Vincent J. Murphy, of Weston Geophysical Engineers, Inc., (personal 

 communication) mentions similar phenomena in other areas. A condition 

 known as "velocity inversion", where a lower-velocity layer is sandwiched 

 between two higher-velocity layers, is the best description of the causative 

 situation for the steps in the time-distance plot shown in Figure 18. The 

 velocity inversion causes the layer with the lower velocity to be hidden. 

 In addition, an error in computing depth to the underlying layers results. 

 The surface wave reaches a limited number of phones in regular succession, 

 and then attenuates, causing succeeding phones to await the arrival of the 

 wave which must pass through the deeper sands and their overlying peat 

 layer . 



Time-distance plots for the remainder of the study area show this step- 

 like arrangement appearing in other localities where extensive peat deposits 

 could be present, especially interior depressions in the barrier islands 

 which now contain brackish or fresh water marshes. Figure 20 shows a 

 time-distance plot and its associated cross section near the Stage Island 

 Pond on southern Plum Island. Peat deposits apparently are associated 

 with the brackish pond and swamp since early stages of the barrier. It 

 is likely that barrier sands now overlie a predune marsh associated with 

 a lower sea level. This is an example of how seismic profiles and nearby 

 drill-hole logs complement each other. 



Plum Island seismic profile line PI-17 (transverse profile. Figure 20) 

 is tied to seismic profiles that run parallel to the beach. This arrange- 

 ment of profiles is desirable wherever possible, because tying three geo- 

 phone arrays together at a common shotpoint with a right-angle relationship 

 between two of them provides an excellent check on the continuity of seismic 

 returns. In this example. Plum Island lines PI-17, PI-18 and PI-19 are 

 tied to a common shotpoint on the beach. Till was calculated at a depth 

 of 90 feet on profiles run parallel to the ocean, but was found between 75 

 and 80 feet on a transverse profile. The time-distance plot also shows 

 the characteristic time steps due to the buried peat. 



Glaciomarine clay deposits are extensive beneath Plum Island. Their 

 presence was known before this study, in part from a study (McCormick, 

 1968; Mclntire and Morgan, 1963) of previous work in the surrounding areas, 

 and from the occasional outcrop of similar clays in the Ipswich quadrangle 

 (Sammel, 1963). SjSgren and Wager (1969), Swedish geologists, discovered 

 a velocity contrast between thick clay sequences and overlying wet sand 

 sequences during a preliminary engineering study for foundation construc- 

 tion on fjord and river deposits in northern Sweden. Although velocity 

 contrast was expected between glaciomarine clay and wet sediments in this 

 study area, none appeared, even in locations where impressive clay thick- 

 nesses underlie estuarine and barrier sands. For example. Figures 21 and 

 22 show one of the seismic time-distance curves and the cross section from 

 the marsh-capped Middle Ground in the Parker River. Sand deposits greater 

 than 60 feet thick overlie glaciomarine clays that extend to at least 100 

 feet, the point at which drilling ceased. The time-distance plots show no 



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