DEEP CRUSTAL SEISMIC REFLECTIONS AT NEAR‐VERTICAL INCIDENCE

Abstract

Seismic reflections from discontinuities deep within the crust (reflection times of 8 to 16 sec) have been recorded along four different lines over a widespread area in southern Alberta, resulting in a total of 90 km of near‐vertical‐incidence profiling. Systems of geophone and hole patterns were designed to form an effective filter against long period surface waves. The data were recorded on FM analog magnetic tape and the results were digitized in order to apply digital processing techniques. Power‐spectra calculations indicate that the energy of the reflected wavelets is concentrated in the range 5 to 15 Hz. Synthetic seismograms were made for comparison with field recordings and they suggest that velocity transition zones within the deep crust are less than one kilometer in vertical extent. Along one profile an expanding spread was utilized and a strong reflection at 11.6 sec was continuously correlated over nearly 25 km. A least‐squares analysis of the [Formula: see text], [Formula: see text] plot gives an average vertical velocity of 6.2 km/sec to a depth of 34 km. Other weaker reflections have been recorded from layers at or near the base of the crust, and some events are evident at shallower depths as well. Continuous profiling along a line 40 km long has resulted in a seismic cross section which shows structural relief of 8 km over a distance of 25 km. These results indicate that the reflection technique with its high resolving power may be used to map complicated structures which even a detailed refraction survey may fail to delineate. It is probable that many continental areas have similar crustal complexities, but these have been undetected due to the inherently lower resolution of the refraction method.