Field observations are tested against modal propagation theory to find the practical limitations upon derivation of layer permittivities and signal attenuation rates from a radar moveout profile over two-layer ground. A 65-MHz GPR pulse was transmitted into a 30-60-cmthick surface waveguide of wet, organic silty to gravelly soil overlying a drier refracting layer of sand and gravel. Reflection profiles, trench stratigraphy, resistivity measurements, and sediment analysis were used to quantify the propagation medium and possible attenuation mechanisms.Highly dispersive modal propagation occurred within the waveguide through 35 m of observation. The fastest phase velocity occurred at the waveguide cutoff frequency of 30 MHz, which was well received by 100-MHz antennas. This speed provides the refractive index of the lower layer, so the near-cutoff frequencies must match a lower layer refraction. A slower, lower frequency phase of the dispersed pulse occurred at about 60-70 MHz, with an average attenuation rate of about 0.4 dB/m. Similar events appear to have reflected back and forth along the waveguide. Modal theory for the average layer thickness shows all primary events to be different aspects of a TE 1 mode, predicts the correct 30-70-MHz phase speeds and low-frequency cutoff phenomenon, but also predicts that the 60-70-MHz group speed should be slightly lower than observed. An Airy phase was apparently out of the bandwidth. Two-dimensional finite-difference timedomain modeling qualitatively simulates the main field results.After accounting for an inverse dependency of amplitude on the square of the range, the high resistivity of the surface layer accounts for the 0.4-dB/m attenuation rate for the 60-70-MHz phase of the pulse. However, erratic amplitudes, interface roughness, and the reflected packets indicate scattering. We conclude that permittivities can be well estimated from dispersive moveout profiles given an average surface layer thickness, and the wide bandwidth of GPR antennas allows the full dispersion to be seen. Attenuation rates appear to be derivable from the higher frequency part of our dispersive event, for which attenuation might be least affected by the waveguide dispersion.
Involvement of the Devonian Jago stock in Cenozoic fold-and-thrust deformation of the northeastern Brooks Range illustrates the influence of a relatively small, isolated crystalline body on the mechanical stratigraphy and subsequent deformational behavior of an otherwise layered sedimentary package. The small size of the stock allowed it and the structurally coupled overlying Mississippian Kekiktuk Conglomerate to deform nonpenetratively as a horse in a regional duplex, in contrast to the semiductile behavior of the nearby but much larger Okpilak batholith. Shear was localized in the upper part of the stock and the conglomerate due to partial detachment of the overlying Carboniferous Lisburne Group. North-vergent thrust-related folds formed in the mechanically layered Lisburne Group carbonates instead of the symmetrical, unfaulted detachment folds more typical of the region because an underlying regional detachment horizon in the Mississippian Kayak Shale is depositionally absent over the stock. Unusually competent contact-metamorphosed pre-Mississippian metasedimentary rocks were thrust over the stock and its cover because a ramp formed at the edge of the stock and cut upsection through the Lisburne Group due to the absence of Kayak Shale. A 40Ar/39Ar age of foliated white mica indicates thrusting of the stock by 61 Ma; fission-track ages indicate cooling at ~44 and ~28 Ma. These ages indicate a cooling history that implies ~11 km of unroofing since ~61 Ma, only ~1.5 km of which can be explained by the inferred duplex structure. The remaining ~9.5 km of unroofing is most likely due to subduplex structural thickening above a deep regional detachment.
We have determined attenuation rates and relative dielectric permittivities of a fine-grained sediment formation, part of which was contaminated by hydrocarbons. Our purpose was to test previous laboratory documentation of the effects of probable adsorption and∕or capillary tension on the electrical properties of water at a common ground-penetrating radar center frequency of [Formula: see text]. The formation is a saturated, and dense glacio-marine diamicton that had a small area partially contaminated by jet fuel. Surface and trench observations showed saturation near 30% by volume, uniform and high density, resistivity at [Formula: see text], and a total hydrocarbon content of less than 0.1% within a small zone. The local hydrocarbon contamination and a spatial gradient in grain size allowed the possibility of seeing if an increase of adsorbed water and∕or relief of capillary tension within interstitial water affected the attenuation rates. We calculated the rates from the amplitude decay of ground surface wave pulses within a series of WARR (moveout) profiles recorded along a single transect. Our rates of [Formula: see text] in the coarser-grained and contaminated zone can be accounted for by the resistivity values, while those of [Formula: see text] in the uncontaminated and finer-grained zone require additional dielectric loss. The relative permittivities varied from about 17 to 22, for which modeling strongly suggests a free water value of dielectric permittivity throughout despite the fine grain content, with a mild trend toward lower values in the finer-grained zone. A strong correlation was found between attenuation rates and clay content, and only a weak correlation with hydrocarbon content. An empirical mixing theory suggests that a [Formula: see text] relaxation frequency can explain the attenuation rates in the uncontaminated zone and that DC resistivity alone falls far short of predicting the correct rates. The hydrocarbon content analysis suggests that if a threshold level for the hydrocarbon attenuation effect exists, it is not higher than about [Formula: see text]. In addition, offset surveys appear to be a useful field approach for measuring field attenuation rates.
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