DESIGN FOR A MANUALLY CONTROLLED MULTIPLE ELECTRODE ELECTRICAL RESISTIVITY ARRAY FOR STUDENT GEOPHYSICAL SURVEYS

To facilitate rapid collection of electrical resistivity data for a groundwater study in karst terrain, we designed and constructed a 56 electrode array controlled with a manual switching box. The switchbox allows directing current to electrodes using Wenner or Schlumberger arrangements for vertical sounding on several centers after only one emplacement of electrodes or Dipole-Dipole arrangements for measuring pseudo-sections.

The rate of data collection improved most with longer node spacing and when operated by fewer students because moving the electrodes was the slowest step in the data collection procedure. Results of data for the same test site using traditional four-electrode Wenner array to determined depth to the base of the plow zone and depth to bedrock are virtually identical to those measured with the big array (R-squared = 0.99).

The spacing between electrodes is 3 meters, allowing a maximum spacing with the Wenner array of 55 meters. Data analysis using a spreadsheet helps students understand the concepts underlying their data interpretation. For undergraduate student hydrogeology projects, the instrument is a low cost alternative to commercial multi-node arrays and all materials are readily available.