Field dielectric measurements of frozen silt using VHF pulses

Abstract

Radiowave propagation experiments utilizing short pulses in the VHF band were conducted in the permafrost tunnel at Fox, Alaska. The purpose was to measure dielectric properties of this naturally occurring, perennially frozen organic silt which is common to much of interior Alaska and for which ice content varies between about 54 and 79% by volume. Transmissions across a septum dividing two drifts gave relative dielectric permittivity values between 3.9 and 7.3. The low values resulted when transmission was predominantly through an ice wedge. Propagation along the septum gave values of 3.3 and 5.0 depending on antenna polarization. This propagation was influenced by the dry, surface silts, as was propagation along a ceiling section, which also gave an approximate value of 3.3. The data from attempted transmissions from the ground surface directly above the tunnel to the tunnel ceiling (approximately 12 m distance) are ambiguous, as signals that propagated indirectly along the transmitter cable through a nearby ventilation shaft may or may not have masked direct transmission through the permafrost. The results agree with previous laboratory investigations conducted at temperatures well below that of naturally occurring materials in interior Alaska suggesting that winter refrigeration of the tunnel by circulated outside air greatly affected the natural conditions at this site.