Design and Testing An Airborne Squid Magnetometer for Uxo Detection

Oak Ridge National Laboratory is conducting a project under SERDP to design, test and field an airborne full-tensor magnetic gradiometer using a Superconducting Quantum Interference Device (SQUID). This is the second year of a multi-year project. In this paper we discuss the basic electronic design, the shielded operations, unshielded operations and orientation noise. UXO signatures and results of ground-based walk over surveys are covered in the associated paper by Doll et al. (this volume). The electronics were designed to optimize the resolution of the digital data given the limitations of 24-bit analog-digital converters (A/DC). This will be achieved to a certain extent through variable gain settings and by halting operations during aircraft turns. The greatest gains in resolution result from forcing the instrument to reset back to zero each time it reaches a saturation threshold. This function requires reconstruction of the original signal from a “saw-tooth” output. Measurements during shielded operations were conducted to determine the absolute minimum noise level in near-ideal laboratory conditions. In the bandwidth of interest, these were found to be quite variable depending on the test facilities. Noise levels at Tristan’s facilities were measured as 14.1pT/√Hz at 1Hz as compared to the manufacturer’s measurements of 0.10pT/√Hz at 1Hz. The difference was attributed to the high noise environment and potentially poorer shielding facilities. While in the shield, a vertical dipole (85 μAm2) was synthesized and mapped using a calibrated loop of wire dragged through the access port in the shield. The results were comparable in character to synthetic models, indicating that the system was performing properly. Unshielded static tests demonstrated an average background noise level of 46.5pT (rms). They also revealed a requirement for additional RF-shielding to remove periodic and random steps in the data. Three levels of shields have been designed and built. Orientation tests measured a maximum instrument slew rate of approximately 42,000nT/s. This corresponds to motion equivalent to ±5° at 1Hz, which is within the normal operating parameters of the helicopter while on line. These tests also revealed several areas that require additional investigation. More detailed measurements are required to calibrate the instrument for gain, orthogonality and linearity. These additional tests are being planned for 2005 in cooperation with the USGS-Denver, using their shielded facilities and calibration algorithms developed with SERDP funding for their fluxgate tensor system.