Surface Nuclear Magnetic Resonance Research Articles

Improved technology using transmitting currents with short shutdown times for surface nuclear magnetic resonance.

Surface nuclear magnetic resonance (SNMR) technology is widely used in the detection of groundwater. However, the dead time arising from the coupling effect of the transmitting circuit on the receiving coil results in partial or complete loss of the SNMR signal. This situation is especially unfavorable for the detection of short relaxation time targets. To solve this problem, we analyzed the shortcomings of the traditional SNMR launch system, and we propose a new transmission method based on an untuned constant voltage-clamped technology to overcome the problems of high resonance voltage, an uncontrollable shutdown process, and long shutdown times. Untuned transmission topology without a matching capacitor, pulse width modulation, and a constant voltage-clamped technique were applied to guide the current rise and shutdown of the system in a controllable way using an integer-period transmission pulse. A simulation experiment comparing the traditional method of transmission and this new method was conducted. The results showed that not only can the new method control the transmission current shutdown process but it can also avoid the delay in response. When the transmitting current drops from 10 A to 0.12 µA, the traditional method requires 2.29 ms and the new method requires only 4 µs. The new transmission system that we have developed based on an untuned constant voltage-clamped technology can improve the level of the transmitting current effectively and shorten the shutdown time.

Magnetic resonance sounding evidence shows that shallow groundwater discharge maintains the lake landscape in the Hunshandake Sandy Land, North China

The Hunshandake Sandy Land (HSL) is a well-known water-rich sandy region in China that is home to many lakes. Most of these lakes are sustained by groundwater inflow. However, due to the limited hydrogeological survey data, the mechanism by which the lakes interact with groundwater is not well understood. Magnetic resonance sounding (MRS) is a geophysical method that has emerged in recent years that can directly detect the distribution of groundwater and determine the lithology of the aquifer medium. Compared with drilling, this method is convenient, rapid, and inexpensive. In this study, to determine how the lakes are recharged by groundwater, a field experiment using MRS was conducted on Haoletuyin Lake, which is located in the central HSL. The results showed that compared with borehole data, the MRS-derived aquifer depth has an uncertainty of 6.88%. Additionally, the lithology of the media derived from the MRS is consistent with that derived from the borehole data. This finding suggests that the MRS-derived results are highly accurate. Based on the MRS-derived results, two hydrogeological profiles were plotted, and they show that the lake in the experimental area is recharged primarily by shallow groundwater and not by deep groundwater. This study is significant because it not only reveals the mechanism by which Haoletuyin Lake is maintained, but also proves that MRS is an excellent tool for studying lake-groundwater relations in desert (sandy land) regions with limited hydrogeological survey data.

Mitigating Narrowband Noise Sources Close to the Larmor Frequency in Surface NMR

A common noise source in surface nuclear magnetic resonance (NMR) is narrowband noise sources that occur at frequencies close to the Larmor frequency, such as power-line harmonics or other sinusoidal signals of unknown origin. These noise sources can lead to significant perturbation on the estimated NMR signal and degrade the accuracy of subsurface characterizations. We demonstrate that the spectral analysis envelope detection scheme, where the envelope is estimated using the discrete Fourier transform for a suite of sliding windows, can be modified to mitigate the effect of narrowband noise sources. Selection of an appropriate length rectangular window is shown to allow one to effectively place a stopband at frequencies very close to the Larmor frequency, where the stopband may be selected to coincide with narrowband noise sources. The result is a significant reduction in the influence of the narrowband noise source on the estimated envelope, without introduction of distorting transients on the signal. Synthetic and field results are presented to illustrate the effectiveness of the proposed approach, as well as identify limits on the required resolution of the narrowband noise source's frequency.

Uncertainty analysis of UMRS parameters and its application for water detection in the tunnel

As a novel geophysical exploration technology, the underground magnetic resonance sounding (UMRS) method has been applied to detect water inrush in underground construction over the past 10 years. Non-invasive UMRS technology has found potential use in preventing water inrush disasters in tunnel due to its direct sensitivity to water molecules. Small water bodies are often the origin of severe water inrush disasters in tunnels. In most cases, UMRS signals have a low SNR, yielding high uncertainties that are important to be correctly estimated for an optimal interpretation of the water distribution. In this paper, we describe a method by which to evaluate the uncertainty of UMRS parameters for advanced water detection in tunnels, calculating for the quasi-whole space model. The key point is that the response caused by water surrounding the tunnel is non-negligible. In addition, we discuss the resolution of water content as the main inverted parameter for UMRS advanced detection. In order to guide the configuration of the acquisition settings, the influences of some parameters, including coil size and turns, tunnel size, water content surrounding the tunnel lane, as well as the noise level, on the uncertainty of the advanced target, are analyzed. Finally, we invert a set of measurement data related to a case of tunnel advanced detection, and analyze the uncertainty of the results.Based on the study results, we have concluded that during the advanced detection the water content surrounding the tunnel may significantly increase the uncertainty of the target. In addition, the increasing coil size and number of turns cause an increase in the amplitude of the signal, and also provide a reduction in the uncertainty of the water content. However, at the same time, these also introduce a higher noise level, which may offset the improvement. For these reasons, it is necessary to conduct the parameter uncertainty analyses according to different tunnel environmental conditions before performing the field test, so as to obtain reliable UMRS results.

