Personal views on the effectiveness of airborne EM in Australian mineral exploration: a client perspective

Abstract

In any attempt to appraise the effectiveness of AEM in mineral exploration, the dominant questions relate to cost and credibility. Risk reduction is a key issue in assessing cost. Bedrock responses can normally be guaranteed with aeromagnetics, but this is not so with AEM. The risk of using AEM, considering that the costs are an order of magnitude higher, is significant enough to act as a deterrent when making decisions on how to explore. One way to reduce risk is to first fly wide spaced reconnaissance lines, and then fly detailed follow up only over those areas judged amenable to the survey system. Another way may be to pool resources through surveys conducted for multiple clients, including governments, so as to popularise conductivity and bedrock conductor maps, in the way magnetic images became popular in the 1980s and are now considered essential. Credibility depends on the application. In ground water and environmental work there is wide agreement that the method is effective. However, in mineral exploration, results are mixed; in vast thinly covered areas such as the Canadian Shield, the record is good, but in areas of ubiquitous conductive cover, which include much of Australia, the record is poor. Credibility is affected by both emotional and technical issues. Emotional issues include scepticism over a lack of exploration successes in Australia, fear of staking ones reputation on a survey that doesn't live up to expectations, hope but without any guarantees, confusion over which AEM system is best which can inhibit decision making, and even the personalities of the people involved. Technical issues include transmitter waveform and opinions on whether the theoretical blindness of impulse systems to highly conductive targets is worth the worry, motion induced false anomalies, the possibilities of which beg for continuous monitoring of bird location and orientation, and interpretation problems which recognises that an important aspect of mineral exploration is still "bump hunting" and that there is a need for more reliable discrimination between bedrock and overburden sources.