dominate, it is not unusual for depths to disagree by 20 to 30 percent 

 particularly i£ the lines are oriented at right angles to one another. 



4. Problems of Interpretation 



The line connecting the travel times for a seismic wave in the first 

 layer rarely passes through the origin on the time-distance plot. Figure 

 16 is an example of this non-zero intercept; the figure is a reproduction 

 of the time-distance plot of seismic line CB-8A east of Crane Beach. The 

 location of this intercept on this time axis closely matches the depth to 

 the water table in the sand over the seismic profile. A comparison 

 between the penetration drill-log for the nearby drill hole CBD and the 

 time-distance plot showed that the water table is about 3 feet beneath 

 the surface. The first segment of the time-distance plot intercepts the 

 time axis at about 6 to 8 milliseconds. Assuming a velocity in dry sand 

 of about 1000 feet per second for the seismic wave, it can be proven that 

 this timelag represents a 3- to 4-foot overburden of dry sand. 



Previous discussion of the slower velocity of the seismic wave in dry 

 sand suggested the complications in placing a geophone spread over dunes. 

 Problems of interpretation can be greatly reduced by seeking a level route 

 through the dunes for the seismic line. In addition, a sketch of dunes 

 should be placed on the field notes if the seismic line is not surveyed. 

 A thickening or thinning of this upper dry layer yields an apparent sedi- 

 ment velocity which can be confusing. Figure 17 is a sketch showing a 

 3-foot thickening over a 330-foot geophone spread. This thickening can 

 change the apparent velocity of (5000 feet per second) water-saturated 

 sediment lying underneath this dry layer. Sediment below the water table 

 rarely yields a seismic velocity less than that of the range 4700 to 5000 

 feet per second. If a lower velocity is found, a situation similar to 

 that in Figure 17 should be sought as an initial explanation. Occasionally 

 there is a velocity variation beneath the water table which can be attri- 

 buted only to sediment texture or to the environment of deposition. Such 

 a horizon could be verified only by drill-hole correlation. 



Plum Island seismic lines PI-7, PI-8 and PI-9 and the nearby drill hole 

 PIC suggest another problem common to seismic work near the shore. Apparent 

 attenuation of the seismic energy takes place along this profile due to a 

 possible thickening of a peat sequence beneath a dry overlying sand (Figure 

 18) . The resulting time-distance plot is unusual (Figures 18 and 19) . 



The presence of peat under a refraction profile does not always create 

 a time-distance plot with a step-like appearance like that in Figure 18. 

 Recent work by the author on Cape Cod beaches near Brewster, underlain by 

 marsh-peat sequences, shows that these layers have little effect on seismic 

 transmission. In the area of Plum Island and Castle Neck, the presence of 

 these step-like breaks in the time-distance plot strongly suggests that a 

 nearby drill hole might show an energy absorbing layer, such as peat, near 

 the surface. 



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