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The seismic refraction method is based primarily on the measurement of the time of advance of a wave front generated by an explosion. This measurement is made by timing the arrival of a wave front at various instruments placed in progression away from the explosion or the “shot point.” Figure 1 represents the advance of a wave front in a two-layer structure consisting of approximately 80 ft of cover material, such as, gravel over bedrock. The wave front travels about 5000 ft per sec in the gravel, and about 20,000 ft per sec in the bedrock. The wave front becomes an approximate sphere of about 50 ft radius 10 milliseconds after it leaves the shot point. Approximately 16 milliseconds after the shot, it reaches the rock surface at point A. From this point of contact a wavelet starts into the rock with a velocity of 20,000 ft per sec, and 20 milliseconds after the shot the wavelet has reached the distance noted by that circle in the diagram, not only vertically below A, but radiating in all directions within the rock so that even along the gravel-rock contact, the wave front has reached point B. However, along the contact, the wave in the rock starts a new series of wavelets that travel in the gravel back to the surface of the ground. This phenomenon is in accordance with Huygen's principle. Wavelets at B and C are but two of many such wavelets present. The envelope of these wavelets is the new refracted wave, DE, at twenty-five milliseconds after the shot. From the explanation of Fig. 1, it is evident that a wave, originating at the shot point and passing through the overburden, will enter the bedrock and emerge through the overburden to the surface.
Director of Weston Observatory, Department of Boston CollegeCentury Geophysical Corp., WestonTulsa, Mass.Okla.