Improved and new resistivity-depth profiles for helicopter electromagnetic data

Abstract

The calculation of apparent resistivities, based on the model of a homogeneous half-space, is commonly the first step in order to evaluate helicopter electromagnetic (HEM) data. Due to the increase in frequencies used in HEM systems, survey results are not only displayed as apparent resistivity maps but also as cross-sections that require resistivity and depth information. After a brief description and discussion of the basic approaches, improved HEM resistivity–depth profiles are derived. The apparent resistivities ρ a are calculated more accurately when better approximations are used. The corresponding depth value, the centroid depth z p*, is newly defined as the sum of the apparent depth and the half of the apparent skin depth. The resulting profile is referred to hereafter as the improved standard sounding curve ρ a( z p*). Several algorithms for deriving “enhanced” resistivity–depth profiles, which are more sensitive to resistivity variations with respect to depth, are presented. Two of these enhanced resistivity–depth profiles are based on algorithms used for the interpretation of MT data. The ρ NB( z s*) sounding curve is derived from the Niblett–Bostick algorithm. It requires multifrequency data because the enhancing is achieved by differentiating the ρ a( f) sounding curve with respect to frequency. The other one, the ρ*( z d*) sounding curve, is computed from each frequency independently of the other frequencies because no differentiation is involved. It is similar to Schmucker's ρ*– z* scheme and it uses the apparent depth d a to enhance the sensitivity of the apparent resistivity. Furthermore, both novel algorithms are able to increase the depth of exploration. All enhanced resistivity–depths profiles are compared with the improved standard sounding curve ρ a( z p*) and with the differential parameter method, ρ Δ( z Δ), published by Huang and Fraser. Since being robust and easy to calculate, the proved ρ a( z p*) method should be used for the standard calculation of resistivity–depth profiles. Besides, it is the basis for the enhanced methods, which, in addition, can be used to derive more sensitive resistivity–depth profiles. Among all methods discussed, only ρ a and ρ NB are relatively independent of the measured sensor height h, which is a serious problem in survey areas with dense forests. The corresponding centroid depth values, z p* and z s*, are not distorted by the vegetation, if they are displayed with respect to the elevation of the HEM sensor which should be given in m a.s.l.