Pulse transmission through frozen silt

An important objective of geophysical exploration in permafrost regions is the delineation of subsurface ground ice. Recent studies have reported on the use of ground-penetrating radar to detect massive ground ice. VHF-band radiowave short pulses were transmitted within the permafrost tunnel at Fox, Alaska, over distances between 2.2 and 10.5 m. The propagation medium was a frozen silt containing both disseminated and massive ice with temperatures varying from -7 C near the transmitter to probably -2 C near the center of the tunnel overburden. The short pulses underwent practically no dispersion in the coldest zones but did disperse and refract through the warmer overburden, as suggested by calculations of the effective dielectric constant. Most significantly the measured frequency content decreased as the effective dielectric constant increased. The results indicate that deep, cross-borehole pulse transmissions over distances greater than 10 m might be possible, especially when the ground is no warmer than -4 C. The information thus gained could be used for identifying major subsurface variations, including ground ice features.