scholarly journals Apsu: a wireless multichannel receiver system for surface nuclear magnetic resonance groundwater investigations

2019 ◽  
Vol 8 (1) ◽  
pp. 1-11 ◽  
Author(s):  
Lichao Liu ◽  
Denys Grombacher ◽  
Esben Auken ◽  
Jakob Juul Larsen
Keyword(s):  
Nuclear Magnetic Resonance ◽  
Magnetic Resonance ◽  
Data Acquisition ◽  
Dead Time ◽  
Noise Cancellation ◽  
Low Noise ◽  
Central Unit ◽  
Long Distance ◽  
Receiver System ◽  
Nuclear Magnetic

Abstract. Surface nuclear magnetic resonance (surface NMR) has the potential to be an important geophysical method for groundwater investigations, but the technique suffers from a poor signal-to-noise ratio (SNR) and long measurement times. We present a new wireless, multichannel surface-NMR receiver system (called Apsu) designed to improve field deployability and minimize instrument dead time. It is a distributed wireless system consisting of a central unit and independently operated data acquisition boxes each with three channels that measure either the NMR signal or noise for reference noise cancellation. Communication between the central unit and the data acquisition boxes is done through long-distance Wi-Fi and recordings are retrieved in real time. The receiver system employs differential coils with low-noise preamplifiers and high-resolution wide dynamic-range acquisition boards. Each channel contains multistage amplifiers, short settling-time filters, and two 24 bit analog-to-digital converters in dual-gain mode sampling at 31.25 kHz. The system timing is controlled by GPS clock, and sample jitter between channels is less than 12 ns. Separated transmitter/receiver coils and continuous acquisition allow NMR signals to be measured with zero instrument dead time. In processed data, analog and digital filters cause an effective dead time of 5.8 ms including excitation current decay. Synchronization with an independently operated transmitter system is done with a current probe monitoring the NMR excitation pulses. The noise density measured in a shorted-input test is 1.8 nV Hz-1/2. We verify the accuracy of the receiver system with measurements of a magnetic dipole source and by comparing our NMR data with data obtained using an existing commercial instrument. The applicability of the system for reference noise cancellation is validated with field data.

scholarly journals Apsu: a wireless multichannel receiver system for surface-NMR groundwater investigations

2018 ◽  
Author(s):  
Lichao Liu ◽  
Denys Grombacher ◽  
Esben Auken ◽  
Jakob Juul Larsen
Keyword(s):  
Data Acquisition ◽  
Dynamic Range ◽  
Dead Time ◽  
Signal To Noise Ratio ◽  
Noise Cancellation ◽  
Low Noise ◽  
Dipole Source ◽  
Central Unit ◽  
Long Distance ◽  
Receiver System

Abstract. Surface nuclear magnetic resonance (surface-NMR) has the potential to be an important geophysical method for groundwater investigations, but the technique suffers from poor signal-to-noise ratio (SNR) and long measurement times. We present a new wireless, multichannel surface-NMR receiver system (called Apsu) designed to improve SNR, field deployability and minimize instrument dead time. It is a distributed wireless system consisting of a central unit and independently operated data acquisition boxes each with three channels that measure either the NMR signal or noise for reference noise cancellation. Communication between the central unit and the data acquisition boxes is done through long distance WiFi and recordings are retrieved in real time. The receiver system employs differential coils with low-noise pre-amplifiers and high-resolution wide dynamic range acquisition boards. Each channel contains multi-stage amplifiers, short settling-time filters and two 24-bit analog-to-digital converters in dual-gain mode sampling at 31.25 kHz. The system timing is controlled by GPS clock and sample jitter between channels is less than 12 ns. Separated transmitter/receiver coils and continuous acquisition allow NMR signals to be measured with zero instrument dead time. In processed data, analog and digital filters causes an effective dead time of 4 ms. Synchronization with an independently operated transmitter system is done with a current probe monitoring the NMR excitation pulses. The noise density measured in a shorted-input test is 1.8 nV/√(Hz). We verify the accuracy of the receiver system with measurements of a magnetic dipole source and by comparing our NMR data with data obtained using an existing commercial instrument. The applicability of the system for reference noise cancellation is validated with field data.

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.

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.

Using the temporal distribution of noise for reference noise cancelation in sNMR surveys

2020 ◽  
Author(s):  
Thomas Kremer
Keyword(s):  
Nuclear Magnetic Resonance ◽  
Magnetic Resonance ◽  
Noise Reduction ◽  
Field Survey ◽  
Coherence Function ◽  
Noise Cancellation ◽  
Nuclear Magnetic Resonance Method ◽  
Primary Loop ◽  
Secondary Loop ◽  
Nuclear Magnetic

<p>The surface Nuclear Magnetic Resonance method is gaining momentum as an efficient geophysical method for the detection and characterization of groundwater. However, the method still suffers from a low signal to noise ratio mostly due to electromagnetic noise of anthropic origin.</p><p>To solve this problem, signal processing in surface nuclear magnetic resonance surveys often relies on the reference-based noise cancellation technique. This method consists of capturing the main characteristics of the noise through a secondary loop ideally located and obtaining an estimate of the noise affecting the primary loop which can be subtracted from the noisy sNMR signal.</p><p>The main problem associated with the method occurs when the spatial distribution of the noise is heterogeneous, which can result in a low correlation between the reference loop and the primary loop, and hence in a poor noise reduction. Difficulties may also arise when the field survey location prevents the display of a reference loop for logistics or physical reasons.</p><p>To remediate these situations we have investigated the possibility of recording noise-only signals through the primary loop, prior to the sNMR measurement, and use those signals as references for subsequent calculation of the local transfer function. The correlation between a series of noise-only signal recorded on a primary loop was analyzed through the computation of the magnitude squared coherence function, and comparison was made with noise records from a secondary loop. The analysis demonstrates that temporal reference noise cancellation (TRNC) can provide more efficient noise reduction results than the classical spatial reference noise cancellation if the temporal noise-only database is large enough.</p><p>Such a technique would be particularly suited for the development of long-term sNMR monitoring systems, where noise records could be acquired for long periods without any difficulty. In addition to two field survey examples, we present a synthetic statistical analysis to estimate the minimal volume of the signal database required for optimal noise reduction.</p>

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