EnviroMT : A new controlled source/radio magnetotoelluric system

Abstract: In the frame of an EU project, a new electromagnetic prospecting instrument was designed and constructed for environmental and engineering applications. The instrument works in the frequency domain in the band 1-250 kHz, whereby the collected data can be used to study the variation of ground resistivity from the surface to a maximum depth of few hundred meters. The system is operational in two modes, the Radio Magnetotelluric (RMT) and the Controlled Source Tensor Magnetotelluric (CSTMT). The RMT method makes use of the signals generated by powerful distant radio transmitters operating in the frequency range 14-250 kHz. The CSTMT technique, with a remotely controllable double magnetic dipole source covering the band 1-100 kHz, is utilized when deeper targets are aimed at or low resistivity terrains are studied. With the aid of user-friendly database software the collected data can be visualized and modeled in1D, directly in the field. In November 1998, the first test survey at the Collendoorn dumpsite in the Netherlands was conducted to map the vertical boundaries and lateral extensions of the pollution plume along four RMT profiles located east of the dumpsite. The results of 1D inversion of the data correlated well with the bore-hole logs, revealing that the estimated resistivities and depths to the top of the pollution plume were realistic. The pollution plume appears to be extending more in the northern part. Another test was carried out north of Uppsala, at Skediga in Sweden, using both RMT and CSTMT techniques, to study the morphology of a clay-sandy aquifer. The depth of the resistive crystalline bedrock as estimated by 1D and 2D inversion of the EnviroMT data is in good agreement with bore-hole data. To the east, towards River Fyris the conductive clay lenses become thicker.