Time Domain Electromagnetic Method Research Articles

Joint use of TEM and MRS methods in a complex geological setting

Transient Electromagnetic (TEM), known also as Time Domain Electromagnetic (TDEM) and Magnetic Resonance Sounding (MRS) methods were applied jointly to investigate variations in lithology and groundwater salinity in the Nahal Hever South area (Dead Sea coast of Israel). The subsurface in this area is highly heterogeneous and composed of intercalated sand and clay layers over a salt formation, which is partly karstified. Groundwater is very saline, with a chloride concentration of 100–225 g/l. TEM is known as an efficient tool for investigating electrically conductive targets like saline water, but it is sensitive to both the salinity of groundwater and the porosity of rocks. MRS, however, is sensitive primarily to groundwater volume, but it also allows delineating of lithological variations in water-saturated formations. MRS is much less sensitive to variations in groundwater salinity in comparison with TEM. We show that MRS enables us to resolve the fundamental uncertainty in TEM interpretation caused by the equivalence between groundwater resistivity and lithology. Combining TEM and MRS, we found that the sandy Dead Sea aquifer filled with Dead Sea brine is characterized by a bulk resistivity of ρ x > 0.4 Ωm, whereas zones with silt and clay in the subsurface are characterized by a bulk resistivity of ρ x < 0.4 Ωm. These observations are confirmed by calibration of the TEM method performed near 18 boreholes.

A time–domain electromagnetic sounder for detection and characterization of groundwater on Mars

A prototype time–domain electromagnetic (TDEM) sounder was developed to technical readiness level (TRL) 5 to detect and characterize deep groundwater on Mars. The TDEM method induces eddy currents in the subsurface by abrupt extinction of a steady current in a large, flat-lying loop antenna, and the subsurface response is measured using the same loop or a separate receiver. TDEM has been widely used in terrestrial groundwater exploration and is ideally suited to sense the high electrical conductivity associated with saline groundwater expected on Mars. The inductive regime of TDEM is distinct from ground-penetrating radar: the latter has higher resolution but smaller depth of investigation. Our Mars-prototype TDEM was tested in the laboratory and at a local field site before the principal test was performed on Maui, Hawaii. This location was chosen because of its analogy to Mars in electrical properties: dry, resistive basalt over saline pore water. Results compared favorably to soundings made with a commercial TDEM, clearly detecting the seawater interface at depths of 250 m. We subsequently developed a ballistic deployment system for the loop antenna suitable for robotic missions. Compressed gas launches two projectiles; each consists of two spools on a guide stick. Payout on one spool is back towards the launcher and on the other toward its twin on the other projectile. In this way a triangular loop antenna is formed. The full system was tested twice, successfully achieving a distance of ∼70 m in both. A system capable of deploying a 200 m loop antenna on Mars would have mass <6 kg (including 0.3 kg electronics) and within one sol could detect groundwater at depths up to 5 km. TDEM can probe to depths not possible for radar and answer the question: does groundwater – and a likely subsurface habitable zone – exist on Mars?

The relationship between saline groundwater within the Arava Rift Valley in Israel and the present and ancient base levels as detected by deep geoelectromagnetic soundings

The time domain electromagnetic (TDEM) geophysical method was employed to detect saline groundwater bodies within and in the close margins of the Arava Rift Valley. The Arava Valley aquifers are known to occupy fresh to saline groundwater. The lateral subsurface inflow to the Arava from west and east is characterized by fresh to brackish waters. The results of the present study indicate that salination of groundwater is controlled by both present day and ancient base levels, namely by the Dead Sea in the north and by the Gulf of Elat in the south. The configuration obtained by the TDEM survey exhibits interfaces and palaeo-interfaces between fresh to brackish waters and underlying seawater or diluted seawater intruded inland from both base levels as well as brines intruded from the northern base level. The central Arava structural and hydrological divide seems to escape seawater or brine encroachment at least to the considerable depth of the TDEM measurements.

