Imaging UXO Using Electrical Impedance Tomography

This paper reports the results of tests where electrical impedance tomography (EIT) was evaluated as a tool for detecting and locating buried unexploded ordnance (UXO). The method relies on the electrolytic polarization induced at the boundary between soil and buried metal. This induced polarization (IP) produces a measurable phase delay between the electric current imposed on the subsurface and the resulting voltage distribution. If natural sources of induced polarization are small compared to those due to buried metal objects, then tomographs of impedance phase may be used to indicate where metal-soil polarization may be present.

Three controlled tests were performed at a field site containing inert UXO buried in known locations. These tests produced a phase anomaly of about 20 milliradians that closely matched the known location of buried UXO objects. A fourth uncontrolled or blind test was performed under a building without prior knowledge of UXO presence. That test yielded phase anomalies as high as 75 milliradians. Limited excavation was performed at some of these anomalies but only a small amount (a few tens of grams) of metal was recovered. More extensive excavations are too costly until the building is razed.