Development of a Sensitive Multi-Axis Magnetic Tensor Gradiometer for Passive and Active Electromagnetic Detection of Underground Facilities

The state-of-the-art in magnetoresistive sensors has improved to a point where they can be used to construct extremely sensitive, room temperature, multi-axis magnetic gradiometers. Typical magnetoresistive elements have a white noise floor of 5 pT/Hz(square root), and operate with flat response from DC to tens of MHz. A light weight, inexpensive, multi-axis gradiometer could be constructed from these devices that would be a key element in airborne electromagnetic surveys for detecting underground facilities. The sensitivity and versatility of the sensors is such that the one instrument could be used in both passive and active electromagnetic surveys; detecting perturbations in the Earth's magnetic field from ferrous materials used in the construction of the facility; magnetic leakage from power used in the facility; and eddy currents and magnetic moments induced in materials within the facility from externally applied ELF/VLF probe signals. These complimentary data sets can be combined in a sensor fusionscheme to minimize sensor clutter and discriminate against false targets.