A Seismic Section of the Sverdrup Basin, Canadian Arctic Islands
Published:January 01, 1967
Seven seismic refraction profiles were completed in 1961 in the Canadian Arctic Islands under the Polar Continental Shelf Project of the Department of Mines and Technical Surveys. These seven profiles were shot along a line extending from a location 30 miles north of Ellef Ringnes Island, along the west coast of this same island to a point 30 miles south of the island into Belcher Channel.
The program was carried out using an S-SS helicopter with the supply support and major moves by Otter fixed-wing aircraft. Dynamite charges were successfully detonated in depths of water to 1,285 ft without the use of “boosters.” A Decca Survey Chain provided regional navigation but this system is not sufficiently accurate for long-range seismic refraction surveying. These seven profiles are presented in a cross section across the Sverdrup Basin, correlated by seismic velocities, indicating a thickness of approximately 40,000 ft of sediment in the center of the Basin to a formation with a horizontal velocity of 20,000 ft/sec “Basement” or the base of the unmetamorphosed sediments may be 64,000 ft deep at this location.
Figures & Tables
Seismic Refraction Prospecting
The seismic method is divided into reflection and refraction techniques, based on whether or not a wave undergoes a reflection at the extent of its travel. Thus, while most refracted events have not been reflected, most reflected events have been refracted, because a refraction occurs across any velocity interface in accordance with the simple and basic Snell’s law. This law states that the sine of the angle of incidence is to the sine of the angle of refraction as the velocity on the first side of the interface is to the velocity on the second side of the interface.
Where the refraction angle is large, and not near to zero as it is in the case of reflection work, there are many considerations concerning the geometry of the raypath that have to be made in refraction interpretation. Basically, the papers in this volume describe various techniques for separating out special raypath solutions and making approximations that give us a structural geologic picture from the study of these approximations or specializations.
The following factors are of extreme importance in refraction surveying’ 1) Distance’ Surveying must be accurate in order to make correct depth determinations of the refractor by the use of the refraction method. 2) Velocity’ The velocity of the various horizons, through which the refracted wave passes, must be known if an accurate structural picture is to be determined. Many of these velocities can be determined from the refraction data, and, in fact, the refraction method is a good means of establishing many of the velocities needed for these calculations. 3) Time’ Accurate time information is a prerequisite, although this is no more the case in refraction than in reflection work. In most instances, refraction information is to be recorded to the nearest 1/1,000 sec for exploration purposes.
The distance parameter will be discussed first. In many surveys the distance between the shot and receiver may be extremely long (25 to 50 miles), and the requirement for accuracy is just as vital as if this distance were very short (a few hundred feet). Because of the differential velocities involved, distance errors can cause errors in depth greater than the distance errors themselves. For somecases, in the experience of the editor, the depth error may be three times the distance error. The velocity is very critical in refraction information. Of particular important is the refractor velocity, which is often used to determine the time to be subtracted from the total time path to determine that amount of time which is near verticalor can be converted to a vertical path time.