DHEM at Las Cruces, Spain - successes and failures

Abstract

Summary Downhole electromagnetic (DHEM) logging was undertaken within four holes drilled at the Las Cruces VMS body in Spain. The geology of the deposit is geophysically complex inasmuch as the background and overlying geology is conductive. A number of loop positions were pre-modelled in order to optimise coupling with the sulphides. The objective in all cases was to locate additional off-hole massive sulphides, belonging to both the main sulphide body and a smaller satellite body. The DHEM was undertaken with a variety of loop positions and at a number of base frequencies. Modelling was undertaken using Maxwell EM modelling software. In three of the holes, by using a number of thick plates and various conductive layers and half-spaces, the DHEM results could be explained by the known sulphides. In the case of a fourth hole, off-hole anomalies were located at depth. A new borehole was recommended based on these data. This borehole was later drilled and an extension to the satellite body intersected.