Mapping Groundwater and Soil Moisture Using Multi-Depth Electrical Conductivity Data from Towed TEM Carts

Abstract

Groundwater presence and salinity both correlate with electrical conductivity, a property readily mapped over a swath of depths by various geophysical instruments. Thus, it would seem then that exploration for groundwater could be a simple matter, however, for most groundwater investigations the level of detail of geological complexity demands detailed acquisition and multi-dataset interpretation.

Towed or airborne electromagnetic survey permits multi-layer map coverage of a site as is necessary for revealing geological detail that would make little sense when viewed as individual transects. Further, because electrical conductivity responds to so many natural variables related to groundwater, multi-disciplinary information must be included in the interpretation. In many cases, the footprint of an airborne system will be too large to sufficiently resolve complexity of geology and cultural interference to successfully site bores. In other cases the mobilization cost of an airborne system will, on its own, exceed the exploration budget. In these cases, towed electromagnetic devices including AgTEM4TM cart have a niche.

Exploration depth and resolution of practical towed devices is very limited by their practical size constraints so great care must be taken in other design aspects including effective, robust primary field nulling and maximizing of transmitter loop – receiver loop separation.

Case studies of groundwater exploration demonstrate these geological interpretation and physical design challenges and limitations.