NEGATIVE INDUCED‐POLARIZATION EFFECTS FROM LAYERED MEDIA

Abstract

Negative induced‐polarization responses from lateral resistivity distributions such as near‐surface vertical contacts and dikes, three‐dimensional buried bodies, etc., have been well known for years. Recent comprehensive computations of resistivity and induced‐polarization response functions over a three horizontally layered earth have revealed that negative induced‐polarization effects can also occur whenever the geoelectric section is of K type (ρ1, < ρ2 > ρ3) or Q type (ρ1 > ρ2 > ρ3). For such sections, the induced‐polarization function of the first layer (as defined by Seigel) can be negative. As a result, if the first layer is polarizable the response from deeper layers can be severely masked solely as a function of the resistivity distribution, thus limiting the depth of detection of buried sources. It is shown that for an m‐layered earth, negative induced‐polarization functions can be expected for those layers lying above a K or Q type sequence in the resistivity section. Any application of induced‐polarization surveys must thus be preceded by a thorough evaluation of the resistivity data. The same applies for the quantitative interpretation of IP data.