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Electromagnetic and gravity methods of mineral exploration

The research focused on frequency domain electromagnetic (FEM) techniques both for surface measurements and borehole measurements. The objectives were:
to develop a methodology for measuring and interpreting results obtained by the various FEM techniques and systems (MELIS, a surface to surface multifrequency electromagnetic (EM) system, REMI, a surface to borehole 1-axis EM receiver probe, and a surface to borehole 3-axis EM system now manufactured and marketed by IRIS-Instruments under the BORIS mark, to assess their ability to detect and characterize conductive deposits in terms of location, depth, thickness, conductance and limits.

For the 2 EM methods studied (MELIS and REMI), the measurement procedures have been optimized and a method of interpretation has been developed using 2-dimensional modelling. The 2 methods are now operational and ready to be marketed.
Compared to classical CSAMT, measurements of magnetic only components, such as performed with MELIS, are less sensitive to superficial inhomogeneities.
Compared to time domain transient electromagnetic (TEM), FEM is easier in terms of field operation (stronger signal to noise radio, lighter equipment) and of numerical modelling for interpretation. Although its depth of investigation is limited by the transmitter to receiver distance, the MELIS system appears to be one of the most efficient technique for deep massive sulphide exploration.
It has been possible to study the borehole 3-axis EM method only briefly (as the probe was still being developed). The modelling achieved on this method has shown the direction for continuing research. A promising tool, BORIS, has been finalised and is being promoted by IRIS-Instruuments (Orleans, France).

Reported by

Bureau de Recherches Geologiques et Minieres
45060 Orleans
France
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