Transient Electromagnetic Soundings Research Articles

Hydrogeological Investigation of a Goaf and Subsidence Area Based on a Ground-to-Air Transient Electromagnetic Sounding Method

Water hazards in coal mines have always been the main geological hazard that restricts the safety of coal mine production. The traditional electromagnetic method is difficult to accurately detect the hydrogeological conditions of underground goafs due to accuracy and interference problems. The ground-to-air transient electromagnetic method is an electromagnetic detection technology with strong adaptability to various terrains, a large detection depth, a wide coverage, a high resolution, and fast speed. This paper mainly applies the ground-to-air transient electromagnetic method to conduct a hydrogeological engineering geological survey in a mine subsidence area. By using this method effectively, changes in hydrogeological and engineering geological conditions in the mine subsidence area are identified and water filling conditions below section 310 and section 250 of the roadway are determined. This study provides accurate and reliable basic data support for rescue operations and post-disaster reconstruction efforts, while also offering valuable insights for similar exploration projects.

Integration of EM techniques for mapping buried historical harbours: a case study from the archaeological site of Tell el-Rub'a, Egypt

AbstractThis study focuses on the integrated application of the frequency-domain terrain conductivity (TC) and transient electromagnetic (TEM) methods to map an ancient riverine harbour (port) at the Egyptian archaeological site of Tell el-Rub'a (prehistoric Mendes). Fifty-one TC profiles were measured using six EM frequencies and 16 TEM soundings were performed at the selected site. The apparent conductivity maps derived from the TC survey enabled defining the lateral extension of the harbour basin and its edges to penetration depths ranging between 12 and 30 m. The vertical extension of the harbour basin was determined using the TEM inversion. Three layers were defined: the topsoil, the conductive silty clay infill and the resistive sand bedrock. The bedrock is found at depths ranging from 19 to 31 m. The TC and TEM results were combined to form a 3D image of the harbour basin. This study confirms the existence of Mendes harbour and demonstrates the effectiveness of the TC and TEM methods for such archaeological studies.

Combined geophysical methods to investigate seawater intrusion in the Souss-Massa coastal area, Morocco

Groundwater are strategic water resources for irrigation and drinking purposes in the Souss-Massa semi-arid region (Morocco). These resources have been threatened during the last decades by different factors such as drought, over-pumping and marine intrusion. The latter is responsible for the degradation of the groundwater quality along the coastal area, which is reducing its usability. Therefore, the localisation of the freshwater/saline water interface is essential for better sustainable management of groundwater resources in the area. The objective of the present study is to assess the temporal changes, the extent, and the depth of the seawater intrusion in the Souss-Massa coastal area. To fulfil this objective, two monitoring sites were equipped with water conductivity measurement devices (Subsurface Monitoring Device – SMD) to observe the temporal changes of the salinity at a local scale, as well as to support the interpretation of a transient electromagnetic (TEM) survey. This survey consisted of 83 TEM soundings, along thirteen profiles located perpendicularly to the ocean and the two rivers delineating the study area. The data from the SMD stations highlight the temporal changes of electrical conductivity (EC) with depth, clearly illustrating the freshwater/saltwater interface location in the central part of the study area (−35.55 m NGM at the Tifnit location, ca. 1 km from the coast). In addition, the results of the TEM measurements show that the marine intrusion is already advanced towards the land, over a maximum distance of 2500 m. Finally, some new monitoring sites have been proposed to improve the current monitoring network for seawater intrusion and assess whether mitigation solutions lead to a slower progression of the seawater intrusion. Decision-makers can use these results as a basis for the development and implementation of effective policies to ensure a sustainable and integrated water resources management in this region.

