resistivity anomaly identified in the joint inversion of the combined geo-electric array. Subsequent analysis of a complex synthetic geo-electrical model analogous to the present field situation proves the efficacy of the joint inversion of combined arrays. As a whole, joint inversion of multi-array data is established as an efficient technique for better subsurface imaging in case of the complex subsurface structure. In-depth analyses confirm the possibility of sulfide mineral-rich deposit (quartz reef/limonitic quartz reef) which requires further geological drilling and lithogeochemical sample analysis, for possible future exploration.

Control of fracture networks on a coastal karstic aquifer: a case study from northeastern Yucatán Peninsula (Mexico)

The Yucatan Peninsula karst aquifer in southeastern Mexico is important because it is the only source of freshwater supply in the region. Along the eastern coast, the aquifer behaves as a shallow unconfined aquifer, and one of its main characteristics is the development of a complex network of karstic conduits. Electrical resistivity tomography (ERT) is a geophysical technique that provides reliable information on aquifer properties that can be applied to karst. With this in mind, ERT surveys (with dipole-dipole and Wenner arrays) together with vertical profiles of groundwater electrical conductivity were obtained in the northeast of Yucatan Peninsula in the Akumal zone. The objective was to evaluate the characteristic structure of the unsaturated and saturated areas, and to determine the importance of factors at a local level that affect the thickness of the freshwater lens, as well as groundwater flow directions near the coast. The results of the ERT sections show a thin vadose zone with resistivity values greater than 1,052 Ωm and a saturated zone with resistivity values that vary from 1 to 1,052 Ωm, and show the presence of dissolution conduits and fracture zones that are affecting the freshwater lens of the aquifer. In addition, water-table measurements show the role of fracture networks in the groundwater flow. The overall results indicate conditions for a possible microbasin in the Akumal zone, highlighting the importance of local fractures, which could work as groundwater divides along the eastern coast of the Yucatan Peninsula.

Integrated geophysical techniques for subsurface imaging of active deformation across the Himalayan Frontal Thrust in Singhauli, Kala Amb, India

Non-invasive geophysical techniques employed across the Himalayan Frontal Thrust (HFT) at Singhauli in the north-western Frontal Himalaya to understand shallow subsurface geological structures and their nature in the context of active tectonics. The combination of high-resolution Ground Penetrating Radar (GPR), Electrical Resistivity Tomography (ERT) and Multi-channel Analysis of Surface Waves (MASW) techniques with the different acquisition parameters found to be beneficial in gaining high-resolution images of shallow subsurface geological structures. The contact zone of resistivity obtained between <134 and >134 Ωm suggests the juxtaposition of dissimilar lithological units explained as two faults dipping to the northwest related to the HFT. The average separation between Fault 0 and Fault I is 12m which in general agreement with the trench excavation survey. The GPR radargram over a length of 45m show offset reflectors at a horizontal distance of ~12–15m close to the active deformation zone. The horizontal reflectors abruptly truncate across two northerly dipping planes. The 2D Vs section (MASW1&2) exhibited dipping attitude of the litho units (with different velocity zones) in the northeast direction which are also consistent with the local geological setting. Prominent liquefaction feature identified in the ERT section specifically with the 1 and 1.5m electrode spacing show explicit expression of its existence due to contrasting resistivity ranging between 100 and 450 Ωm. A strong correlation has been established between the field based geological observations, carried out earlier in this area through a trench excavation survey, with the present work using geophysical techniques. The integrated approach found to be highly beneficial in those areas where contrasting sub lithological units exist in terms of their physical properties. This study argues for judicious use of the ERT, GPR and MASW techniques to delineate shallow subsurface geology across various active faults in the Himalayan terrain to comprehend the active deformation related to large magnitude paleoearthquakes and earthquake hazard assessment.

IMAGING TROPICAL PEATLAND AND AQUIFER POTENTIAL IN SOUTH SUMATERA USING ELECTRICAL RESISTIVITY TOMOGRAPHY

Indonesia has one of the largest tropical peatland areas in the world. In Sumatra Island, peatland is spread over 11 areas and it was mainly found in Riau (60.1%) and South Sumatra (19.6 %) Provinces. This study investigates the subsurface of tropical peatland in Ogan Komering Ilir Regency, South Sumatra Province. Data was recorded using Electrical Resistivity Tomography (ERT) method based on the sub-surface images of tropical peatland. This study was conducted based on dipole-dipole configuration with 72 channels spread. This paper also studies the physical properties (thickness and electrical resistivity) of peatland and its substrate using ERT. In this study the ERT section and the geological map identified Alluvium (Qs), Kasai Formation, and the Basement. Result shows the ERT is applicable for imaging the thickness of tropical peatland and other geological features (Aquifer, geological structures, and stratigraphy). The electrical resistivity of peat varies from 20 ohm meter to 120 ohm meter and the thickness of peat varies from 2–5 meter. In some ERT sections, the basement was identified from 130 meter to 170 meter beneath the surface. The aquifer sweet spots were located from ERT Sections combined with the hydrogeological map. The aquifer was identified in Kasai Formation. The thickness of the aquifer layer is 2–20 meter. These physical properties may support the peatland conservation (forest fire mitigation) and geotechnical analysis purposes.

