Case Studies - Mineral Exploration

Forzando Coal Mine Bankpan Area, South Africa

The following map shows an apparent resistivity map of a portion of a coal mine in South Africa using the DIGHEM system. 

Bankpan Area, South Africa
Mapping Dikes in Coal Seam
7200 Hz Resistivity

The coal seam is cut by diabase (dolerite) dikes, and a sill lies at a varying distance under the coal. Not all of the dikes appear on the magnetic data. The heat of the intrusions devolatilises the coal, reducing its economic value, and the hard dikes are a hazard to the coal mining equipment. Drawn on the geophysics map are the locations of the previously known dikes and the areas of de-volatilised coal (grey). The map shows a parallel pair of resistivity anomalies straddling each dike, the dual anomaly representing the dual peak normal for coplanar coil pairs over a narrow vertical conductor. The coaxial resistivity responses show single peaks over each dike, but are more sensitive to flight direction/strike coupling. The dual track anomaly of the coplanar data tends to be more obvious. Both data sets are collected on a normal DIGHEM survey.

Several previously unknown dikes are apparent by their dual parallel anomalies, some of which have since been defined by drilling. Many areas have much more subtle anomalies due to thin dikes or non-magnetic, cooler felsic dikes. Extensive data enhancement and careful interpretation are necessary to detect and map the dikes.

The broad, non-linear resistivity highs match those locations where the underlying sill comes in close proximity to the coal (< 20m), and devolatilises the coal. Again there is good correlation to the information gained from drill holes. A DIGHEM survey in advance of the drilling could help design the drilling pattern, after which the drill results and the HEM data could be combined into an accurate map of the problem areas.

This data is courtesy of Forzando Coal Mine.