Using surface nuclear magnetic resonance and spontaneous potential to investigate the source of water seepage in the JinDeng Temple grottoes, China

The JinDeng Temple grottoes are large-scale grottoes that were dug in natural cliffs more than 500 years ago. The grotto art provides an important legacy and is of high historical, cultural, and artistic value. Over the course of more than 500 years, some areas of the grottoes have been seriously affected by seepage damage. Non-destructive detection methods (surface nuclear magnetic resonance and spontaneous potential) were used to investigate the cause of the seepage. The surface nuclear magnetic resonance method was successfully used to detect the spatial occurrence of groundwater and the water content per unit volume in the mountain above the grottoes. The results of spontaneous potential revealed the recharge and flow directions in the upper part of the mountain. The two methods used were complementary and provided mutually consistent results regarding the cause of water seepage in the grottoes, and the relationship between the positions of the water-bearing areas in the upper part of the mountain and the seepage. In addition, the two methods provided important basic data for the formulation of a work plan to protect and preserve the cultural relics.

Application of adiabatic pulses for magnetic Resonance Sounding – Pulse shapes and resolution

Magnetic Resonance Sounding (MRS) can image the spatial distribution of hydrologically relevant parameters in in the subsurface. However, the application of MRS is often limited by its low signal-to-noise ratio. The use of adiabatic excitation pulses show promising features to overcome this limitation. In this work, we study practical considerations when applying adiabatic pulses for MRS, i.e. calculation of the sensitivity kernel for varying pulse shapes and vertical resolution.The pulse shape is crucial for the performance of adiabatic pulses. We investigate the shapes of adiabatic pulses recorded during a MRS and observe small systematic deviations from the theoretical predicted pulse shape and variations between different pulse strengths. We show that the overall impact on the obtained sounding curve and inversion result was small. This enables to limit the time consuming modelling of the spin dynamic to one representative pulse shape, which significantly speeds up the calculation of the sensitivity kernel, necessary for the interpretation of MRS.Additionally, we show that on-resonance excitation generally outperforms adiabatic excitation concerning vertical resolution and depth of investigation (both up to a factor of two). This is true for a wide range of noise conditions. For a very shallow depth interval compared to the loop size, however, adiabatic excitation features improved imaging capabilities.

Tunnel Magnetic Resonance Tomography for 2-D Water-Bearing Structures Using Rotating Coil With Separated Loop Configuration

In tunnel construction, to prevent the occurrence of water inrush, the geological conditions of faults and underground rivers must be determined in advance. As a direct detection method of groundwater, magnetic resonance sounding (MRS) has been applied for the advanced detection of water-related hazards in tunnels and mines recently. However, the results of conventional 1-D MRS cannot correctly reflect the spatial distribution characteristics of complicated water-bearing structures. In this article, we propose a measurement scheme using a rotating coil with separated transmitter and receiver loop configuration (SEP) for the 2-D imaging of water-bearing structures, such as faults and conduits. In this scheme, the receiver coil rotates several times, while the transmitter coil, which is separated from the receiver coil by a certain distance, remains stationary. Moreover, all the observed data participate in the inversion to achieve the 2-D magnetic resonance tomography (MRT). Numerical simulations of water-bearing faults are performed, and we compare and analyze the 2-D sensitivity and inversion results of two measurement schemes for a rotating coil, i.e., an overlapping transmitter and receiver loop configuration (OVE) and SEP. The results showed that the imaging results of the SEP are better than those of the OVE because the OVE imaging has a symmetric artifact. Finally, we discuss the influence of the transceiver distance, the resistivity, and the environmental noise on the imaging results. Moreover, the imaging results of the water-bearing conduit at different locations were obtained to validate the effectiveness of the SEP rotating coil measurement scheme.

Magnetic resonance sounding dataset of a hard-rock headwater catchment for assessing the vertical distribution of water contents in the subsurface.

