Low noise, multichannel surface NMR receiver system with wireless connections to receiver coils

Abstract

Surface NMR is a unique tool in groundwater geophysics due to its ability to directly measure the depth-resolved water content in the near sub-surface and provide depth-resolved estimates of the hydrological properties. The method is still in its infancy and several challenges must be overcome for the method to be generally applicable. These challenges include a low signal to noise ratio limiting measurements in many places of interest, a low production rate making the method costly in field campaigns and expensive instruments. We present a new multichannel, low noise surface NMR receiver system where receiver coils are connected to a main unit with wireless connections. The system works as a completely independent add-on to existing surface NMR transmitter systems. The receiver system are built as individual operated data acquisition boxes with timing synchronized by GPS and connected through WiFi to the main unit. The wireless feature allows for placement of reference receiver coils without any restriction imposed by cable lengths. The internal noise of the system operated in differential mode is 1.8 nV/sqrt(Hz) measured on a single stack. We verify the performance of the system by measurements of a controlled source and by comparing our measurements with measurements performed with a commercial surface NMR system.