An overview of new efficient electromagnetic methods to resolve the geological heterogeneity on hectare scale surveys

Abstract:

Efficient high resolution imagining of the geological heterogeneity of the top 30 m of the subsurface is of outermost importance when working with problems like artificial recharge, nitrate retention capacity, preferential flow etc . However, there is a lack of geophysical mapping techniques in this zone when working on real scale problems from a few hectares to more than 10 000 hectares. Smaller 2D sections can be produced by e.g. ERT or georadar but the efficiency is not sufficient when the need is to build full 3D models of the subsurface with spatial resolution in the meter scale over large areas. For this reason we have developed two new systems both towed by an ATV.

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.

Figure Caption

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