Time Domain Electromagnetic Method Research Articles

Study of Thermal Water Resources in Sri Lanka Using Time Domain Electromagnetics (TDEM)

Thermal springs can be utilized as an environmental friendly source of renewable energy, as well as for other purposes. Time domain electromagnetic (TDEM) method is used as an accessory tool in geothermal exploration to investigate the local heterogeneities in geology. The study consists of two of the thermal springs in Sri Lanka, Wahawa and Mahaoya, both in the same crustal unit. Major structural discontinuities of the area vary between two peculiar fracture sets striking ENE and NW. Close relationships between these individual springs have been witnessed by their approximate major cation chemistries, which provoke the idea of geochemical provinces. Survey reveals the pathways of the heated water to the surface, at Wahawa, although the path is not clear at Mahaoya. Near surface resistivity diagrams can be used only to interpret the immediate depths of the springs. The expected structural relationships may be identified with a grid of TDEM soundings encompassing both the spring systems.

Geological environment of karst within chalk using airborne time domain electromagnetic data cross-interpreted with boreholes

The ability of airborne Time Domain ElectroMagnetic (TDEM) to image plurikilometric chalk heterogeneities and its implications for the development of a karstic system is addressed in this study. A heliborne TDEM survey was conducted around Courtenay (France) over the Paris Basin Upper Cretaceous chalk. This aquifer is known as a highly weathered and karstified horizon both strongly modify chalk petrophysical properties. Numerous boreholes and one recently reprocessed seismic line were used in order to strengthen TDEM interpretations. We performed cross statistics between boreholes and the resistivity model. This allowed defining empirical resistivity ranges corresponding to the main geological formations within the area. We were therefore able to map large scale heterogeneities in the chalk over the study area. First, the TDEM method highlighted probable weathering corridors in the chalk, related to the tectonic activity, consistent with faults previously interpreted in the seismics at deeper levels. Second, it was possible to image a large scale undulating geometry in the chalk with a SW-NE orientation, this direction is consistent throughout the Paris Basin, and well defined on the cliffs of Normandy (Channel coast, north of France). This geometry has revealed two separate chalk deposits C1 and C2 in Courtenay area: C1 is more resistive than C2. The resistivity model has then been compared to piezometric measurements acquired as part of previous hydrological studies. The karstic drainage appears to be developped within C1 chalk deposit and most of the piezometric domes seem to be associated to intermediate resistivity zones in C1, interpreted as weathered. According to the results obtained from this study, we were able to suggest a geological framework for the development of Courtenay karstic system.

Detection of Saline Groundwater Bodies between the Dead Sea and the Mediterranean Sea, Israel, Using the TDEM Method and Hydrochemical Parameters

A Time Domain ElectroMagnetic (TDEM) survey coupled with hydrogeological information was carried out along the Be’er Sheva valley between the Mediterranean Sea and the Dead Sea base levels, aiming to obtain the subsurface distribution of the different fresh and saline water bodies. Hydrochemical considerations and previous TDEM results hinted that the previously accepted model of an upper fresh water body underlain by solely Dead Sea (R-type) brines in the east and C-type brines in the west does not match the actual observations. Thus, the suggested working hypothesis of additional intruding seawater from the Mediterranean Sea to the Dead Sea base level was checked by the TDEM method. The results, indeed, exhibit an upper high resistivity fresh water body, an underlying low resistivity brine body and a moderate resistivity body in between. The origin of this body is not uniquely determined based on the geophysical measurements alone. Analysis of borehole data testifies that hydrochemical parameters of the body cannot be solely interpreted as a mixture of the above two end members, but rather calls for an additional contribution of intruding seawater. The suggested configuration consists of an upper fresh water body flowing to both base levels in the west and the east. Underneath there is a saline body resulting from seawater intrusion reaching from the Mediterranean to the Dead Sea base level. At the bottom there is a third body of the westward density driven Dead Sea brine. The entire configuration is supported by results of subsurface flow modeling.

