The objective of the project was to prove the concept of an active tensor magnetic gradiometer system (ATMGS) using physics-based models and systems-based real-world simulations.
Three distinct but related methods were used in this feasibility study. First, numerical computer models and simulations of various configurations of an ATMGS were developed and implemented. Simulation data were used as the basis for developing data processing and analysis routines. Second, theoretical approaches to analyzing ATMGS data were investigated. Third, engineering analyses were undertaken of sensors and interface electronics to determine those modifications necessary to implement a prototype ATMGS.
This research demonstrated the advantages of an ATMGS which operates as a dual-mode survey system. In the first (static) mode, the ATMGS performs as a magnetic gradient system that provides both vector and total field magnetic measurements. These data can be processed using conventional software based upon potential fields theory. In the second (active) mode, the ATMGS measures the secondary magnetic fields produced by a primary field generator. These secondary fields are caused by induced magnetic moments, which themselves are functions of a target’s composition, location, and shape. Spheroidal ferrous targets closely approximate intact unexploded ordnance (UXO). Therefore, if a target produces secondary fields distinctive of a spheroidal body, it can be classified as UXO-like, as opposed to rod-like or plate-like bodies. This work has shown that the ATMGS data can be inverted to obtain spheroidal dimensions, and that it is possible on this basis to distinguish between the different types of body geometry. However, this study has also shown that it is not possible to separate the induced and remanent moments of a magnetically permeable body using ATMGS data.
Implementation of the ATMGS in hardware is feasible, but placing fluxgate magnetometers inside a powerful primary field coil poses difficult engineering problems. Foremost is the need to partially or fully cancel the primary field at the location of the fluxgate cores. The most sensible approach is to add a cancellation current directly to the fluxgate drive coil. Whether this can be done successfully with existing feedback and interface electronics is unknown. Designing and fabricating a solenoid centered on a large sensor array that can maintain structural rigidity during field deployment would require a major effort. Furthermore, the primary coil/sensor array combination would have to be accurately calibrated to compensate for inorthogonalities and asymmetries. Calibrating the TMGS has proven to be both troublesome and time-consuming. Calibration data must be acquired at a controlled magnetic site using specialized experimental assemblies, and data processing to derive accurate calibration coefficients has not been streamlined.