Magnetic Resonance Sounding (MRS) measurements are acquired at 16 stations in the Strengbach headwater catchment (Vosges Mountains – France). These data, rendering the vertical distribution of water contents in the subsurface, are used to show their potential in conditioning a hydrological model of the catchment, as described in the article “Magnetic resonance sounding measurements as posterior information to condition hydrological model parameters: Application to a hard-rock headwater catchment” – Journal of Hydrology (2020). Acquisition protocols follow a free induction decay scheme. Data are filtered by applying a band-pass filter at the Larmor frequency. A filter removing the 50 Hz noise is also applied with the exception of data at a Larmor frequency close to the 50 Hz harmonic. The signal envelopes are then fitted by a decaying exponential function over time to estimate the median characteristic relaxation time of each MRS sounding.

Cancellation of varying harmonic noise in magnetic resonance sounding signals

Magnetic resonance sounding (MRS) is a promising geophysical method for direct detection and quantification of groundwater. However, practical application of MRS is greatly limited because it is vulnerable to electromagnetic interference. Most traditional denoising methods for MRS assume that the harmonic noise is stable within one measurement period. In this work, the harmonic noise in MRS is analyzed based on the short-time power spectral density (PSD). In the primary and reference coils, the harmonic noise is often found to be nonstationary, and its variations in both coils may be asynchronous. A frame-based denoising method is introduced based on multichannel Wiener filtering in the frequency domain. By establishing harmonic noise models with different variabilities, the frame-based denoising method is compared with traditional denoising methods when dealing with a variety of noise types. The results show that the frame-based method improves denoising when the harmonic noise has asynchronous variations and multiple sources. In addition, we analyzed the factors influencing this new approach and found that when the harmonic noise is stable, a longer frame length produces better denoising. When the harmonic noise changes asynchronously or multiple noise sources with different fundamental frequencies are present, there is an optimal range of frame lengths from approximately 0.08–0.2 s. The deviation between the Larmor frequency and the power frequency also affects denoising. This study has demonstrated improvements in denoising during short-time variations in noise and with multiple noise sources that will undoubtedly expand the scope of MRS applications.

Improving the accuracy of 1D surface nuclear magnetic resonance surveys using the multi-central-loop configuration

Surface nuclear magnetic resonance is a near-surface geophysical method for characterizing the spatial distribution of liquid water in the top 100 m of the subsurface. The recovered water content models are obtained through the solution of an ill-posed inverse problem that is a function of acquisition parameters, including location and shape of the transmitter and receiver coils. In this paper, we introduce the multi-central-loop acquisition and inversion strategy where one or several smaller receivers coils are placed in the center of the larger transmitter loop and where all the data sets synchronously recorded through each loop are inverted simultaneously. We investigate the attributes of this acquisition and inversion strategy including the ability to provide improved resolution, accuracy and reduced uncertainty on the estimated subsurface models compared to single channel acquisition methods. Using widely-adopted inversion methods and introducing a new data interpretation technique called Bayesian Evidential Learning 1D imaging, we show that the multi-central-loop configuration provides improved recovery of synthetic models and reduced levels of inverted parameter uncertainty. A field case is also presented where the multi-central-loop results appear to better match the lithologic knowledge of the area compared with single channel configurations, again providing smaller uncertainties.

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.

Coupled magnetic resonance and electrical resistivity tomography: An open-source toolbox for surface nuclear-magnetic resonance

Nuclear-magnetic resonance (NMR) is a powerful tool for groundwater system imaging. Ongoing developments in surface NMR, for example, multichannel devices, allow for investigations of increasingly complex subsurface structures. However, with the growing complexity of field cases, the availability of appropriate software to accomplish the in-depth data analysis becomes limited. The open-source Python toolbox coupled magnetic resonance and electrical resistivity tomography (COMET) provides the community with a software for modeling and inversion of complex surface NMR data. COMET allows the NMR parameters’ water content and relaxation time to vary in one dimension or two dimensions and accounts for arbitrary electrical resistivity distributions. It offers a wide range of classes and functions to use the software via scripts without in-depth programming knowledge. We validated COMET to existing software for a simple 1D example. We discovered the potential of COMET by a complex 2D case, showing 2D inversions using different approximations for the resistivity, including a smooth distribution from electrical resistivity tomography (ERT). The use of ERT-based resistivity results in similar water content and relaxation time images compared with using the original synthetic block resistivity. We find that complex inversion may indicate incorrect resistivity by non-Gaussian data misfits, whereas amplitude inversion shows well-fitted data, but leading to erroneous NMR models.

