Sign changes in DHEM surveys for cindered coal in the Sydney basin

Abstract

Coal seams adjacent to post-depositional igneous intrusives may show very high conductivities due to presence of graphite in the cindered (metamorphosed) coal. A downhole TEM survey through the Bulli coal seam, in the south Sydney basin shows the conductance of a 4 metre intersection of cindered coal to be about 50 S, indicating that the TEM method, both surface and downhole, has applications in the identification of areas of cindered coal. The anomaly due to the highly conductive cindered coal shows a sign-reversal at intermediate times which is attributable to the influence of overlying conductive sediments and which demonstrates the importance of considering the influence of conductive overburden or host rock before attempting interpretation of target geometry using free-space models.