Magnetic Resonance Sounding Data 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.

Multitype Noise Suppression in Magnetic Resonance Sounding Data Based on a Time–Frequency Fully Convolutional Neural Network

Magnetic resonance sounding (MRS) is one of the technical applications of nuclear magnetic resonance (NMR) used to directly detect and quantify groundwater content. MRS suffers from a low signal-to-noise ratio (SNR) due to the low amplitude of free induction decay (FID) signals and an inability to shield environmental noise. In this paper, a time-frequency fully convolutional neural network (TFCN) was proposed to suppress random, harmonic, and spike noise from MRS data. The TFCN parameters were trained with the time-frequency spectrum obtained by the short-time Fourier transform (STFT) of the MRS datasets as the input and the noise-free FID signals as the output. Based on the results of synthetic and field data experiments, the TFCN was compared with existing denoising methods. The results showed that the TFCN extracted the envelope of the FID signals from low-SNR random noise with higher accuracy than other methods. Moreover, the TFCN simultaneously suppressed multiple types of noise and exhibited high computational efficiency.

Monitoring unsaturated water flow using magnetic resonance soundings

We present an innovative hydrogeophysical approach for non-invasive quantification of the unsaturated water flow. For the water content measurements, we apply the Magnetic Resonance Sounding (MRS) method in the time-lapse mode. For inversion of MRS measurements, we approximate the subsurface by a horizontally stratified media. Laterally, the MRS estimated water content is averaged over the area below the measuring setup. During this study, we used a square loop of 75 m side-length. The time-lapse inversion allows visualizing variations in the water content in the vadose zone down to twenty meters. We tested our approach at the Villamblain test site (France). For that, we carried out the MRS monitoring in 1999–2000 and 2020–2021. During these two observation windows, MRS shows seasonal variations of the water content but also a strong dependence of the water content on the rainfall that was different between two data sets. Comparison of MRS results with the rainfall records shows a good correspondence in-between. For measuring and processing MRS data, we used commercially available MRS instrument and interpretation software.

Harmonic Noise-Elimination Method Based on the Synchroextracting Transform for Magnetic-Resonance Sounding Data

Magnetic-resonance sounding (MRS) is a direct, quantitative, accurate, and efficient technology that has been applied in the fields of groundwater detection, regional water-resource investigation, and geological-disaster-risk early warning. However, owing to strong environmental noise, the signal-to-noise ratio (SNR) of data collected by MRS instruments is low; thus, providing reliable results for subsequent hydrological interpretation is difficult. To solve this issue, we propose a method based on the synchroextracting transform (SET) to eliminate harmonic noise from MRS data in an environment with strong noise. Compared with the harmonic modeling cancellation (HMC) and three existing data processing algorithms, the SET can effectively eliminate the harmonic noise in a single record and process multiple types of harmonic noise in stacked data at a time. The calculation amount and time are much lower than those of the HMC algorithm, and the calculation accuracy is similar to that of the HMC algorithm. More importantly, after the harmonics are eliminated by the SET method, the impact on the free induced decay (FID) signal is small, whereas the other three methods severely distort the FID signal. Considering that FID signals may exhibit the characteristics of multiexponential decay and that its Larmor frequency is close to a certain harmonic frequency, we study the applicable scope of the SET. Finally, by processing the results of field-measured data, the validity and practicability of SET are further verified. We also discuss the advantages of the SET method and existing problems to provide future research directions.

Electromagnetic Noise Suppression of Magnetic Resonance Sounding Combined with Data Acquisition and Multi-Frame Spectral Subtraction in the Frequency Domain