Preliminary results of clay soils state monitoring using transient electromagnetic sounding apparatus

The background for creating an electromagnetic soil sounding apparatus with receiving and transmitting antennas separated by a distance of up to 100 meters is considered in this paper. The temperature and frequency dependences of the complex dielectric permittivity of multilayer clay soil are studied in conjunction with the results of numerical simulation of ultra-wideband log-periodic antennas immersed in the soil to a depth of 1.5 m. The choice of a high-voltage generator and pulse receiver is studied, and a structure of apparatus prototype is given. Based on the results of oscillograms’ and signal spectra measurements with 30 m antennas separation, and with reference to the soil temperature, estimates of the possible limits of change in the electrophysical parameters of the clay soil at a geophysical site are provided. Conclusions are drawn about ways to improve the apparatus in order to create a new technology for transient electromagnetic monitoring of permafrost.

Imaging Arctic Permafrost: Modeling for Choice of Geophysical Methods

Knowledge of permafrost structure, with accumulations of free natural gas and gas hydrates, is indispensable for coping with spontaneous gas emission and other problems related to exploration and production drilling in Arctic petroleum provinces. The existing geophysical methods have different potentialities for imaging the permafrost base and geometry, vertical fluid conduits (permeable zones), taliks, gas pockets, and gas hydrate accumulations in the continental Arctic areas. The synthesis of data on cryological and geological conditions was the basis for a geophysical–geological model of northern West Siberia to a depth of 400 m, which includes modern permafrost, lenses of relict permafrost with hypothetical gas hydrates, and a permeable zone that may be a path for the migration of gas–water fluids. The model was used to model synthetic seismic, electrical resistivity tomography (ERT), and transient electromagnetic (TEM) data, thus testing the advantages and drawbacks of the three methods. Electrical resistivity tomography has insufficient penetration to resolve all features and can run only in the summer season. Seismic surveys have limitations in mapping fluid conduits, though they can image a horizontally layered structure in any season. Shallow transient electromagnetic (sTEM) soundings can image any type of features included into the geological model and work all year round. Thus, the best strategy is to use TEM surveys as the main method, combined with seismic and ERT data. Each specific method is chosen proceeding from economic viability and feasibility in the specific physiographic conditions of mountain and river systems.

Quantification of soil textural and hydraulic properties in a complex conductivity imaging framework: Results from the Wolfsegg slope

We present an approach that permits to predict hydraulic conductivity from extensive, multi-methodical geophysical data collected on a hillslope affected by landslides in Austria. The objective of the investigation is the spatial characterization of a slope affected by mass movements to derive hydrogeological structures and preferential flow paths. The geophysical data sets presented in this study consists of 24 densely distributed complex conductivity (CC) imaging profiles, collocated to these profiles we also collected 517 transient electromagnetic (TEM) soundings and 10 refraction seismic tomography (RST) profiles. Additionally, we also present well-logging data (namely electrical resistivity, natural gamma logs) collected in five boreholes. Cores recovered during the drilling of the boreholes are used to aid in the interpretation of the geophysical units, while analysis of the sediments was conducted to obtain grain size distributions, cation exchange capacity and mineralogy of the subsurface materials. While all geophysical data was processed initially independently, the final complex conductivity imaging results are based on the incorporation of structural constraints about the electrical units obtained from the TEM soundings. The interpretation of the resolved CC units is then sustained by the RST images and the lithological information from the boreholes. We estimate then the hydraulic conductivity of the subsurface derived from the CC images using a two-step approach. In a first step, we investigated the link between the complex conductivity and the different soil volume fractions of gravel, sand, silt and clay. In a second step, we applied a pedo-transfer function, namely the well-known Rosetta model, that permits to predict the hydraulic conductivity from the estimated grain size volumes. This approach allowed a quantitative interpretation of the geophysical data and thus a 3-dimensional (3D) representation of the grain size distribution and hydraulic conductivity in the investigated slope section. Thereby, we observed well-determined site-specific relationships (R2 > 0.7) from the comparison of the complex resistivity images and grain size analysis. The obtained hydrogeophysical 3D model permits to delineate the geometry of an aquiclude, and, thus, the analysis of preferential water-flow paths. In particular, we can identify a spatial correlation between the aquiclude interface and morphological features.

