Electrical Resistivity Imaging is now a proven application for shallow to intermediate depth Coal. Its application band is for 5 to 10 meter thick seams for depths from the surface to about 50/60 metres. It can show seam structure, faulting and weathering in a wide variety of conditions.

Research by Sub-Surface Imaging has developed resistivity imaging so it can be usefully employed for the shallower open cut coalmines, and also for underground mining leases. Its most useful in areas where surface conditions preclude the use of seismic. This includes mapping dykes, sills, faults, coal seam elimination, coal seam oxidation and others. It has applications in the case of deeper deposits for imaging of structures between borehole and mine drives The best application for this purpose is Cross hole ERI. Its has been shown as feasible and demonstrated at several sites.

For technical and economic reasons Resistivity imaging is very easy to apply to shallow coal seams. The situation where coal seams are less than say 70 metres in depth is also precisely the target depth for open cut coalmines. ERI works best where seismic imaging is challenged, is in areas where a variably weathered surficial layer (e.g. basalt) creates static shifts. Here, crosshole borehole resistivity imaging is shown to be a viable options.

The application of resistivity imaging relies on the fact that the coal seam usually stands out as an electrically resistive layer. The technique itself is essentially a combination of the old sounding and profiling techniques, and is achieved by implementing computer controlled data acquisition from arrays of electrodes and much more sophisticated computer algorithms to image the data. The output is an image, which is effectively a blurred rendition of the true electrical structure of the subsurface. Cross borehole mode in a very similar way.

Coal mapping using ERI requires some very specific procedures to get good results. The coal is usually thin relative to its depth. The coal signature can be masked by higher resistivity material above or below it. If the coal is faulted this can give a very confusing picture to the uninitiated and weathering / oxidation of the coal requires specific handling.SSI have developed methods for coal imaging that work reliably for up to 60 and in some cases 100 metres depth. These include array layout, electrode design, equipment and power requirements. It’s possible to image deeper but the thinner coal seams are not seen reliably. Its better to use ERI for fault and structure mapping at greater depths. The exception to this if the feature scale is a significant of percentage of its depth.

One aspect of ERI for coal that does work well is structure and fault mapping. There are specific signatures for faults and structure that are unique. SSI have demonstrated these methods on a number of mine sites and the results are available for review.

The images below are specific examples of ERI in use for images shallow to intermediate depth coal.

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