Discrimination of buried plastic and metal objects in subsurface soil

Abstract

The electrical conductivity object locator (ECOL) uses electric conductivity maps to distinguish buried foreign objects from the regular soil in the subsurface. Assuming that foreign objects and the regular soil have different electrical conductivities, when an electrical current is induced into the subsurface, the difference in conductance causes an electrical field distortion. Theoretically, one can measure the outside field distortion to solve the conductivity profile. Because the problem is highly nonlinear and has noisy field conditions, mapping the conductivity profile is an interesting and challenging task. In addition, the high contrasts in conductivity values among metallic and nonmetallic objects and soil and the high correlation within the model parameters add to the level of difficulty. The high contrast causes the computational instability in the inversion; the high correlation is due to locating the small objects. The ECOL technology utilizes several techniques to overcome the difficulties and locate the mine-like small objects. ECOL applies a low-amplitude (100-/spl mu/A to 500-/spl mu/A) electric alternating current, single or multiple frequency. The impressed AC current generates AC potentials and magnetic fields throughout the site; these are measured at the surface and the boundary of the site. Also, ECOL establishes a finite element model to compute the surface and boundary values from the amount of current, physical structure, and assumed or previous estimated conductivity profile of the subsurface. ECOL estimates the conductivity profile of the subsurface and the characters of the buried object by minimizing the sum of the square of the differences between the measured and the computed values. The minimization is based on a gradient approximation technique, namely, simultaneous perturbation stochastic approximation.