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Discussion papers | Copyright
https://doi.org/10.5194/gi-2018-1
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 19 Feb 2018

Research article | 19 Feb 2018

Review status
This discussion paper is a preprint. A revision of the manuscript is under review for the journal Geoscientific Instrumentation, Methods and Data Systems (GI).

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

Lichao Liu1, Denys Grombacher1, Esben Auken1, and Jakob Juul Larsen2 Lichao Liu et al.
  • 1Hydrogeophysics Group, Department of Geoscience, Aarhus University, 8000 Aarhus C, Denmark
  • 2Department of Engineering, Aarhus University, 8200 Aarhus N, Denmark

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 31.25kHz. The system timing is controlled by GPS clock and sample jitter between channels is less than 12ns. 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 4ms. 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 1.8nV/√(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.

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Latest update: 19 Jul 2018
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Short summary
This paper introcudes the design workflow and test approaches of a surface-NMR receiver. But the method and technqiues for instance signal loop, acqusition board, GPS synchronization, WiFi network could be employed in other geophysical instruments.
This paper introcudes the design workflow and test approaches of a surface-NMR receiver. But the...
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