Early geophysical papers of the Society of Exploration Geophysicists earlygeophysical00soci Year: 1947 13 24 EUGENE McDERMOTT mined as shown in Figure 6A. The method is not very accurate, as second order quantities in time are being used. All of these methods, however, combined with experience, permit a fair estimate of the ve- locity. The accuracy of the method requires that allowance be made for the thin weathered layer at the surface, as shown exaggerated in Figure 7. A representative thickness for this weathered layer is approximately 30 feet. As the velocity in it is very low—2,000 feet
Early geophysical papers of the Society of Exploration Geophysicists earlygeophysical00soci Year: 1947 13 24 EUGENE McDERMOTT mined as shown in Figure 6A. The method is not very accurate, as second order quantities in time are being used. All of these methods, however, combined with experience, permit a fair estimate of the ve- locity. The accuracy of the method requires that allowance be made for the thin weathered layer at the surface, as shown exaggerated in Figure 7. A representative thickness for this weathered layer is approximately 30 feet. As the velocity in it is very low—2,000 feet per second as com- pared with 8,000 in the unweathered layer below it—it can not be av- eraged with this layer but must be separated, as it varies considerably in thickness. This is accomplished by shooting a small charge at the shot and recorder points. The thickness of this layer and the time con- sumed in it are thus determined bv the refraction obtained from the un- SffOT PECO&DER Fig. 7.—Weathered layer (exaggerated). 42
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