As an advanced groundwater detection method, magnetic resonance sounding (MRS) has received more and more attention. However, the biggest challenge is that MRS measurements always suffer with a bad signal-to-noise ratio (SNR). Aiming at the problem of noise interference in MRS measurement, we propose a novel noise-suppression approach based on the combination of data acquisition and multi-frame spectral subtraction (DA-MFSS). The pure ambient noise from the measurement area is first collected by the receiving coil, and then the noisy MRS signal is recorded following the pulse moments transmitting. The procedure of the pure noise and the noisy MRS signal acquisition will be repeated several times. Then, the pure noise and the noisy signal are averaged to preliminarily suppress the noise. Secondly, the averaged pure noise and the noisy signal are divided into multiple frames. The framed signal is transformed into the frequency domain and the spectral subtraction method is applied to further suppress the electromagnetic noise embedded in the noisy MRS signal. Finally, the de-noised signal is recovered by the overlap-add method and inverse Fourier transformation. The approach was examined by numerical simulation and field measurements. After applying the proposed approach, the SNR of the MRS data was improved by 16.89 dB and both the random noise and the harmonic noise were well suppressed.

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.

Magnetic resonance sounding measurements as posterior information to condition hydrological model parameters: Application to a hard-rock headwater catchment

In headwater catchment, the calibration of hydrological models is complex due to the scarcity of data in mountainous areas. Here, an innovative methodology is developed to condition hydrological model parameters by using magnetic resonance sounding (MRS) measurements in combination with stream flow rate data. MRS has the specificity in the various geophysical imaging techniques of being mainly sensitive to the vertical distribution of water content among the subsurface. In a way very similar to hydraulic head observations, these local distributions of water content may serve as information in a hydrological model to pattern subsurface flow by seeking model parameters. Simulations are run with different sets of parameters of a hydrological model. Each simulation provides as an output a 4-D map (3-D spatial plus time) of the vertical water content distributions over the whole catchment and their fluctuations over time. This output is then used to simulate the MRS signal that would be produced by the estimated water content. The simulated MRS signal is compared to measured MRS data to determine which hydrological simulations (which model parameters) are close to observations. The approach is applied on a hard-rock headwater catchment housing a very shallow and thin aquifer where an MRS survey covers the whole studied site. Hydraulic parameters of an integrated hydrological model of the catchment are spatially distributed by zones with uniform values, the prior delineation of the zones being guided by pedological studies. As MRS measurements supply local but spatially distributed information, the method conditions the various zones on their parameter values in a much better way than the classical (in headwater catchments) measure of the stream flow rate at the outlet of the system. Finally, hydrological simulation and time-dependent MRS forward calculations can help identifying possible locations for MRS stations to monitor the transient behavior of the hydrological state of the catchment.

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.

Embankment dam leakage detection by joint use of Magnetic Resonance Sounding and Electrical Resistivity Imaging

Embankment dam leakage may cause devastating consequences and its early detection plays a significant role in the stability of embankment dam. Conventional dam leakage detection methods, such as piezometric tube or osmometer, are restricted by limitations of spatial distribution and high costs of instruments. This paper introduces the method of joint use of Magnetic Resonance Sounding(MRS) and Electrical Resistivity Imaging(ERI) to investigate embankment dam leakage. MRS and ERI are carried out to reveal embankment dam leakage in the same site and the stratigraphy of electrical conductivity obtained by ERI is carefully used for the inversion process of MRS data. And then the amount of groundwater in the subsurface can be directly determined by the technique of MRS. The MRS and ERI results show good consistency with yield data obtained by geological survey. Finally, the detecting capability of joint use of MRS and ERI is verified.

Random Noise Suppression of Magnetic Resonance Sounding Data with Intensive Sampling Sparse Reconstruction and Kernel Regression Estimation

The magnetic resonance sounding (MRS) method is a non-invasive, efficient and advanced geophysical method for groundwater detection. However, the MRS signal received by the coil sensor is extremely susceptible to electromagnetic noise interference. In MRS data processing, random noise suppression of noisy MRS data is an important research aspect. We propose an approach for intensive sampling sparse reconstruction (ISSR) and kernel regression estimation (KRE) to suppress random noise. The approach is based on variable frequency sampling, numerical integration and statistical signal processing combined with kernel regression estimation. In order to realize the approach, we proposed three specific sparse reconstructions, namely rectangular sparse reconstruction, trapezoidal sparse reconstruction and Simpson sparse reconstruction. To solve the distortion of peaks and valleys after sparse reconstruction, we introduced the KRE to deal with the processed data by the ISSR. Further, the simulation and field experiments demonstrate that the ISSR-KRE approach is a feasible and effective way to suppress random noise. Besides, we find that rectangular sparse reconstruction and trapezoidal sparse reconstruction are superior to Simpson sparse reconstruction in terms of noise suppression effect, and sampling frequency is positively correlated with signal-to-noise improvement ratio (SNIR). In one case of field experiment, the standard deviation of noisy MRS data was reduced from 1200.80 nV to 570.01 nV by the ISSR-KRE approach. The proposed approach provides theoretical support for random noise suppression and contributes to the development of MRS instrument with low power consumption and high efficiency. In the future, we will integrate the approach into MRS instrument and attempt to utilize them to eliminate harmonic noise from power line.

