"wavelength" of the seismic pulse. We recognize this complication from 

 acoustic theory. It has been demonstrated vividly by Woods (1956) in a series 

 of graphic results obtained from experiments with a one dimensional model. 

 The model consisted of a 300-foot length of 2-inch pipe, at one end of which were 

 placed a small loudspeaker (electrically pulsed to simulate the shot) and a 

 microphone (the seismometer). Geologic formations were modeled by variations 

 in the cross sectional area of the pipe, using pieces of wood cut to desired shapes 

 and introduced into the pipe. Any desired bed sequence could thus be simulated. 

 The results of some of his work are shown in Figure 26-4. Whereas the model 

 cannot be expected to yield quantitative results, its value is best expressed by 

 Woods, "These records have been used to advance the argument that each 

 reflection seen on a seismic record is nearly always a composite of several 

 reflections. In seismic exploration, a constant source of error is the naive idea 

 that each line-up on the record represents a single interface which is to be 

 plotted on a cross section at a definite depth. There is the companion idea that 

 each line-up on a seismic record is to be correlated with some single sharp kick 

 on the well resistivity log. It is to dispel these ideas that the experiments were 

 made with the acoustic model." Despite the widespread tendency to correlate 

 a particular reflection with the top or bottom of a particular geologic member, 

 we must realize that this may not be the case and temper our belief accordingly. 



GEOMETRY OF RAY PATHS A line drawn to represent a particular path 



along which a wave propagates is called a ray 

 path. In an isotropic medium the ray paths are perpendicular to the wave fronts. 

 In seismic prospecting ray-path geometery is of much importance. The pictures 

 used to illustrate the geometry of ray paths are known as ray-path diagrams 

 (figs. 26-5, 26-6, 26-7, 26-8). The assumption of a constant velocity within each 

 individual layer requires that the ray paths be straight within each layer, and 

 thus simplified computing equations result. In many cases, the use of straight- 

 ray paths is justified because the results obtained through their use fall within 

 the limits of error established for the particular prospect. If the simpler methods 

 prove inadequate, more refined techniques (such as curved-ray methods) are 

 indicated. Standard reference books on geophysical prospecting (Broughton 

 Edge and Laby, 1931; Dix, 1952; Dobrin, 1952; Eve and Keys, 1954; Heiland, 

 1946; Jakosky, 1950; Nettleton, 1940) develop equations relating distance, time, 

 and velocity as used in most routine seismic prospecting. These equations must 

 be expressed in terms of the observable or derived quantities obtained from 

 the records and particular time-distance plots. 



Travel-time (or time-distance) curves are plots of arrival times versus the 

 shotpoint-to-seismometer horizontal distance (figs. 26-5, 26-6). Through analysis 

 of travel-time curves, the geophysicist may recognize the type of the returned 



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