Overhauser Sensor Array Based 3-D Magnetic Gradiometer for the Detection of Shallow Subsurface Unexploded Ordnance

The information on the high-precision and multidirectional magnetic gradient is highly relevant to detecting and locating shallow subsurface unexploded ordnance (UXO). In this article, the design of a 3-D Overhauser magnetic gradiometer is presented. Taking the sensor array interferences and working efficiency into account, a time slice overlapping based operating time sequence is devised. A precision narrowband tuning capacitance network based on a cascaded subdivision strategy is constructed to improve the signal-to-noise ratio (SNR) of free induction decay (FID) signals. Additionally, a frequency measuring approach combining multiphase clock and multichannel is developed to suppress counting errors and improve the frequency measuring accuracy of FID signals. The 3-D Overhauser magnetic gradiometer’s effectiveness was validated through extensive laboratory and field experiments. The laboratory test results show that the magnetic measuring performance of each sensor from the Overhauser magnetic sensor array is consistent. The proposed 3-D Overhauser magnetic gradiometer is able to measure magnetic fields from 20 to $100~\mu \text{T}$ with an indicating error of less than 0.1 nT. The field exploration demonstrates the applicability of the gradiometer for shallow subsurface UXO detection within the urban complex electromagnetic surroundings. Overall, this study can provide support for the accurate detection and localization of shallow subsurface UXO under strong interference surroundings.