Resistivity Depth Sounding & Profiling

Resistivity Depth Sounding & Profiling

Electrical surveys undertaken by a dc resistivity device involve placement of electrodes in the ground. There are various geometry's for the layout of the electrodes, but most have all four electrodes in line. The Wenner, Slumberger, and Dipole Dipole arrangements are the most popular. The two outer electrodes are the current source and sink; current is driven by self contained batteries. The two inner potential electrodes sense at the surface the electrical potential while current is flowing between the outer electrodes. The potential measured varies with electrode spacing in a predictable way, and also changes as the strata and contained fluids vary laterally and vertically. And from this data set, layers of differing properties can be determined.

Two modes of operation are common, namely, a) depth sounding and b) profiling. In the depth sounding mode, all four electrodes are initially placed in the ground with a very short spacing between adjacent electrodes. A reading is taken, and then the array is reset with an incremental increase in spacing. Another reading is taken, and the array is, in turn, progressively expanded in this manner until the maximum depth to be investigated is reached. The current and potential sense progressively deeper layer as the array is expanded.

In the profiling mode, a constant electrode spacing is selected that senses the subsurface geology to the depth of interest, and this constant array is "leap frogged" along a profile line to measure lateral variations along the profile that have geologic meaning.

Paleo Shoreline

Paleochannel Location

Municipal Landfill

Landslide

Southwest has used resistivity to:

  • Map faults and landslides
  • Map fracture zones for water well sighting
  • Map large karst voids and sea caves
  • Find depth to groundwater, in certain specific situations
  • Generally characterize subsurface geology.
  • Aid environmental contamination flow projects when used to evaluate locations of paleo stream channels or fault zones.
  • Find buried bedrock valleys, which can be an important source of groundwater in unconfined aquifers.
  • Vertical joints or cracks, which can indicate subsidence features.