Examination of Airborne FDEM System Attributes for UXO Mapping and Detection

摘要

In this project, we have conducted computer modeling and mock-up tests to assess the benefits of several airborne frequency-domain electromagnetic system configurations for mapping and detection of UXO, and have projected their likely performance in comparison to an existing time- domain airborne system. The goal of the first year of the project was to identify preferred configurations that would be analyzed in greater detail during the second year of the project. Most of the designs that were evaluated had both transmitter and receivers mounted on booms that are attached directly to the helicopter. Two of these designs were shown to have low sensitivity to vibration and flexure and modest response amplitudes. Both had horizontal coplanar transmitters and receivers. Comparisons of the FDEM models with TEM models have used the Battelle-TEM-8 system for reference, as this is the only known airborne TEM system for UXO mapping and detection. This suggested that the S/N performance of the quadrature output of the two FDEM designs would be similar to the observed S/N of TEM systems, though differences in the numerical modeling procedures for the two methods adds some uncertainty to the comparison. Mock-up testing of FDEM systems and ground-based offset measurements of ordnance, conducted as part of the TEM-8 development and assessment, indicated that FDEM measurements may yield up to 4 times more signal than TEM measurements, with noise levels being comparable between the two methods. A S/N improvement of 4X for FDEM relative to TEM designs would enable detection of standard ordnance items at approximately 1 m higher altitude than with current TEM systems. The diameter of the transmitter loop was shown to be a key factor in determining the S/N performance of these systems, with larger diameters than have been flown to date yielding better S/N due to lower rates of primary field strength attenuation at the target level.