A 2-channel wideband SQUID system for high-frequency geophysical applications

A prototype radiomagnetic sounding system for measuring the conductivity of the shallow subsoil down to some 10 m depth was built. With this system, the horizontal magnetic field and field gradient of radio transmitters in the frequency range 10-2000 kHz are detected in small bore-holes of /spl lsim/1 m depth using two wideband SQUIDs. The SQUIDs are operated at 4.2 K in a flux-locked loop having a bandwidth of 11 MHz, a white flux noise level of 1.6/spl times/10/sup -6/ /spl Phi//sub 0///spl radic/(Hz), a feedback range of /spl plusmn/610 /spl Phi//sub 0/, and a maximum slew rate of 2/spl times/10/sup 6/ /spl Phi//sub 0//s above 500 kHz increasing to 5/spl times/10/sup 7/ /spl Phi//sub 0//s at 13 kHz. The system was successfully tested in a first field measurement under urban environmental conditions. The system design allows one in principle to use nitrogen-cooled sensors.