Examination of the Potential for Time Domain Electromagnetic Method for Monitoring Infiltration and Recharge in Arid Areas

إن عملية تغذية المياه هي من أهم مركبات حساب وقياس ميزانية المياه لتخطيط مصادر المياه في المناطق الصحراوية. إن الطرق الجيوفيزيقية السطحية لها عدة مميزات لمراقبة تغذية المياه، حيث إنها لا تحتاج لحفر آبار وغير مكلفة ويمكن استخدامها لمراقبة مساحات واسعة. يمكن استخدام الطرق الكهربائية والكهرومغنطيسية لمتابعة تصريف المياه وذلك لاعتماد الخواص الكهربائية على المحتوى المائي ومسامية الوسط، لكن من غير الواضح هل يمكن استنتاج التغير في كمية المياه أثناء تصريف المياه من القياسات الجيوفيزيقية فقط أما لا؟ في هذه الدراسة قمنا بدراسة مدى كفاية الطرق الكهرومغنطيسية في النطاق الزمني لمتابعة تصريف تسرب المياه في وسط جاف. وقد قمنا بهذه الدراسة المبدئية كتحليل لحساسية المتغيرات حيث قمنا بتغيير المسامية، ومحتوى المائي الأولى، والتوصيلة المائية في الأرض، وحساب التأثر الكهرومغنطيسي مع الزمن أثناء عملية تصريف المياه لباطن الأرض. نحن نعتبر هذه الدراسة مرحلة أولى لدراسة مدى إمكانية استخدام الطرق الكهرومغنطيسية كوسيلة لمتابعة تغذية المياه للخزانات الجوفية مما يفتح البابا لدراسات أخرى في هذا المجال .

Are the lower subaquifers of the Mediterranean coastal aquifer of Israel blocked to seawater intrusion? Results of a TDEM (time domain electromagnetic) study

Kafri, U. and Goldman, M. Are the lower subaquifers of the Mediterranean coastal aquifer of Israel blocked to seawater intrusion? Results of a TDEM (time domain electromagnetic) study. Isr. J. Earth Sci. 55: xx-xx. The TDEM (time domain electromagnetic) method was employed to delineate a low resistivity seawater intrusion into the multiple aquifer system of the Mediterranean coastal aquifer of Israel. The upper (A, B) subaquifers of the aquifer have been known for a long time to be intruded by seawater along the entire coastal plain. It was sug- gested in the past and supported by sparse hydrogeological information that the lower (C, D) subaquifers are blocked to the sea and thus might escape seawater intrusion. The results of the survey, in conjunction with existing borehole information, al- lowed delineating large areas along the central and southern coastal aquifer, where the lower subaquifers solely occupy relatively fresh water (below 1000 mg/l Cl) close to the seashore. The largest of these areas is an approximately 20 km strip between north Ashdod and Bat Yam. According to the low TDEM resistivities obtained at appropri- ate depths, the lower subaquifers at other areas seem to be locally blocked to the sea or vice versa intruded by seawater.

Les sondages électromagnétiques temporels comme outil de reconnaissance du gisement phosphaté de Sidi Chennane (Maroc) : apport à la résolution d'un problème d'exploitation

Exploitation of the phosphatic layers in Sidi Chennane deposit (Morocco) collides frequently with problems bound to the existence, in the phosphatic series, of sterile bodies qualified as derangements. Our study shows that these bodies, masked by the Quaternary cover can be mapped using the Time-Domain ElectroMagnetic Soundings method (TDEM). It is based on the acquisition and the interpretation of a series of tests carried out above a visible derangement in an old trench of exploitation and on 2500 TDEM soundings carried out in a virgin area of the deposit. The article concerns to the analysis of the results and of the proceeding for a possible large geophysics survey. To cite this article: A. Kchikach et al., C. R. Geoscience 338 (2006).

Groundwater development in Lemoa, Guatemala

In the rural Guatemalan Central Highlands, drinking water wells are a rare commodity due to lack of data regarding depth and sustainability of groundwater. At a study well in Lemoa, aquifer sustainability and other characteristics were estimated using hydrograph data and Theis’s recovery method for pumping tests. These tests showed that while specific capacity remained fairly constant, transmissivity increased, indicating development of the well subsequent to installation of the study well. The time domain electromagnetic (TDEM) method was evaluated at the site as a prospective groundwater prospecting technique. Characterization of subsurface conditions using TDEM is feasible with local subsurface data for correlation; however, it is limited to delineating low electrical resistive zones without geologic data for correlation. Data showed that groundwater produced from volcanic alluvium at approximately 79 m in depth is sustainable at the study area at current use rates.

