High Resolution Magnetics in the Presence of Intense Near Surface Magnetic Noise

Abstract

The general problem of magnetic exploration in a region of near surface magnetic noise interference has been studied with particular attention being devoted to the specific example of the Elura orebody underlying a maghemitic noise source. Prime considerations were the relative merits of ground level and airborne surveys, data sampling requirements, and filter processes by which optimum enhancement of a deep source signal could be achieved with least distortion. The primary objectives of the research program were to define and quantify the signal and noise waveform parameters, to investigate the filtering operations that could be applied to distinguish between signal and noise waveforms, and to quantify the signal to noise ratio that could be achieved from both airborne and ground level magnetic surveys in the presence of intense near surface noise. The characteristics of noise from the most commonly encountered near surface sources were described and defined. A similar generalization of the signal characteristics could not be made because of the dependence upon the definition of each individual exploration target. In this study a limited definition of 'signal' was used. The anomaly arising from a large subsurface geological structure such as a lithological boundary, fault, or base metal deposit of economical dimension was considered as 'signal'. The Elura orebody was adopted as a practical model of a signal source because it was of economic but relatively small size and it was of magnetic susceptibility representative of many base metal sulphides. From this particular model a magnetic signal could be precisely defined for varying depths of burial of the source. The geological environment of the Elura orebody was described as a case example where signal and noise sources could each be related to their geological origin.