New developments in high resolution airborne TEM instrumentation

Abstract

During the last decade, approximately 40,000 ground-based, single-site TEM soundings have been carried out for groundwater investigations in Denmark (an areas of 42,000 km2). In future the investigations will be even more intensified, and therefore the use of airborne TEM measurements has been proposed. Detailed groundwater investigation in Denmark calls for high-resolution, continuous TEM measurements in the decay time interval from approximately 10 microseconds to 10 milliseconds. In order to meet these requirements, a new airborne TEM system, HETEM, has been developed. The transmitter coil, together with the Rx coil, is mounted on a 10 ´ 10 m frame carried by a helicopter at an altitude of 30 m above ground and a speed of 20 km/hr. The transmitter pattern is segmented, and the highest moment is 16,000 Am2. The waveform, together with other controlling parameters, is measured ``on the fly' to ensure high data quality. Off-time decay signals are measured in the time interval of 15 microseconds to 10 milliseconds. The data quality from the system is comparable to the data quality obtained by conventional ground-based systems.