A HTS rf SQUID vector magnetometer for geophysical exploration

We report on a highly sensitive rf SQUID vector (x, y, z) magnetometer system operating at 77 K and suitable for electromagnetic geophysical exploration. We achieved stable long-term operation of all three channels during field trials outside magnetic shielding in urban and rural environments. With sensors on 10/spl times/10 mm/sup 2/ substrates, the white noise level in earth's field was 40 fT//spl radic/Hz above 100-300 Hz, in all three channels. The new system meets the high requirements for slew rate (>2 mT/s), dynamic range (>130 dB) and bandwidth (20 kHz) which must be fulfilled in geophysical measurements, especially the TEM (transient electromagnetic) method. We also present results of geophysical TEM field trials over various test sites. We obtained good reproducibility of SQUID data and a good correlation between output signals of reference induction coils and the derivative of SQUID signals. Results from TEM measurements indicate that for late time signals, originating from deeper geological structures, the SQUID magnetometer provides a better signal-to-noise ratio than an induction coil.