Electrical Resistivity Surveys
Collection of electrical resistivity data in the field (or in a laboratory) can be done with different instruments and electrode geometries. The term electrical resistivity survey is used both to refer to the process of data collection as well as the resulting measurements. Historically (until about the mid 1980s) data were collected using basic four electrode systems in modes known as profiling and sounding. Note that electrical resistivity data has been collected in boreholes for oil and gas exploration purposes using borehole geophysical methods since 1927 (see here for some links to borehole geophysical methods) but for the method which is the focus of this primer (i.e. electrical resistivity collected using electrodes which are semi permanently emplaced in the ground) profiling and sounding were the two primary configurations. Since the mid 1980s (enabled by improvements in instrumentation, and specifically the introduction of multi electrode systems) numerous other configurations have been developed. The main ones are
- Collection of 2D profiles using a linear array of electrodes located on the surface.
- Collection of data using 3D grids of electrodes (so called 3D surveys)
- Collection of data using electrodes placed along boreholes (either in a single borehole or in multiple boreholes)
- Collection of data with electrodes towed behind a boat (marine resistivity) or behind a vehicle using a landstreamer. This is a special case of the 2D profiles: basically at every position of the tow vehicle we collect a small 2D profile
- Collection of time-lapse data (also known as 4D data), which can be done using all the configurations listed above.