Electrical Resistivity Primer

OutlineMotivationGeophysical ImagingSubsurface Electrical PropertiesThe Electrical Resistivity MethodSurveys and Data DisplayTimelapse Electrical Resistivity

2D profiles

One popular survey type which was made possible by multi electrode instruments is the 2D profile. For a 2D profile data is collected using a multi electrode system over an array of generally equidistantly spaced electrodes laid out along a straight line on the surface. This configuration remains one of the most popular ones used to date. The reasons for this popularity are threefold:

  • This kind of configuration can be easily collected (just set up a straight line with equally spaced electrodes) using current hardware;
  • The data resulting from this kind of survey can be relatively easily processed on a standard PC using both commercial and open source resistivity processing software;
  • The processed data looks like a fence diagram of electrical properties - something which is intuitively clear to interpreters. Note that the operative word here are "looks like": unless the electrical properties are laterally invariant there will be 3D effects which need to be taken into consideration in the acquisition and processing, and ignoring this (as some processing codes do) can lead to artefacts. However, in many cases the resistivity profile is "sufficiently close" to a fence diagram that the advantage conferred by the 2D profile outweigh the inaccuracies.

Associated with 2D profiles is a choice of how to collect data (the so called acquisition geometry). The acquisition geometry describes the relative position of the current and potential electrodes. It is clear that as we increase the number of electrodes the number of possible ways we can collect data increases rapidly. Different standard acquisition geometries have been developed over the years which are generally believed to provide reasonable quality data. An overview of the most commonly used geometries is given here. Specific acquisition geometries are selected for three reasons:

  • Completeness of the resulting dataset (more measurements are better up to a point - note that there is a finite number of independent measurements which can be collected using a fixed number of electrodes);
  • Sensitivities of the different measurements to subsurface electrical resistivity distributions of interest ;
  • Signal/Noise ratios.

There are multiple papers and studies which discuss the sensitivity of different array types and the respective value of different arrays. This includes e.g a a paper in Vadose Zone Journal by Furman et al (note that a pdf of this paper can be found on the internet through google scholar) and the course notes from Loke on resistivity imaging.