Apsu — A new compact surface nuclear magnetic resonance system for groundwater investigation

Geophysics ◽  
2020 ◽  
Vol 85 (2) ◽  
pp. JM1-JM11 ◽  
Author(s):  
Jakob Juul Larsen ◽  
Lichao Liu ◽  
Denys Grombacher ◽  
Gordon Osterman ◽  
Esben Auken
Keyword(s):  
Nuclear Magnetic Resonance ◽  
Magnetic Resonance ◽  
Larmor Frequency ◽  
Low Noise ◽  
Quality Data ◽  
Subsurface Water ◽  
Near Surface ◽  
Peak Current ◽  
Multiple Receivers ◽  
Nuclear Magnetic

Surface nuclear magnetic resonance (NMR) is emerging as a competitive method for aquifer exploration due to its direct sensitivity to subsurface water, but the method still has several shortcomings, for example, a signal-to-noise ratio that is often poor, long survey times, and bulky equipment. We have developed Apsu, a new surface NMR system designed for near-surface groundwater investigations. It provides several features such as a compact transmitter unit, separated, small receiver coils, wireless connections between multiple receivers, quasi-zero dead time, and robust phase determination. The transmitter unit is powered by a lightweight generator, and it drives a triangular current in an untuned [Formula: see text] transmitter coil. The peak current of the triangular waveform is up to 145 A, with an effective peak current of 105 A at a Larmor frequency of 2 kHz, corresponding to a 30 m depth of investigation. The frequency and amplitude in each half-oscillation of the transmit pulses can be modulated independently, which gives great flexibility in the pulse design. The receiver uses low-noise preamplifiers and multiple receivers linked to a central unit through Wi-Fi. The use of small receiver coils and wireless connections to multichannel receivers greatly improves the layout configuration flexibility and survey efficiency. The performance of the system under field conditions is demonstrated with high-quality data collected near Silkeborg, Denmark, using on-resonance and numerically optimized modulation pulses.

Frequency cycling for compensation of undesired off-resonance effects in surface nuclear magnetic resonance

Geophysics ◽  
2016 ◽  
Vol 81 (4) ◽  
pp. WB33-WB48 ◽  
Author(s):  
Denys Grombacher ◽  
Mike Müller-Petke ◽  
Rosemary Knight
Keyword(s):  
Nuclear Magnetic Resonance ◽  
Magnetic Resonance ◽  
Larmor Frequency ◽  
Forward Model ◽  
Resonance Excitation ◽  
Entire Volume ◽  
Resonance Effects ◽  
Aquifer Properties ◽  
The Impact ◽  
Nuclear Magnetic

To produce reliable estimates of aquifer properties using surface nuclear magnetic resonance (NMR), an accurate forward model is required. The standard surface NMR forward model assumes that excitation occurs through a process called on-resonance excitation, which occurs when the transmit frequency is set to the Larmor frequency. However, this condition is often difficult to satisfy in practice due to the challenge of accurately determining the Larmor frequency within the entire volume of investigation. As such, in situations where an undesired offset is present between the assumed and true Larmor frequency, the accuracy of the forward model is degraded. This is because the undesired offset leads to a condition called off-resonance excitation, which impacts the signal amplitude, phase, and spatial distribution in the subsurface, subsequently reducing the accuracy of surface NMR estimated aquifer properties. Our aim was to reduce the impact of an undesired offset between the assumed and true Larmor frequency to ensure an accurate forward model in the presence of an uncertain Larmor frequency estimate. We have developed a methodology where data are collected using two different transmit frequencies, each an equal magnitude above and below the assumed Larmor frequency. These data are combined, through a method we refer to as frequency cycling, in a manner that allow the component well-described by our estimate of the Larmor frequency to be stacked coherently, whereas the component related to the presence of an undesired offset is combined destructively. In synthetic and field studies, we have determined that frequency cycling is able to mitigate the influence of an undesired offset providing more accurate estimates of aquifer properties. Furthermore, the frequency-cycling method stabilized the complex inversion of surface NMR data, allowing advantages associated with complex inversion to be exploited.

