How to Measure Bedrock Resistivity

Design a simple topographic survey of the area that you will be testing for electrical resistivity. Since you cannot measure every point in the area you are testing, determine the number of samplings needed and from where in the field the samples will be taken. Mark these points on the topographic survey. Using that map as a guide, place easily visible field markers in the field where you will be taking the actual readings.

Insert the four electrodes of the electrical resistivity meter in the ground. Probes should be equally spaced 3 feet to 30 feet apart in a straight line, with each probe the same distance from the probe next to it. The farther apart the probes, the deeper in the ground the soil resistivity will be tested. This is why it is important to take multiple readings from the same location. Electrodes should be inserted at shallow depths, typically 6 inches, 1 foot, 1 1/2 feet, 2 feet, 2 1/2 feet. All four probes should be inserted at the same depth for each individual reading.

Move the electricity resistivity meter with electrodes to a second location, using your topographic survey as your guide. Repeat Step 2, taking and recording additional readings at the new location. Continue using your plan to take additional readings until you have completed all samples. Because no sub-surface geography is homogeneous, it is important to take multiple readings at various locations in order to get an accurate picture of the subsurface bedrock.