Features of high-density transient electromagnetic sounding data inversion under oil and gas prospecting in the Nepa-Botuoba anteclise area

The data of near-field transient electromagnetic sounding, which are widely used for the purposes of oil and gas exploration within the Nepa-Botuoba anteclise, are typically interpreted in terms of quasi-horizontally-layered models of media. The purpose of this study is to develop an approach to the inversion of transient electromagnetic sounding curves obtained via high-density 3D observation networks. The study was based on mathematical modeling, whose results allowed to estimate the lateral spatial parameters of a non-stationary electromagnetic field as well as to understand the differences between the results of one-dimensional inversion of the transient electromagnetic sounding data and the true parameters of the target horizon. As a result, the characteristic of the electromagnetic field described by an exponential function and used in the lateral-constraint inversion of high-density electromagnetic sounding was obtained. The proposed approach was tested on the practical data within the site under investigation located on the slope of the Nepa-Botuobа anteclise. It is shown that the use of spatial discrepancy under TEM sounding data inversion makes it possible to obtain geoelectric models characterized by lateral consistency of section geoelectric parameters. The application of the spatial stacking-based approach in the inversion process allows to increase the solution stability of the inverse problem of the near-field TEM sounding data.

The Structure of Permafrost in Northern West Siberia: Geophysical Evidence

The permafrost of Arctic West Siberia stores extremely rich resources of hydrocarbon fuels that remain a key energy source and an important element of the global economy. A large amount of natural gas in permafrost is bound in gas hydrates which may become an alternative fuel of the future. Shallow subsurface in the permafrost area of northern West Siberia has been studied by transient electromagnetic (TEM) soundings to estimate the permafrost thickness and to detect faults as channels for fluids and heaving features as possible indicators of gas hydrate accumulations. The shallow transient electromagnetic (sTEM) surveys were conducted in discontinuous and continuous permafrost at two sites in northern West Siberia (the northeastern Yamal Peninsula and the southern Gydan Peninsula), with a focus on the vicinities of lakes and cryogenic landforms (frost mounds). The sTEM method resolves well the heterogeneous permafrost structure, with faults, numerous unfrozen zones (taliks), and frost mounds marked by resistivity and seismic anomalies. Some lakes are located above faults, and their origin may be related to deformation, whereas the frost mounds may mark deep-seated fluid dynamic processes. Local thickening of permafrost may be associated with the presence of gas hydrates. The detected features may trace gas migration pathways and reveal possible sites of gas emission.

A new Stefan equation to characterize the evolution of thermokarst lake and talik geometry

Abstract. Thermokarst lake dynamics, which play an essential role in carbon release due to permafrost thaw, are affected by various geomorphological processes. In this study, we derive a three-dimensional (3D) Stefan equation to characterize talik geometry under a hypothetical thermokarst lake in the continuous permafrost region. Using the Euler equation in the calculus of variations, the lower bounds of the talik were determined as an extremum of the functional describing the phase boundary area with a fixed total talik volume. We demonstrate that the semi-ellipsoid geometry of the talik is optimal for minimizing the total permafrost thaw under the lake for a given annual heat supply. The model predicting ellipsoidal talik geometry was compared to talik thickness observations using transient electromagnetic (TEM) soundings in Peatball Lake on the Arctic Coastal Plain (ACP) of northern Alaska. The depth : width ratio of the elliptical sub-lake talik can characterize the energy flux anisotropy in the permafrost, although the lake bathymetry cross section may not be elliptic due to the presence of near-surface ice-rich permafrost. This theory suggests that talik development deepens lakes and results in more uniform horizontal lake expansion around the perimeter of the lakes, while wind-induced waves and currents are likely responsible for the elongation and orientation of shallow thermokarst lakes without taliks in certain regions such as the ACP of northern Alaska.

