2.5-D forward modeling and inversion of time domain IP method

Abstract

The time-domain induced polarization (IP) method is commonly used for mineral exploration. A new 2.5-D FEM program in which Cole-Cole parameters are incorporated was developed for realistic analysis of time-domain IP data in this study. Using the program, transient changes of a resistivity pseudo section can be calculated, while traditional methods can only use pseudo sections of apparent resistivity and chargeability. The program will be useful for analysis of time-domain IP data because it can simulate the transient changes of a resistivity pseudo section due to the changes of time constant and frequency exponent in addition to chargeability.

A 2.5-D inversion program that estimates resistivity and three Cole-Cole parameters from observed IP data was also developed in order to characterize the differences of ore type. The inversion process contains two steps. First, ordinary 2.5-D resistivity inversion is done at each time step to calculate the resistivity of each rectangular block. Then, resistivity and three Cole-Cole parameters of each block are calculated using transient changes of block resistivity. As a result of the test inversion using the IP model with different IP parameters, individual IP parameters were estimated correctly. Additionaly it was found that two IP anomalies with the same resistivity and chargeability could be characterized by time constant and frequency exponent. This 2.5-D inversion, which can estimate time constant and frequency exponent, in addition to resistivity and chargeability, will be widely used for mineral exploration.