Pilot‐scale field validation of the long electrode electrical resistivity tomography method

Abstract

Field validation for the long electrode electrical resistivity tomography (LE-ERT) method was attempted in order to demonstrate the performance of the technique in imaging a simple buried target. The experiment was an approximately 1/17 scale mock-up of a region encompassing a buried nuclear waste tank on the Hanford site. The target of focus was constructed by manually forming a simulated plume within the vadose zone using a tank waste simulant. The LE-ERT results were compared to ERT using conventional point electrodes on the surface and buried within the survey domain. Using a pole-pole array, both point and long electrode imaging techniques identified the lateral extents of the pre-formed plume with reasonable fidelity, but the LE-ERT was handicapped in reconstructing the vertical boundaries. The pole-dipole and dipole-dipole arrays were also tested with the LE-ERT method and were shown to have the least favorable target properties, including the position of the reconstructed plume relative to the known plume and the intensity of false positive targets. The poor performance of the pole-dipole and dipole-dipole arrays was attributed to an inexhaustive and non-optimal coverage of data at key electrodes, as well as an increased noise for electrode combinations with high geometric factors. However, when comparing the model resolution matrix among the different acquisition strategies, the pole-dipole and dipole-dipole arrays using long electrodes were shown to have significantly higher average and maximum values than any pole-pole array. The model resolution describes how well the inversion model resolves the subsurface. Given the model resolution performance of the pole-dipole and dipole-dipole arrays, it may be worth investing in tools to understand the optimum subset of randomly distributed electrode pairs to produce maximum performance from the inversion model.

Enhanced resolution for long electrode ERT

“…A few of the buried electrodes were nested at a couple of positions. The present analysis limits the discussion to the results from the long electrodes, but the short electrode information is presented in Rucker et al (2012a).…”

“…5b). Rucker et al (2012a) reportedly used 4546 measurements after data reduction for final inversion.…”

“…For our work in imaging high ionic strength waste plumes beneath nuclear waste storage facilities, we purposely opted for the 2‐pole array to ensure the highest signal strength for the very conductive environment. Of note, additional testing was conducted in the pilot‐scale study of Rucker et al (2012a) with randomized 3‐pole and 4‐pole LE‐ERT pairs of measurements, but these array types were not seen as viable as the target was significantly offset from the known location. Ramirez et al (2003) also stated that a randomized 4‐pole LE‐ERT produced anomalies that least resembled a modelled target.…”

“…To demonstrate the increased resolution for vertically oriented long electrodes, we reopen the 4‐pole LE‐ERT pilot‐scale analysis from Rucker et al (2012a) and show that (1) acquiring 2‐pole long electrode data is the preferred approach; (2) a complete 2‐pole data set can be used to reconstruct any subset of 4‐pole long electrode data; and (3) inversion of 4‐pole long electrode data is the preferred approach. Through this demonstration we combine the best of both array types to image targets with much greater fidelity than with any one array type alone.…”

Geophysical Characterization of Serpentinite Hosted Hydrogeology at the McLaughlin Natural Reserve, Coast Range Ophiolite

“…The method has been commonly used to investigate the near surface by creating 2-D or 3-D models of subsurface resistivity that is representative of observed geologic formations and structure (Dunbar et al, 2015;Griffiths & Barker, 1993;Loke et al, 2013;Rucker & Glaser, 2015). We selected the dipole-dipole array to conduct all of our 2-D ERT profiles because of its higher sensitivity to vertical structures (Aizebeokhai, 2010) and high model resolution (Rucker et al, 2012;Wilkinson et al, 2012). For CROMO sites and Serpentine Valley surveys we used an Advanced Geosciences, Inc. Supersting R8 instrument.…”