III. SEISMIC PROFILE INTERPRETATION 



Although time breaks on the type 47 Polaroid film records can be picked 

 and plotted in the field, it is necessary to replot these data at a larger 

 scale in the laboratory. Time breaks can be picked only to the nearest 

 millisecond. A plotting scale of 10 milliseconds per inch on the vertical 

 axis and 50 feet per inch on the horizontal axis has proved to be a conven- 

 ient layout for the time-distance plots. After plotting the time points, 

 a line (or series of lines) is dravm by visual fit through these points. 

 Velocities for the various layers are computed from these lines. Velo^ 

 cities can be computed to a high degree of accuracy and later rounded. 

 When drawing these lines through the data points, slopes should be con- 

 structed on the time-distance graph in accordance with reasonable seismic 

 velocities. Topographic relief or subsurface irregularities occasionally 

 will cause unusually high or low velocities. 



1. Formulas and Nomograms 



Selection of an appropriate interpretation method for seismic refraction 

 data has long been discussed by geophysicists. There is a choice between 

 two formulas: the critical -distance formula, and the time-intercept formula. 

 The critical -distance formula is an excellent determinant for the classical 

 two-layer system. However, this formula performs poorly in the multilayer 

 situation common to the shore environment. The time -intercept formula 

 (there are several variations) has significant shortcomings, but is more 

 successful in multilayer situations. For this reason, it is used most 

 frequently in data interpretation in this study. Nomograms for both 

 critical-distance and time -intercept methods were obtained. When results 

 were checked, the calculated thicknesses and depths usually agreed within 

 5 percent . In addition to the nomograms for depths , a nomogram was also 

 used to compute offset distances and critical angles. Refraction geo- 

 physicists are concerned about the appropriate location of the depth that 

 has been calculated by the formulas . An offset distance was calculated 

 as a function of velocity and depth, then the calculated depth was placed 

 away from the shotpoint by an amount equal to this offset distance. 

 Figure 13, a sketch of a typical two-layer case, shows the offset distance 

 with respect to the overlying shotpoint. 



2. Seismic Velocities 



When determining intercept times or the difference of intercept times 

 for use in the time-intercept formula, apparent velocities should not be 

 confused with true velocities. Velocities associated with water -saturated 

 sediments tend to cluster around 5,000 feet per second with a plus or 

 minus deviation of about 300 feet per second. Velocities other than those 

 centered around 5,000 feet per second can be related to the degree of satur- 

 ation or to the texture of the sediment. Apparent sediment velocities also 

 may be created in much the same manner as apparent bedrock velocities. This 

 situation will be discussed later. Apparent and true bedrock velocities 



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