The use of the time domain electromagnetic method to delineate saline groundwater in granular and carbonate aquifers and to evaluate their porosity

The use of surface geoelectric (GE)/geoelectromagnetic (GEM) methods to determine the porosity of freshwater saturated aquifers using Archie's equation leads in many cases to significant inaccuracy. This is caused by the uncertainties in determining both fluid and bulk resistivities required for calculating porosity of a rock sequence in the case of a known rock type. In order to solve the problem in coastal and other saline-water-intruded aquifers, it is proposed herein to apply the time domain electromagnetic (TDEM) method to detect first the fresh/saline water interface and then to determine the fluid and bulk resistivities of the saline water saturated portion of the aquifer. It is shown that calculating porosity of this portion of the aquifer is much more accurate than that of the freshwater saturated part. Since the porosity of the aquifer is generally invariant with regard to the location of the interface, the proposed method in many cases is applicable to the entire aquifer. The method was tested in both granular and carbonate coastal aquifers of Israel saturated with normal seawater as well as with Dead Sea concentrated brines. The cases of granular coastal aquifers exhibit a good agreement between measured porosities and those calculated using TDEM results. In the case of carbonate aquifers, the situation is more complicated due to their heterogeneity and much wider range of porosities stemming from their karstic and fractured nature.

Development of TDEM Data Acquisition System Based on High Tc SQUIDs for Mineral Exploration

The Time Domain Electromagnetic Method (TDEM) survey is one of the several geophysical exploration methods. In the conventional TDEM survey, an induction coil is used as the magnetometer. However, the measurement depth is limited to about 500 m. Using high Tc SQUIDs, there are expectations of large bandwidth and high sensitivity for the TDEM. We developed the high T c SQUID TDEM system. We have reduced the system noise by developing a 20 mm x 20 mm step-edge type direct coupled SQUID and a low noise direct readout flux locked loop (FLL) circuit. We have also improved the slew rate, optimizing the parameter of the FLL circuit. Consequently, the system noise of less than 0.2 pT/Hz 1/2 at 1 kHz was achieved in the earth's magnetic field. The slew rate was 7.3 mT/sec. We conducted field trials and confirmed that the TDEM using high Tc SQUIDs obtains information of deeper region with high precision compared with the TDEM using induction coils.

Geophysical monitoring and evaluation of coastal plain aquifers.

We use time domain electromagnetic (TDEM) soundings to monitor ground water conditions beneath the coastal plain in eastern North Carolina. The TDEM method measures the earth's response to an induced electromagnetic field. The resulting signal is converted, through a complex inversion process, to apparent resistivity values, which can be directly correlated to borehole resistivity logs. TDEM soundings are used to map the interface between fresh and salt water within coastal aquifers, and estimate depth to basement when siting new monitoring wells. Focused TDEM surveys have identified areas of salt water encroachment caused by high volumes of discharge from local supply wells. Electromagnetic sounding, when used in tandem with the state's network of monitoring wells, is an accurate and inexpensive tool for evaluating fresh water/salt water relationships on both local and regional scales within coastal plain aquifers.

