Early geophysical papers of the Early geophysical papers of the Society of Exploration Geophysicists earlygeophysical00soci Year: 1947 64 0. C. LESTER, JR. After completion of this profile, a hole was dug at the shot point and the top of the water table was determined to be at a depth of slightly less than 8 feet. On the assumption that below the water table all air in a free state is displaced by water, whose density and elasticity are of the same order of magnitude as the earth under consideration, and hence has no material effect on the velocity of transmission of sound through such substa
Early geophysical papers of the Early geophysical papers of the Society of Exploration Geophysicists earlygeophysical00soci Year: 1947 64 0. C. LESTER, JR. After completion of this profile, a hole was dug at the shot point and the top of the water table was determined to be at a depth of slightly less than 8 feet. On the assumption that below the water table all air in a free state is displaced by water, whose density and elasticity are of the same order of magnitude as the earth under consideration, and hence has no material effect on the velocity of transmission of sound through such substance, both the shot point and recorders were lowered to the top of the water table. At this depth the shorter or 'weathered' portion of the profile was re-shot with the result shown in curve (B), Figure 2. DISTANCE IN FEET Fig. 2 On examination, this curve indicates an 'unweathered' velocity (5,200 feet per second), which passes through the origin with only that curvature which is to be expected with a slight increase of velocity with depth. At the same recording distances from the shot point, the 'weathered' portion of the time-distance curve has been eliminated with the elimination of the 'aerated layer.' The obvious conclusion is that where the water table is sufficiently shallow the thickness of the aerated layer is determined by its position. However, if the water table be at considerable depth, the 'weathered' thickness is deter- mined by the depth to which any appreciable amount of air penetrates. This explains, in a measure, the sometimes extreme variation of 'weathered' thickness with different surface materials. It would also predict a change of weathered thickness with wet and dry seasons. Though these experiments have not been carried out to such an extent as to make the conclusions indisputable, it may be said that all the experimental data obtained favor the theoretical predictions resulting from the assumption of an 'aerated' layer. 394
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