3 km but with less resolution than the transducer system. ^or penetrations 

 greater than 3 km, explosive source or other high power, low frenuency 

 systems are used. The sparker and other like systems may record only a 

 dark line at the water-sediment interface, therefore obscuring any layer- 

 ing in these upper tens of meters of surface sediments. The system selected 

 should be based on the requirements of the survey. 



The greatest advantage of seismic reflection profiling over conven- 

 tional methods is the continuity of the record. The marine construction 

 engineer is supplied with a more accurate and complete understanding of 

 the subbottom geology by these seismic profiles than he could obtain 

 using only borings and depth sounding techniques. Sediment and rock units 

 may be correlated from a few known points, eliminating the need for an 

 extensive boring or coring program. 



Borehole data may be correlated with the seismic record. Thus, a 

 survey may proceed from the borehole into unknown areas with an extra- 

 polation of the known strata. Additional borings or cores may be taken 

 in questionable regions or those requiring additional sediment analysis. 

 Sediments, bedrock, and competent strata are mapped simultaneously. For 

 foundation studies, slumping, large- and small-scale faulting and other 

 geologic hazards are located. Differential compaction of overburden, 

 sediment erosion and deposition (scour and fill) , eroded channels filled 

 with unconsolidated sediments are among the other geologic features which 

 may be delineated. Continuous seismic reflection profiling provides infor- 

 mation from which a detailed and accurate geologic map of a potential con- 

 struction site could be constructed. Additionally, the records are useful 

 in the planning of dredging, excavation, and other operations requiring 

 knowledge of the "rippability" and traf f icability of an area. 



In short, continuous reflection profiling can be a most useful tool 

 for the marine construction engineer. Significant subbottom phenomena 

 often overlooked by the use of only conventional depth soundings plus 

 borings are detected. 



DATA INTERPRETATION 



For a proper geologic and engineering interpretation of the continuous 

 reflection profile, an understanding of the factors which influence the 

 record is necessary. In this report, the interpretation of the data is 

 divided into two categories: the interpretation of the geologic setting 

 (or the qualitative interpretation) and the quantitative interpretation 

 of the acoustic signal. 



To interpret accurately the geologic setting of an area from the 

 reflection record, some knowledge of the local and regional geology and 

 the operating characteristics of the system is necessary. Conversely, 

 much information about the physical properties of the sediment can be 

 inferred from the acoustic properties (reflectivity, acoustic velocities, 

 and losses) of the bottom. 