THE APPLICATION OF THE TRANSIENT EM METHOD INTO THE GEOTHERMAL FIELD EXPLORATION

In this work we examine the potential of the time-domain electromagnetic method (Transient EM) in the exploration of geothermal field and the study of the geological environment. The method was applied in the geothermal field of Aristino (Alexandroupolis), westwards of the Traianoupolis, where the known hot springs exist. The EM method and more specifically magnetotellurics (MT), it has been applied in the past in Greece by various researchers, but it is the first time the TEM method is applied for geothermal exploration purposes. As it has been shown by previous works, in the wider region of Aristino an important geothermal field is developed, with geothermal fluid temperature of 30°-90° and high content of dissolved salts of 10gr/lt. These factors result to the appearance of highly conductive geoelectrical anomalies which are directly related to the geothermal field. The existence of two drillholes in the region allowed the calibration and the control of the effectiveness of the TEM method. The geophysical survey has delimited the area of local geothermal interest that is formed by hot aquifer at a relatively shallow depth, extended North of Aristino and between the main faults of the region. The two main faults are responsible for the creation of a tectonic graben in the intermediary region and the development of a highly conductive formation which correlates well with the hot aquifer tank. The geothermal fluid circulates through the faulting system having high temperature caused by the contact with magmatic masses. The application of TEM proved the operational advantages of the method in the geothermal field exploration. The relatively limited number of TEM soundings presented here does not allow the export of safe conclusions regarding the geothermal field of the wider region, although the evaluation of results based on the geological and geoelectrical data of the region is extremely encouraging and justifies the use of the TEM method in the systematic study and mapping of geothermal fields.

A Study on One‐Dimensional Forward for Airborne Time‐Domain Electromagnetic Methods

AbstractBased on the theory of electromagnetics, the forward computing formulae and algorithm are derived for airborne time‐domain electromagnetic methods (ATEM) with dipole‐dipole array under the condition of stratified earth. The computer program for computing the ATEM response is worked out and the forward computing is completed for some typical geoelectrical sections. The forward computing results provide scientific basis for analyzing the detecting capability and applied conditions of the ATEM and for designing the helicopter ATEM system.

Tensor time domain electromagnetic resistivity measurements at Ngatamariki geothermal field, New Zealand

Experimental measurements in the Ngatamariki geothermal field, North Island, New Zealand were made to test the applicability of the time domain electromagnetic method for detailed investigation of the resistivity structure within a geothermal field. Low-frequency square wave signals were transmitted through three grounded bipole current sources sited about 8 km from the measurement lines. Despite high levels of electrical noise, transient electric field vectors could be determined reliably for times between 0.02 and 3.3 s after each step in the source current. Instantaneous apparent resistivity tensors were then calculated. Apparent resistivity pseudosections along the two measurement lines show smooth variations of resistivity from site to site. Over most of the field the images consistently show a three-layer resistivity structure with a conductive middle layer (3–10 Ωm) representing the conductive upper part of the thermal reservoir. A deep-seated region of low resistivity in the northwest of the field may indicate a conductive structure at about 1 km associated with a deeper diorite intrusion. Measurements sited closer than about 100 m to drillholes appear to have been disturbed by metallic casing in the holes. A change in resistivity structure in the east of the field may indicate a major geological or hydrothermal boundary.

A comparison of time domain electromagnetic and surface nuclear magnetic resonance sounding for subsurface water on Mars

The time domain electromagnetic (TDEM) method has enjoyed wide success in terrestrial groundwater exploration, and the contrast in electrical conductivity between dry overburden and groundwater containing even a small amount of dissolved solids on Mars will yield a robust response. However, moist clays or even ores (e.g., massive hematite) will also be electrically conductive and could be mistaken for aquifers on Mars if proper geologic context is lacking. Surface nuclear magnetic resonance (SNMR) is the only noninvasive geophysical method that responds nearly uniquely to water. As the measured EMF is proportional to the proton‐precession frequency, which in turn is proportional to the planet's static magnetic field, SNMR signals are comparatively weak. Using small systems of several kilograms and several watts, the exploration depth of SNMR is one to two orders of magnitude smaller than TDEM: the latter can detect water to depths up to a few kilometers, whereas the former is limited to depths of a few tens of meters. There is no improvement in SNMR signal‐to‐noise with increasing static field where penetration is controlled by aquifer salinity (skin depth). As reasonable integration times cannot substantially increase the exploration depth, much larger transmitter current or loop mass (either requiring system masses of tens to hundreds of kilograms) are the only way to implement SNMR for exploration to depths of at least hundreds of meters. In spite of some ambiguity in target identification, TDEM is recommended for the first generation of in situ active‐source EM measurements for groundwater on Mars.

