Cross-borehole sensing: identification and localization of underground tunnels in the presence of a horizontal stratification

This paper addresses the detection and localization of a buried two-dimensional (2D) dielectric object in the presence of an air-Earth interface. The techniques used are modifications of the well-known backpropagation operator, including plane-wave angular spectral filtering and detection of the cross-polarized scattered field. Cross-correlation of the received signal with a known target signature is included for comparison and found to be useful for detection. It is shown that use of the positive spatial frequency components of the fields scattered by the buried target, together with the backpropagation of vertical cross-polarized fields, yield results that are relatively insensitive to the presence of the horizontal stratification. Examples illustrate that backpropagation can be used for detection and localization purposes in a cross-borehole configuration. Some experimental results utilizing a 2.5 GHz laboratory scale model and a tunnel-like target are also included for comparison.