The thickness of the unconsolidated sediment or overhurden can he 

 determined from the acoustic reflection record. Since the speed of 

 sound generally increases with increasing depth of burial, such thick- 

 nesses are usually minimal. Since the speed of sound in unconsolidated 

 sediments is approximately that in seawater, the correct magnitude of 

 the sediment thickness is indicated when read directly from the record 

 (Moore, 1960). To calculate accurately the thickness of rock units, 

 however, the acoustic velocity for the rock type present must be known, 

 since this velocity will be considerably higher than that for the water. 

 The relationship between acoustic velocities and properties of sediment 

 and rock will be discussed in detail in a later section. 



Thus, reflection records are very effective in the planning of exca- 

 vation and dredging work and traf ficability studies. The amount of material 

 and degree or difficulty in removing it ("rippability") are the kinds of 

 engineering information that can be inferred from the record (Moore and 

 Palmer, 1967; Patterson and Meidav, 1965). Very detailed geologic pro- 

 files, useful in many marine engineering applications, can be constructed 

 from the reflection record plus borings or cores for lithologic control. 



It must be pointed out that there are problems and limitations with 

 continuous reflection profiling. Since the acoustic signal is propagated 

 omnidirectionally or in a conical pattern (-60°) from the ship, the first 

 signal received by the hydrophone may come from a point not normally 

 beneath the ship. These signals which appear as "side" or "ghost" echoes 

 are reflected from promontories off to one side, ahead, or behind the ship. 



Also, caution must be exercised in identifying multiple bottom reflec- 

 tions. These reflections, seen in the bottom profile, are generally caused 

 by the entrapment of the signal in the water column, bouncing between the 

 air and seafloor interfaces. Multiples become complex when reverberations 

 between subbottom interfaces are also recorded. 



The limitations of the system used must always be considered in 

 interpreting the record. The system's apparent efficiency, for example, 

 is a function of the fundamental frequency present in the received signal 

 and recorder scanning rate. The impedance contrast of the bottom material 

 also has an influence on the apparent efficiencv of a system. 



Other factors, less controllable, such as ship and water noise, and 

 extraneous 60 cps signals may cause spurious signals on the record. 



It is apparent from the above discussion that the interpretation of 

 a continuous seismic reflection record requires the skill and experience 

 of individuals familiar with geologic properties and the capabilities and 

 limitations of the system used. 



A certain amount of quantitative information about the nature and 

 physical properties of marine sediments and rocks can be derived from an 



