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

Abstract

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 tool for mapping large areas. The first test flights were flown in June 2000 using the GEOTEM system. Traditional approximate interpretation tools for air-borne 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 50-70 m. The ground-based PROTEM 47 system has a maximum penetration of approximately 170 m, 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.