Groundwater Imaging for Coal Explorers

Applications for Coal Explorers
Towed TEM is very effective at resolving spatial detail across coal deposits both vertically and horizontally. It principally detects changes in moisture presence and salinity and does respond significantly to the moisture and salt bound up in clay within the substrate sediments. Since coal measures typically are somewhat clayey throughout except for the coal horizons themselves, they are typically of low resistivity. Thicker dipping resistive layers within such strata typically stand out and faults are clear from discontinuity of such strata in the 3D images produced. Cells of rainfall recharge and baseflow discharge often are evident from the observed substrate resistivity distribution. Degree of weathering and overburden alluvium also are indicated. In cases where the alluvium is derived from the same material as the underlying weathered horizon on the coal measures, and the alluvium is not well sorted, it may be effectively the same as the underlying weathered top of the coal measures and therefore not geophysically or otherwise distinguishable. In other cases the coal measures contrast with alluvium because they are not significantly weathered or because the alluvium is sorted into horizons of different grain size and different resistivity. Alluvium then is generally distinguished from consolidated coal measures by recognition of the 3D features within it. For example, the alluvium may contain meandering point bar sand deposits which are revealed while the coal measures may reveal a criss-crossing network of faults in otherwise planar layers that are not typical of modern day geomorphological features in the area.

If there is a waterway over a coal mining area, then the lightweight HERBI (Hydrographic Electrical Resistivity and Bathymetry Imager) can be floated down the waterway to image detail pertinent to study of the connection between the waterway and the substrate to as deep as 30m.

By towing our land based AgTEM sytem across a proposed mining area we can conduct multi-depth imaging to between 50 to 100m deep (or more with higher power gear). 20 km per day is typically surveyed in coal mining areas as there typically are lots of fences, gates and other barriers as well as multiple land owners to negotiate. In open paddocks 40 km per day is more typical. Due to the detail of structures typically found in coal mining areas we recommend survey at nominal 50m line spacing or less.

See our ‘Land Based’ page for examples of AgTEM data.

Coal mines often have groundwater monitoring requirements. With buried cables of sensors we can conduct electrical resistivity tomography at various scales to detect changes in earth moisture and salinity over time. Perhaps connectivity between a stream and advancing underground workings is to be monitored – an ERT system could continually image a transect across the underground workings and geometrically define changes in moisture within the profile so that geological reasoning can be applied to identifying the actual meachanism and location of any aquitard failure.

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