Magnetic Resonance Sounding Method Research Articles

Evaluating the applicability of magnetic resonance sounding for groundwater exploration at selected locations in Aswan, Egypt

ABSTRACT Magnetic resonance sounding (MRS) was performed to evaluate its applicability for characterising groundwater at selected locations in Aswan Governorate, Egypt. The MRS mainly detects a weak but noticeable alternating magnetic field as MRS signals when the proton spin in groundwater produces non-magnetic moments and rotates around the geomagnetic field, indicating the detection of groundwater. The MRS system was first correlated at a piezometer with a known water table depth (~80 m). Subsequently, three MRS sounding stations with unknown water tables were tested at Qism Aswan at ~ 18 km from the piezometer using NUMIS Auto magnetic resonance sounding system following the MRS site common practices. The MRS data were subjected to adaptive notch filtering to remove spiky noise and potential power line harmonics, and finally enhance the MRS signals at the selected stations. The water depth (~75–95 m) estimated from the MRS data around the piezometer largely coincides with the actual depth (~80 m) at the piezometer itself. A transient electromagnetic (TEM) profile performed near the measured MRS stations indicates no water content. Unlike the MRS data for NNL-1, the data for NNL-2 and NNL-3 match well with TEM data at shallow depths. The evaluation generally indicates that the MRS method is applicable for groundwater exploration at the selected piezometer and the other three stations. However, the method necessarily requires additional denoising efforts to comprehensively characterise groundwater occurrence and aquifer parameters.

Research on the Detection Method of MRS Signal Initial Amplitude Based on Chaotic Detection System

When using magnetic resonance sounding (MRS) method for groundwater, the detection accuracy is often affected by electromagnetic noise. The chaos method can achieve signal extraction at a lower signal-to-noise ratio. However, this method has many problems when used to obtain MRS signals. Aiming at the attenuation characteristics of MRS signals, in this paper we propose a pre-segmentation method of MRS signals to improve the accuracy of chaos detection. In addition, a chaotic detection system model suitable for MRS signal recognition was constructed, and a calculation method for center-covered was used to improve the efficiency of determining the chaotic state. Through a large amount of simulation data, we have verified that this method can reliably extract MRS signals with a signal-to-noise ratio (SNR) of -30 dB, which provides technical support for the MRS method to work with a relatively high electromagnetic noise.

Harmonic noise cancellation of magnetic resonance sounding signal based on wavelet modulus maximum

Magnetic resonance sounding (MRS) signals are often severely distorted by noise, primarily the power-line harmonics which originates from anthropogenic interferences. Noise cancellation is a particularly important issue for the application of the MRS method. In order to reduce the harmonic peaks as well as the noise floor between the 50 Hz harmonics effectively, a method based on the wavelet modulus maximum is proposed. The method is based on marking the local modulus maximum points in the wavelet transform coefficients and removed the modulus maximum contributed by noise for MRS signal reconstruction. The optimum wavelet basis for noise cancelling is determined through simulations with synthetic signals. The accuracy and stability of the proposed method are investigated using synthetic signals embedded in both artificial and field-recorded noise. In addition, the results of the test on field data show the proposed method can effectively improve the fitting accuracy of the signal parameters and obtain stable inversion results.

Contribution of Electromagnetic Shielding and the Bloch–Siegert Effect to Magnetic-Resonance Sounding