Integrated use of electrical resistivity tomography and ground penetration radar for identifying subsurface sinkholes in Rufa Graben, Central Riyadh

Two-dimensional electric resistivity tomography and ground-penetrating radar (GPR) study was carried out in the eastern Rufa Graben, Riyadh area. Dipole–dipole configuration was used with a total length of 360 m and an electrode spacing of 5 m. In addition, the same number of ground-penetrating radar profiles was conducted in the same location of the electrical resistivity tomography (ERT) profiles. The goal of this study was to map and delineate the subsurface karstic features in the study area such as sinkholes, cavities, and fractured zones. The two methods showed the efficiency to detect the near-surface cavities and their distribution in the study area. The detected cavities were of different sizes. The air-filled cavities have appeared in ERT sections as anomalies that have high resistivity values while the cavities filled with wet clay were characterized with low-resistivity values. In GPR sections, the karstic features were seen by either hyperbola diffractions or uplift and discontinuity in horizontal reflection events. The results obtained from this study will contribute in solving the geotechnical problems for any expected future constructions planned to be done in the study area.

Assessing chromite ore processing residue (COPR) waste dump site using electrical resistivity tomography (ERT): a case study from Umaran, Kanpur, India

Leaching of chromium ions causes a serious threat to groundwater around chromite ore processing residue (COPR) dump sites in many countries. As a result, detailed subsurface characterization of the affected region is crucial for assessing the associated risks as well as initiating remedial measures. Though the conventional approaches (e.g., drilling and water sampling) provide important information but are expensive and unable to decipher detailed subsurface scenario. Thus, in the present study, electrical resistivity tomography (ERT) (a cost-effective and faster approach) method has been employed to assess the effect of unplanned COPR waste dump beside agricultural land at Umaran, Kanpur, India, in conjunction with the available geochemical information. Inverted 2-D ERT sections depicted resistivity variation in the subsurface, and its correlation with previous geochemical results reveals the resistivity boundary between contaminated and clean zones as ~ 15 Ω·m. The study also depicts that the contamination plume is slowly migrating towards NE direction below the agriculture land but rate of migration is faster along southern direction. Therefore, the agriculture land and corresponding groundwater at ~ 50 m away from the dump site in NE direction are not affected by COPR leachate. Vertically, the COPR leachate has affected mostly up to ~ 20 m depth in the region inside the dump boundary; however, at some places, it is migrating further downward. Thus, the study demonstrates the efficacy of ERT method in characterizing COPR dump site and provides crucial information in managing safe agriculture practices over the region as well as for initiating scientific remedial measures.

Identification of gypsum karstification using an electrical resistivity tomography technique: The case-study of the Sivas gypsum karst area (Turkey)

The Sivas district is the most important gypsum karstified area in Turkey. The oldest evaporites in the region were formed during the sedimentation process of the Oligocene period. Generally, these units extend along the southern part of the Sivas basin. After the Early Miocene Sea drained from this region, extensive evaporite deposition developed. An electrical resistivity tomography (ERT) study was carried out between the Hafik and İmranlı districts of the Sivas gypsum karstified area. In this research, the prior aim was to determine the general characterization, geometry, sedimentary cover and resistivity properties of gypsums. In addition, the study includes the synthetic modelling of different gypsum karstification types. The evaluation and interpretation were based on ERT data that was collected from the Hafik-İmranlı region. First, the different karstifications were identified, together with the characteristic features of gypsum in the area. The ERT studies showed that there are large and old karst structures between the Hafik and Zara district. However, the karst structures that are located between the Zara and İmranlı regions were interpreted as smaller and younger. The characteristics of both of the regions are different in terms of the morphometric properties. It is certain that the Kızılırmak River has an important impact on the formation of these structures. ERT studies have shown that the cover- and bedrock-type collapse sinkholes are very spread out across the area, and their geometries are different. Throughout time, the suffosion type of cavities formed inside the cover-type sinkholes as a consequence of geological and geomorphological events. The ERT sections showed that certain parts of these structures collapsed, and then alluvial materials filled in. The shallow groundwater level in the drilling results and the conductive zone in the ERT results are compatible with each other. ERT studies clearly determined the various karstification typologies of gypsums in the region, which also contributed to the creation of a detailed conceptual geological model.

