Using Geophysics for Groundwater Exploration and Aquifer Characterization

By: Andy Parsekian

Brady sets up the NMR instrument.

The objectives of this project are to demonstrate the effectiveness of combined time-domain electromagnetic (TEM) and surface nuclear magnetic resonance (NMR) for groundwater exploration in the APY lands of northwest South Australia, and to identify possible new groundwater resources.  The TEM measurement has been used for decades both in a ground-based acquisition mode and attached to aircraft.  This method is particularly effective at imaging the subsurface over long distances due to the rapid speed of data acquisition, although it is sometimes limited due to ambiguous interpretation of electrical properties of the subsurface.  The surface NMR method has also been used for decades, however in the past ten years, this method has seen particularly exciting developments and improvement in instrumentation.  Surface NMR is special among geophysical measurements because it is exclusively and unambiguous to liquid water.  The most common limitations for surface NMR are either not enough water in the aquifer to generate a signal or too much ambient “noise” that masks the signal we desire to detect.  Fortunately, at our field sites in the APY lands, there were regularly aquifers present within the top   50 m of the subsurface and the noise was very low enabling easy collection of high-quality data. 

The surface NMR instrument was set up on the bed of a truck.
Here, Denys sets up the system for data acquisition.

Aaron and Kevin making
a TEM sounding.

Tim Munday (dark shirt) discusses an
airborne geophysics dataset with
the group.

Our general strategy was to first look at large scale airborne geophysical maps that are available from Geoscience Australia.   We were able to identify certain targets that seemed like a possible aquifer from these maps.  Then, we took the TEM and surface NMR instruments to the field to analyze the targets in high details.  The TEM measurement was used at many sites along our target transect to get a high-resolution spatially semi-continuous image of subsurface structure, with particular focus on the position of the base of the aquifer.  Then, we visited several of the TEM sites with the surface NMR instrument that has a much longer data acquisition time and therefore less total measurements were possible.  The next step is, after more rigorous data processing, to interpret the TEM data using the direct water observations from the NMR to best characterize the aquifer. 

Denys using the magnetometer
before starting the NMR survey.

The preliminary results of our efforts have demonstrated the both the NMR and TEM methods are very well suited to the geologic conditions of the APY lands.  One of the possible limitations in this region is the magnetic geologic materials that are present that could limit the ability of surface NMR to reliably detect water.  Fortunately, we found that although the effect of magnetic materials was observable in our signals, due to the low noise conditions at the site we were still able to analyze those data.  Furthermore, we also were also able to utilize an NMR data acquisition mode that has been newly implemented in the surface NMR instrument that allowed us to minimize the contribution of magnetic effects to the signal.