James R. Rozen

High-T/sub c/ SQUID gradiometer for mobile magnetic anomaly detection

The prototype for a nitrogen-cooled high-T/sub c/ SQUID gradiometer has been developed and is being evaluated for magnetic anomaly detection of underwater targets in mobile surveys. The prototypes design is based on the concept of the Three-Sensor Gradiometer (TSG). In the TSG approach, balance of two independent SQUID magnetometers is more difficult to attain than for conventional low-T/sub c/ gradiometers in which signal subtraction occurs prior to a single SQUID stage. Experiments have been conducted using a platform-motion simulator to evaluate performance of this gradiometer for mobile operation. Sensor configuration, experimental procedures, approaches for improved performance, and empirical results are reported. Interesting results of predictions to estimate detection range obtained from matched-filter calculations are included. The paper concludes with description of current preparations for a sea test of this sensor and a perspective of future developments.

Enhanced magnetic anomaly detection using a nitrogen-cooled superconducting gradiometer

During the 1980s the Superconducting Gradiometer/Magnetometer Sensor was demonstrated in the Magnetic and Acoustic Detection of Mines Advanced Technology Demonstration to provide effective mine detection, localization, and classification capabilities, especially against buried mines, and to reduce significantly acoustic false alarms arising from bottom clutter. This sensor utilized Superconducting Quantum Interference Devices manufactured using the low critical temperature (low Tc) superconductor niobium and liquid helium for sensor cooling. This sensor has most recently bee integrated into the Mobile Underwater Debris Survey System and has been demonstrated successfully in a survey to locate unexploded ordnance in coastal waters.

Effects of radio frequency radiation on the dc SQUID

The effects of radio frequency radiation on the dc SQUID are examined. Simulations show how the shape of the SQUID transfer characteristic is distorted by radio frequency interference (RFI). How this affects three commonly used SQUID modulation methods is discussed, and the results explain why we experimentally observe the bias current reversing readout method to be the least susceptible to RFI. The commonly seen increase in the low frequency flux noise power spectrum of dc SQUIDs in unshielded environments is also explained.