Case Studies - Mineral Exploration

MEGATEM and GEOTEM Comparison in Canada: Reid-Mahaffy Test Site

Introduction

The Reid-Mahaffy airborne geophysical test site is located approximately 40 kilometres northwest of Timmins, Ontario. As part of the Ontario Government's Operation Treasure Hunt (OTH), the site was established by the Ontario Geological Survey (OGS) with the assistance and co-operation of Falconbridge Limited. The site offers a variety of rock types, discrete bedrock conductors at various depths and with a broad range of conductances. There is also some variation in overburden conductivity and thickness.

Survey Specifications

The GEOTEM system, mounted in a CASA 212-200 aircraft, flew the Reid-Mahaffy Test Site as part Fugro Airborne Surveys' contractual obligations with the OGS. Independently of OTH, Fugro flew the site with its Dash 7 aircraft equipped with the MEGATEM system. The flights were conducted in January 2000 and in both cases the systems used a waveform frequency of 90 Hz and a pulse width of 2 ms. 

Results

Illustrated are segments of profiles showing the B-field results flown along line 15. The ones on the left are from the X coil; on the right from the Z coil. The curves of the same colour indicate comparable delay times. Because the pulse widths are slightly different, the correspondence in delay times is approximate at early times, but very close at late delay times.

B-Field Results flown along Line 15 from the X-coil 

Anomalous responses at comparable delay times have amplitudes about twice as large on the MEGATEM data. This is true on the X and Z components. The late-time small amplitude noise seems to be comparable on the Z component, but for the X component is slightly less on the MEGATEM data.

B-Field Results flown along Line 15 from the Z-coil 

Both airborne systems reveal an unmistakable discreet conductor near the centre of the profile. Ground truth information and results of borehole EM at this conductor location are presented in the figure at lower left. The large amplitude positive response at early time and very small amplitude cross-over at late time are interpreted to infer that the borehole has intersected one of the conductors near its top edge. 

Cross section (simplified). Line 15 OTH Test Site

Acknowledgement

Information illustrated on the cross section diagram has been compiled from data kindly provided by Falconbridge Limited, Timmins Exploration Office, who assumes no liability or responsibility for the accuracy of the geological and geophysical information shown.

Reference

Ontario Geological Survey 2000. Airborne magnetic and electromagnetic surveys, Reid-Mahaffy Airborne Geophysical Test Site Survey; Ontario Geological Survey, Miscellaneous Release Data (MRD)-55.