Introduction to the Time Domain Electromagnetic Special Issue of JEEG

The idea to propose a special issue on time domain electromagnetic (TEM) methods was based on my perception that recent innovations and developments, both in instrumentation and processing, deemed it time to perform a checkup on the state-of-the-art of the method. Many of these advancements address offshore and airborne applications rather than ground-based ones. For example, the popular use of the marine controlled source electromagnetic (MCSEM) method in the oil industry, undoubtedly a real “renaissance” for EM, marked a surge in the development of new devices and approaches, together with the continuous improvement in technology and processing of airborne EM data. As shown in this issue, the application of ground-based TEM has also grown, with advances in instrumentation, data processing, and its application. The papers by Xue et al. and Li …

The Ancient Roman Aqueduct of Karales (Cagliari, Sardinia, Italy): Applicability of Geophysics Methods to Finding the Underground Remains

ABSTRACTDetection of the buried antique Roman aqueduct, which supplied fresh water to the ancient town of Karales (Cagliari, Italy), is not a trivial problem because of the small size of its cross section, its depth (about 10 m), and of the presence of shallow conductive layers. In order to determine the best geophysical method to use in the research of the conduit, a test was carried out over a well‐known section of the underground aqueduct in its extra‐urban part. Taking into account the geological features of the site, time‐domain electromagnetic (TEM), very low‐frequency (VLF) and electrical tomography methods were chosen. The test was conducted over several profiles. The results showed that among the electrical resistivity arrays, the Wenner–Schlumberger is the most suitable for the detection of the conduit some metres in depth, whereas the other electrical methods did not have enough resolution to distinguish between the aqueduct and other non‐related anomalies at the requested depth. The TEM method also showed good capability to detect the presence of the aqueduct in spite of very conductive superficial layers. With the exception of one profile, all VLF profiles showed anomalies that could be correlated to the aqueduct. Copyright © 2014 John Wiley & Sons, Ltd.

Relevance of AEM and TEM to Detect the Groundwater Aquifer at Faiyum Oasis Area, Faiyum, Egypt

This study investigates the groundwater aquifer located in Fayuim oasis. In this study, two of the electromagnetic measurement methods have been used in determining the hydrological situation in the Fayoum oasis. The first is airborne electromagnetic (AEM) which, sometimes is referred to as Helicopter electromagnetic (HEM) and the second is ground Time-domain Electromagnetic method (TEM). The subsurface consists of four geoelectrical layers with a rough slope towards the center. The third and the fourth layers in the succession are suggested to be the two-groundwater aquifers. The third layer saturates with fresh water overlying saline water which exists in the bottom of the second one. It is worth mentioning that the depth of the fresh water surface undulates between the surface level in two lakes in the study area and 57 meters below the ground, whereas the thickness of the fresh water aquifer varies from 13 to 36 meters. The depth of the saline water surface undulates between 59 and 81 meters below the ground. In general, airborne electromagnetic surveying has the advantage of fast resistivity mapping with high lateral resolution. Groundbased geophysical surveys are often more accurate, but they are definitely slower than airborne surveys. It depends on targets of interest, time, budget, and manpower available by the method or the combination of methods that will be chosen. A combination of different methods is useful to obtain a detailed understanding of the subsurface resistivity distribution.

Investigation of groundwater resources in the Komadugu Yobe Valley (Lake Chad Basin, Niger) using MRS and TDEM methods

Groundwater resources quantification and management is a key issue for agricultural development in the Komadugu Yobe (KY) River valley region in the semiarid part of the Lake Chad Basin. To improve the knowledge of available groundwater resources in this poorly-documented area, a geophysical survey across the river valley was conducted near the town of Diffa, southeast Niger. The goal was to estimate the hydrogeological properties of Quaternary formations to a depth of ∼100m. Numerical modeling showed that sedimentary deposits composed of thin clayey, loamy, and sandy layers could not be accurately resolved in detail by non-invasive geophysical methods due to the limited spatial resolution of the methods. The use of the Time Domain Electromagnetic (TDEM) method alone was not sufficient to estimate aquifer parameters and the Magnetic Resonance Sounding method (MRS) was used to supplement the geophysical dataset. A twelve kilometer long profile (117 TDEM and 11 MR soundings) was surveyed across the valley to evaluate changes in hydrogeological properties of the Quaternary aquifer from the middle of the river valley to the surrounding sandy plain area. Our results show that the Quaternary aquifer below the KY valley differs from its surroundings and it can be described as follows: (a) the thickness of the KY aquifer ranges from 30 to ∼60m and is limited at depth by electrically conductive clay layer, (b) at a depth of ∼70–80m, TDEM soundings show a continuous conductive substratum (resistivity is ∼2Ωm), attributed to Pliocene clayey formations, (c) the KY valley aquifer may have a larger volume of unconfined groundwater than the surrounding plain aquifer area (MRS water contents of 20–25% and ∼15% respectively), (d) the KY aquifer transmissivity estimated from MRS data is higher than values derived from pumping tests conducted outside the river valley. This study confirms that the Komadugu Yobe valley aquifer represents a significant resource for future agricultural development. In addition, our study shows that the aquifer is not protected by shallow clayey layers; therefore, precautions against contamination should be taken to preserve the quality of this resource.

