Jean Lemieux

Resistive‐limit, time‐domain AEM apparent conductivity

On-time measurements, which can be obtained with existing airborne transient electromagnetic (AEM) systems, are used to map the apparent conductivity of the ground. Such measurements can extend the conductivity aperture of these systems substantially at the low end of the conductivity range. A theoretical analysis is given that provides a simple, approximate, resistive-limit solution for any transient EM waveform. The approximate result is compared to the full-numerical result for the specific case of the GEOTEM system over a conductive half-space. Examples of GEOTEM data reduced to apparent conductivity using the approximate analytical solution are presented from surveys for kimberlites and massive sulfide deposits.

Application of a modified GEOTEM® system to reconnaissance exploration for kimberlites in the Point Lake area, NWT, Canada

Airborne geophysical surveying with electromagnetic (EM) and magnetic methods is an effective reconnaissance exploration tool for kimberlite pipes because the target can have an associated EM and magnetic anomaly. The EM response of kimberlite pipes is most often attributed to weathering alteration in a near‐surface layer, whereas the magnetic response is attributed to magnetite and ilmenite within the deeper unweathered kimberlite pipe. The discrete shape of kimberlite diatremes results in an easily identifiable anomaly pattern. Diamondiferous kimberlites have recently been found in the Northwest Territories (NWT) of Canada, an area glaciated in the Pleistocene and therefore devoid of a strongly weathered zone. By configuring the GEOTEM® airborne EM system to operate at high frequencies (270 Hz) and to take measurements while the transmitter is switched on, weakly conductive bodies may be detected because there is an adequate contrast with the surrounding highly resistive country rock. System modificatio...

Case histories illustrating the characteristics of the HeliGEOTEM system

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.

Geological mapping with power line fields measured with MEGATEM data

As an experiment, we extracted the intensity of the EM fields at the power-line frequency from raw MEGATEM survey data. This was done to determine if additional geological information could be extracted from EM secondary fields due to power-lines. We selected a survey near Chibougamau, Quebec where there are many strong powerlines and some conductive features in an otherwise resistive environment. The estimated half-space apparent conductivity mapped the strong conductors, but gave poor information in the resistive areas. Better geological information was obtained in the resistive areas by imaging the powerline field amplitudes after removal of the long wavelength features. Correlations with known geology show that power line fields can help in geological interpretation.