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. BACKGROUND Locating UXO is a partially solved problem, meaning that many solutions exist but none work universally. UXO is typically detected using magnetometers, metal detectors, ground penetrating radar or controlled source electromagnetic induction. Except for the radar, these techniques provide little information regarding the depth of burial of potential targets and are sensitive to cultural artifacts such as metal fences, power lines or buildings. Factors that affect performance of traditional methods include soil moisture content, depth of burial or non-metallic targets. In some cases, the soil itself generates signals that can confuse the diagnostic method and data interpretation. In this work, we evaluate another technology, electrical impedance tomography (EIT, also referred to as complex resistivity tomography or induced polarization tomography; Oldenberg and Li, 1994; Weller et al., 1996; Shi et al., 1998; Ramirez et al., 1999), for locating buried UXO. EIT produces a tomograph or image of the subsurface electrical properties which can provide information regarding the target size, shape and depth. This information can be useful not only for detecting and locating the target but also for planning and monitoring remediation. The electrodes required to make EIT measurements can be deployed beneath and around buildings or other structures, so that surveys can be performed to sense under a building. Most other detection methods are blinded by interference from the metal in a building. Surveying under buildings is the primary focus of the work reported herein. Electrical impedance, long used to probe the subsurface, has two components: magnitude and phase. Impedance magnitude, or resistivity, defines the distribution of electrical current because flow is concentrated where resistivity is low and flow is sparse where resistivity is high. Because ordnance composition is typically metallic we expect buried ordnance to be of low electrical resistivity. Impedance phase, another measure of induced polarization, describes the behavior of current flow as it depends on frequency (Telford et al., 1976). The metalfluid boundary causes a frequency dependant impedance due to the transition from electronic to ionic current flow at the boundary. Therefore, we expect buried ordnance to produce induced polarization or a phase signal in the electrical impedance. This polarization arises from the same mechanism responsible for the effect in metallic ore deposits (Keller and Frischknecht, 1966). Mare Island, located near Vallejo, California, was a naval base beginning at the Civil War era and its mission included the manufacture, stockpiling and distribution of naval ordnance (Fig. 1). The Department of the Navy closed the base in 1996 and plans to return it to civilian control. However, large amounts of buried UXO have been discovered on site, delaying this transfer. Although some times the material is found scattered, a large accumulation 12 Journal of Environmental and Engineering Geophysics of UXO is shown in Fig. 2. As a result, the U.S. Navy has supported an active remediation program to render the site safe for civilian use. Figure 1. Location of the former Mare Island Naval Shipyard. Numerous areas are currently under investigation for buried UXO (after SSPORTS, 1997). The primary method for locating UXO has been magnetic surveys using portable magnetometers (U.S. Army Report, 1997). The method is fast, economical and easy to use. However, due to interference from ferromagnetic and ferrimagnetic metal, the magnetometer cannot be used near or under buildings. At Mare Island there are many buildings which may have been built over buried UXO, and many of these buildings must be removed. Worker safety could be improved if any UXO could be located and emoved before a building is razed. 13 Daily et al.: Imaging UXO using Electrical Impedance Tomography Figure 2. Buried mass excavated in dredge ponds at Mare Island.