Imaging Regional Geology and Au – Sulphide mineralization over Dhanjori greenstone belt: Implications from 3-D Inversion of Audio Magnetotelluric data and Petrophysical Characterization

The 2.1-Ga-old Paleoproterozoic Dhanjori Group of rocks covers 800 sq. km area within the Singhbhum Crustal province of eastern India, and is composed of siliciclastic metasedimentary sequence interlayered with mafic to ultramafic rocks. The meta-volcanosedimentary sequence is deformed and is of low-grade greenschist metamorphic facies. The gold occurrences within the study area are confined to the sulphide hosted mafic/ultramafic volcanic rocks and also in the basal quartz pebble conglomerates (QPC) within the Dhanjori Formation. Gold mineralization within the mafic/ultramafic meta-volcanics formations occurs within the quartz ± carbonate vein i.e. lode gold and as detrital gold within the QPC (0.5−7 ppm) of Phuljhari Formations. Regional Audio Magnetotelluric (AMT) measurements across the geological domains of Dhanjori Group and Iron Ore Group (IOG) were carried out in the frequency range of 10 kHz–10 Hz. This work discusses the conductivity model from 3-D inversion along with geology and petrophysical properties in mapping the regional geological features, associated sulphide mineralization and influence of texture, grain and bulk properties on mineralization. Dimensionality and strike analysis of the AMT data indicate complex conductivity structures. 3-D inversion of off-diagonal component of impedance was performed for obtaining an optimum value of uniform half space initial resistivity and error floor for off-diagonal component of impedance data. These were used to perform 3-D inversion of full impedance to produce a conductivity model of the study area. The conductivity model delineated major geological boundaries and the sulphide bearing mineralized zone. A major conductor is observed which extends to a depth of ∼250 m within meta-volcanics. On comparison with the available borehole adjacent to this, it is confirmed that the conductor corresponds to sulphide mineralization. The resistivity low at a depth of 150 m coincides with metabasalts (pyrite present which extends to 159 m with arsenopyrite). Another mineralized zone is observed at the boundary of iron ore group volcanics and Proterozoic-gabbro-anorthosite-ultramafics (PGAU), it starts appearing from a depth of ∼400 m and can be seen which extends to a depth of ∼800 m. It is worth mentioning that there lies Kundarkocha gold mine in the same geological domain. In addition, near surface conductor to a maximum depth of ∼50 m is observed all along the AMT profile. The sediments and fluids could account for this conducting feature. This is in agreement with available Electrical Resistivity Tomography section. The borehole within Dhanjori Volcanics also detected sulphide mineralization for ∼50 m. Thus, it can be inferred that the near surface conductor indicates the presence of gold in association with other sulphide minerals. The petrophysical studies helped to estimate the concentration of minerals within the rocks. High density and low susceptibilities in the area is an indicative of low-grade metamorphism and low level of serpentenization. Combined density and susceptibility scatter diagram and component proportion phase diagram suggests 20 vol%, 78 vol% and 2 vol% of haematite/sulphides, barren rocks and magnetite respectively. The high value of chargeability and negligible value of susceptibility indicates the presence of disseminated sulphides rather than haematites and/or massive sulphides in the rocks. Ternary plot suggest that the mineralization is controlled by texture and grain properties. Petrophysical analysis suggests that the lithotypes of the study area are similar to those of greenstone terrain in the Abitibi Subprovince. Integrating inverted section, geology and petrophysical properties has a substantial impact on the study area in terms of predicting the mineralization prospect and at the same time substantially reduces the drilling cost. Thus, the study provides a new orientation and advancement to the exploration strategy for deeper deposits.

The contribution of geophysics to archaeology: a case study of an ancient canal of the Oc Eo culture in the Mekong Delta, Vietnam

Generally, underground ancient canals are infilled with alluvial materials, with the canal bed and substrate often having different resistivity values. This study aimed to determine the location and morphology of Malleret’s ancient canal 16 located to the southeast of the Ba The mountain, Mekong Delta, Vietnam by means of geophysical techniques. Two geophysical methods were used: electromagnetic profile and electric resisitivity tomography. A geoelectric structure 70 m long with 70–95 mS/m of apparent conductivity was found. On the electrical resistivity tomography section, a resistivity zone of 10–20 Ω∙m, 1–4 m deep, 70 m wide corresponds to the mentioned above geoelectric structure, which is in an asymmetric U-shape extending toward the southeast-ern bank of canal 16. Hand-augering confirmed that the canal bed is fully incised into Holocene sediments as a substrate which stretches down to the Pleistocene. The sediments are composed of loams mixed with plant re-mains with a resistivity ρ ~ 10–15 Ω∙m. Both of the canal banks at a depth of 5 m are made up of Holocene sedi-ments (ρ ~ 4–10 Ω∙m). The 14C measurements determined the age of the organic matter in the canal as being equal to 1210 ±85 BP, suggesting canal 16 ceased being operational at that time. The precise positioning of canal 16 on the ground surface, as well as identifying the morphology of the canal bed, were corroborated by geophysical techniques. The obtained results are of considerable value to archaeologists.

