The objective of this project was to provide new capabilities for wide-area mapping and detection of unexploded ordnance (UXO) by developing airborne frequency-domain electromagnetic (FDEM) systems as an alternative to magnetometer and time-domain electromagnetic (TEM) systems. At the first level, these could be used to assess ordnance density in areas where site conditions are unfavorable for magnetometers. In the longer term, it is anticipated that an airborne FDEM system could consistently perform better than magnetometer systems (analogous to the performance of ground-based electromagnetic [EM] systems vs. magnetic systems), even at sites where geologic conditions are less problematic for magnetic surveys. The system assessment would focus on mapping and detection, rather than discrimination, of UXO.
The researchers conducted computer modeling and mock-up tests to assess the benefits of several airborne FDEM system configurations for mapping and detection of UXO, and have projected their likely performance in comparison to an existing TEM airborne system. The goal of the project was to identify preferred configurations.
The majority of designs that were evaluated had 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 signal-to-noise (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 four times more signal than TEM measurements, with noise levels that are comparable between the two methods. A S/N improvement of four times 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.
Another potential route to improvement in S/N was found for configurations that use a ground-based transmitter in combination with airborne receivers. Systems which use this approach for mineral prospecting are referred to as “semi-airborne”. This approach would have many practical advantages over configurations with helicopter-mounted transmitters. First, these configurations would involve less weight and associated moment of inertia on the helicopter, making them easier and safer to fly. The ground-based transmitter would allow for receivers to be placed across the entire array (sidebooms and foreboom) so that they would be more operationally efficient than the current TEM-8 system. The emplacement of a large (up to 15 km square) ground-based transmitter loop for the semi-airborne Turair and FLAIRTEM mineral prospecting systems was reported to be efficient, sometimes requiring as little as an hour to deploy.
Given the results, the researchers recommend that future research be focused on evaluating the semi-airborne configuration. Important questions remain unanswered and would need to be addressed in order for this to prove a viable design.