Abstract —Calibration experiments of magnetic-resonance sounding (MRS) were carried out on the ice of the Ob’ Reservoir. For the first time, a comparative quantitative analysis of the contribution of electromagnetic shielding and the Bloch–Siegert effect to the MRS signal was carried out, and the asymmetry of this contribution when detuning from the resonant frequency was investigated. The results of theoretical calculations without regard to the Bloch–Siegert effect differ about three times from the experimental data at the maximum intensity of the radio frequency pulse for the experimental data. At the same time, when taking into account the interference of electromagnetic shielding and the Bloch–Siegert effect, the results of calculating the amplitude and phase of the signal best approximate the field experimental data. Based on the good agreement between the experiment and the theoretical calculations taking into account the interference of electromagnetic shielding and the Bloch–Siegert effect, it is demonstrated that this model should be used to solve the direct and inverse problems of MRS of underground aquifers. The results obtained allow us to calibrate the MRS method and use it not only as an indicator of underground water but also as a means of quantitative measurement. The necessity of taking into account the Bloch–Siegert effect in real field conditions is demonstrated by the example of MRS studies in the Ebro River valley (Spain). When taking into account only electromagnetic shielding (without regard to the Bloch–Siegert effect), the signal amplitude differs nearly twice at the maximum pulse intensity for experimental data, and the signal phase differs 12 times at the maximum pulse intensity. Therefore, in previously published works, approximation of the experimental data using the model without regard to the Bloch–Siegert effect was made by adding layers at depths close to the maximum for this type of antenna (approximately 50 m). In reality, however, these layers do not exist; they appear only as an artifact of the model without regard to the Bloch–Siegert effect. The best approximation of the experimental data can be achieved by taking into account the interference of electromagnetic shielding and the Bloch–Siegert effect.

Magnetic resonance sounding for the estimate of hydrogeologic characteristics of chalk aquifers (North of France)

The hydrogeological characteristics are required for investigating the modelling processes. The transmissivity and hydraulic conductivity values which are obtained from the inverse methods and pumping tests are not representative in the northern part of France where the major aquifer is composed of the chalk deposits. In addition, these methods are costly and time-consuming. In the north of the Paris Basin and by using the magnetic resonance sounding (MRS), the chalk aquifer reveals the vertical distribution of the porosity, the hydraulic conductivity, and the transmissivity. The decay constants of the MRS signal which have been identified from the MRS analysis are considered parameters which control the permeability and the transmissivity values. Five MRS surveys were realized and performed by the electrical resistivity tomography (ERT) aiming to characterize the lithology variation and the thickness of the hydrogeological units. This comparison shows the power of MRS methods of the chalk aquifer in the characterization of the hydrogeological system in the north of France. The water content which is deduced from the MRS highlights a heterogeneous distribution in the aquifer and the water content can reach 35%. Results which are deduced from the pumping test and the MRS are consistent. Indeed, the transmissivity values vary generally between 10−3 and 10−2 m2/s. This comparison shows the utility of the multidisciplinary approaches in the characterization of the chalk system in the north of France.

Geophysical Prospecting for Groundwater Resources in Phosphate Deposits (Morocco)

The Moroccan phosphate deposits are the largest in the world. Phosphatic layers are extracted in open-pit mines mainly in the sedimentary basins of Gantour and Ouled Abdoun in Central Morocco. The purpose of this study was to prospect and evaluate the water potential of aquifers incorporated in the phosphatic series using the following geophysical methods: Magnetic resonance sounding (MRS), electrical resistivity tomography (ERT), time-domain electromagnetics (TDEM), and frequency-domain electromagnetics (FDEM). The objective was, on the one hand, to contribute to the success of the drinking water supply program in rural areas around mining sites, and on the other hand, to delimit flooded layers in the phosphatic series to predict the necessary mining design for their extraction. The use of geophysical methods made it possible to stratigraphically locate the most important aquifers of the phosphatic series. Their hydraulic parameters can be evaluated using the MRS method while the mapping of their recharge areas is possible through FDEM surveys. The results obtained in two selected experimental zones in the mining sites of Youssoufia and Khouribga are discussed in this paper. The application of the implemented approach to large phosphate mines is in progress in partnership with the mining industry.