Detecting groundwater sources for water supplies using magnetic resonance sounding in arid areas with scarce hydrogeological data: a case study on the Mongolian Plateau

In most arid areas, due to scarce hydrogeological data, it is a challenge to locate groundwater sources and to meet water demand for residential, irrigation, and mining uses. In this study, an innovative method is presented, using magnetic resonance sounding (MRS), to detect areas suitable as groundwater sources on the Mongolian Plateau. First, a target investigation area was identified with a small number of MRS surveys of potential areas by determining whether aquifers exist, whether the aquifers have relatively large water contents and relaxation times, and whether there are hydraulic connections among the aquifers. Next, an intensive MRS survey (158 points in total) was conducted in the target investigation area, and eight boreholes were drilled. A comparison of the borehole data and MRS data showed that when the MRS data had a high signal-to-noise ratio, the aquifer depth and transmissivity estimated by MRS were associated with a deviation of only 4.85 m from the measured depth, and an uncertainty in transmissivity of 15.53%, respectively. These values indicated that the proposed method is highly accurate. Finally, a kriging interpolation method was used to construct distribution maps of groundwater levels, aquifer thickness, transmissivity, and water yield, based on the borehole and MRS data. The reliability of the results was assessed from several perspectives. The findings showed that this step-by-step approach is an effective method of groundwater source detection in arid areas with scarce hydrogeological data.

Estimation of hydraulic transmissivity from MRS by varying the porosity exponent in detrital aquifers of the Iberian Peninsula

ABSTRACTTo this date, estimation of hydraulic transmissivity with magnetic resonance sounding (MRS) requires transmissivity values from pumping tests in nearby wells in order to determine a calibration constant from the relationship between both types of data. The use of this technique is limited due to the fact that reported values of this calibration constant are highly variable, even within the same type of aquifer. In order to minimize this shortcoming, a new methodology, which has been developed and presented here, is used to estimate the transmissivity of detrital aquifers with heterogeneous behaviour based only on the results obtained by MRS. The exponents applied to decay time and free porosity used in the expression to compute MRS transmissivity have conventionally been fixed values. In contrast, the work presented here aims to improve the applicability of this technique using a variable exponent for free porosity in each type of aquifer, obtained only from MRS results. In order to achieve this goal, previously published MRS data from four detrital aquifers in the Iberian Peninsula have been used and a relationship between the porosity exponent and a function of decay times and free porosities has been developed. This approach eliminates the need to drill and perform pumping tests in additional boreholes, which has cost and time implications, once the correct expression is determined for a given type of aquifer. The range of values thus obtained for the porosity exponent is similar to that reported in the literature. The deviation of the results obtained with this methodology regarding the results of pumping test is similar to the deviation with conventional process. This suggests that the proposed methodology is appropriate for the aquifers considered, and the approach may be expanded to study other types of aquifers. The drawbacks of the proposal related to the conventional equivalence are explored.

