An HTS dc SQUID system for geomagnetic prospection

We have developed a highly sensitive de SQUID system for geomagnetic prospe ction operating at 77 K. Because of the large voltage swing of our hip chip magnetometers it is possible to use a directly coupled SQUID electronics. Thus, the requirements for TEM measurements can be fulfilled: slew rate >1 mT s(-1) and dynamic range >120 dB. In measurements above real geological targets the significantly better perf ormance of the SQUID compared to conventional coils has been demonstrated. The lower noise of the SQUID system at late times allows us to measure the transients for 5 to 10 times longer, which results in an increase of the in vestigation depth by a factor of 2-3. For the same data quality a reduction of the measurement time between 25 and 100 compared to coils is possible.