A 2-CHANNEL WIDE-BAND SQUID SYSTEM FOR HIGH-FREQUENCY GEOPHYSICAL APPLICATIONS

A prototype radiomagnetic sounding system for measuring the conductivi ty of the shallow subsoil down to some 10m depth was built. With this system, the horizontal magnetic field and field gradient of radio tran smitters in the frequency range 10-2000 kHz are detected in small bore -holes of less than or equal to 1m depth using two wideband SQUIDs. Th e SQUIDs are operated at 4.2 K in a flux-locked loop having a bandwidt h of 11 MHz, a white flux noise level of 1.6x10(-6) Phi(0)/root Hz, a feedback range of +/-610 Phi(0), and a maximum slew rate of 2x10(6) Ph i(0)/s above 500 kHz increasing to 5x10(7) Phi(0)/s at 13 kHz. The sys tem was successfully tested in a first field measurement under urban e nvironmental conditions. The system design allows one in principle to use nitrogen-cooled sensors.