Reply to “Cooperative constrained inversion of multiple electromagnetic data sets” (Michael S. McMillan and Douglas W. Oldenburg, 2014, Geophysics, 79, no. 4, B173–B185)

Abstract

We thank Andre Revil for his interest in our paper (McMillan and Oldenburg, 2014) and for raising some concerns, and we would like to add a few comments in reply. Mr. Revil mentions in his discussion two assumptions that have been made in our paper. The first is that the electric conductivity of rocks containing metallic ores, such as sulfides, is frequency dependent. The second assumption states that we expect to see conductive anomalies in the presence of sulfide and gold mineralization, which goes against theoretical and laboratory studies outlined in Revil (2014). These assumptions are closely connected, and their relevance in interpretation depends upon the total chargeability of the material, the range of electromagnetic (EM) frequencies used in the analysis, and the conductivity values of the various rock units. The frequency dependence of sulfide-bearing rocks is a well-researched phenomenon, and for completeness it could have been addressed in our original paper (e.g., Pelton et al., 1978; Mahan, 1986). The goal of our paper was to present a practical strategy for cooperatively/jointly inverting data from multiple EM surveys to delineate conductive and resistive structures that have important consequences for a field exploration program. Although frequency dependence of conductivity is potentially a factor, when compared with the cumulative effect of other assumptions that are necessary to invert field data (e.g., precise knowledge of survey parameters, uncertainties assigned to data, and stopping criteria for the inversion), we believe that formally including this complicating factor would not have substantially altered the outcome of the paper. It also would not have aided the original goal of presenting …