THREE-DIMENSIONAL ELECTRIC MISE-A-LA-MASSE SURVEY OF AN IRREGULAR LEAD-ZINC-COPPER DEPOSIT IN CENTRAL SWEDEN *

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  • *

    Paper presented at the Twenty-Ninth Meeting of the European Association of Exploration Geophysicists held in Stockholm, June 1967.

ABSTRACT

The principle of the mise-á-la-masse method is to earth one current electrode of a pair in a conducting mineral show (in a borehole, in an outcrop etc.) and measure the resulting distribution of electric potentials. The distribution will, to some extent, reflect the geometry of the ore mass of which the mineral show forms a part.

In a mise-á-la-masse survey of a lead-zinc deposit in Central Sweden electric potentials were measured on the surface of the ground as well as in some 25 drillholes, in either case with earthings (successively) in three different parts of the irregular ore deposit. Besides this, measurements were made in drillholes with earthings in two further drillholes.

Geologic correlation between the drillholes is difficult in the present case on account of the irregular geometry of the ore deposit. However, the mise-á-la-masse measurements clearly showed the dip and the pitch of the ore body, established connections between the different ore widths encountered in the various holes, and yielded information about the shape of the ore mass.

In particular, the survey showed that the ore lenses must be crescent-shaped rather than tabular, and the dip was indicated to be westerly, instead of easterly as originally presumed.

Three-dimensional models of equipotential surfaces were constructed from the observed drillhole and surface potentials (using transparent plastic sheets and thin copper wire) and these helped to elucidate the mass geometry further.

The surface and underground potential data collected in the present case should be of nterest to geophysicists working on analytic continuation problems.

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