New Method for Detecting Risk of Tunnel Water-Induced Disasters Using Magnetic Resonance Sounding

A new method for detecting risk of tunnel water-induced disasters using magnetic resonance sounding (MRS) is proposed in this letter. The method utilizes magnetic resonance signals that are generated directly from hydrogen protons to achieve the purpose of detecting risk of tunnel water-induced disasters directly and quantitatively. This letter evaluates the potential of this method based on a systematic study involving forward modeling, numerical experiments, and a large-scale physical model test. The relationship between the magnetic resonance signal response in the tunnel and the position and water content of water-bearing structures is obtained by the forward modeling. In the numerical examples, the inversion results are in agreement with the synthetic model. In the physical model test, the inversion results can accurately locate the water-bearing structure, and the water content linearly decreases with the water level of the water-bearing structure, which verifies the feasibility and effectiveness of predictions made based on the MRS data. These results demonstrate that tunnel MRS can be used to anticipate water-induced disasters.

Joint use of singular value decomposition and Monte-Carlo simulation for estimating uncertainty in surface NMR inversion

We propose a simple and robust approach for investigating uncertainty in the results of inversion in geophysics. We apply this approach to inversion of Surface Nuclear Magnetic Resonance (SNMR) data, which is also known as Magnetic Resonance Sounding (MRS). Solution of this inverse problem is known to be non-unique. We inverse MRS data using the well-known Tikhonov regularization method, which provides an optimal solution as a trade-off between the stability and accuracy. Then, we perturb this model by random values and compute the fitting error for the perturbed models. The magnitude of these perturbations is limited by the uncertainty estimated with the singular value decomposition (SVD) and taking into account experimental errors. We use 106 perturbed models and show that the large majority of these models, which have all the water content within the variations given by the SVD estimate, do not fit data with an acceptable accuracy. Thus, we may limit the solution space by only the equivalent inverse models that fit data with the accuracy close to that of the initial inverse model. For representing inversion results, we use three equivalent solutions instead of the only one: the “best” solution given by the regularization or other inversion technic and the extreme variations of this solution corresponding to the equivalent models with the minimum and the maximum volume of water. For demonstrating our approach, we use synthetic data sets and experimental data acquired in the framework of investigation of a hard rock aquifer in the Ireland (County Donegal).

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.

Bloch–Siegert Effect in Magnetic-Resonance Sounding

The magnetic-resonance sounding (MRS) method is intended for groundwater prospecting. The Earth’s magnetic field used in MRS is of the order of 5 × 10−5 T. The radiofrequency (RF) field produced in MRS by the surface antenna is linearly polarized and may be comparable with the geomagnetic field. The use of RF pulses at Larmor frequency leads to the appearance of the mean resonance offset in rotating frame (Bloch–Siegert shift) which is caused by the influence of the off-resonance counter rotating component of the RF field. The solution of the inverse problem for ground water distribution must take account of the proton spin resonance frequency shift, because an inaccurate kernel of the respective integral equation may lead to significant errors in the estimation of aquifer characteristics. The value for Bloch–Siegert effect with one order of approximation higher than usual has been calculated. The MRS technique with 100-m-diameter antenna has been used at the frozen Ob reservoir near Novosibirsk, Russia, to detect subice water to prove the improved physical model of MRS. There is a good agreement between measured and calculated data taking into account the Bloch–Siegert and the next-order effects. New physical model accounting the Bloch–Siegert effect allowed a significant improvement of the reliability of the MRS data interpretation.

Interpretation of magnetic resonance measurements in the varying earth’s magnetic field

At the scale of a magnetic resonance sounding (MRS) field setup, the earth’s magnetic field in the subsurface may vary laterally with depth and over time. These variations can be caused by different natural factors and generally cannot be compensated for by accurate tuning of the measuring device. The varying geomagnetic field (GMF) causes nonresonance conditions of excitation that affect the amplitude and phase of the MRS signal. Usually, variations of the GMF do not exceed a few hertz and their effect on the amplitude is relatively small, permitting us to assume near-resonance conditions for inversion. However, in some cases, the results may be erroneous if a varying GMF is not taken into account. Motivated by possible improvements in MRS inversion, we have developed a procedure for measuring and interpreting MRS data that considers a varying GMF. Our results showed that it is relatively easy to take a time-varying GMF into account. As a demonstration, we have developed the inversion of MRS data measured in Benin (West Africa). A depth-varying GMF is a more complex problem, and to consider this, we have developed an algorithm of nonlinear inversion. We have tested this approach on synthetic data, which resulted in an improved inversion. Field validation of this procedure awaits the discovery of a suitable test site with known variations of the earth’s magnetic field in the subsurface.

