Depth of detection of highly conducting and volume polarizable targets using induced polarization

We define the apparent frequency effect in induced polarization (IP) as the relative difference between apparent resistivities measured using DC excit ation on the one hand and high-frequency excitation (when the IP effect van ishes) on the other. Assuming a given threshold for the minimum detectable anomaly in the apparent frequency effect, the depth of detection of a targe t by IP can be defined as that depth below which the target response is low er than the threshold for a given electrode array. Physical modelling shows that for the various arrays, the depth of detection of a highly conducting and volume polarizable target agrees closely with the depth of detection o f an infinitely conducting and non-polarized body of the same shape and siz e. The greatest depth of detection is obtained with a two-electrode array, followed by a three-electrode array, while the smallest depth of detection is obtained with a Wenner array when the array spread is in-line (i.e. perp endicular to the strike direction). The depth of detection with a Wenner ar ray improves considerably and is almost equal to that of a two-electrode ar ray when the array spread is broadside (i.e. along the strike direction).