SQUID gradiometers for a fundamental study of underwater magnetic detection

An LTS SQUID gradiometer measuring six elements of first order magnetic gradient was produced for a fundamental study of magnetic detection and localization under water. It was installed at the sea bottom shallower than 30 m and we measured magnetic noise which might degrade its detecting performance. Noise caused by the imbalance of the gradiometer pickup coils was suppressed by using an auxiliary three-axis SQUID magnetometer output. The imbalance parameters were determined by measuring geomagnetic variations surrounding the sea bottom. A test for the trial detection of an iron boat was conducted with the use of this gradiometer. The results obtained showed a high signal-to-noise ratio for the gradiometer.