An overview of a highly versatile forward and stable inverse algorithm for airborne, ground-based and borehole electromagnetic and electric data

We present an overview of a mature, robust and general algorithm providing a single framework for the inversion of most electromagnetic and electrical data types and instrument geometries. The implementation mainly uses a 1D earth formulation for electromagnetics and magnetic resonance sounding (MRS) responses, while the geoelectric responses are both 1D and 2D and the sheet’s response models a 3D conductive sheet in a conductive host with an overburden of varying thickness and resistivity. In all cases, the focus is placed on delivering full system forward modelling across all supported types of data. Our implementation is modular, meaning that the bulk of the algorithm is independent of data type, making it easy to add support for new types. Having implemented forward response routines and file I/O for a given data type provides access to a robust and general inversion engine. This engine includes support for mixed data types, arbitrary model parameter constraints, integration of prior information and calculation of both model parameter sensitivity analysis and depth of investigation. We present a review of our implementation and methodology and show four different examples illustrating the versatility of the algorithm. The first example is a laterally constrained joint inversion (LCI) of surface time domain induced polarisation (TDIP) data and borehole TDIP data. The second example shows a spatially constrained inversion (SCI) of airborne transient electromagnetic (AEM) data. The third example is an inversion and sensitivity analysis of MRS data, where the electrical structure is constrained with AEM data. The fourth example is an inversion of AEM data, where the model is described by a 3D sheet in a layered conductive host.

Three-dimensional Inversion of Magnetic Resonance Sounding (MRS) for Groundwater Detection

For this paper, we consider the resulting 3-D inversion using inversion modeling, which is motivated by developing theory and the recent application of the Magnetic Resonance Sounding (MRS) technique in detecting and mapping of subsurface groundwater. MRS is a non-invasive method which directly detects the groundwater’s existence from surface measurements. A pulse current, at a proper frequency, is transmitted into a loop. After hydrogen atoms of water molecules in the subsurface are energized by pulses of alternative currents, the magnetic resonance field is produced by the H protons is measured within the same loop. Generally, MRS has two observable factors: initial amplitude and decay time. The aim of threedimensional inversion is to extract the information, i.e., the value and distribution of two physical parameters of the subsurface conditions: water content and subsurface properties (pore and grain size). Additionally, we present a general formulation for inverting the initial amplitude and decay time of the MRS data to recover a 3-D distribution of groundwater. The forward problem was solved using an integral equation method in the spatial domain. An improved Levenberg-Marquardt strategy was employed to solve the inverse problem. Two synthetic examples are illustrated to determine the basic functionality of the inversion algorithm. The real data results show applicability and relevance in larger-scale field examples.

Noninvasive Estimation of Water Retention Parameters by Observing the Capillary Fringe with Magnetic Resonance Sounding

The magnetic resonance sounding (MRS) method is usually applied for delineation and characterization of aquifer system stratification. Its unique property, distinct from other hydrogeophysical methods, is the direct sensitivity to water content in the subsurface. The inversion of MRS data yields the subsurface water content distribution without need of a petrophysical model. Recent developments in instrumentation, i.e., decreased instrumental dead times and advanced noise cancellation strategies, enable the use of this method for investigating the vadose zone. A possible way to interpret MRS measurements with focus on water retention (WR) parameters is an inversion approach that directly provides WR parameters by modeling the capillary fringe (CF inversion). We have developed this kind of inversion further to account for different WR models and present a sensitivity study based on both synthetic and real field data. To assess the general applicability of the CF inversion, we analyzed the resolution properties for different measurement layouts and the parameter uncertainties for different realistic scenarios. Under moderate noise conditions and if the water table position is known, all WR parameters except the residual water content can be reliably estimated. The relative accuracy of the estimated pore distribution index estimation is better for larger CF. Small measurement loops of 5‐m diameter achieve the best resolution for shallow investigation depths of <10 m.