Random noise suppression and parameter estimation for Magnetic Resonance Sounding signal based on maximum likelihood estimation

Magnetic resonance sounding (MRS) is a promising geophysical method for direct detection and quantification of groundwater. However, the application of MRS is considerably limited due to its vulnerability to electromagnetic interference. In this paper, the statistical method of maximum likelihood estimation (MLE) is introduced to estimate nuclear magnetic resonance (NMR) parameters and suppress random noise in MRS. Using synthetic NMR signals and noise models, we analyse the factors that influence the denoising effect of MLE. The results show that MLE denoising can be effective even when the random noise deviates from a Gaussian distribution. A multi-exponential NMR signal may reduce the accuracy of MLE-based parameter estimation if a bad initial guess is used, but it has little effect on the suppression of random noise. Harmonic noise can reduce the effect of MLE, while impulsive noise has little effect on MLE. In addition, increasing the recording time or frequency increases the amount of data of the MRS signal, thus improving the effectiveness of the MLE method. Field examples show that this method can effectively suppress random noise interference with less stacking compared with traditional methods, thus greatly shortening the working time of MRS and improving its working efficiency.

Numerical Simulation of 2-D Underground Magnetic Resonance Tomography by Using Rotating Antenna and Sector Scanning

Magnetic resonance sounding (MRS) has been applied to underground constructions, such as tunnels and mines, to detect and forewarn groundwater sources hidden in front of the mine face, also named disaster water sources. Disaster water sources are mostly found in 2-D or 3-D structures; as such, their spatial distribution characteristics are difficult to reflect accurately by conventional 1-D MRS results. This letter proposes a method for sector-scanning magnetic resonance tomography (MRT) measurement using rotating antennas for 2-D water-bearing structures, such as water-filling conduits, faults, and goafs to obtain the 2-D distribution image of water content and relaxation time ( $T_{2}^{*}$ ) in front of the face by inversion. Through the numerical simulation of conduits, we analyzed the 2-D sensitivity, resolution, and inversion results of the rotating antennas. The imaging result at a high noise level was improved by increasing the number of antenna rotations. We also determined the effects of 2-D MRT on the faults and goafs and discussed the minimum cross-sectional area and maximum distance for reliable imaging of conduits.

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 fast multi-exponential inversion of magnetic resonance sounding using iterative Lanczos bidiagonalization algorithm

Due to multi-exponential decay-time properties of the subsurface volume units or layers, magnetic resonance sounding (MRS) relaxation data exhibit a multi-exponential behavior. MRS inverse problem in a multi-exponential modeling framework brings about a very large size of the parameter space which is computationally costly. In this paper, a fast and memory efficient inversion algorithm to retrieve the aquifer properties in terms of water content and relaxation time is presented. The original nonsymmetric linearized forward matrix is projected onto a Krylov subspace with smaller dimension using an iterative Golub-Kahan-Lanczos bidiagonalization (GKL) method. Because of ill-conditioning of the projected linearized forward matrix a regularized damped least squares equation is applied at each step of the GKL factorization method to extract the best possible approximation of the partial water content. Numerical experiments based on synthetic and field data demonstrate that the proposed inversion method provides a good estimation of the water content and relaxation time compared to the standard algorithm with computationally more efficient functionality.

Investigating the impact of artificial environmental watering on a semi‐arid, highly saline floodplain using electromagnetics and surface nuclear magnetic resonance

Investigating the impact of artificial environmental watering on a semi‐arid, highly saline floodplain using electromagnetics and surface nuclear magnetic resonance

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.

Seasonal Subsurface Thaw Dynamics of an Aufeis Feature Inferred From Geophysical Methods

AbstractAufeis are sheets of ice unique to cold regions that originate from repeated flooding and freezing events during the winter. They have hydrological importance associated with summer flows and winter insulation, but little is known about the seasonal dynamics of the unfrozen sediment layer beneath them. This layer may support perennial groundwater flow in regions with otherwise continuous permafrost. For this study, ground penetrating radar (GPR) were collected in September 2016 (maximum thaw) and April 2017 (maximum frozen) at the Kuparuk aufeis field on the North Slope of Alaska. Supporting surface nuclear magnetic resonance data were collected during the maximum frozen campaign. These point‐in‐time geophysical data sets were augmented by continuous subsurface temperature data and periodic Structure‐from‐Motion digital elevation models collected seasonally. GPR and difference digital elevation model data showed up to 6 m of ice over the sediment surface. Below the ice, GPR and nuclear magnetic resonance identified regions of permafrost and regions of seasonally frozen sediment (i.e., the active layer) underlain by a substantial lateral talik that reached >13‐m thickness. The seasonally frozen cobble layer above the talik was typically 3‐ to 5‐m thick, with freezing apparently enabled by relatively high thermal diffusivity of the overlying ice and rock cobbles. The large talik suggests that year‐round groundwater flow and coupled heat transport occurs beneath much of the feature. Highly permeable alluvial material and discrete zones of apparent groundwater upwelling indicated by geophysical and ground temperature data allows direct connection between the aufeis and the talik below.