Feasibility study of a surface-borehole NMR method

We present results of a feasibility study of a borehole induction-coil sensor for surface-borehole NMR (SBNMR) investigations. This sensor of 7 cm diameter and 180 cm length is connected to a standard MRS (Magnetic Resonance Sounding) instrument. Thus, SBNMR is a cost-effective extension of the MRS method. Using a downhole sensor increases the depth of investigation and the resolution of MRS. In the near-horizontal Earth's magnetic field, the sensitive area of the sensor is represented by a cylinder of a few meters in diameter. A blind zone of 0.5 to 1 m around the borehole is due to the disturbance of the Earth's magnetic field by the magnetic core of the sensor. The relatively large volume investigated with SBNMR and the blind zone around borehole may represent an advantage of SBNMR over the NMR (nuclear magnetic resonance) borehole tool investigating a narrow zone around the borehole. However, using the Earth's magnetic field renders the SBNMR performance site dependent with an inherently low signal-to-noise ratio. Our first results show a good correspondence between SBNMR, MRS and borehole data.

Time-lapse magnetic resonance sounding measurements for numerical modeling of water flow in variably saturated media

We presented an innovative hydrogeophysical approach that allows numerical modeling of water flow in a variably saturated media. In our model, we approximated the subsurface by horizontally stratified porous media. The model output was a time varying water content profile. Then, we compared the water content provided by the model with the water content measurements carried out using the time-lapse Magnetic Resonance Sounding (MRS) method. Each MRS sounding provided a water content profile in the unsaturated zone down to twenty meters. The time shift between the profiles corresponded to the time lapse between individual MRS soundings. We minimized the discrepancy between the observed and the modeled MRS signals by varying hydraulic parameters of soil layers in the water flow model. For measuring and processing MRS data, we used NUMIS MRS instrument and SAMOVAR software. We carried out water flow modeling with HYDRUS-1D software. This paper reports our results and summarizes the limitations of the MRS method applied to water content measurements in the unsaturated zone.

A Theoretical Study of Underground Magnetic Resonance Sounding for the Advanced Detection of Water Influxes in Tunnels

Based on surface magnetic resonance sounding (MRS), a relationship is proposed to express the MRS response signal with a vertical coil for the MRS method for a whole underground space model. Firstly, the declination and inclination characteristics of the Earth's magnetic field and the coil normal angle are studied by deriving the angle rotation matrix. Surprisingly, the results indicate that the MRS signal can be effectively improved by changing the normal direction of the coil perpendicular to that of the Earth's magnetic field. The advanced detection distance of the underground MRS method is closely related to the exciting pulse moment and the receiving sensitivity. Hence, a larger pulse moment and high receiving sensitivity correspond to a longer advanced detection distance. However, the limited transmitting moment will reduce the advanced detection distance. In the research coincident loop with the coil 2 m by 2 m square is employed. In order to overcome the noises with 1 nV and 100 nV level, the turns of loop should be 100 turns and 400 turns, respectively. Finally, the numerical simulation results verify the feasibility of underground whole MRS theory and resolution analysis method for the advanced detection of water-inrush disasters in mines and tunnels.

Numerical simulation and analyze of magnetic resonance sounding with adiabatic pulse for groundwater exploration

Magnetic resonance sounding (MRS) technology used to detect groundwater directly and quantificationally, which owns the advantages of rich information and low cost. In these years, it has shown significant potential applications in hydrological detections. Considering the traditional measurements with the geomagnetic field, the nano-valt MRS signals are easy to suppress to environmental noise. As one of the MRS signal enhancement methods, the adiabatic pulse was quite popular recently. It is transmitted with variable frequency and amplitude pulse satisfying the adiabatic condition, which can enhance the signal amplitude and signal-to-noise ratio several times. However, there are only a few reports about this method, especially its geophysical modeling. Thus, we introduce the calculating progress of transverse magnetization and kernel function for this method. By employing the interpolation, the computation cost of forward modeling is obviously reduced. Moreover, several different settings of adiabatic transmitting factors are also proceeded to obtain certain results. In conclusion, a pulse moment with 80 ms transmitting duration and 30 quality factor produces a maximum signal amplitude that is enhanced 16.56 times for deep areas. The research in this paper could provide powerful support for MRS method used in noisy environments.