Non-invasive characterization of water-bearing strata using a combination of geophysical techniques

Hydrogeological investigations were carried out in an arid area of Inner-Mongolia to determine the locations of future water supply resources. The first geophysical survey was conducted near Baiqi to identify favorable boreholes using magnetic resonance sounding (MRS). The yield capacities of 43 sites were investigated, and the extent of the potential groundwater storage was determined. Previous studies have indicated that a major tectonic structure may have a significant impact on the groundwater flow and well yield in the study area. Therefore, high-resolution seismic surveys were applied in the second stage of the investigation to determine the fault locations. After the regional identification, a major structure was investigated in detail to map the fracture patterns. Based on the assumption that the hydraulic conductivity of this formation is similar along the entire strike of the fracture, we proposed drilling a borehole (BQ3) in the zone. However, this well has a yield of only 0.8 L/s, falling short of the required flow rate of 3.0 L/s. Therefore, the objective of the final stage of exploration was to accurately define the attitude and extension of the aquifer and to select a more favorable borehole site that would meet the required water flow rate. The geophysical exploration was carried out using time-domain electromagnetic (TDEM) and MRS methods. The MRS results suggest optimal locations for water supply boreholes within the subsurface structures mapped by the TDEM inversion method. The data obtained by drilling and coring are in agreement with the predicted aquifer thickness from the TDEM data. Pumping tests indicate that the water discharge of borehole BQ4 was 3.5 L/s. Our results demonstrate that the delineation of the groundwater body using a combined application of three geophysical methods (the MRS, TDEM and 2D seismic methods) was successful. ► A well was determined using 2D seismic method, but, it has a yield of only 0.8 L/s. ► TDEM provides geoelectrical distribution in fracture zones identified by 2D seismic. ► Joint use of MRS and TDEM is useful for determining aquifer layers when SNR is low. ► QT inversion of MRS can get optimum spatial resolution of water content distribution. ► A combined use of three geophysical methods was successful to delineate groundwater.

Application and evaluation of electromagnetic methods for imaging saltwater intrusion in coastal aquifers: Seaside Groundwater Basin, California

Developing effective resource management strategies to limit or prevent saltwater intrusion as a result of increasing demands on coastal groundwater resources requires reliable information about the geologic structure and hydrologic state of an aquifer system. A common strategy for acquiring such information is to drill sentinel wells near the coast to monitor changes in water salinity with time. However, installation and operation of sentinel wells is costly and provides limited spatial coverage. We studied the use of noninvasive electromagnetic (EM) geophysical methods as an alternative to installation of monitoring wells for characterizing coastal aquifers. We tested the feasibility of using EM methods at a field site in northern California to identify the potential for and/or presence of hydraulic communication between an unconfined saline aquifer and a confined freshwater aquifer. One-dimensional soundings were acquired using the time-domain electromagnetic (TDEM) and audiomagnetotelluric (AMT) methods. We compared inverted resistivity models of TDEM and AMT data obtained from several inversion algorithms. We found that multiple interpretations of inverted models can be supported by the same data set, but that there were consistencies between all data sets and inversion algorithms. Results from all collected data sets suggested that EM methods are capable of reliably identifying a saltwater-saturated zone in the unconfined aquifer. Geophysical data indicated that the impermeable clay between aquifers may be more continuous than is supported by current models.

