A vector handheld frequency-domain sensor for UXO identification

Abstract

The GEM-3D⁺ sensor developed by Geophex, Ltd. is a new incarnation of their widely known GEM-3. The sensor provides the analyst with all three vector components of the secondary magnetic field over a wide range of frequencies. The GEM-3D⁺ features an innovative "beacon-based" positioning system that provides a full description of its location and orientation at every point without requiring any on-sensor hardware beyond an electronic compass. This enhances the usefulness of the instrument for dynamic surveying, This paper presents some methods and results related to UXO identification using the GEM-3D⁺. Our analyses exploit data provided by the sensor in both grid-based and dynamic measurements to characterize different objects, including metal spheres and actual UXO. For the data analysis we alternate between the dipole model and the more rigorous standardized excitation approach. We review some ill-conditioning issues encountered with the latter model and the different approaches that we use to overcome them. In applications, the availability of horizontal field components in the data allow us to identify UXO vs. non-UXO items while minimizing the nonnegligible effects of ground response.