A quantitative appraisal of airborne and ground-based transient electromagnetic (TEM) measurements in Denmark

The last decade has seen growing use of ground-based transient electromagnetic (TEM) methods in Denmark for hydrogeological purposes. Due to an intensified mapping campaign, airborne TEM methods were proposed as a possible toot for mapping large areas. The first test flights were flown in June 2000 using the GEOTEM system. Traditional approximate interpretation tools for airborne data are insufficient in hydrogeological investigations where a quantitative model specifying model parameter reliability is needed. We have carried out full nonlinear one-dimensional inversion on the field amplitude of airborne synthetic and field data and compared the airborne method with the traditional ground-based PROTEM 47 system that has found extensive use in Denmark. An improved measuring procedure for airborne systems is suggested to facilitate the estimation of noise that is necessary in a quantitative inversion. The analyses of synthetic data demonstrate the differences in resolution capability between ground-based and airborne data. Ground-based data typically resolve three- or four-layer models and occasionally up to five layers. Airborne data resolve three layers as a maximum, one or two layers being common. The airborne GEOTEM system detects layers to depths of more than 300 m, bearing only little information about the top 5070 in. ne ground-based PROTEM 47 system has a maximum penetration of approximately 170 in, with higher resolution capabilities in the top 100 m. Coupling to man-made conductors is a serious problem for all TEM methods in densely populated areas and results in distorted data. Coupling influences the airborne data from Denmark on two-thirds of the area covered. These data must be eliminated to avoid misinterpretation.