Recent demonstrations have shown that magnetic and electromagnetic induction technology can reliably detect most buried unexploded ordnance (UXO). Unfortunately, current technology is unable to reliably discriminate between UXO and other items that pose no risk, and typical survey methods currently deployed have an excessive level of false alarms. Technologies are needed that can cost-effectively discriminate UXO, which range in size from 20-mm shells to 2000-lb bombs, from other subsurface items.
The objective of this SERDP Exploratory Development (SEED) project was to demonstrate that a simple seismic device, consisting of an impact source and a few receiver elements, could be used to discriminate between buried UXO and clutter.
The Spectral Analysis of Surface Waves (SASW) seismic test is routinely used to determine mechanical properties of soil and pavement structures. The test instrumentation consists of an impact source and a pair of seismic receivers. In the standard test, soil mechanical properties are estimated from the phase velocity dispersion relation that is calculated from the frequency-dependent phase shift between the signals measured at the two receivers. When there is a buried object close by, interference between the primary and backscattered waves results in fluctuations of the apparent phase speed as a function of frequency and location of the sensors relative to the object. This project attempted to demonstrate that, when measured over a range of aspect angles relative to a suspected target, the frequency-dependent phase shifts among the signals, measured at the receivers, contain a unique “signature” of the target that can be exploited to discriminate between buried UXO and clutter.
Researchers measured phase velocity fluctuations at low frequencies in front of UXO and clutter targets. The frequency and space resolution of the measurements (50 Hz and 10 cm, respectively) were not adequate to resolve the pattern or obtain quantitative estimates of reflection coefficients. Consequently, the results are inconclusive. Although there is qualitative evidence of an exploitable phenomenon, the researchers were not able to quantitatively evaluate the potential for UXO/clutter discrimination with the configuration.
Presently, it is estimated that 75% of the costs associated with remediating UXO-contaminated land are derived from digging non-ordnance targets. This technology, if it can be developed to reach its theoretical potential, could reduce these costs significantly.