Interpretation of Crone pulse electromagnetic data

Abstract

Time‐domain electromagnetic (TEM) prospecting systems, including the Crone pulse electromagnetic (PEM) system, are becoming more widely used in the search for massive sulfides. We have developed interpretation aids for the moving‐source configuration of the Crone PEM system using the PLATE and SPHERE programs developed by the University of Toronto. PLATE models a thin rectangular sheet in free space and SPHERE models a two‐layer sphere in free space. The effects of varying the model parameters, particularly the dip, depth, and conductance for large plates, are shown. As the dip of a plate goes from 0 to 90 degrees, the negative side lobe over the conductor becomes less pronounced. The depth can be estimated from the amplitude of the positive peak over the edge of a conductor. The depth of exploration for the Crone PEM system is approximately twice the coil separation for shallow dipping plates, and one coil separation for steeply dipping plates when there is not a significant half‐space or overburden response. The ratio of the on‐conductor and off‐conductor side‐lobe areas gives an estimate of the dip if the depth of the target has been determined. The resolution of dip for plates dipping more than 60 degrees is poor. The conductance of a plate can be determined from either channel‐ratios or the estimated time constant. Channel‐ratios sometimes provide better estimates, because they have less dependence on the size of the conductor at earlier times. At early times, the target response can be obscured by conductive overburden. Only at late times would interpretation aids be useful in evaluating the target parameters. A dip‐depth nomogram and channel‐ratios gave good first estimates of the target parameters of three field examples interpreted by computer modeling.