How deep beneath the surface can I see using Electrical Resistivity Imaging?

Short Answer: You can expect a depth that is roughly 18% of the distance separating the two dipoles.

Long Answer: The depth of penetration is dependent on the distance we separate the transmitter and receiver electrode. As the transmitter and receiver dipoles are separated by an increasing distance, the signal strength lessens and approaches the background noise. The depth of penetration has been empirically found to be around 18% that of the distance separating the two dipoles. Please see the scientific paper, A Modified Pseudosection for Resistivity and IP by L.S. Edwards (1977).