A Superconducting Magnetic Tensor Gradiometer for Underwater UXO Detection

Abstract

Magnetic surveys are routinely used for detection of discrete magnetic targets, such as unexploded ordnance (UXO). Gradient tensor measurements have a number of benefits over measuring total field intensity or its vector gradient: they provide detailed information about a target in a single pass, without necessarily passing directly over the target; they determine the location of the target using a direct method, rather than the indirect inversion method required with other measurements; and they determine directly the magnitude and orientation of the magnetic moment, as well as the location of the target, aiding discrimination between sources and characterization of the munitions. The CSIRO is building a magnetic tensor gradiometer, designed for underwater deployment, based on high temperature superconducting quantum interference devices (SQUIDs). Technical challenges for the project include: designing a high temperature SQUID device that can achieve the required gradient sensitivity in motion; developing a gradiometer housing that can deal with the boil-off gas from the liquid nitrogen cryogen while underwater; identification and removal of wave-induced magnetic noise; compensation for magnetic noise caused by motion and the platform itself; and developing a dipole-tracking algorithm for gradient tensor measurements that is both robust and computationally undemanding. This paper presents the progress to date on this project including an evaluation of gradiometer performance in laboratory conditions, and describes some simple methods for localization and characterization of compact magnetic sources using gradient tensor measurements.