Controlled Source Electro-Magnetic Mapping - development and evaluation ofits application to mineral exploration

Mineral exploration in Europe is increasingly directed towards the discover y of blind orebodies, and this necessitates the improvement of existing geo physical methods and the development of new techniques. A project was launc hed to develop Controlled Source Electro-Magnetic Mapping (CSEMM) as a geop hysical method with a depth penetration of 50 to 1000 m and to evaluate its performance and limitations by field testwork. A prototype high-resolution, electromagnetic, geophysical system of hardwar e and software, based on the theory of controlled-source magnetotellurics w as developed. The electrical source field is fed into the earth by dipoles at preselected frequencies and the CSEMM multi-channel receiver measures fi ve electric and three orthogonal magnetic fields. The transmitter and recei ver are synchronized by highly stable global positoning system clocks. Suit able mapping algorithms and spatial filters are applied to the data, and a new calibration technique permits calibration during measurement without lo ss of time. The CSEMM system was field-tested in the Aguas Tenidas area of the Iberian Sulphide Belt over a blind massive sulphide body, which had been outlined b y drilling, at depths from 250 m to 550 m. Following an initial calibration survey ten lines, totalling 12 000 m of profile length, were completed in seven days using Two electrode arrays at each receiver site. After processi ng the data were displayed as two-dimensional profiles, as plan views for s pecific depths and as three-dimensional visualizations. The results identif ied the massive sulphide body and provided additional information regarding the distribution of geological structures and Lithologies. Testwork was also undertaken in the Irish Midlands to detect massive zinc-l ead bodies in sedimentary strata, but the results were confused by very str ong electromagnetic interference from cultural sources. Overall, it is conc luded that CSEMM can be a cost-effective exploration and appraisal tool, pa rticularly in areas adjacent to deep (200-1000 m) massive sulphide bodies. The project was coordinated by Metronix GmbH & Co. KG (Germany) with the CS A Group, Ltd. (Ireland), as partner.