ＳＱＵＩＤ磁力計を用いた金属鉱床探査のための電磁探査法測定システム開発

I report on the research and development of a data acquisition system, based on a time domain electromagnetic (TDEM) method using a highly sensitive high-temperature superconductor (HTS) SQUID vector magnetometer operating at 77 K, which is suitable for mineral exploration by the Metal Mining Agency of Japan (MMAJ). The MMAJ has achieved stable long-term operation of all three channels during field trials. The SQUID system meets high requirements for slew rate (7.3 mT/s), dynamic range (100 dB) and bandwidth (DC∼100 kHz). It offers deeper penetration of depth than the induction coil system because it is capable of recording the step response that decreases with time slower than the impulse response of the induction coil system. The MMAJ's SQUID system will perform gradient observations that can provide much better resolution of deeper conductive targets than conventional EM field component observations. We have obtained good reproducibility of SQUID data and good correlation between the output signals of the reference induction coils and the derivatives of the SQUID signals in TDEM field trials.

Experimental evaluation of hydrocarbon detection with the Long-Offset Time-Domain Electromagnetic Method in the Cretaceous carbonates of the Tampico–Misantla basin, Mexico

In Mexico, an experimental study with a Long-Offset Time-Domain Electromagnetic Method (LOTDEM) was carried out in the Tampico–Misantla basin of northeastern Mexico. The main objective was to evaluate this method for hydrocarbon exploration. The selected area is suitable for LOTDEM experiments because high quality seismic data, geological information and well logging data are available. The results obtained are excellent, and allow us to determine the types of fluids present in the rock pores and fractures of the studied geological structures. Resistivity anomalies correlate in position and depth with the geological structures observed in the seismic sections where the LOTDEM survey was carried out, and with well logging data from the nearby Franco Española oil field. In the studied area, these structures are 800–1200 m deep, in carbonate rocks with high clay content and are invaded by salt water. Consequently, the resistivity and primary porosities are very low. The main reservoirs of oil and gas in this region are a naturally fractured basinal facies, consisting of fine-grained limestones and shaly limestones, that corresponds to the Cretaceous units San Felipe, Agua Nueva, Tamaulipas Superior and Tamaulipas Inferior. The fractured rocks reservoirs are very difficult to detect, even with well logs. However, the results of this survey show the higher resolution, the greater depth of investigation, and the advantages that the LOTDEM method present compared to the traditional frequency domain electric and electromagnetic (FEM) methods.

Integrating TDEM and MT methods for characterization and delineation of the Santa Catarina aquifer (Chalco Sub-Basin, Mexico)

Magnetotelluric (MT) and time domain electromagnetic (TDEM) surveys were undertaken in the region of Santa Catarina, located in the Chalco Sub-Basin of the Mexico Basin. The objective was to constrain the geometry of the fresh water aquifer and confirm the continuity of the basaltic flows between the volcano and the sedimentary basin. In order to define the stratification at depth with an emphasis on the geometry of the main aquifer, 11 MT and 5 TDEM soundings were recorded along a north–south profile. Interpretation of MT soundings show that the bedrock is located at a depth of at least 800–1000 m. Using TDEM apparent resistivity curves to constrain the high frequency MT data, three main layers were defined overlying the bedrock. These layers are, from the surface to bottom, a 20- to 40-m-thick layer of sands, ash and clay, followed by a very conductive 200-m-thick layer of sand and ash, saturated with highly mineralized water and, finally, a zone with gradually increasing resistivities, corresponding to the main aquifer. The TDEM data, the magnetic transfer functions and the 2D MT model also indicate that a shallow resistive structure is dipping, from the northwest, into the lacustrine deposits of the basin. This feature is likely to be highly permeable fractured basaltic flows, evidenced also in one of the water wells. To verify the presence of fractured basalts below the volcano ranges, 38 TDEM soundings were collected on the flanks of the Santa Catarina range. Layered models obtained from the TDEM soundings enabled an assessment of a major conductive zone (1–10 Ω m) at depth. Two hypothesis are envisaged and the nature of this zone is attributed either to a clayey layer or to fractured basaltic flows. If the latter possibility is confirmed, this continuous zone could provide a channel by which the water contaminated by the Santa Catarina landfill may leak into the basin.

