. Early geophysical papers of the Society of Exploration Geophysicists . Fig. 4. It may be objected that the foregoing analysis is based on the slope- effect and not on dip. To a certain extent this is true. In extreme cases, like that shown in Fig. 4 where the high velocity layers are unusually thick and clearly defined, the interpretation can be based purely on the theory of refraction shooting. But in a majority of cases the layers of higher velocity are relatively thin, rather closely spaced, and seldom recognizable in the outcrops. Thus it seems to be the aggregate effect of many sloping
. Early geophysical papers of the Society of Exploration Geophysicists . Fig. 4. It may be objected that the foregoing analysis is based on the slope- effect and not on dip. To a certain extent this is true. In extreme cases, like that shown in Fig. 4 where the high velocity layers are unusually thick and clearly defined, the interpretation can be based purely on the theory of refraction shooting. But in a majority of cases the layers of higher velocity are relatively thin, rather closely spaced, and seldom recognizable in the outcrops. Thus it seems to be the aggregate effect of many sloping layers which generally produces asymmetrical velocities such as those tabulated in the latter part of this paper. Consequently, there is no choice but to make the interpretation in terms of dip rather than in terms of slope. Case 2 In Fig. 5a the wave is shown traveling in an up-dip direction. Exaggerat- ing conditions somewhat for the sake of clearness, we may assume that the wave, taking the path of least time, descends fairly deeply into the shale and is steadily refracted on a curved path until the critical angle is reached, when it follows the bedding and finally emerges at the surface giving the time interval A/j. In shooting down dip, the principle of reversibility of wave paths tells us that, between identical surface points, the path will be the same from either direction and there can be no difference in the recorded times. Simi- larly, if the subsurface dip is everywhere uniform, equal shooting distances will give identical paths even though the surface points are not the same. 221
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