High-Resolution Subsurface Imaging of Deeply Submerged Targets Based on Distributed Near-Ground Sensors

A high-resolution subsurface imaging technique based on distributed near-ground sensor networks that utilize ultrawideband waveforms in the very high frequency (VHF) range is presented. An accurate scattering model for a target buried in realistic subsurface environment, modeled as a vertically stratified medium, is presented first. Then an inversion technique that uses ultrawideband near-field focusing is described. The signal penetration depth as a function of frequency and various subsurface parameters is calculated based on the developed forward model. The imaging resolution as it relates to the accuracy of background retrieval is also analyzed. A semi-empirical soil dielectric model that is originally developed for the ultra high frequency band is modified and validated at the VHF range with measured results available in the literature. For a given soil textural composition and frequency, the model predicts the real and imaginary parts of the dielectric constant as a function of soil moisture content. This soil dielectric model is utilized to make the inversion more efficient. To address the challenge associated with the design of compact and ultrawideband VHF antennas, a scheme utilizing multiple antennas and reduced number of frequency points is proposed. The sensor arrangement both in terms of spatial distribution and polarization of each antenna as it relates to the lateral resolution, as well as minimizing the direct coupling between the Tx and Rx antennas, is analyzed. The proposed subsurface imaging approach is validated based on numerical techniques and a laboratory scale model measurement results.