Design of Magnetic Resonance Sounding Antenna and Matching Circuit for the Risk Detection of Tunnel Water-Induced Disasters

The magnetic resonance sounding (MRS) method can noninvasively, directly, and quantitatively detect groundwater and forecast the potential for a water-induced disaster during tunnel construction. However, due to the spatial limitations of underground engineering, the signal-to-noise ratio (SNR) of the receiving signal is low (<; 1), which severely limits the reliability of the disaster warning. Thus, the question of how to design the meter-scale antenna and improve the SNR has become the bottleneck that restricts the successful application of the underground MRS method. To address this problem, we comprehensively consider the factors of detection depth, signal amplitude, and antenna size and weight using theoretical calculations and model simulations in this paper. We determine that an 8-turn transmitting antenna and a 30-turn receiving antenna with both side lengths of 6 m constitute an effective combination for achieving reliable detection of large water-bearing structures. Meanwhile, we propose a front-end matching circuit for a high-inductance antenna to solve the problem of attenuation of the signal amplitude due to high impedance and successfully increase the SNR from 1.12 to 5.13. Finally, in the tunnel of the Shanghai-Kunming high-speed railway of China, we perform the field measurement of an underground river and validate the practicability of the proposed antenna and matching circuit.

Noise analysis and cancellation for the underground application of magnetic resonance using a multi-component reference antenna – Case study from the rock laboratory of Mont Terri, Switzerland

Because of their principle capability to non-invasively detect dangerous water bodies and water-filled decompaction zones, underground magnetic resonance sounding (MRS) measurement are expected to have a great potential for the safety of underground facilities. However, a general applicability of underground MRS is questionable. The small antenna sizes reduce the sensitive volume and thus the signal-to-noise ratio. Additionally, the receivers must be installed in the nearest vicinity (i.e. reactive near field) of electromagnetic (EM) noise-emitting installations necessary for the logistics and safety inside the tunnels. This study focusses on the characterisation of underground EM noise characteristics including their spatial and temporal variability as well as proper strategies for noise cancellation. Passive EM measurements using conventional MRS equipment were conducted in the rock laboratory of Mont Terri, Switzerland. The reference-based noise cancellation (RNC) method yielded the best performance, given that all orthogonal spatial components of the noise field were considered and combined on the remote multi-component reference antenna. The multiple coherence between the primary and the reference channels is highest and stable against temporal and spatial noise variations, if the reference antenna combines square and figure-of-eight coils. Using a primary figure-of-eight coil, final noise levels of <0.1 nV/m2 were achieved after a common averaging time. An MRS noise level at this magnitude allows the detection of non-clay-bound and therefore dangerous water deposits inside the shale formation. This is demonstrated with data from a surface MRS experiment in an urban test area, which exhibits similar noise characteristics as the Mont-Terri rock laboratory. Under such conditions, the underground MRS method would be able to provide at least a yes/no criterion for further activities to maintain on-site safety. Future research should focus on strategies for a data acquisition that allow at least a rough 3-D mapping of potential water reservoirs.

Geophysical investigation of a freshwater lens on the island of Langeoog, Germany – Insights from combined HEM, TEM and MRS data

