The study gathered data with various acoustic and electrical
resistance intruments over a two year period. The shallow water and shoreline
portions of the study around Tuktoyaktuk and Richards Island required the
unique working platform provided by ARKTOS Beta. The purpose of the study
was to determine required depths for possible pipeline installation into the
Above Photograph Reproduced with the
permission of the Minister of Public Works and Government Services Canada,
2003 and Courtesy of Natural Resources Canada, Geological Survey of
Photograph Reproduced with the permission of the Minister of Public Works
and Government Services Canada, 2003 and Courtesy of Natural Resources
Canada, Geological Survey of Canada.
Caption: Canadian Coast Guard's ARKTOS BETA returning to Polar Continental Shelf Project via a lagoon, Tuktoyaktuk, Northwest Territories (August 1990).
Publisher: Ottawa : Geological Survey of Canada , 1990 08
Photographer: Aumento, Lara S.
Collection: Arctic Exploration (Arctic Scenery)
Image Size: 500 x 329 (32.94)
Quotation From: Canada Geological Survey Commission, Polar Continental Shelf Project.
Acoustical and Electrical Stratigraphy of an Inundated Thermokarst-Rich Coast, Beaufort Sea, Northwest Territories.
Solomon, Steven M. - Geoligical Survey Of Canada
Scott, William J. & English G. - Centre for Cold Oceans Resource Engineering, Memorial University of Newfoundland.
The Canadian Beaufort Sea has been the site of hydrocarbon exploration for the past two decades and has proven reserves of oil and gas. However, development of these reserves has not yet taken place. This is due in part to our lack of understanding about the distinctive set of physical processes which control the environment. The Northern Oil and Gas Program (NOGAP) is a federal program charged with acquiring information for regulation of hydrocarbon-related developments in the north. It involves numerous government departments and includes work on issues as diverse as archaeological resources, socioeconomic impacts, and oceanographic processes occurring in the coastal zone particularly in the vicinity of Richards Island, a location chosen as a potential pipeline landing site for oil and gas from the Amauligak field offshore.
Objective of this study:
To collect data on shallow stratigraphy in the Beaufort Sea coastal zone with particular emphasis on characterizing the onshore to offshore transition.
In order to survey in the extremely shallow water of the Beaufort Sea coastal zone the Arktos Beta, an amphibious vehicle designed and manufactured as an Arctic escape and rescue craft, was used. The vehicle is owned and operated by the Canadian Coast Guard. It uses a combination of tracks and jets to propel itself through ice-infested waters and over ice, shoals or on land. Preliminary sea trials in August 1990 established its ability to carry out high resolution geophysical surveys in shallow water. For this survey, high resolution acoustic instruments were supplemented by the MicroWIP (for MICROprocessor-controlled Waterborne Induced Polarization), a marine electrical resistivity system mobilized and operated under contract by personnel from the Centre for Cold Oceans Resources Engineering, St. John's Newfoundland.
Resistivity Measurement Technique:
The MICRO-WIP system determines the electrical properties of the water and seabed materials by injecting electrical current, and measuring the surface voltage distribution which results. The transmitter used in this survey was a 7.5 KVA Huntec M4, which produced a current of about 15 amperes. Power for the transmitter was provided by a motor-driven alternator. Connection to the water was through two electrodes at the front end of the streamer. Seven non-polarizing electrodes connect the MICRO-WIP receiver to the water. The data acquisition and control system is based on an IBM-compatible personal computer. Values of apparent resistivity were calculated in real time during the survey. The resulting values were recorded both on the hard drive in the computer and on 3.5-inch floppy diskettes.
To prepare an interpretation, the resistivity pseudosection for each line was reviewed, and data sets were selected for inversion in terms of a one-dimensional (1-D) layered model, to provide estimates of the thickness and resistivity of successively deeper layers under the location of the field dataset. For each inversion the first layer thickness was set to the observed water depth, and the first layer resistivity to the observed water resistivity. If necessary, the model values gained from the inversion process were then adjusted by cut-and-try procedures until the error of fit was reduced to about one percent. In many cases mare than one appropriate model can be determined for which the fit between field and model data sets is acceptable. These models are said to be equivalent.
