in the field. Figiire 9 shows the recorder with its Polaroid camera in 

 position for recording. Figure 10 shows the geophone attached to its 

 cable takeout. Phones are always planted in holes below the vegetation 

 or loose soil layer. Figure 11 is a typical time record as shown on the 

 type 47 film. Time breaks are usually sharp due to the excellent shock- 

 wave transmission in water-saturated sediment . 



The Geospace refraction system used yields 12 recording traces and 

 one time break. The recorder provides 100 hertz timing lines (10 milli- 

 seconds) on the record. Recording time of the instrument can be selected 

 as .2, ,3, or .4 second. These times allow bedrock deeper than 500 feet 

 to be located, assuming a two- layer case and speeds of 5,000 and 15,000 

 feet per second for the upper and lower layers, respectively. 



Lightweight Geospace geophones with a maximiom sensitivity of 14 hertz 

 were attached to the takeout cable at 30-foot intervals. The use of slip- 

 on type connectors required preventing sand grains from being stuck between 

 the connector and the takeout on the wire. Whenever lines were run on 

 dry sand or salt marsh, it was necessary to dig the phones into more com- 

 pact sand or peat. On the low-tide terrace, or beach face of the barrier 

 beaches, and on the intertidal bars and tidal deltas, the phones could 

 simply be pressed into the sand, and with this efficiency as much as 1 

 mile of beach can be profiled in 1 day using 30-foot phone spacings. 



Figure 12 is a diagram for a typical seismic refraction line layout 

 employed in this study. Two 12 -phone takeout cables were laid down with 

 a two-phone overlap. By convention, phones are numbered away from the 

 center of the spread. A shot is made at the shotpoint with phones on 

 section A. Then the phones are moved to section B and two more shots are 

 made. This yields a total of four records, two representing sediment 

 returns with a shot near the "12-phone" end and two representing bedrock 

 returns with a shot over 330 feet from the "one" end. When plotted, two 

 times are common to each pair of records; this duplication allows for 

 corrections due to loosening of the sediments surrounding the shotpoint . 



Anq)lifier potentiometers are adjusted on the recorder depending on the 

 gain setting required to eliminate background noise. Background noise 

 includes vehicles, wind, pedestrian traffic, and the surf. A heavy surf 

 can seriously affect coastal seismic operations. This gain setting varied 

 with the proximity of the phones to the shot hole. The GT-2A system has 

 an individual gain control for each channel; phones near the shot can have 

 decreased gain on their amplifiers, and more distant phones can have in- 

 creased gain on their amplifiers. 



Problems were encountered in the intertidal area where the insulated 

 parts of the phone takeouts or connectors touched the wet sand. A phenom- 

 enon known as "shot feed" almost obliterated most time traces where several 

 connectors touched the wet sand. This is probably caused by the electric 

 blaster current (90 VDC) being conducted through the salt water in the sand 



14 