Characterization of Valley-Fill Alluvial Aquifers in Plutonic and Volcanic Semi-arid Andes Using Electromagnetic Methods: The Case of the Limarí Catchment (Chile)

Many areas of the world are facing sustained periods of water resource stress during which the enhanced exploitation of groundwater is key to maintaining irrigation and drinking water supplies. A good knowledge about groundwater resources is therefore essential to develop sustainable water management strategies. In this study, we aimed to characterize a mountainous watershed in the semi-arid Chilean Andes. The area of interest is distinguished by a high topographic gradient and narrow valleys filled with sedimentary deposits of various origins and surrounded by plutonic and volcano-sedimentary rocks. To characterize the hydrostratigraphy of this complex sedimentary system and to estimate the volume of groundwater stored, we implemented a multidisciplinary approach integrating geophysical data from transient electromagnetic sounding (TEM), hydrogeological, geological, geomorphological and groundwater quality information. The results indicate the presence of two aquifer layers in the majority of the investigated area: a superficial unconfined aquifer and a deeper confined (or semi confined) aquifer. We found that the width and depth of the sedimentary deposits increase with decreasing topography, while the proportion of fine material increases, in coherence with the sedimentation processes. Finally, we quantified the groundwater contribution of the different areas of the catchment and identified the main aquifer potential area in the pediplanes of the coastal mountain range (storing approximately 67% of the water available for extraction). The main contributions to the total uncertainties on the groundwater storage (ranging between 30 and 80% of the estimated volumes) are due to the propagation of the uncertainty on the thickness and porosity/specific yield of the modeled hydrostratigraphic layers. Due to the large spacing between TEM soundings and the limited number of stratigraphic bore logs in part of the studied area, the obtained characterization should be integrated with additional data for precise borehole sittings. Nevertheless, the implementation of TEM allowed to cover an extensive area and to reach large depth of exploration, so that it was possible to extract general information about the hydrostratigraphy of the different areas of the catchment.

Recharge mechanism and salinization processes in coastal aquifers in Nam Dinh province, Vietnam

In Nam Dinh province, in the Red River delta plain in Northern Vietnam, groundwater in the shallow Holocene aquifer shows elevated total dissolved solids up to 35 km from the coastline, indicating a saltwater intrusion from the Gulf of Tonkin. High groundwater salinities have been encountered below and adjacent to the Red River in the deep Pleistocene aquifer. Since 1996, large-scale groundwater abstraction was initiated from the deep aquifer, and the observed elevated salinities now raise concerns about whether the groundwater abstraction is undertaken sustainably. We have conducted a study to obtain a fundamental understanding of the recharge mechanisms and salinization processes in the Nam Dinh province. A holistic approach with multiple methods including transient electromagnetic sounding and borehole logging, exploratory drilling, sampling and analyzing primary ion and stable isotope compositions of water and pore water, groundwater head monitoring, hydraulic experiments laboratory of clay layers, and groundwater modeling by using the SEAWAT code. Results reveal that saline river water is leached from the Red River and its distributaries into the shallow aquifers. The distribution and occurrence of salty pore water in the Holocene aquitard clay shows that meteoric water has not been flowing through these low permeable clay layers. Marine pore water has, however, been leached out of the Pleistocene clay. When this layer is present, it offers protection of the lower aquifer against high salinity water from above. Salinity as high as 80 % of oceanic water is observed in interstitial pore water of the transgressive Holocene clay. Saltwater is transported into the Pleistocene aquifer, where the Holocene clay is directly overlying the aquifer.