Delineation of saltwater intrusion zones using the time domain electromagnetic method: the Nabeul–Hammamet coastal aquifer case study (NE Tunisia)

AbstractThe time domain electromagnetic method (TDEM) is applied to monitor, to delineate and to map the saltwater intrusion zones in the Mediterranean Plio‐Quaternary aquifer. Forty‐two TDEM soundings were carried out in the coastal plain of Nabeul–Hammamet region (NE Tunisia). TDEM resistivity data were correlated with the existing borehole logging data to assign them to a particular lithology and to provide information about the position of the freshwater–seawater transition zone. The geoelectric sections showing the vertical configuration of seawater intrusion, with the brackish‐salty‐saturated zones, have a resistivity ranging from ~0.1 to 5 Ω⋅m and are detected at a depth lower than 1.5 m. The salinized zones are located at Nabeul (Sidi Moussa, Sidi El Mahrsi, Al Gasba and Mrazgua) and at Hammamet (Touristic zone of Hammamet north and south, Baraket Essahel) and reached a distance of 4 km from the coastline, indicating a severe state for the aquifer in these zones. These TDEM results are confirmed by the increase of chloride concentration content in the analysed water samples of monitoring wells. Moreover, in the northeastern part, the presence of a saltwater front located far from the coast and along the NW–SE major surface fault can be explained by two hypothesis: (i) this fault seems to provide a conduit for seawater to move readily towards the water wells and (ii) the clay and gypsum infiltration of marine Messinian deposits through the fault plane leads to low resistivities. Finally, it comes out from this study that TDEM survey has successfully depicted salinized zones of this coastal aquifer. Copyright © 2012 John Wiley & Sons, Ltd.

Time Domain Electromagnetic Differential Equation Methods

As the time domain electromagnetic differential equation methods, FDTD, FIT, TDFEM have some relations in the mesh generations, discrete equations and hodge operator.

TDEM survey in urban environmental for hydrogeological study at USP campus in São Paulo city, Brazil

Abstract In this work, some TDEM (Time Domain Electromagnetic) results at USP ( University of Sao Paulo ) campus in Sao Paulo city, Brazil, are presented. The data were acquired focusing on two mains objectives: (i) to map geoelectrical stratigraphy of Sao Paulo sedimentary basin, emphasizing on hydrogeological studies about sedimentary and crystalline aquifers, and (ii) to analyze the viability of TDEM data acquisition use in urban environment. The study area is located in Sao Paulo basin border, characterized by Resende and Sao Paulo formations, which are constituted by sand-clays sediments over a granite-gneissic basement. Two equipments were used in order to acquire database: Protem47 (low power), and Protem57-MK2 (high power). Capacitive noise affect obtained data with Protem47 due to the presence of metal pipes buried at IAG/USP (Institute of Astronomy, Geophysics, and Atmospheric Science) test site at USP. On the other hand, capacitive noise did not affect acquired data with Protem57-MK2, and the data present high signal to noise ratio. Surveys helped in determining sedimentary and crystalline aquifers, characterized by a fracture zone with water inside basin basement (conductive zone). Results show good agreement with local geology obtained from lithological boreholes located in the study areas. Moreover, it shows that TDEM method can be used in urban environments with a countless potential in hydrogeological studies, offering great reliability. Studies showed that main TDEM-method limitation at USP was the lack of space for opening the transmitter loop. Results are very promising and open new perspectives for TDEM-method use in urban environments as this area remains unexplored.

A Bird Calibration Device of Helicopter‐Borne TEM with a Concentric Bucking Loop

AbstractThe multiple instrument calibrations of flying helicopter‐borne time‐domain electromagnetic method (HTEM) can eliminate the environmental time‐varying effect on the system during flight. Using the bird calibration device to solve the problem that the wire‐loop method can't achieve, this paper considers the bird model of HTEM with the concentric bucking loop, bringing forward the bird device space models and the coil circuit models, and analyzing the transmitter magnetic field distribution, the bucking loop's effect and the relationship between it and the receiving coil, the space information of the calibration device and its signal detected method. Conclusions are as follows: (1) The bucking loop that loses 0.89% of the transmitting area can increase 44.5 dB of the signal dynamic range. (2) The calibration device is horizontally placed between the bucking loop and the transmitting coils. If it has more turns and greater radius and is closer to the bucking loop, its signal response becomes stronger. (3) When the inductance of calibration coil, with less resistance, becomes greater, its exponential decays more slowly. The signal detection time point is directly decided by damping character of receiver coil which had matching resistance. We did experiments about the calibration curve characters and the device coil displacement. The relative error about the time constant of the device characteristics is less than 1.3%, which proves the validity of the coil circuit models and device space models. The proposed models and approaches are also the same with the bird calibration device research about HTEM with the eccentric bucking loop and the known wire‐loop static tests on the high‐resistance formation.