Integrated characterization of ancient burial mounds using ERT and limited drillings at the Hepu Han Tombs, in coastal area of Southern China

The Hepu Han Tombs, located on the south coast of Guangxi Zhuang Autonomous Region, in southern China, bordering the Beibu Gulf, can date back to the Han dynasty (206 BCE–220 CE), when Hepu was an important international trade port of the maritime Silk Road, spreading strong and persistent cultural influence to neighboring countries from ancient China, and also connecting China with African, European and other Asian countries in national migration, religious communication, political, trade, technical and cultural exchanges, and so on. The research of the Hepu Han Tombs can offer opportunities to understand the burial customs and the Han culture in the Chinese Han Dynasty. With the aim of detecting the location, depth and geometry of two burial mounds at Jinjiling Site of Hepu Han Tombs, an electrical resistivity tomography (ERT) survey was performed in cooperation with archaeological team to calibrate the results with detailed information from the limited drillings in the area. Besides, series of 2-D ERT sections, acquired above the larger mound, were combined into a “Pseudo 3-D” volume, and iso-resistivity surfaces were further calculated to emphasize the location and lateral variations within the data volume, expected towards a more detailed and quantitative interpretation. Both the 2-D and 3-D resistivity imaging gave a clear evidence of structural details of the burial mounds. The results have shown a general consistency between the geophysical work and traditional archaeological drilling explorations, improving our knowledge on the un-excavated mounds, and allowing detailed pre-excavation plan at the Hepu Han Tombs.

Integrated application of 2D resistivity and electromagnetic methods to investigate a metallic-sulfide deposit in Soap Gulch, Montana. A case study

Geoelectrical methods involving electrical resistivity tomography (ERT), self-potential (SP), frequency domain electromagnetic (FDEM), and very low frequency (VLF) methods have been used to provide valuable information in locating a known sulfide ore body in Soap Gulch, Montana. The study develops basis of comparison for the geophysical techniques employed. Ranges of resistivity along the area have been established using interpreted ERT which can help to understand the subsurface distribution of sulfides in the area. A sulfide body was delineated from the survey area corresponding to anomalously low resistivity values on the ERT section, negative SP, and high apparent current density zone in VLF. Depth to the localized ore zone ranges approximately from 10 to 20 m. FDEM data reflect the conductivity distribution of the shallow subsurface (less than 6 m deep); hence, the delineated sulfide zone had minimal contribution to FDEM measurements. The results of the study show that SP, VLF, and ERT methods provide significant information in localizing ore bodies. The survey revealed that the resistivity values obtained from ERT profile corroborate the FDEM, SP, and VLF from the area.

Magnetotelluric exploration of a deposit scale prospecting over a proterozoic volcanics, Eastern India

North Singhbhum Mobile Belt (NSMB) is a metallogenic province with dominant rock types of carbon phyllites, graphitic schists, quartzites, tuffs, rhyolites dominated with Dalma basalts of Proterozoic ages. Earlier 2-D Magnetotelluric (MT) studies across Dalma Volcanics (DV), lying almost at the center part of NSMB, hinted towards 3-D behavior at MT short periods for sites at the boundary of DV and Singhbhum Group of Metapelite (SGM). This tempted to acquire Audio MT (AMT) data sets over DV and SGM. The data was acquired at an interval of ~ 500 m for a profile length of ~7.5 km. The 3D joint inversion of MT + AMT data was performed with impedance, tipper and combination of impedance and tipper. Resistivity cross-sections from the 3D joint MT + AMT inversion of the combination of impedance and tipper are in broad agreement with previous 2-D results but with modified dimension and location. It delineated both near surface and deep conductor and establishes the link between these conductors. The near surface conductors are in agreement with the coincident Electrical Resistivity Tomography section. Near surface conducting heterogeneities mostly represents carbon phyllites, graphitic schists and tuffs - indicators of gold deposits. These mostly lie at the northern fringes of DV are surrounded by shallow resistive subsurface rocks. Relatively deep conducting volcanics are mostly dominant below DV. The presence of alternating conductive and resistive rocks below DV also validates the bimodal character of Dalma Volcanics. The sensitivity analysis suggest that the modeling results are mostly sensitive up to the depths of ~ 15 km. Conductivity anomalies of deposit scale both at shallow and deep levels, possibly enriched with sulfide-mineralization representing the metallogeny are associated with the evolution of Proterozoic Volcanics. The obtained depths of DV and SGM are 4–5 km and ~1.5 km, which is in agreement with the gravity data.