TRANSIENT ELECTROMAGNETIC SOUNDING IN 2D, 3D, AND 4D MODES: SEQUENCE OF GEOLOGICAL EXPLORATION ACTIVITIES

From 1980s, electromagnetic prospecting is an integral component of geophysical techniques for oil and gas exploration in many regions of the Russian Federation. Electromagnetic methods are used at all stages of geological exploration, including reconnaissance, discovery, prospecting and appraisal, and economic mining. In oil and gas exploration, the most commonly used technique is the near-field transient electromagnetic sounding (TEM). On the other hand, for each of these stages, specific tasks of electromagnetic prospecting have not been clearly defined yet. Furthermore, there are no standard requirements to the volume of a priori geological and geophysical information, which is sufficient for solving a geological problem. If such information is lacking or insufficient, a geological problem may be incorrectly stated and/or improperly correlated with the current stage of works. Our study aims to define which geological problems should be addressed by electromagnetic survey at the different stages of geological exploration, and to specify the requirements to the availability of a priori geological and geophysical information. We have analyzed the electromagnetic data from the regions of East and West Siberia, which have different geological settings. The main geological problems that can be solved by the seismic and electromagnetic techniques are determined. We propose a set of geophysical survey operations, which is optimal for oil and gas exploration. An effective sequence of electromagnetic prospecting stages is determined, and preliminary and accompanying surveys are specified. The requirements to the availability of a priori information are proposed, and its volume is defined. Geological problems are defined with respect to the corresponding stages of geological exploration. Special attention is given to approaches to the mineral resource estimation and calculation of hydrocarbon reserves, taking into account electromagnetic survey results. It is proposed to use the electromagnetic data at the stage of prospecting and appraisal, which aims to assess the resources of categories D l and D 0 , as well as at the exploration stage for estimating the reserves of category C 2 in new and explored fields. Approaches to the economic assessment of using electromagnetic surveys as part of the geological exploration are discussed. Our study shows that a correct sequence of electromagnetic prospecting operations at each stage can ensure obtaining an original geological dataset of the given stage and thus providing a proper basis for the next stage of oil-gas field investigation.

A Flexible Single Loop Setup for Water-Borne Transient Electromagnetic Sounding Applications.

Water-borne transient electromagnetic (TEM) soundings provide the means necessary to investigate the geometry and electrical properties of rocks and sediments below continental water bodies, such as rivers and lakes. Most water-borne TEM systems deploy separated magnetic transmitter and receiver loop antennas—typically in a central or offset configuration. These systems mostly require separated floating devices with rigid structures for both loop antennas. Here, we present a flexible single-loop TEM system, the light-weight design of which simplifies field procedures. Our system also facilitates the use of different geometries of the loop antenna permitting to adjust the depth of investigation (DOI) and the minimum sounding depth in the field. We measure the turn-off ramp with an oscilloscope and use the DOI to assess the minimum and maximum exploration depth of our single-loop TEM system, respectively. A reduction of the loop-antenna size improves early-time TEM data due to a reduced length of the turn-off ramp, whereas an increase of the loop-antenna size enhances the signal strength at late times, which allows to investigate deeper structures below the lake bed. We illustrate the capabilities of our system with a case study carried out at Lake Langau in Austria. Our results show that our system is capable of reaching a DOI of up to 50 (with a maximum radius of the circular loop of ), while it also resolves the water layer down to a minimum thickness of (when the radius is reduced to ).

A comparison of 2D inversion of RMT and 1D weighted joint inversion of ground-based and helicopter-borne electromagnetic data

A Pleistocene buried valley, the Cuxhaven Valley, located in Northern Germany, is extensively studied by three different electromagnetic methods. In order to enhance the resolution of the very shallow subsurface structures, the radiomagnetotelluric (RMT) measurements were conducted on the buried valley in addition to the helicopter-borne electromagnetic (HEM) and transient electromagnetic (TEM) data, which were available from the study area along the same profile. First, the RMT soundings are interpreted using the conventional 1D and 2D inversion algorithms. The RMT interpreted models inferred two-layer structure of the subsurface up to a depth of 30 m, which comprises a resistive layer at the top. Available lithological data is used for correlating the resistivity values derived from the 2D inversion of the RMT data with the subsurface formation. Subsequently, a 1D weighted joint inversion algorithm is utilized to interpret these three data sets jointly. The 2D interpreted models of RMT soundings are compared with the 1D weighted joint interpreted models of RMT, HEM, and TEM soundings.

Coupled geophysical and geochemical approach to detect the factors affecting the groundwater salinity in coastal aquifer at the area between Ras Sudr and Ras Matarma area, South Sinai, Egypt

