Technical Papers - Airborne Electromagnetics

Case Study: The Evolution of Airborne Time Domain Electromagnetic Applications for Geologic Mapping; a Noranda Perspective

Michael J. Schaefer, Noranda Mining and Exploration
1101 W. Grant Road, Suite 202 Tucson, Arizona 85705, USA

John Gingerich, Noranda Mining and Exploration
1 Adelaide St. E., Suite 2900 Toronto, Ontario M5C 2Z6 Canada

Jean Lemieux, Fugro Airborne Surveys
2060 Walkley Road, Ottawa, Ontario, Canada, K1G 3P5 

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 interpretation challenges that the use of airborne time domain electromagnetics (TDEM) technology presents to exploration geologist and geophysicists. Following 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 geological models to properly interpret the data. A need for lower noise and better calibrated EM systems was also recognised. 

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. 

GEOTEM_DEEP surveying in 1997 over Noranda's San Jose, Arizona and adjoining Sol porphyry properties reliably identified known subsurface bedrock features ranging in depth of 150m to 200m. Diamond drilling results suggests that airborne TDEM mapping locally defined the bedrock interface to depths in excess of 300m. 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.

Exploration Geophysics (1998) 29, 204-210.

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