Allan J. Delaney

Ground-penetrating radar reflection profiling of groundwater and bedrock in an area of discontinuous permafrost

We have used ground‐penetrating radar to profile the depth of permafrost, to groundwater beneath permafrost, and to bedrock within permafrost in alluvial sediments of interior Alaska. We used well log data to aid the interpretations and to calculate dielectric permittivities for frozen and unfrozen materials. Interfaces between unfrozen and frozen sediments above permafrost were best resolved with wavelet bandwidths centered at and above 100 MHz. The resolution also required consideration of antenna configuration, season, and surface conditions. Depths to subpermafrost groundwater were profiled where it was in continuous contact with the bottom of the permafrost, except near transitions to unfrozen zones, where the contact appeared to dip steeply. The complexity of the responses to intrapermafrost bedrock, detected at a maximum depth of 47 m, appears to distinguish these events from those of subpermafrost saturated sediments. The relative dielectric permittivity ranged between 4.4 and 8.3 for the permafro...

Ground and airborne resistivity surveys of permafrost near Fairbanks, Alaska

In permafrost regions investigations for such geotechnical endeavors as route selection for roads and pipelines and site investigations for buildings and dam construction often require that a careful assessment be made of the presence or absence of frozen ground, of the ice content of frozen ground, and of the depth of frozen ground. In the vicinity of Fairbanks, Alaska, where the permafrost is discontinuous, ground and airborne methods of mapping electrical resistivity using radiowaves were tested as means of delineating permafrost. When the resistivity maps are compared with surficial geological data, the following conclusions are reached: (1) In areas of fine‐grained sediments, where the near surface sediments are relatively uniform, VLF resistivity delineates permafrost. (2) In areas where surface sediments vary widely (flood plains), VLF resistivity shows little information on permafrost conditions but can provide other important geotechnical information, such as, depth to bedrock, surface soil type,...

Delineation of a complexly dipping temperate glacier bed using short-pulse radar arrays

We have defined the complex bed topography for a section of a small temperate glacier using 50 MHz monostatic short-pulse radar data and a synthetic-aperture array-processing method. The data were collected on a 100 m by 340 m array grid in the upper stem of Gulkana Glacier, central Alaska, U.S.A. The array processing was based on a modified three-dimensional (3-D) Kirchhoff migration integral and implemented with a synthetic-aperture approach that uses sequences of overlapping sub-arrays to generate depth images in vertical planes. Typical sub-array beam patterns are generally <5° at the -6 dB level, giving a flashlight-like searching capability without distorting the wavelet shape. The bed topography was constructed using normal reflections picked from 3-D array depth images. In some instances reflections were imaged outside the data-coverage area. The bed surface dips steeply, both parallel and transverse to the direction of ice flow. The maximum observed depth is roughly 140 m. The 3-D method resolved bed dips up to 45°. In regions of steepest dip, it improved depth accuracy by 36% compared with raw data, and by 15% compared with standard two-dimensional (2-D) migration. Over 12 dB of signal-to-noise improvement and improved spatial resolution was achieved compared to raw data and 2-D migration. False bottom layering seen in the raw data and in 2-D migrations is not observed in the 3-D array results. Furthermore, loss of bottom reflections is shown by the 3-D migration to be attributable to the dip and curvature of the reflector, and not scattering losses or signal clutter from englacial inclusions.

Electrical resistivity of frozen and petroleum-contaminated fine-grained soil

Abstract Both positive and negative resistivity anomalies associated with contaminated soil have been reported in the recent literature. By positive we mean expected increases of resistivity with decreasing water content, increasing hydrocarbon content, and freezing. In this investigation, we tested these findings by measuring the electrical resistivity of samples of fine-grained soils contaminated with petroleum and by performing a field survey of an abandoned petroleum storage site where drilling and sampling confirmed soil contamination. In the laboratory, the cylindrical silt samples demonstrated a three-order-of-magnitude decrease in resistivity as water content increased. A corresponding resistivity increase was observed when water content was held at a constant level and petroleum percent was increased incrementally to saturation. Resistivity and temperature were also measured along a vertical profile as both the frost and petroleum front advanced beneath the surface of a large, unsorted silty-sand sample. Resistivity of the uncontaminated silty sand increased from 400 to 4000 Ω m as the soil froze; resistivity further increased to 10,000 Ω m when the soil was both frozen (−3.0°C) and contaminated. In the field we contoured resistivity, determined by Wenner array profiling within the containment berm of a storage tank where spills occurred sporadically during the 25-year operation period. Increasing values of apparent resistivity compare favorably with documented residual petroleum contamination and corroborates our laboratory measurements. We conclude that residual petroleum on soil particle surfaces results in a permanent increase in soil resistivity and that positive anomalies should be expected in most common situations.

