DHEM surveys of the 2K area, North Mine, Broken Hill

Abstract

Drillhole EM (DHEM) surveys have been used at the North Mine in Broken Hill, NSW to help determine the size and attitude of the 2K mineralisation, which is a separate lens of high-grade massive sulphides some 200 m away from the mine workings and approximately 2 kms below the surface. The 2K zone was surveyed from an underground drillhole collared at 1650 m below the surface, using several surface, and one underground, loops. The results are presented as a case history of deep DHEM in a hot and hostile environment, where the results were used to assist the structural inter-pretation of the area and help decide whether to further investigate some high grade ore intersections. However, the prime purpose of the paper, which is presented after the history, is firstly to demonstrate how an apparent or false response may occur through drillhole deviation, and secondly, to show that the interactive effects caused by a conductive overburden cannot be ignored during an interpretation, especially with transmitting loops which are poorly coupled to the target. To recognise the former and to allow for the latter, requires use of the 'true trace' of the probe rather than straight line approximations and 'complete curve' modelling instead of using stripped data.