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The common understanding in surface nuclear magnetic resonance (NMR) is that the excitation pulse should be kept much shorter than the NMR relaxation times for an effective excitation. We present analytic, numerical, and experimental evidence demonstrating that this need not be so, and surface NMR signals can be retrieved even for absurdly long excitation pulses. We solve the Bloch equations in the limit of infinitely long excitation and use this result to demonstrate that a measurable magnetization can be obtained with long excitation pulses. We perform numerical simulations of the Bloch equations incorporating effects of relaxation‐during‐pulse, and our results suggest that long excitation pulses can be used to increase the depth of investigation, but that the spatial resolution of long pulse excitation is low. Data collected in Kompedal, Denmark using up to 3 s long excitation pulses provide experimental proof for the feasibility of long pulse surface NMR. Plain Language Summary: Surface nuclear magnetic resonance (NMR) is the only non‐invasive geophysical method with a direct sensitivity to groundwater. A surface NMR measurement is carried out by pulsing a resonant current in a large coil laid out on the surface. This generates an electromagnetic field, which perturbs the nuclear spin of hydrogen nuclei in groundwater. The subsequent relaxation of the nuclear spins gives rise to a measurable signal with an amplitude proportional to the amount of groundwater and NMR relaxation times controlled by the host material of the water. The common understanding is that the excitation pulse should be much shorter than the NMR relaxation times for effective excitation. We present analytic, numerical, and experimental evidence demonstrating that this need not be so, and signals can be retrieved even for absurdly long excitation pulses. A numerical analysis of long pulse excitation suggests that this can be used to increase the depth of investigation. Experimental proof is given by a field data set where we retrieve NMR signals with a 200 ms relaxation time following excitation with a 3 s long pulse. Key Points: The influence of the nuclear magnetic resonance (NMR) excitation pulse duration in surface NMR is investigated in the long pulse limitEven if the NMR excitation pulse is much longer than the NMR relaxation times, surface NMR signals from groundwater can still be detectedThe depth of investigation in surface NMR measurements can be increased using long excitation pulses [ABSTRACT FROM AUTHOR]

The integration of non-invasive geophysical technologies has been popular in recent years for hydro-geophysical applications. Electrical resistivity tomography (ERT) and surface nuclear magnetic resonance (surface NMR) are geophysical methods for classifying subsurface soil media up to 100 m below the soil's surface. The main focus of the present study is to estimate the seasonal changes in the subsurface aquifer properties like water content and hydraulic conductivity using the combination of 2D/3D ERT and surface NMR methods, and these results were compared with the pulse moment time inversion (QTI) scheme. It was observed that small-scale variability in aquifer properties was found in the ERT and surface inversions. However, these models were developed for the field scale. Thus, small-scale variability can be neglected. During the post-monsoon and monsoon seasons, the size of low resistivity (4–14Ohm-m) and very low resistivity (23–33Ohm-m) zones are as shown in Fig. 5. Similarly, surface NMR measurements revealed a significant increase in the water content during these seasons. The chi-square value for the QTI scheme was 1.0672, which was quite near to 1. As a result, the QTI model is considered to be accurate for this study. The subsurface soil samples particle size distribution curves were studied, and soil components were classified using the transverse relaxation time. Thus, the combination of ERT and surface NMR approach promises to be a powerful tool for estimating subsurface characterization. [ABSTRACT FROM AUTHOR]

Groundwater is a critical freshwater source for billions of people (Klee, 2013), but resources are increasingly stressed by climate change, pollution, and overexploitation (Frappart & Ramillien, 2018, https://doi.org/10.3390/rs10060829). Locating groundwater sources and monitoring their sustainable use is a difficult task. One method that shows great promise is surface nuclear magnetic resonance (NMR), as it is the only technique capable of non‐invasive direct measurements of subsurface water content and pore‐space properties. Unfortunately, surface NMR often suffers from low‐amplitude signals that limit mapping speeds and may prohibit measurements in noisy environments. We demonstrate a steady‐state scheme for surface NMR, enabled by recent breakthroughs in transmitter capabilities and numerical modeling that delivers orders of magnitude signal enhancements. The new technique is a highly efficient measurement that is both narrowband and utilizes the full measurement interval in contrast to traditional approaches. Our results demonstrate that high‐fidelity groundwater measurements are now possible in hitherto inaccessible areas. Plain Language Summary: Surface nuclear magnetic resonance (NMR) is a non‐invasive technique that allows one to locate groundwater and quantify its abundance. The technique shows great promise to help enhance understanding of groundwater systems, but commonly suffers from low signal qualities due to low‐amplitude signals and high noise levels. To address the signal quality issues, a novel data acquisition strategy for surface NMR is presented, which involves measuring the signal produced by long pulse trains composed of identical repeated pulses. The proposed scheme's advantage over traditional workflows are an order of magnitude increase in stacking rates, and a dramatic reduction of noise bandwidths. The presented scheme shows great potential to expand the conditions where surface NMR can be employed. Key Points: Steady‐state surface nuclear magnetic resonance offers significant signal enhancements compared to traditional approachesSteady‐state processing enables a significant reduction of the noise bandwidth [ABSTRACT FROM AUTHOR]