SHORT-PULSE RADAR DETECTION OF GROUNDWATER IN THE SAGAVANIRKTOK RIVER FLOODPLAIN IN EARLY SPRING

Short-pulse radar operating in the UHF band was used to search for unfrozen water beneath ice blisters and within the gravels of the Sagavanirktok River floodplain near Prudhoe Bay, Alaska. The investigations were carried out in early April 1991, when daily mean air temperatures still were below −20°C. The radar pulse spectrum was centered near 400 MHz and the radar time range was set to cover about 10 m depth. The flat, snow-covered surface allowed good antenna-ground coupling, and the ice and gravels provided a low-loss propagation medium that allowed detail of the gravel structure to be seen in the data. Grids of several traverses were profiled at three sites, all within an area of about 0.5 km2. Unfrozen water was inferred from the strength and polarization of the radar reflections. One site contained a large icing blister beneath which an extensive reservoir was mapped and seen to extend several tens of meters beyond the mound. Data from two other sites indicated taliks 5–7 m deep near a frozen mound and within the gravels. One of the profiles extended 700 m across the floodplain and revealed other high-reflectivity targets, which were presumed to be water, within about 7 m of the surface; a deeper sounding confirmed this depth limit. It is concluded that taliks exist in the floodplain throughout the winter and are a probable source for the numerous icing mounds seen along most Arctic rivers east of Prudhoe Bay.

Multidimensional GPR array processing using Kirchhoff migration

We compare the ability of several practical ground-penetrating radar (GPR) array processing methods to improve signal-to-noise ratio (SNR), increase depth of signal penetration, and suppress out-of-plane arrivals for data with SNR of roughly 1. The methods include two-dimensional (2-D) monostatic, three-dimensional (3-D) monostatic, and 3-D bistatic Kirchhoff migration. The migration algorithm is modified to include the radiation pattern for interfacial dipoles. Results are discussed for synthetic and field data. The synthetic data model includes spatially coherent noise sources that yield nonstationary signal statistics like those observed in high noise GPR settings. Array results from the model data clearly indicate that resolution and noise suppression performance increases as array dimensionality increases. Using 50-MHz array data collected on a temperate glacier (Gulkana Glacier, AK), we compare 2-D and 3-D monostatic migration results. The data have low SNR and contain reflections from a complex, steeply dipping bed. We demonstrate that the glacier bed can only be accurately localized with the 3-D array. In addition, we show that the 3-D array increases SNR (relative to a 2-D array) by a factor of three.

Dielectric measurements of frozen silt using time domain reflectometry

Abstract Dielectric properties of ice-rich frozen silt from the permafrost tunnel at Fox, Alaska, have been measured both in the field and in the laboratory, using time domain reflectometry (TDR). Undisturbed field samples obtained with a modified CRREL core barrel were placed in a specially adapted rigid coaxial line mated to the TDR unit. The volumetric water content of the undisturbed samples varied between 65 and 81% and the sample temperature was approximately −7.0°C. The laboratory samples were reconstituted with volumetric water content between 0 and 55%. Temperature was varied between +25° and −25°C. The data were processed to cover the frequency range of 0.05–1.0 GHz. For the undisturbed samples, dispersion tended toward a maximum between 0.3 and 1.0 GHz. The range for the real part of the dielectric coefficient was 3.8–5.3 at the low frequency end, while the imaginary part varied between 0.01 and 0.42 for the entire frequency range. These results from the field studies agree with laboratory measurements and other field observations, indicating that the TDR core barrel sampler is an effective technique for measuring dielectric properties of undisturbed samples. This method could easily be applied for in situ dielectric testing of frozen fine-grained soils and ice.

GPR images of hidden crevasses in Antarctica

We have used airborne and ground-based GPR at 400 - 500 MHz to image hidden crevasses in Antarctica. The radar antennas were either pushed in front of a snowmobile, or cantilevered from a helicopter flying at about 6 m altitude and speeds near 15 m/s. We used a high trace acquisition rate, long time ranges, and migration to show snow bridge depth, crevasse width, septums between crevasses, connecting cavities and associated folded strata in the ground-based profiles. The crevasse images are formed by the discontinuities in reflections from the stratified firn. Diffractions from crevasse walls are strongly single-sided, originate from point and linear discontinuities, and provide advance warning. Airborne recorded images are similar, but less detailed than surface images. An increased trace rate could improve airborne imaging and allow greater profile speeds.

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.

Stratigraphic profiling in the Antarctic dry valleys

We have obtained the first ground-penetrating radar profiles of permafrost in the Dry Valleys of Antarctica. The area has long been thought of as a possible analog for Martian conditions. We interpret deltaic glacio-fluvial outwash, the disposition of lodgement till, and eroded out-wash previously thought to be terminal moraine from 100- and 400-MHz profiles at selected sites in McKelvey, central Wright, and lower Taylor Valleys, respectively. The relative permittivity appears to range between about 4 - 5.5 , which is consistent with the dense silty and sandy gravel, mineralogy and low ice content. Maximum depth of stratigraphy profiled was about 30 m. Weak interface reflectivity may limit interpretation of maximum penetration because absorption should be low, and scarce diffractions at depth imply weak scattering at 100 MHz. Future GPR systems in the Dry Valleys and on Mars should use colinear antennas to accommodate the rocky surfaces, and lower frequencies to search for deeper reflectors such as bedrock.