Classification of Magnetic Susceptibility Anomalies and Their Relevance to UXO Detection

The attention given the natural geologic background of a site and its influence on geophysical sensors has grown considerably over the past several years, primarily because of the governmentsupported unexploded ordnance (UXO) technology demonstrations and government mandated UXO cleanup activities. Explosive and ordnance detection (EOD) teams have encountered difficulties at sites thought to be conducive for geophysical surveying, as well as at geologically challenging sites. Geophysical anomalies thought to be UXO have been dug to reveal rusty soils, “hot” rocks, animal burrows, and unexplained occurrences. No longer can a site be considered typical and its geoenvironmental setting ignored. The physical aspects and geologic makeup of a site must be evaluated prior to initiating a survey to ensure proper sensor selection for an area. The primary geophysical methods used for UXO detection are magnetometry and electromagnetics (EM). To understand how the geologic environment affects these sensors, it is necessary to understand what physical property or properties the sensors respond to and how these properties are manifested within the geo-environment. The constitutive properties relevant to magnetometry and EM methods are magnetic susceptibility, electrical conductivity, and dielectric permittivity. It becomes evident how challenging a UXO geophysical survey can be after viewing the range of values these parameters can exhibit for different soil types and moisture conditions (Table 1). The time domain EM induction sensors (TDEM) are primarily influenced by the magnetic susceptibility, whereas the frequency domain EM induction sensors (FDEM) are influenced more by the electrical conductivity but also magnetic susceptibility. Ground penetrating radar (GPR) instruments are affected by all three parameters, dielectric permittivity, electrical conductivity, and magnetic susceptibility. The electrical conductivity of a soil or rock is controlled by the pore fluid, moisture content, pore connectivity, and ions in solution. As the latter three factors increase, so does the electrical conductivity. Clay-enriched soils, particularly those containing montmorillonite, have the capacity to absorb significant amounts of water, thus increasing the electrical conductivity. As the electrical conductivity increases, the detection capabilities of FDEM and GPR sensors decrease. The three properties collectively affect the power attenuation of a GPR sensor. Figure 1 shows how the attenuation varies for different parameter values. Magnetic susceptibility influences all sensors used for UXO detection and discrimination. This paper concentrates on magnetic soil occurrences and their influence on UXO detection.