An experienced crew can drill up to 50 feet per day. The early ap- 

 pearance of clay in a hole allows speedier progress. A crew can occasion- 

 ally drill and sample a 100-foot hole in one long day. But usually, it 

 takes 2 1/2 to 3 days to do a good job on a hole. This includes setup and 

 takedown time for the rig and transportation from the previous site. 



Estuarine (barge -mounted) operations are slower, but unlimited washing 

 water (over the side of the barge) is an obvious advantage. Anchoring a 

 rig against tidal currents is not too difficult. Drilling is then done 

 through a well in the center of a barge with the casing extending up to 

 deck level. This casing not only facilitates re-entry but also helps to 

 anchor the barge. A site on an intertidal bar is desirable because drill- 

 ing can be done at low tide while the drilling base is grounded. 



2. Seismic Refraction Study 



Seismic refraction principles were used in earthquake seismology 

 before their application in shallow refraction surveys. The principles 

 of refraction remain the same whether the layers being studied are thou- 

 sands of feet thick or merely tens of feet thick. Refraction surveys en- 

 able the scientist to gather data on the geometry and composition of sub- 

 surface materials. The simplest case is a two-layer problem consisting 

 of a high-speed layer overlain by a low-speed layer, such as crystalline 

 bedrock overlain by glacial material. 



In the simple two-layer case, a seismic wave is generated at the sur- 

 face and travels away from its propagation point in all directions at a 

 velocity unique to the upper layer. On crossing the interface with the 

 lower layer, the wave moves at a higher velocity and overtakes the surface 

 wave. Geophones placed in a linear fashion away from the point of energy 

 propagation measure the arrival time of the direct (surface) and indirect 

 (subsurface) waves. A time-distance plot of these arrival times will 

 determine when the indirect wave overtakes the direct wave, allowing 

 determination of the depth to the subsurface layer. 



The field techniques of the shallow-refraction phase are -similar to 

 those of the drilling phase. In both phases, equipment must be transport- 

 ed to sites not easily accessible. Dependable four-wheel drive vehicles 

 and rugged boats are needed. Seismic study requires the use of bulky, 

 but delicate, rolls of wire and explosives; both can prevent transportation 

 problems. 



The seismic work was done with the Geospace Model GT-2A portable re- 

 fraction system. This unit works well near the shore because it is weather- 

 proof when the instrument cover is closed, and is rugged and dependable 

 over a broad range of temperatures and humidities. All time records are 

 taken on type 47 Polaroid film, allowing rapid review of seismic results 



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