Magnetic and electromagnetic (EM) surveys are two effective geophysical techniques for unexploded ordnance (UXO) investigations. Both have a well-documented ability to locate UXO but are prone to excessive false alarms, which significantly increases the cost of cleanup. For instance, at some sites up to 75 percent of excavated items are non-UXO. The key to reducing these false alarms is the development of algorithms that can discriminate between an intact UXO and other items such as scrap metal or geological features. Recent developments have enabled the recovery of dipole or ellipsoidal parameters through inversion of magnetic data. Additionally, frequency and time-domain electromagnetic measurements can be inverted to recover parameters in reduced modeling formulations. These recovered parameters can then be used to discriminate between UXO and other items. For example, the recovered parameters in a two-dipole model for time-domain electromagnetics can be used to determine whether the object is magnetic (possibly UXO) or not and whether the object is plate-like (non-UXO) or rod-like (UXO).
The overall objective of this SERDP Exploratory Development (SEED) project was to determine how magnetic soils affect the recovered parameters from magnetic and EM inversions and, if significant, to explore possible techniques to mitigate their effects. Specific objectives included: (1) quantitatively evaluate the effects of the background susceptibility and thereby determine under what conditions the inversion algorithms can work (while ignoring these effects) and (2) determine how the data can be modified so that the current inversion algorithms will work in the presence of background susceptibility.
In order to understand how different distributions of background susceptibility affect magnetic and EM data, the two data types were forward modeled for UXO items embedded in media with arbitrary distributions of susceptibility. This allowed the characterization of "magnetic noise" and thus a determination of situations where the susceptibility has a significant impact on the UXO response. The effects that magnetic susceptibility has on the recovered parameters arising from the inversion of magnetic and time-domain EM data were then investigated. At some point, the background susceptibility has a strong enough effect on inversion results such that the problem cannot be ignored. Two options are then available: (1) perform some "removal" of the noise and continue to use the inversion algorithms or (2) modify the inversion algorithms to recover information about the background and UXO. This project investigated methods for modifying the data and using the current inversion algorithms.
In a follow-on SERDP project (MR-1414), researchers are now focusing on practical questions regarding the physical understanding of magnetic soils, how to generate useful site characterizations of magnetic properties, and how to use information from the site characterization to process magnetic and EM data.
The resulting technology will increase the accuracy of UXO detection in the presence of strong magnetic susceptibility in the soil and enhance the reliability of UXO discrimination, with the follow-on effect being significantly reduced cleanup costs. (SEED Project Completed - 2003)