An overview of new efficient electromagnetic methods to resolve the geological heterogeneity on hectare scale surveys
The first system (Figure b) is based on a commercial electromagnetic induction (EMI) instrument in the frequency domain, measuring 6 data points with three coil separations. When towing the system on sledges the data quality is extremely high and with negligible data drift. This justifies full non-linear inversion of the data instead of the commonly used approximate transforms such as the Low Induction Number (LIN), often used with these systems. The depth of investigation (DOI) is 5 – 8 m with detailed information right from the surface, sounding distance 2 – 3 m, line distance 10 – 20 m and a daily production on farm fields in the rage of 70 – 80 line km. A typical average resistivity map is shown in Figure a. The heterogeneity is obvious showing patches of sand in an otherwise clay and till dominated geology.
Getting a larger depth of investigation is for several reasons hardly possible with an EMI system and we have therefore developed the first prototype of a transient electromagnetic (TEM) system (Figure c) with the same production capability as the EMI system but with a larger DOI of 30 – 50m. The transmit energy comes from a 2 x 4 m single turn coil with currents of 4 Amp and 30 Amp in a low and high moment. The first gate is measured 1 us after a turn-off of 2.4 us, which allows us to resolve even the first meter of the subsurface.
In the presentation, we discuss pros and cons of the two methods and demonstrate their usage for extremely high spatial resolution of the critical zone on hectare scale surveys.
a) GCM mapping at a 4 hectare large field. Mean-resistivity map, interval of 1-1.5 m. b) GCM system c) prototype TEM system