There may be as many as one million acres of the marine environment that are potentially contaminated by unexploded ordnance (UXO). These environments vary significantly with respect to water depth, sea floor morphology, and geologic regimes. Current deployment modes place sensors 1 to 2 m above the sea floor, restricting detection capabilities, equivalent to that of airborne platforms in terrestrial applications.

The objective of this project was to develop a High Temperature Superconducting Tensor Gradiometer (HTSTG) for detection and discrimination of UXO in the marine environment (in water depths up to 20 m).

The first milestone of this project was to demonstrate a sensitivity of 2 pT /m /√Hz at 10 Hz in laboratory conditions when measured in a mu-metal shielding enclosure. The second milestone was to demonstrate full tensor measurements while the system was both stationary and in motion under laboratory conditions.

For the first milestone, measured sensitivities of six gradiometers ranged from 1.25 pT rms/ m / √Hz to 2.28 pT rms/ m / √Hz at 10 Hz with four of the six achieving sensitivities of < 2 pT /m /√Hz at 10 Hz. 

The second milestone proved more difficult to demonstrate:

• The total system was operated unshielded in the Earth’s field. Four of the six gradiometers met the target sensitivity while sitting motionless in the Earth’s field. The two remaining gradiometers’ noise performance was believed to have been by influenced by wind-induced noise; i.e. by motion effects.

• The issue of the sensitivity of the complete gradiometer in terms of the individual device sensitivities is somewhat complex. However, it was determined that as the gradient falls off as the fourth power of the distance, the uncertainty in the measurement of the gradient is proportional to the fifth power of the distance; i.e. falls off rather slowly. If each of the devices was 14% less sensitive than the specification, the detection range would only drop from 4 m to 3.87 m; i.e. by 3%. This degrading of performance is conservative, since most of the devices have better than the specified sensitivity.

• Magnetometers used to provide compensation for the motion effects in the gradiometer signals have been shown to be capable of removing 99.4% of the gradiometer’s motion induced signals.

• Accurate calculation of the system’s compensation coefficients is critical to ensure that calculated tensor components are free of motion induced signals.

• These compensation coefficients need to be determined in a low-gradient field environment.

• A further improvement of the system’s common mode performance by a factor of up to 100 to 500 is required to ensure that the intrinsic sensitivity of the system can be realized from a moving platform.
• For limited platform movement, such as might be expected in an underwater capsule, use of a global feedback field cancellation coil may provide a means to meet this improvement.

The project team recommends further research to determine the compensation coefficients and to examine the benefits of operating a global feedback scheme using feedback coils with open coil structure. These developments could support an underwater demonstration of the extended detection range and characterization capabilities of the HTSTG system.

The system has inherently higher sensitivity and immunity to external noise than conventional magnetic sensor systems, thus improving detection performance in the difficult marine environment. The system also has enhanced characterization abilities as the location and magnetic moment of a target can be determined definitively by gradient tensor measurements at a few locations along a profile. This characteristic provides an advantage in the marine environment, which is difficult to regularly sample due to the effects of wind, waves and currents on the tow-vessel or deployment platform.