Blocking borehole conductivity logs at the resolution of above-ground electromagnetic systems

Abstract

Borehole conductivity logs provide an in situ measurement of the electrical conductivity of the subsurface. Despite the measurements being a proxy for the true earth structure, they are often used as ground truth when inferring subsurface electrical conductivity boundaries between lithologies. Borehole conductivity measurements are therefore commonly used to plan and benchmark electromagnetic (EM) surveys and to establish the credibility of a given inversion technique. A consequence of the diffusion physics of EM prospecting is that not all subsurface features present in a conductivity log can be resolved by an EM system, nor can they be recovered by a subsequent inversion. Quantification of the ability of an EM system to determine layer boundaries in the subsurface is therefore an issue meriting investigation. We have developed a reversible-jump Markov chain Monte Carlo (RJMCMC) method to segment borehole conductivity logs at the scale recoverable by a given EM system as the foundation for an objective comparison between the inversion results and conductivity logs. A common consequence of RJMCMC inversions for EM problems is that few layers are required to fit the data. Similarly, we find that a borehole log blocked at the scale sensed by an EM system consists of a limited number of segments. Segmentation of borehole conductivity logs is determined by the physics of EM prospecting and by factors such as base frequency, number of gates, system geometry, and noise levels. For a survey line intersecting a borehole near Carnarvon, Western Australia, we see that different inversion schemes result in images of the subsurface that are consistent with a borehole conductivity log segmented according to the mechanics of the EM system and accounting for the physics of EM prospecting.