Seismic and Gravity Profile Across the Northern Wasatch Trench, Utah
Published:January 01, 1967
The present paper describes some of the geophysical results that ultimately led to the discovery of the Wasatch trench, Utah, as a great structural feature extending along the west side of the Wasatch Range for a distance of about 160 miles (Cook and Berg, 1956, 1958, 1961; Cook, 1962, p. 325; Cook et al., in preparation for publication). The investigation was part of a broad geophysical study of the eastern part of the Basin and Range Province.
Intermittently during 1956 to 1958, a seismic and gravity profile was taken across the area which is now known to constitute the northern part of the Wasatch trench. The profile extends in a generally eastward direction between Little Mountain South and the northern Wasatch front in the vicinity of Ogden and North Ogden, Utah (Figure 1). In June 1958, 28 seismic stations were taken along the profile for a distance of 15 miles between Little Mountain South and North Ogden. The immediate object of the survey was to determine the major subsurface geologic features of the area with special emphasis on the existence of Basin and Range faulting. The long-range objective of the survey was to attempt to perfect seismic techniques to chart effectively and most efficiently the location and character of Basin and Range faults and the thickness of rocks of Cenozoic age in the basins.
In 1956, a gravity profile, with approximately one-mile spacing between stations, was established across the valley from Little Mountain South to the Wasatch front,
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.