The integration of geophysical and hydrochemical approaches successfully reflects the hydrogeological vulnerability of the arid coastal aquifer in the study area. Both techniques defined the high and low groundwater potentialities regarding water quality and quantity and delineated the fresh/saline water interface.The groundwater exploration comprises 36 transient electromagnetic (TEM) sounding and 4 Two-dimensional electrical resistivity imaging (2DERI) lines. The inversion of both geophysical methods reveals four consequent subsurface layers; 1) the surficial unsaturated alluvial sand deposits layer has a resistivity range of 50–538 Ωm, and thickness <10 m, 2) the partially saturated alluvial sand, 3) the saturated alluvial sand and silt with fresh/brackish groundwater, and 4) the lower saturated alluvial sand and gravel layer with saline groundwater and clay sheet intercalations. The third water-bearing layer acts as a thin fresh/brackish groundwater lense of a moderate resistivity (0.8–55 Ωm) floating over deeper saline groundwater with a very low resistivity range (0.8–5.8 Ωm).The relatively moderate to higher resistivity of the fresh/brackish groundwater zone recorded in the eastern side at the downstream portion of the study area indicates promising subsurface recharge. The thickness of this zone ranges from 4 to 24 m (Groundwater Group I; 507 ≤ TDS ≤3920 mg/L & Groundwater Group II; 4031 ≤ TDS ≤4717 mg/L). While the lower resistivity values characterize the northern and southern portions of higher groundwater salinity (Group III; 5144 TDS 6771 mg/L & Group IV 7452 TDS 27573 mg/L), suggesting mixing with deeper saline/seawater and intercalations of clay sheets. Regarding water quality, it is recommended to excavate the groundwater only for the freshwater/brackish zone to avoid up conning of deep saline groundwater. Finally, flash floods are the primary source of groundwater in this vulnerable area. With a recent intensive expansion of groundwater excavation, a continual monitoring program or groundwater modelling is highly recommended.

Subsurface Geological Characterization of the Late Neogene–Quaternary Argive Basin, Peloponnese, Greece Using Transient Electromagnetic Data and Vintage Stratigraphic Logs

The onshore and offshore clastic deposits of the Argive Basin and the Argolic Gulf, respectively, in Peloponnese, Greece, form a Late Neogene–Quaternary half-graben that connects with the Aegean Sea. The onshore Late Neogene–Quaternary sequence, comprised of chaotically intercalated cohesive and granular clastic deposits, is in angular unconformity with bedrock comprised of Triassic–Upper Cretaceous strongly-weathered, highly-fractured karstic limestones thrusted against Paleogene flysch deposits. While the surface geology of the Argive Basin is well-known, the subsurface geology remains both poorly mapped and understood. We utilized transient electromagnetic (TEM) soundings coupled with 185 vintage stratigraphic logs, current surface geology knowledge, and insights from available geophysical surveys to characterize the subsurface conditions of this sedimentary basin. We estimated the thickness of the young deposits (the depth to bedrock) and detected potential subsurface tectonic structures. The TEM-FAST 48HPC data acquisition system with integrated inversion and visualization software package was used with a single-loop dimension of 50 m × 50 m to collect a total of 329 TEM soundings at 151 stations scattered throughout the basin. The TEM station spacing varied from 200 to 750 m allowing the mapping of 80 km2. The total depth of investigation with the inverted TEM data and the lithology logs was 130 m and 183 m, respectively. The joint interpretation produced several quasi-two-dimensional electrical resistivity profiles that traverse the sedimentary basin in various azimuths and depth slices of average electrical resistivity covering the basin. The depth slices and the vintage stratigraphic logs revealed an uneven bedrock topography overlain by an irregularly thick (over 180 m) Late Neogene–Quaternary heterolithic sediment cover.

FIRST RESULTS OF STUDIES IN THE GORLOVSKAYA VALUES BY GROUND METHODS OF ELECTRIC EXPLORATION WITH CONTROLLED SOURCES

The article presents the first results of the study of the Gorlovskaya depression in the area of coal deposits of the Novosibirsk region by methods of ground electrical prospecting using transient electromagnetic sounding and electrical tomography. Near-surface models were obtained to depths of 100 m, as well as deep geoelectric characteristics of the sedimentary filling of the depression and the eastern fault boundary.