The Magnetic Resonance Sounding (MRS) method is based on the resonance behaviour of proton magnetic moments in the geomagnetic field. The main distinction between MRS and other geophysical methods is that it measures the magnetic resonance signal directly from groundwater molecules, making it a selective tool sensitive to groundwater. As the signal generated by the protons is very small, the method is also sensitive to electromagnetic interference (noise) and this is one of the major limitations for practical application. The frequency of the magnetic resonance signal (the Larmor frequency) is directly proportional to the magnitude of the geomagnetic field and varies between 800 and 2800 Hz around the globe. Whilst natural noise within this frequency range is generally not very large (excepting magnetic storms or other temporary disturbances), the level of cultural noise (electrical power lines, generators, etc.) may be very high. In order to improve performance, three existing filtering techniques were adapted to processing MRS measurements: block subtraction, sinusoid subtraction and notch filtering. The first two are subtraction techniques capable of suppressing stationary power-line noise without distorting or attenuating the signal of interest, both involve subtracting an estimate of the harmonic component but differ in the way the component is estimated. The block subtraction method consists of ascertaining the power-line noise (or “noise block”) from a record of the noise alone, and then subtracting this block from a record containing both the noise and the signal. The sinusoid subtraction method is based on the calculation of the amplitude, frequency and phase of power-line harmonics using noise records. The notch filtering method does not require knowledge of the power-line harmonic parameters but it may cause distortion of the measured signal. During the study, it was found that, in the investigated frequency range, the electromagnetic noise produced by electrical power lines was much less stable and regular than expected. The proportion of 50 Hz harmonics (regular part) in the noise energy is site-dependent and may vary between 20% and 50%. Whilst the power-line harmonics are seen clearly on the noise spectra, the amplitude and frequency of each harmonic may vary significantly from one record to another. Under these conditions, any block subtraction scheme based on a high noise regularity cannot be used systematically. The sinusoid subtraction is generally more efficient than the block subtraction and its application allows noise reduction by a factor of up to nearly 5. The notch filtering technique was studied further using a synthetic signal mixed with real noise and the results show that the noise can be reduced by factors of 2–10. The efficiency of the investigated filtering techniques is site-dependent. Three important factors define how successfully noise can be filtered from the signal record: the difference between the Larmor frequency and the nearest power-line harmonic frequency; the relaxation time of the magnetic resonance signal; and the proportion of the regular part of the noise spectrum. In most cases though, the improvement achieved in the critical signal-to-noise ratio (S/N) (5- to 8-fold on average) enables MRS application to be extended towards more noisy areas. The efficiency of the notch filtering technique applied to MRS measurements is demonstrated by a field example from France. [Copyright &y& Elsevier]

The proton Magnetic Resonance Sounding (MRS) is a geophysical technique specially designed for hydrogeological applications. It is based on the principle of Nuclear Magnetic Resonance (NMR) and allows the non-invasive detection of free water in the subsurface.As with many other geophysical methods, MRS is site-dependent. Modeling results show that MRS performance depends on the magnitude of the natural geomagnetic field, the electrical conductivity of rocks, the electromagnetic noise and other factors. For example, the maximum depth of groundwater detection for currently available equipment can vary from 45 to 170 m depending on measurement conditions, although an average depth of investigation is generally considered to be about 100 m. The processing of MRS data can provide the depth, thickness and water content of aquifers.Based on the water content and the relaxation times T1 and T2* provided by MRS, in association with calibration using borehole pumping test data, it is possible to estimate the aquifer''s hydrodynamic properties, namely permeability, transmissivity, and specific yield. In this aim, experience gained through NMR logging has been applied to MRS data interpretation and a comparison between the results of borehole pumping tests and those of MRS experiments reveals a good correlation.An example of an MRS survey in Saudi Arabia is presented. The study area is some 1.3 km2 and underlain by an aquifer composed of fractured diorite. The results of 7 borehole pumping tests and 13 MRS measurements show good agreement. [Copyright &y& Elsevier]