Calibration and testing of a HTS tensor gradiometer for underwater UXO detection

To address the problem of discarded military munitions and unexploded ordnance in the marine environment, we have developed a highly sensitive tensor gradiometer based on HTS materials. The main motivation for this work is to enhance and improve the ability to detect, localise and characterise small projectiles (typically >; 40 mm calibre) where conventional magnetic surveying techniques fail to deliver adequate performance. The sensors provide a gradient field sensitivity of ~1-2 pT_rms m^-1 Hz^-1/2, which would, in theory, enable detection of ordnance above ~40 mm (minimum dipole moment of 0.01 Am²) at a standoff distance of 4 m. We discuss here results from our most recent full system field trial in a remote low magnetic gradient environment. We use pre-calibrated balancing routines to compensate for the common mode signals due to the gradiometers finite balance. We have determined the tensor components for a moving magnetic dipole target while the system itself is undergoing motion.