Loop source transient electromagnetics in an urban noise environment: A case study in Santiago de Chile

We have developed a transient electromagnetic (TEM) study in the noisy urban environment of the megacity Santiago de Chile. Our investigation characterizes the electrical conductivity structure of the Santiago Basin down to 300 m depth, providing key information about the sedimentary infill, hydrogeologic aspects, and geomorphological units. In total, 52 TEM soundings were recorded over roughly 900 km2. We identified different transient characteristics and noise patterns, spatially correlated to the investigation area. This step allowed classification of the soundings into three groups: highly distorted data, transients severely affected by cultural electromagnetic noise sources, and good-quality data with low noise levels. Conventional 1D inversion techniques were then used to derive resistivity depth models. The models were interpreted using sedimentary units, alluvial fan, static groundwater level information, and also were verified using borehole data. Based on 1D models, different minimum sedimentary thicknesses are observed, ranging between 50 and 300 m in depth. The average derived sedimentary thickness is approximately 210 m. In addition, seven soundings indicated the basement. These are mostly located toward the periphery of the Santiago Basin. We found a high conductive zone toward the north and a low-conductive area toward the south of the basin. The southern area is characterized by the absence of conductive layers at a depth between 100 and 150 m. This is likely related to the presence of a thick gravel layer and the absence of shallow clay layers acting as aquitards. The resistivity distribution and the sedimentary thickness obtained for this noisy and urban environment are key inputs for groundwater research as the water supply becomes more limited, specifically in central Chile. The overall consistency of the derived subsurface models highlights the suitability of the TEM method for investigating urban noise environments.

Geological and geophysical investigations of an engineering site characterization for construction purposes in Western Saudi Arabia

Characterizing subsurface site conditions and understanding main geological features are critical components of any safety assessment for any construction, particularly defining subsurface flow paths and weakness (i.e. fracture and fault) zones. Poor definition of flow paths and weakness zones may lead to serious hazards during construction. This study, integrating geophysical and geological investigations, is undertaken to enhance the understanding of both surface and subsurface complexities of an engineering site characterization located in western Saudi Arabia near Red Sea. Two geophysical investigations, Electrical Resistivity Tomography (ERT) and Transient Electromagnetic (TEM), were carried out to characterize the subsurface of the study area. A total of three ERT profiles and sixteen TEM sounding measurements was conducted in the study area. Geological investigation was mainly focused to understanding geological features and bedrock conditions of the two opposite river's abutments at the study site. Geophysical results show NNE-SSW and NNW-SSE fault zones which were confirmed by in-situ geological inspections and by satellite images. The results also reveal the presence of geothermal fluids validated by the observation of hot water flowing in the river along the measurements. This study demonstrates that the integrated geophysical and geological investigations provide means to infer geothermal fluids, locate fractures, and fault zones. This study enabled us to characterize and locate the geological structural framework as well as possible geothermal fluids existence in the study area.

Investigating the less ambiguous hydrogeophysical method in exploring the shallow coastal stratified-saline aquifer: a case study at West Port Said Coast, Egypt

Recently, groundwater resources of the coastal aquifers attract the attention of arid countries such as Egypt. West Port Said coast is one of those areas, which is depending on groundwater for urban and industrial expansions. Usually, groundwater exploration in such coastal areas is considered as one of the most difficult challenges due to its highly conductive substrata and seawater intrusion. Thus, the main objective of this research is determining the best hydro-geophysical method, which shows the high accuracy results with less ambiguity in exploring the coastal aquifer in the west Port Said coastal study area, which was a part of Manzala Lagoon as inferred from the processed satellite images. Meanwhile, the vertical electrical sounding (VES) and transient electromagnetic (TEM) have become the most popular hydro-geophysical techniques, which are widely used to explore and locate groundwater resources. In this study, a total of five VES and eight TEM soundings were carried out and correlated with the nearby drilled wells. The results showed that the electrical current of the VES method cannot penetrate deeper than 10 m due to the presence of saline materials that obscured the current to pass through and cannot distinguish clearly between the subsurface conductive layers and suffers from high ambiguity. While the TEM method has provided fast, relatively accurate results, deeper (300-m depth) and has the ability to distinguish clearly between the subsurface conductive layers compared with VES technique. Consequently, the TEM method showed many comparable and less ambiguous results to the drilled wells and suitable for such coastal environments.