scholarly journals Surface Nuclear Magnetic Resonance Monitoring Reveals Karst Unsaturated Zone Recharge Dynamics during a Rain Event

Water ◽  
2020 ◽  
Vol 12 (11) ◽  
pp. 3183
Author(s):  
Naomi Mazzilli ◽  
Konstantinos Chalikakis ◽  
Simon D. Carrière ◽  
Anatoly Legchenko
Keyword(s):  
Nuclear Magnetic Resonance ◽  
Magnetic Resonance ◽  
Unsaturated Zone ◽  
Low Noise ◽  
Water Dynamics ◽  
Rain Event ◽  
Experimental Site ◽  
Initial State ◽  
Before And After ◽  
Nuclear Magnetic

Understanding karst unsaturated zone (UZ) recharge dynamics is crucial for achieving sustainable management of karst hydrosystems. In this paper, we provide the first report of the application of surface nuclear magnetic resonance (SNMR) monitoring of a karst UZ during a typical Mediterranean rain event. This 79 days’ SNMR monitoring is a part of a more than 2 years of SNMR monitoring at the Low Noise Underground Laboratory (LSBB) experimental site located within the Fontaine de Vaucluse karst hydrosystem (southeastern France). We present eight SNMR soundings conducted before and after the rain event that accumulated 168 mm in 5 days. The obtained results demonstrate the applicability and the efficiency of SNMR for investigating infiltration dynamics in karst UZs at the time scale of a few days. We present the SNMR amplitudes that highlight strong signal variations related to water dynamics in the karst UZ. Infiltrated water cause increased SNMR signal during 5 days after the rain event. A significant draining process of the medium starts 15 days after the main event. Finally, after 42 days, the SNMR signal returns close to the initial state.

Algorithms for removing surface water signals from surface nuclear magnetic resonance infiltration surveys

Geophysics ◽  
2016 ◽  
Vol 81 (4) ◽  
pp. WB97-WB107 ◽  
Author(s):  
Samuel Falzone ◽  
Kristina Keating
Keyword(s):  
Nuclear Magnetic Resonance ◽  
Surface Water ◽  
Magnetic Resonance ◽  
Water Content ◽  
Water Resource Management ◽  
Measurement Techniques ◽  
Subsurface Water ◽  
Aquifer Storage ◽  
Water Signal ◽  
Nuclear Magnetic

Surface nuclear magnetic resonance (surface NMR) is a geophysical method that directly detects water and can be used to determine the depth profile of water content within the subsurface. Although surface NMR has proven useful for investigating groundwater in the saturated zone, its use to study the vadose zone is still in development. A recent study for the South Avra Valley Storage and Recovery Project (SAVSARP) demonstrated that surface NMR can be used to monitor infiltrating water associated with aquifer storage and recovery, a water resource management method in which surface water is stored in local aquifers during wet periods for use during dry periods. However, one of the major issues associated with using surface NMR to monitor infiltrating water is the influence of large bodies of surface water. We have examined the effect that large bodies of surface water have on the surface NMR signal, and we have developed three algorithms (the a priori, late-signal, and long-signal-inversion [LSI] algorithms) to remove this signal. Using synthetic data sets, we have assessed the efficacy of each algorithm and determined that, although each algorithm is capable of suppressing the signal from a water layer with a thickness [Formula: see text], the LSI algorithm provides the most accurate and consistent results. Using a field example from the SAVSARP survey, we have evaluated the use of the LSI algorithm to suppress the surface water signal. Our results have indicated that the signal from surface water detected in a surface NMR survey can be suppressed to obtain the subsurface water content without the use of new measurement techniques or additional equipment.

Nuclear magnetic resonance for near-surface applications — Introduction

Geophysics ◽  
2016 ◽  
Vol 81 (4) ◽  
pp. WBi-WBii ◽  
Author(s):  
Kristina Keating ◽  
Mike Müller-Petke ◽  
Ahmad A. Berhoozmand ◽  
Jean-François Girard ◽  
Lin Jun
Keyword(s):  
Nuclear Magnetic Resonance ◽  
Magnetic Resonance ◽  
Near Surface ◽  
Nuclear Magnetic
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