Target interaction with stratigraphy beneath shallow, frozen lakes: Quarter-wave resonances within GPR profiles

Resonances within ground-penetrating radar (GPR) reflection profiles recorded over shallow, frozen lakes located on an artillery and bombing range in interior Alaska originate near ice/bottom and freeze/thaw interface horizons and are similar to those caused by shallow utilities. The transmitted pulse spectra were centered near 280 MHz. Low-pass filtering differentiates the resonances from background scattering. The transit times between their onset and that of the overlying interface reflections correspond with spectral peaks predicted for layers with thickness of one in-situ quarter-wavelength, which we call quarter-wave resonance. The peaks are below 160 MHz, which is well below the transmitted pulse bandwidth and may occur as a modulation of a reflection sequence. Although the transit times cannot resolve both layer thickness and permittivity, reasonable values of permittivity give overburden layers above the targets on the order of 10 cm, which is consistent with the persistence of the resonances. Numerical models of metal objects beneath an interface between materials of strong permittivity contrast reveal that significant energy at the predicted frequencies requires cylinder diameters too large to be ordnance. However, strong resonances at the predicted frequencies result from flat metal sheets of 0.5-1 m dimension, and so we interpret the resonances to be caused by nonhazardous targets. Late winter is an ideal time to survey for targets in this area because the ice affords repeatable positioning for the antennas and thermal high-dielectric-contrast interfaces are then well developed. The bandwidth of the pulse we used appears ideally suited for both resolution of the shallow strata and detection of the resonances, thereby allowing the resonances to be checked against quarter-wave theory.