A multi-method geophysical survey, including helicopter-borne electromagnetics (HEM), transient electromagnetics (TEM), and magnetic resonance sounding (MRS), was conducted to investigate a freshwater lens on the North Sea island of Langeoog, Germany. The HEM survey covers the entire island and gives an overview of the extent of three freshwater lenses that reach depths of up to 45m. Ground-based TEM and MRS were conducted particularly on the managed western lens to verify the HEM results and to complement the lithological information from existing boreholes. The results of HEM and TEM are in good agreement. Salt- and freshwater-bearing sediments can, as expected, clearly be distinguished due to their individual resistivity ranges. In the resistivity data, a large transition zone between fresh- and saltwater with a thickness of up to 20m is identified, the existence of which is verified by borehole logging and sampling. Regarding lithological characterisation of the subsurface, the MRS method provides more accurate and reliable results than HEM and TEM. Using a lithological index derived from MRS water content and relaxation time, thin aquitard structures as well as fine and coarse sand aquifers can be distinguished. Complementing the existing borehole data with the lithology information estimated from MRS, we generate a map showing the occurrence of aquitard structures, which significantly improves the hydrogeological model of the island. Moreover, we demonstrate that the estimates of groundwater conductivity in the sand aquifers from geophysical data are in agreement with the fluid conductivity measured in the boreholes.

Glimpse into the design of MRS instrument

ABSTRACTThe Magnetic Resonance Sounding method (MRS) was developed in the former USSR in the late 1970s. Nowadays, available MRS instruments are more compact and reliable, and enormous progress has been made in electronics, computers and materials. Therefore, we can hope that it may be possible to increase the maximum depth of water detection and to improve the resolution of the method by using a larger current in the loop. Quite naturally, the questions arise: what are the practical limits of the MRS method and how much should be transmitting power to get the maximum depth of investigation? In this paper, we analyse the depth of groundwater detection and the vertical resolution of the MRS assuming different loops and different power levels of the current generator. The originality of our approach consists of a joint analysis of the maximum depth of investigation using accept able loop voltage and the modifications in the instrument design necessary for the improvements. We show that even under very favourable conditions it would be difficult to get significant improvement in the depth of investigation using currently available instruments. For example, under favourable noise conditions when rocks have low electrical conductivity and are non‐magnetic, a 20 m thick water saturated sand layer can be detected at a depth of about 325 m using an existing instrument (4 kV of the loop voltage) with a 400 × 400 m2 square loop. A 20% increase in the detection depth (390 m instead of 325 m) requires more powerful electronic equipment (16 kV instead of 4 kV) thus rendering the MRS system larger and heavier. However, using a 16 kV instrument allows us to increase the resolution depth by about 80% (from 120 m to 215 m). When rocks are electrically conductive, the screening of the MRS signal limits the depth of investigation and allows for only minor improvements even with a much more powerful current generator.

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.

Statistical stacking and adaptive notch filter to remove high‐level electromagnetic noise from MRS measurements

ABSTRACTTraditionally, one of the major limitations for magnetic resonance sounding (MRS) measurement is that the weak signal generated by subsurface water molecules is prone to be disturbed by high‐level electromagnetic noise. In China, the power grid coverage is 94.6% and spiky noise and powerline harmonic noise are always present when utilizing MRS measurement in suburban areas or towns. In order to improve the performance of the MRS method, two new techniques, statistical stacking and adaptive notch filter, are introduced to remove spiky noise and power‐line harmonic noise. Firstly, four stacking procedures are analysed to suppress the natural noise and spiky noise. It could be found that statistical stacking can be utilized in the areas with serious spiky noise and can improve the signal‐to‐noise ratio by a factor of 4 to 7. Moreover, the stacking number is less than other stacking procedures and the measurement time may decrease by nearly 50% in some suburban areas or towns. Secondly, there are a variety of filtering procedures available to suppress power‐line harmonic noise, which are all based on analogue or digital notch filtering. But nearly all of them may cause distortion. An adaptive notch filter is applied here to remove power‐line harmonic noise because harmonic frequencies are away from and (or) close to the Larmor frequency, even when the frequency offset between them is zero. From simulation results, it could be noted that the signal can be recovered after adaptive notch filtering because it is not irretrievably distorted but proportionally attenuated. Thus, the amplitude attenuation can accurately be compensated. The effectiveness of the two techniques applied to MRS measurements is demonstrated by field testing with the prototype of the MRS system developed by Jilin University, China. The results show that the statistical stacking and adaptive notch filter are effective methods to remove high‐level electromagnetic noise from MRS measurements.

