Apsu: a wireless multichannel receiver system for surface nuclear magnetic resonance groundwater investigations
<p>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...
| Main Authors: | , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2019-01-01
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| Series: | Geoscientific Instrumentation, Methods and Data Systems |
| Online Access: | https://www.geosci-instrum-method-data-syst.net/8/1/2019/gi-8-1-2019.pdf |
| Summary: | <p>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<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M1" display="inline" overflow="scroll" dspmath="mathml"><msup><mi/><mrow><mo>-</mo><mn mathvariant="normal">1</mn><mo>/</mo><mn mathvariant="normal">2</mn></mrow></msup></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="21pt" height="11pt" class="svg-formula" dspmath="mathimg" md5hash="8e57d446a1516cf3e237c6eff1bfa487"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="gi-8-1-2019-ie00001.svg" width="21pt" height="11pt" src="gi-8-1-2019-ie00001.png"/></svg:svg></span></span>. 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.</p> |
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| ISSN: | 2193-0856 2193-0864 |