Equivalent models can vary widely in the thickness of a given layer, and unless independent evidence, such as depths from seismic profiles, can be obtained, all equivalent models may be equally acceptable in terms of error of fit. In order to allow appraisal of the equivalence associated with each sounding, plots were prepared for each profile, showing the sounding inverted and the range of resistivity-depth functions which provide field curves which fit the field data within the indicated error.
Interpretations of resistivity in terms of whether or not the sediments are ice bonded (or are permafrost in the loose sense of the word) are based in large part on laboratory studies of the behavior of unconsolidated sediments as their temperature varies. The accompanying graph illustrates this relationship of the unfrozen material by about an order of magnitude.
Interpretation of the acoustic stratigraphy in the near shore areas around Richards Island is hampered by shallow water multiples of the seabed reflection within the sub bottom reflectors. In addition, penetration of the acoustic signal into the seabed is inhibited by the presence of gas, by highly attenuating silt and by internal multiples resulting from the fine sediment layering.
In the interpreted sections illustrated here, an uppermost acoustic reflector (R1) is present within the upper 20 ms (two way travel time - about 15 m) of the seabed. In some cases it is enhanced by the gas concentrating beneath it, in other areas it is partially or completely obscured by gas and multiples. The acoustic unit (unit I) above R1 generally consists of conformably draped well-stratified material which thins or pinches out over highs and as the shoreline is approached. In some locations, the unit contains prograding reflectors which infill depression and in rare locations, reflectors which onlap drowned topographic features. Acoustic strata immediately below R1 (unit II) consists of discontinuous reflectors which are in some cases, disconformable, suggesting large scale cross-stratification. In a few locations where penetration is sufficient, acoustic permafrost (APF) is present within this unit.
Geology And Thermal Conditions In The Study Area:
The onshore surficial geology of the North Head area consists of discontinuous till underlain by sands of the Pleistocene Kittigazuit and Kidliut formations. Well within the zone of continuous permafrost, the area contains abundant evidence of active thermal processes. Permafrost reaches thicknesses up to 700 m. Ground ice (as ice bonding or visible ice) occurs in most of the sediments and may represents up to 13% by volume of the upper 8 m of ground.
Degradation of massive ice results in the development of thermokarst topography. On Richards Island and the Tuktoyaktuk Peninsula the land surface is mottled with thermokarst lakes. The lakes enlarge progressively by melting at their edges producing distinctive retrogressive thaw flow slides. Taliks (unfrozen zones) tens of meter thick form beneath the lakes as a result of thawing of the permafrost underneath the accumulated water. The coastline adjacent to thermokarst-dominated areas is convoluted, attesting to recent relative sea-level rise and consequent breaching and drowning of the lakes.
The inundation of permafrost-bearing coastal sediments by the sea results in increased annual mean seabed temperatures and gradual thawing when the water depth is greater than about 1.5 m (Taylor 1991). Under these conditions, the seabed is separated from overlying ice by an insulating layer of water. The extensive distribution of onshore permafrost and ground ice and its impact on the regional geomorphology suggest that thermal conditions within the coastal and nearshore zones may play a significant role in the distribution of sediments and their geotechnical properties.
The combination of electrical resistivity measurements and conventional high resolution acoustic profiling enhances our ability to interpret the shallow subsurface geology along the Beaufort Sea coast. For instance, the resistivity data are instrumental in the presence of interpreted shallow ice bonding in shallow water provides information which cannot be acquired continuously or as cost-effectively by other means (e.g. drilling).
There are obvious discrepancies between the two geophysical methods, part of which stem from the differences in the physical properties being measured (i.e. acoustic reflections depend upon density and acoustic velocity whereas resistivity depends on pore geometry, pore water salinity, and mineralogy). The resistivity interpretations presented here were made without using the acoustic data as a control on layer thicknesses. In some cases, this additional information may be necessary to use 2-D models rather than the 1-D inversions which were used here to obtain satisfactory fits to the data.
The presence of ice bonded sediments is obviously related to water depth but the exact relationship has not yet been clarified. Future work will be directed to mapping the depth to ice bonding and its relationship to environmental factors such as water depth, exposure to waves, and relict topography. More subtle resistivity variations within and between layers are interesting in that they may represent lithological variations which identify potential granular resources. More closely spaced 1-D modeling will help to define the character of these changes. Finally a drilling program is planned to provide groundtruth in critical area so that the interpretations can be further calibrated and refined.
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