Application Of Frequency Response Towards Travel-Time Tomography For Detecting Soil Disturbance

Soil disturbance evaluation can be used as an indirect tool to detect buried objects, which is in turn of interest in forensic and security applications to local police departments, the Federal Bureau of Investigation and the Department of Homeland Security. Heterogeneity and variations in soil stratigraphy due to finite size rocks and boulders, etc. may be mistaken with disturbed zones. Besides, even uniform precipitation can affect the nonuniform environment unevenly and locally. The moisture difference is the key to creation of dielectric property contrast between disturbed and undisturbed zones. The research on the sandy soil in the pilot scale facility of Northeastern University (referred to as SoilBED) has shown that the disturbance effect may fade by vibrations or after cyclic drainage and recharge due to precipitation over time and up to some extent, but this effect is minimal. Cross-Well Radar (CWR) and Ground Penetrating Radar (GPR) are among the most promising potential detection techniques. The SoilBED facility is used to experimentally simulate the problem along with other numerical and analytical simulations. To study and evaluate the feasibility of disturbance detection using radar-based geophysical techniques, PVC-cased borehole monopole antennas were installed on a circle around the area suspected to be disturbed. Then, cross-tomography measurements are collected and images of the disturbed zone were reconstructed. Obviously, CWR may not be the perfect tool to evaluate soil disturbance over a large area, but can be employed to study the potential of using radarbased techniques to detect soil disturbance. The results can be used for detection of other anomalies, such as tunnels, landmine, etc. Introduction GPR and CWR are radar-based geophysical techniques that can be implemented in a vast variety of applications such as forensic, safety, security, civil infrastructure, and geo-environmental engineering. One application in the forensic field is to detect bodies or objects buried underground. During the experimental application of traveltime tomography to detect DNAPLs (dense nonaqueous phase liquids) in soil, the background soil was removed and then replaced back using the same compaction effort and moisture content, to place objects in soil. Precise provisions were employed to assure achieving the same soil density and moisture content. The background soil was poorly graded uniform silty sand, with minimal dependency of mechanical strength on the grain-size and packing structure. Despite these provisions, a strong scattering effect on the electromagnetic (EM) field by the soil disturbance was observed. The aim of this study is to experimentally and theoretically evaluate the disturbance in the soil. Then, the outcome will be used to evaluate different techniques to eliminate this disturbance and replicate the same soil background for various experimental applications. Besides, detecting the disturbance in soil using radar-based techniques can still be helpful in different forensic or homeland and national security to lead to indirectly detecting difficult to detect buried objects (due to weak property contrast with the background). The disturbance can fade gradually due to rainfalls, drainage, and recharge of groundwater, but to some extent and not completely.