. Early geophysical papers of the Society of Exploration Geophysicists . iftooo rr/nc Fig. 6.—Typical stratigraphic column and velocity curve in Oklahoma. various depths give a first approximation to this velocity-depth rela- tionship. Velocities determined by refraction shooting will naturally almost invariably be too high. Since it is obvious that an error in the assumed thickness of the surface zone will reproduce itself in the computed depth to a horizon below the Pennsylvania^ multiplied by a factor of over six, determi- nation of the thickness of this low velocity surface layer is a crit
. Early geophysical papers of the Society of Exploration Geophysicists . iftooo rr/nc Fig. 6.—Typical stratigraphic column and velocity curve in Oklahoma. various depths give a first approximation to this velocity-depth rela- tionship. Velocities determined by refraction shooting will naturally almost invariably be too high. Since it is obvious that an error in the assumed thickness of the surface zone will reproduce itself in the computed depth to a horizon below the Pennsylvania^ multiplied by a factor of over six, determi- nation of the thickness of this low velocity surface layer is a critical factor. This is best done by shooting a small charge close to the last seismometer on a spread and computing this zone from the resulting observed time-distance curve. Figure 7 demonstrates such a case; 806
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