Case histories illustrating the characteristics of the HeliGEOTEM system

Abstract

The HeliGEOTEM system was introduced in 2005 to provide higher resolution data than fixed-wing electromagnetic (EM) systems. The characteristics of HeliGEOTEM are illustrated by comparing the system with other airborne EM systems. A comparison with previous versions of HeliGEOTEM shows that, since 2005, the early-time information has improved allowing rapidly decaying responses to be identified. An improvement in the signal-to-noise ratio means the system is able to detect bodies at greater depth. A height attenuation test over the Nighthawk conductive body indicates that the latest system could see that target if it were buried 380 m below surface. Another target that is difficult to detect (Caber) is clearly seen on the HeliGEOTEM data.

A comparison of field data at the Maimon deposit indicates that the helicopter DIGHEM frequency-domain system and the HeliGEOTEM time-domain system both acquire data with similar spatial wavelengths. Data collected away from the main ore body and along strike indicate that the HeliGEOTEM sees a less attenuated response from a deeper part of the body. Also, the HeliGEOTEM is able to estimate the conductivity, whereas the DIGHEM system cannot discriminate the conductance, it can only indicate that the body is highly conductive. The DIGHEM data, however, is better able to resolve the near-surface conductivity, and the spatial form of the DIGHEM data is simpler. The data acquired with multiple transmitter-receiver coil pairs (DIGHEM and HeliGEOTEM) provides information superior to single-component data.

Tools used to display fixed-wing airborne EM data have been modified to work with HeliGEOTEM data. These tools can image the structure in cases where the ground is assumed to be comprised of a) horizontal layers or b) discrete conductors.

A comparison of HeliGEOTEM with the helicopter RESOLVE and fixed-wing GEOTEM systems shows that the HeliGEOTEM is able to map most of the shallow features seen on the RESOLVE and to image the resistivity to depths comparable to the GEOTEM (in a moderately conductive environment).

A traverse line from Australia over what is considered a difficult target (the Nepean Mine) demonstrates that the HeliGEOTEM system provides good signal-to-noise ratios. Using the z- and x-component data does a better job of defining the geometry of the target than using the z-component data alone.