The contribution of MRS and resistivity methods to the interpretation of actual evapo‐transpiration measurements: a case study in metamorphic context in north Bénin

ABSTRACTA quantitative budget estimate of actual evapo‐transpiration is a key issue for enhanced hydrological modelling in northern Bénin. Actual evapo‐transpiration is estimated using large aperture scintillometer equipment, devoted to sensible heat flux measurements. However, a previous study reported that the actual evapo‐transpiration cycle is not fully understood. Indeed, the actual evapo‐transpiration depends strongly on several factors such as climate, vegetation pattern, soil water storage and human activities. The respective contributions of the aquifer and vadose zone to the actual evapo‐transpiration budget are not known. When using piezometric variations of the water table, the aquifer contribution is not easy to quantify since the specific yield may vary in the investigated area, located in a metamorphic rock environment. In the present study, we investigate whether significant differences in the aquifer’s specific yield could exist within the large aperture scintillometer measurement area, leading to different actual evapo‐transpiration water losses. We use joint frequency electromagnetic resistivity mapping, geological surveys and magnetic resonance sounding (MRS) to delineate the effective porosity of the regolith around the scintillometre measurement area. Thirteen MRS soundings implemented in key areas reveal a clear classification of the main geological units on the basis of their water content. The MRS water content varies between 1.5–3% for amphibolite and micaschists formations to more than 12% for quartzitic fractured formations, whereas the MRS relaxation time is less discriminating (150–250 ms), indicating a small variation in pore size. Then, as a first modelling exercise, we assumed that the MRS water content (the effective porosity) maximizes the specific yield. The actual evapo‐transpiration budget given by a previous study (Guyot et al. ) is then re‐interpreted using geophysical data: we found that a) the measured water table depletion can explain the actual evapo‐transpiration value providing enough water for the transpiration process and b) the significant discrepancies in actual evapo‐transpiration signals observed between the eastern and western parts of the watershed can be explained by the respective effective porosity of the geological units. Even if further research is needed to link MRS water content to the specific yield and to evaluate a possible role of the deep vadose zone, the hydrogeophysical mapping presented in this study highlights the role of the MRS method for providing relevant information to understand hydrological processes in this complicated geological context of north Bénin.

Application of the magnetic resonance sounding method to the investigation of aquifers in the presence of magnetic materials

It has been previously reported that the magnetic resonance sounding (MRS) method does not produce reliable data in areas where magnetic rocks perturb the geomagnetic field. The applicability of the MRS can be extended by using the spin echo (SE) measuring technique instead of the commonly used measuring scheme based on recordings of the free induction decay (FID) signal. Modifications to the MRS method are presented for measuring and interpreting SE signals. Field results obtained in Cyprus (1999), Canada (2008), and India (2008) reveal that in test sites MRS measurements in the SE mode make it possible to apply the MRS method where the subsurface is composed of sand and gravel that contain magnetite or basalt and in aquifers composed of nonmagnetic sand overlying a magnetic basement. Con-sidering the widespread occurrence of magnetic rocks, this development increases the area where MRS can be applied. However, experience shows that it is more time consuming to measure the SE and more complicated to interpret the field data than it is to work with FID measurements. Numerical results show that the MRS method in the SE mode is less efficient than the FID technique because of the smaller amplitude and wider band of the SE signal. Due to instrumental limitations and unknown distribution of the magnetic fields within the investigated volume, accuracy of the presented MRS-SE approach is site dependent. In a general case, MRS-SE in its current implementation is not able to provide robust estimates of the initial amplitude, which renders MRS estimate of the water content qualitative. For accurate estimate of the water content, more sophisticated approaches need to be developed.

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.