Soil has a remarkable ability to mask geologic features relevant to bore siting that might otherwise outcrop. A person trying to site a bore typically has just existing nearby bore records, soil variation and the lay of the land as clues. Without a knowledge of what is beneath a farm it is not possible to tell if nearby bores have any relevance, soils commonly are spread horizontally from their source and mask what is beneath and lay of land often supplies insufficient clues.
Electromagnetic mapping reveals bulk electrical resistivity which is (apart from in deep mineral exploration) always related in some way to groundwater and the ions within it. Electromagnetic mapping at various depths gets past the masking soil and other shallow strata and usually reveals a bewildering complexity of hydrogeology that can be difficult to interpret. This typically is found to be far more complicated than any conceptual models envisioned in relative ignorance beforehand.
Around ten thousand line kilometres of multi-depth electromagnetic data have been collected across farmland using the AgTEM towed time domain electromagnetic system and experience gained from interpreting this data is summarized into a set of steps in this presentation. The interpretation procedure is relevant also to other systems including airborne systems but the scale of investigation must be different.
In this presentation, a series of logical steps is presented, for planning, executing and interpreting electromagnetic surveys for shallow bore siting. At the end of the process it is important to be able to separate out the new knowledge gained from the ambiguities that remain. Only then can recommendations on siting of bores be logically made. There is an excellent case for encouraging farmers to make their electromagnetic mapping data public and for mosaicking of those maps to be conducted so that neighbours and regulators can see a bigger picture.