Quasi-2D inversion of DCR and TDEM data for shallow investigations

Joint or sequential inversion of direct current resistivity (DCR) and time-domain electromagnetic (TDEM) data commonly are performed for individual soundings assuming layered earth models. DCR and TDEM have different and complementary sensitivity to resistive and conductive structures, making them suitable methods for the application of joint inversion techniques. This potential joint inversion of DCR and TDEM methods has been used by several authors to reduce the ambiguities of the models calculated from each method separately. A new approach for joint inversion of these data sets, based on a laterally constrained algorithm, was found. The method was developed for the interpretation of soundings collected along a line over a 1D or 2D geology. The inversion algorithm was tested on two synthetic data sets, as well as on field data from Saudi Arabia. The results show that the algorithm is efficient and stable in producing quasi-2D models from DCR and TDEM data acquired in relatively complex environments.

Hydrogeophysical Methods for Analyzing Aquifer Storage and Recovery Systems

Hydrogeophysical methods are presented that support the siting and monitoring of aquifer storage and recovery (ASR) systems. These methods are presented as numerical simulations in the context of a proposed ASR experiment in Kuwait, although the techniques are applicable to numerous ASR projects. Bulk geophysical properties are calculated directly from ASR flow and solute transport simulations using standard petrophysical relationships and are used to simulate the dynamic geophysical response to ASR. This strategy provides a quantitative framework for determining site-specific geophysical methods and data acquisition geometries that can provide the most useful information about the ASR implementation. An axisymmetric, coupled fluid flow and solute transport model simulates injection, storage, and withdrawal of fresh water (salinity ∼500 ppm) into the Dammam aquifer, a tertiary carbonate formation with native salinity approximately 6000 ppm. Sensitivity of the flow simulations to the correlation length of aquifer heterogeneity, aquifer dispersivity, and hydraulic permeability of the confining layer are investigated. The geophysical response using electrical resistivity, time-domain electromagnetic (TEM), and seismic methods is computed at regular intervals during the ASR simulation to investigate the sensitivity of these different techniques to changes in subsurface properties. For the electrical and electromagnetic methods, fluid electric conductivity is derived from the modeled salinity and is combined with an assumed porosity model to compute a bulk electrical resistivity structure. The seismic response is computed from the porosity model and changes in effective stress due to fluid pressure variations during injection/recovery, while changes in fluid properties are introduced through Gassmann fluid substitution.

The use of Time Domain Electromagnetic method and Continuous Vertical Electrical Sounding to map groundwater salinity in the Barotse sub-basin, Zambia

This paper describes the results from the application of two geophysical exploration techniques, Time Domain Electromagnetic (TDEM) and Continuous Vertical Electrical Sounding (CVES) that have proved effective in mapping groundwater salinity variations within the sedimentary formations of the Barotse sub basin in the Western Province of Zambia. TDEM was used to map groundwater salinity variations on a regional scale, whereas CVES was used at the local scale to investigate freshwater–saltwater distribution in an ephemeral river valley. On a regional scale, salt water occurrence was shown to be present mainly on the south-eastern portions of the basin, which are situated in a rift that forms a tripe junction with the East African Rift Valley. The general geophysical model indicates an aquifer with saline water with a thickness of about 40m with resistivity variations less than 35Ωm (more than 500mg/l of Cl− based on a formation factor of 5), overlain by an unconfined freshwater aquifer of about 10m thickness with resistivities in excess of 70Ωm (i.e. less than 250mg/l of Cl− based on a formation factor of 5). The origin of the saline water is hypothesized to be related to the evapo-concentration of salts in interdune deposits, which were subsequently buried due to dune migration about 32 to 4 thousands of years ago or kilo annums (ka). The occurrence of saline groundwater could also possibly be linked to evaporation of a former Lake Paleo Makgadikgadi, an extensive endorheic lake system that once covered large parts of Southern Africa. Locally, a thin freshwater aquifer was observed in an ephemeral river valley, indicating recent recharge of river water into a pre-existing saline environment.

