An application of reverse coupling to increase signal strength beneath conductive sediments-Miitel Mine, Kambalda, W.A.

Abstract

Downhole electromagnetics (DHEM) is the principle geophysical tool used at Kambalda Nickel Operations for the detection and delineation of sulphidic ore zones. The case study presented here is from Miitel, a relatively new mine site, which expects to begin production by January 1999. Exploration at Miitel is a challenging proposition as mineralisation occurs in discrete blocks, which, due to limited drilling, are not yet well defined. For this reason, most holes drilled at Miitel are surveyed with DHEM to increase the investigation area and to assess the size of intersected conductors. DHEM logging at Miitel faces a significant problem. Exploration targets are not only overlain by a conductive overburden but are also overlain by a layer of thick pyrrhotitic sediments, positioned l00 m into the hanging ball. The consequence is a low amplitude response from the target and substantial overburden interference at the target area, below the sediments. These effects decrease the signal-to-noise ratios and increase the ambiguity in interpretation. In DHEM logging, usually the optimum transmitter loop position is where coupling is maximal with the target and minimal with all other conductors. Typically this is accomplished by placing the transmitter loop in a normal coupled position where the field lines traverse a path from the centre of the loop through the hanging wall side of the target ore horizon. Reverse coupled loops couple poorly with the sediments because field lines couple through the footwall beneath them, while still coupling well with the targeted ore blocks. Logging results from a surface drillhole at Miitel used both normal and reverse coupled transmitter loops to test the effectiveness of this non-conventional survey design. The results demonstrate how reverse coupling can successfully be used in this adverse environment to overcome the effects of amplitude reduction, current channelling, and drive delay thus improving data quality and interpretation reliability.