shape, orientation, and the degree of cementation; nature of grain-to-grain con- 

 tact, and moisture content (Palmer, 1967; Van Overeem, 1978). 



The amount of acoustical energy reflected from an interface depends largely 

 on the impedance (sediment density times acoustical velocity) of the materials 

 above and below the interface boundary. The greater the impedance differential, 

 the stronger the resulting reflection will be. It is for this reason that the 

 strongest reflection horizons on seismic profiles are the water sea floor bound- 

 ary, contacts between compact sands and soft muds, organic-rich sediments with 

 high concentrations of natural gas, and unconformities between recently deposited 

 sediments and underlying hard bedrock. 



2. Types of Energy Sources . 



Since continuous seismic reflection equipment first came into wide use in 

 the 1960 's, a wide variety of different types and configurations have been 

 developed. However, all the seismic devices consist of three basic components: 



(a) An energy source or transducer that emits acoustical pulses at 

 specific power and frequency levels; 



(b) one or more receivers that pick up the transmitted acoustical 

 "echoes" after they are reflected back from the seabed and subbottom; and 



(c) a recording instrument that converts the reflected acoustical 

 signals to electrical signals, which in turn are converted into a more 

 permanent record (Palmer, 1967) . 



Seismic data are normally recorded on graphic strip chart- type profiles; however, 

 the data may also be entered onto magnetic tapes suitable for computer processing 

 and enhancement following the survey. There are of course many pieces of elec- 

 tronic equipment other than these three basic components but they are specific 

 in function and a detailed discussion is beyond the scope of this report. 



All seismic reflection profiler systems produce vertical profiles of the 

 seabed and subsea floor geological character under the path of a moving survey 

 vessel. However, each system yields a slightly different record of subbottom 

 sediment penetration and degree of resolution, depending on the frequency and 

 power of the emitted signal. A summary of the primary seismic systems is included 

 in the Table. In general, higher power and lower frequency systems will penetrate 

 farther into the sea floor subbottom but will have less resolution; whereas, a 

 relatively low power and high frequency transducer will be able to resolve more 

 detail of the subbottom bedding and geological character, but depth of penetra- 

 tion will be limited. A trade-off between frequency and power is always a 

 problem; therefore, it is important to carefully match the selection of seismic 

 equipment with survey objectives and needs. Another solution is to use several 

 complementary geophysical systems simultaneously during a survey. For most 

 seismic systems the best results are obtained when the seas or the swells are 

 less than 3 feet (1 meter) . Higher wave conditions will significantly deterio- 

 rate the quality of the seismic profiles. 



a. Pinger System . The Pinger is a relatively high frequency (2 to 14 kilo- 

 hertz) , relatively low power piezoelectric transducer that is useful in bathy- 

 metric surveying and for delineating very fine detail of the sea floor and 

 subbottom to depths of about 100 feet (30 meters) in softer materials (see Fig. 

 2) . In areas where the seabed consists of firm fine-grained sand or semicon- 

 solidated coastal plain strata, the pinger signal may not have enough power 



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