An electromagnetic system for planetary geophysical exploration

The paper presents a study, carried out in the context of TESEO project (Sistema di prospezione TDEM per l’Esplorazione Geofisica dei pianeti), for the design of a TDEM (Time Domain Electromagnetic Method) system suitable for planetary subsurface exploration. The TDEM technique has been selected since it is based on the parameter of electrical conductivity, which is generally capable of discriminating geological layered structures in a fully non-invasive manner, down to depths ranging several hundred meters. Furthermore, TDEM doesn’t need any material sample and intrinsically satisfies the planetary protection and cross contamination requirements. Finally, since the TDEM is a time-domain technique, it is in principle possible to retrieve information from different depths with single measurement. This last peculiarity makes the TDEM technique much more expeditious and flexible than analogous techniques operating in the frequency domain. The paper describes the purposes of TESEO project, the requirements due to space application and the preliminary system design.

TDEM for Martian in situ resource prospecting missions

This paper presents a TDEM (Time Domain Electromagnetic Methods) application, addressed to the search for water on Mars. In this context, the opportunities for a TDEM system as payload in a future mission are investigated for different in situ exploration scenarios. The TDEM sounding capability is evaluated with respect to the expected Martian environment, and some considerations are made about the many unknown variables (above all the background EM noise and the subsoil composition) altogether with the limited resources availability (mission constraints in mass, time and power) and the way they could represent an obstacle for operations and measurements.

Tem-fast: a useful tool for hydro-geologists and environmental engineers

Time Domain Electromagnetic (TDEM) methods have proven to be efficacious in many studies involving environmental and engineering problems, producing better results than the traditional galvanic techniques. In fact, the major advantage of this method is that the soundings can be performed in a relatively short time and in a small place. TDEM sounding appears to be particularly useful for discriminating between layers having low resistivity, but interpretative limitations arise when intermediate and deep resistive layers occur. So the method appears complementary to the traditional electrical soundings. This method has been experimentally tested in well characterised test sites where geophysical investigations (reflection seismic, SEV, IP, gravimetry) and drillings, as well as chemical and isotopic analyses had already been conducted. The results confirm that the method is practical, economic and perfectly reliable, not only at large depths, but also in shallow research, using fast acquisition devices working in the range from 4 ms to some ms.

Hydrogeophysical exploration of three-dimensional salinity anomalies with the time-domain electromagnetic method (TDEM)

The time-domain electromagnetic method (TDEM) is widely used in groundwater exploration and geological mapping applications. TDEM measures subsurface electrical conductivity, which is strongly correlated with groundwater salinity. TDEM offers a cheap and non-invasive option for mapping saltwater intrusion and groundwater salinization. Traditionally, TDEM data is interpreted using one-dimensional layered-earth models of the subsurface. However, most saltwater intrusion and groundwater salinization phenomena are characterized by three-dimensional anomalies. To fully exploit the information content of TDEM data in this context, three-dimensional modeling of the TDEM response is required. We present a finite-element solution for three-dimensional forward modeling of TDEM responses from arbitrary subsurface electrical conductivity distributions. The solution is benchmarked against standard layered-earth models and previously published three-dimensional forward TDEM modeling results. Concentration outputs from a groundwater flow and salinity transport model are converted to subsurface electrical conductivity using standard petrophysical relationships. TDEM responses over the resulting subsurface electrical conductivity distribution are generated using the three-dimensional TDEM forward model. The parameters of the hydrodynamic model are constrained by matching observed and simulated TDEM responses. As an application example, a field dataset of ground-based TDEM data from an island in the Okavango Delta is presented. Evaporative salt enrichment causes a strong salinity anomaly under the island. We show that the TDEM field data cannot be interpreted in terms of standard one-dimensional layered-earth TDEM models, because of the strongly three-dimensional nature of the salinity anomaly. Three-dimensional interpretation of the field data allows for detailed and consistent mapping of this anomaly and makes better use of the information contained in the TDEM field dataset.