Case study: The evolution of airborne time domain electromagnetic applications for geologic mapping; a Noranda perspective

Abstract

The advances in airborne time domain EM over the last seven years have facilitated its evolution into an effective mapping system from what has been ostensibly considered primarily a massive sulphide targeting tool. During this period Noranda undertook a series of programs which provided insights into the technical issues and internretation - challenges that the use airborne time domain electromagnetics (TDEM) technology presents to exploration geologists and geophysicists. a model study and test flying program over a sediment hosted copper prospect on Victoria Island in the Canadian Arctic, a series of covered terrain porphyry copper exploration mapping programs were undertaken in southwest Arizona and Sonora Mexico between 1993 and 1997 using both the QUESTEM and GEOTEM commercial airborne TEM systems. The results of this work were encouraging but underscored the need for ground validation programs and robust geologic models to properly interpret the data. A need for lower noise and better calibrated EM systems was also recognized. The thrust of Airborne TDEM technology development has been to improve conductor (massive sulphide) detection to greater depth in ever increasing complex geologic environments. This application is often referred to as bump finding in reference to discrete anomaly amplitude signatures superimposed along ostensibly flat background, profiled channel data. However, as processing software improved (i.e., conductivity-depth inversion/transformation algorithms, CDI/CDT) and integrated exploration models were applied, it became clear that this technology could be applied as a broad geologic mapping tool. This required a shift in how the data was viewed; instead of "anomaly picking' which normally utilised around 1% of the data, the previously ignored background data which contained useful geologic information became the focus of analysis. As airborne TDEM instrumentation and interpretation software improved, airborne TDEM has become a much more effective tool for geologic mapping. GEOTEMDEEP surveying in 1997 over Noranda's San Jose, Arizona, and adjoining Sol porphyry properties reliably identified known subsurface bedrock features ranging in depth of 150 m to 200 m. Diamond drilling results suggest that airborne TDEM mapping locally defined the bedrock interface to depths in excess of 300 m. Evidence also suggests that lithologic discrimination under cover is locally possible. Survey results over the Poston Butte, Arizona, porphyry copper deposit show how airborne TDEM technology can be used as a direct targeting tool. The signature is unfortunately not unique, and similar to inliers of conductive cover. For this reason ground validation programs are essential components of any exploration program. While airborne TDEM technology has evolved to the point where pseudo 3-D mapping (spatial analysis of 1-dimensional CDTs) capabilities are possible, more reliable and robust inversion software, improvements in system noise levels and stability, plus a better understanding of the geologic models are still needed to provide reliable interpretation.