Design and Testing An Airborne Squid Magnetometer for Uxo Detection

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