being towed astern. The distance from the navigational antenna to the towfish 

 should be subtracted when plotting positions of features from the records onto 

 maps. 



III. CONTINUOUS SEISMIC REFLECTION PROFILING EQUIPMENT 



1. Principles of Seismic Reflection Operation . 



Seismic reflection systems all function similarly. Each has a sound source 

 or transducer in the water that generates acoustical signals at regular power 

 and frequency outputs, as well as a receiver in the water at a fixed distance 

 from the source that picks up acoustical pulses reflected from the sea floor 

 and from geological boundaries in the subbottom (Fig. 1). The reflected energy 

 picked up by the receiver is then transmitted to a recorder on the survey vessel 

 where it is graphically displayed to yield continuous seismic profiles. The 

 profiles are analogous to geological cross sections. The acoustical reflections 

 are printed out in relation to the traveltime needed for the transmitted signal 

 to be reflected and returned to the receiver. However, an approximate vertical 

 scale can be constructed by approximating sound velocities in water, and in the 

 most common sediments encountered. For seawater, a value of 4,800 feet (1463 

 meters) per second is commonly used, while for unconsolidated sandy materials an 

 average figure is 5,450 feet (1661 meters) per second. Sound velocities in 

 sedimentary rocks tend to increase with the depth of burial below the sea 

 floor, as well as with the rock age, and past influences such as glacial load- 

 ing or subaerial exposure at times of lower sea level. Velocity is generally 

 related to the relative density of the sediments, but other important factors 

 affecting velocity are sediment porosity; the degree of compaction; grain 



Figure 1. Schematic showing the principles of continuous seismic 

 reflection profiling (from McClelland Engineers, Inc., 

 1981). 



