velocity contrast for the glaciomarine clay. 



The plots show apparent sediment velocities that are probably due to 

 a thickening of the peat sequence towards the center of the sand body. 

 Middle Ground has a bedrock core that rises to within 100 feet of the 

 surface and then drops away to greater depth beneath the present estuary. 



Leet (1950) and Hawkins (1960) presented nomographs for the determina- 

 tion of thickness associated with an arrangement of velocities and depths 

 known as the 'Talind zone" in refraction seismic work. Because it is cus- 

 tomary to base interpretation on "first breaks", or first arrivals, of 

 seismic waves at a geophone, there is a subsurface zone in the three-layer 

 case which is not represented on the time-distance plot. Figure 23 is a 

 graphical explanation for the travel times of a three-layer profile, in 

 which the sediments have velocities of 5,000, 8,000 and 15,000 feet per 

 second. These three layers are shown as a bedrock surface overlain by 

 a till layer which is in turn blanketed with a water-soaked sediment layer. 

 Seismic wave rays that take a path through the intermediate layer do not 

 contribute to the time-distance plot. The arrival that took a path along 

 the deeper but faster bedrock surface registers at the geophone before 

 the arrivals that traveled through the intermediate layer. 



The thickness of the 'Tslind zone" is dependent on the respective veloc- 

 ities of the three layers (Figure 24). In addition, the amount by which 

 V3 (velocity of lowest layer) exceeds V2 (velocity of middle layer) is prob- 

 ably the most significant relationship. Figure 23 shows that 50 feet of 

 8,000 feet per second sediment can be masked beneath 80 feet of 5,000 

 feet per second sediment if the bedrock has a velocity near 15,000 feet 

 per second. According to Leet's nomogram, a maximum factor Y, where 

 Y = H2 (thickness of middle layer) /Hi (thickness of upper layer), equal to 

 .85, can be achieved with this velocity distribution. In other words, as 

 much as 68 feet of till could be masked from the seismic record by 80 feet 

 of overlying sand. 



Thus, the presence of a "blind zone" could explain the absence of a 

 discrete velocity for the clay sequence beneath Plum Island. It appears 

 unusual that data were gathered only from locations where this "blind zone" 

 could mask a higher clay velocity, but it is a possibility due to the 

 thick blanket of uniform sand that almost always covers the clays. 



Drill-hole control linked with close interpretation can often help 

 to reduce errors. At locations where two interpretations are possible 

 due to a possible "blind zone", only an additional drill hole can answer 

 the question. Interpretation of bedrock configuration remains the same 

 even with a "blind zone". Only the relative depth of the third-velocity 

 surface changes, not its configuration. 



A recent seismic reflection study (Duane 1969) , performed by CERC to 

 find available sand for beach nourishment also showed the offshore bedrock 



34 



