SQUID gradiometers for a fundamental study of underwater magnetic detection

An LTS SQUID gradiometer measuring six elements of Ist order magnetic gradient was produced for a fundamental study of magnetic detection and localization under the water. It was to be installed at the sea bottom shallower than 30m and to measure magnetic noises which might degrade its detecting performance. Noises caused by the imbalance of gradiometers' pickup coils were suppressed by using auxilialy three-axis SQUID magnetometer output. The imbalance parameters were determined by measuring geomagnetic variations surrounding the sea bottom. A test for trial detection of an iron boat was conducted with a use of this gradiometer. It resulted in obtaining a higher signal-to-noise ratio for the gradiometer than that for the magnetometer.