Sensors for landmine detection are often affected by soil water content, temperature, electrical conductivity, and/or dielectric constant. The most important of these is water content because it influences the three other properties directly. Using HYDRUS-2D, we modeled water distributions around antitank mines buried in six soil textures varying from sandy loam to clay loam under the climatic conditions of Bosnia and Kuwait. The modeling results demonstrate that soil water content regimes around landmines are strongly affected by the interaction between climate, soil type, and landmine geometry. The occasional short-term accumulation or loss of soil water around landmines depends greatly on weather conditions and soil types. Results also show that steady-state analysis of water flow around buried objects and time averaging of observed water contents may lead to unrealistic conclusions regarding the transient behavior of soil water distributions around land-mines.
During the last two decades, several principles and practices of soil science have been employed successfully in the management of hydrological, geophysical, environmental, and archaeological resources. In turn, several principles of the above disciplines and those of chemical and petroleum engineering have been applied to solving problems in soil science. Here we present another example of such synergies-an application of soil science and hydrology used to improve sensor efficiency for detecting buried landmines. Buried landmines are one of the most common and lethal weapons of ground conflict. Today at least 100 million landmines are scattered across more than 60 countries (Bruschini and Gros, 1997). Detection and disposal of landmines is one of the most difficult to control problems faced in ground conflict, and since mines remain lethal long after military actions have terminated, they have also become a long-term humanitarian disaster.
A wide range of new sensors have been developed or are in development for the detection of buried nonmetallic and low-metallic landmines. Two types of landmine sensors can be distinguished: substance-analyzing sensors and imaging sensors. Substance-analyzing sensors are magnetometers, bio-sensors, chemical sensors, and sensors based on principles of thermal neutron activation, X-ray backscatter, and nuclear quadruple resonance. Imaging sensors are based on passive and active infrared, passive and active mm-wave radar, and ground penetrating radar. Although several of these sensors perform quite well under certain conditions, there is general agreement that none of the present technologies can reach good enough detection while maintaining a low false alarm rate (Bruschini and Gros, 1997). One reason for this is the variety of landmines: there are some 2500 mine and "fuse" combinations (Rouhi, 1997). The other important reason is that the environment in which mines are placed is extremely variable because of variable climate, vegetation, soil type, depth of ground water table, and topography. For example, the three countries that have the largest average number of mines deployed per square mile are Bosnia-Herzegovina in a temperate zone, Cambodia in the humid tropics, and Egypt in an arid desert. (Strada, 1996) Variations in the environmental conditions influence sensor performance because, as a rule, landmine sensors exploit soil and environmental conditions to discern between mines and other objects.
Research efforts regarding mine detection are generally geared toward sensor development and sensor fusion. Little effort has been made to evaluate the environmental conditions that affect sensor performance. Changes in soil texture, soil bulk density, soil volumetric water content (θ), and soil salinity affect microwave radar signals (Wang and Schmugge, 1980; Hallikainen et al., 1985; Dobson et al. 1985; Peplinski et al., 1995; Calvet et al., 1995). Soil volumetric water content is known to affect thermal (Van Wijk, 1963) and electromagnetic (Hoekstra and Delaney, 1974; Topp et al., 1980) properties of soil. The performance of sensors based on microwave radar and infrared imaging is expected to vary with soil and environmental conditions.
The effect of soil and climatic conditions on landmine sensor efficiency has rarely been studied. The presence of landmines may affect soil hydraulic regimes in at least three different ways. First, landmines serve as an obstruction to water flow similarly to buried stones, large boulders, cavities, and utility pipes. Steady-state analyses of water flow in soil (Philip et al., 1989; Knight et al., 1989; Philip, 1990; Warrick and Fennemore, 1995) reveal the development of well defined water distribution zones around subterranean cavities and barriers, which result from obstructions to water flow. Water distributions in these zones are characteristically different from soils without such subterranean features. Philip et al. (1989) and Warrick and Fennemore (1995) showed that a dry soil water zone develops at the bottom of cylindrical objects buried in soil. Second, Bouwer and Rice (1984) and Hendrickx et al. (1991) demonstrated that the hydraulic properties of soils containing boulders are different from those of soils without them. Because the length scales of landmines are similar to those of boulders, the presence of landmines in soil may influence the soil hydraulic regime. Finally, emplacement methods for landmines disturb soils around landmines significantly and, in turn, change soil hydraulic properties.
Differential water distribution caused by obstruction of water flow, changes in hydraulic properties, or soil disturbance may have substantial impact on how a particular sensor functions. A clear understanding of soil water distribution around subterranean objects is necessary before the impacts of soil water regimes on sensor efficiency can be studied. To date, most theoretical analyses consider steady-state water flow in homogeneous soils. The results apply to deeper soil depths, where temporal fluctuations in water distribution have damped out to attain a steady or quasi-steady-state condition. However, in all but a few cases, landmines are scattered at shallow depths from soil surface to a few decimeters deep and, hence, occupy a soil zone characterized by highly transient soil water regimes. Therefore, the objective of this study is to model temporal and spatial changes in soil water distributions around landmines under different climatic and soil conditions.