EMMA—A Geophysical Training and Education Tool for Electromagnetic Modeling and Analysis

An interactive modeling and analysis program with a user-friendly graphical interface, for students and professionals in the field of exploration geophysics, has been developed. The ElectroMagnetic Modeling and Analysis program—EMMA—is capable of modeling one-dimensional responses for most electrical and electromagnetic methods and array configurations, including the time-domain, frequency-domain, resistivity, magnetotelluric and borehole methods. EMMA is available on-line, at no charge, from http:∕∕www.hgg.au.dk. An innovation of EMMA is the calculation of the model sensitivity analysis, a feature that is usually only present in inversion codes. The variance of the model parameters depends on the variance of the measured data and the way in which an error is mapped from the data to the model parameters. The measurement situation is realized by ascribing data noise to the model response. This facilitates the calculation of a realistic model parameter analysis. For time-domain data, piece-wise linear waveforms, ramps and low-pass filters are included in the forward modeling scheme. When implemented on a multi-user relational database structure, EMMA is suitable for classroom exercises where the computers are connected through a local network. As an illustration and for motivation, three exercises focusing on the time-domain electromagnetic method are presented: the mapping of noise to model resolution; the effect of filters on the early time response and layer resolution: and an examination of the electric and magnetic field intensity to understand the physical mechanisms of the method. Short courses on the subject of the transient electromagnetic and resistivity methods for hydrogeophysical investigations, offered to technical advisers and to technical policy-makers, have demonstrated EMMA’s succes in providing a productive learning environment.

Application of integrated magnetic resonance sounding and resistivity methods for borehole implementation. A case study in Cambodia

A geophysical survey was conducted in Cambodia to measure the contribution of geophysics to a running drilling programme. The geology of the area (the province of Siem Reap) mainly consists of recent heterogeneous sediments of sand, silt and clay. The thickness of this formation ranges from 20 to 100 m and lies on Jurassic to Tertiary rocks. Surveys were done with direct current methods [one-dimensional vertical electrical sounding (1D VES), and two-dimensional (2D) electrical imaging], time domain electromagnetic (TDEM) sounding and proton magnetic resonance sounding (MRS). To validate the geophysical results, boreholes were drilled and tested with electrical logs and pumping tests. We found that: (1) The resistivity methods (VES, 2D electrical imaging and TDEM) are very sensitive to the groundwater electrical conductivity which is highly heterogeneous within the province. A preliminary relationship between measured groundwater conductivity and aquifer resistivity is proposed. (2) The MRS gives accurate information on groundwater occurrence for the 5- to 60-m-deep layers. A preliminary relationship between MRS data (aquifer transmissivity estimated from MRS field measurements) and hydrodynamic parameters (aquifer local transmissivity and borehole relative specific capacity estimated from borehole pumping tests) is proposed. (3) The resistivity methods and MRS are complementary, and a joint use is recommended. (4) At the survey scale, the borehole success rate was improved from 56% to 90% by the use of geophysics. Crossing the technical and cost analyses, we propose a geophysical methodology to implement boreholes in the province of Siem Reap. This methodology could both increase the borehole success rate and save money at the programme scale. It consists of the use of: 1. MRS, TDEM and electrical methods jointly where the borehole success rate is less than 30%. 2. MRS and TDEM jointly where the borehole success rate ranges from 30% to 50%. 3. Electrical methods (VES and 2D electrical imaging) as standard methods.

Geophysical (Time Domain Electromagnetic Model) delineation of a shallow brine beneath a freshwater lake, the Sea of Galilee, Israel

The Sea of Galilee is a freshwater lake, into which saline water emerges through onshore and offshore springs and through flux from the lake's sediments. The novel surface marine modification of the time domain electromagnetic method was used to map the spatial distribution of brines in the sediments below the lake. Results indicate that electrical resistivities of 1.0 and 0.5 ohm‐m are detected at depths of ∼10 m below the lake bottom in most of the lake area, which are equivalent to ∼11,000 and 22,000 mgCl/L, respectively. Relatively fresh groundwater was detected beneath most of the shoreline. Faulting controls the vertical interfaces between saline and fresh groundwaters. It is hypothesized that salt transport is dominated by molecular diffusion in the central part of the lake and by advection from regional aquifers in the margins.