OCEAN ENGINEERING SIGNIFICANCE OF MARINE SEISMIC REFLECTION PROFILING 

 TECHNOLOGY 



Technical Note N-1157 



YF 38.535.001.01.002 



by 



E. F. Pawlowicz 



ABSTRACT 



Seismic Reflection Profiling (SRP) data of value to the seafloor 

 engineer include: (1) sediment ("soil") thickness, (2) sediment struc- 

 ture, (3) the slope of the bedrock beneath the sediment laver, (4) bed- 

 rock topography, if sufficient tracklines are run to permit contouring, 

 (5) bedrock structure, if penetration permits, and (6) acoustical data 

 from which certain physical, engineering, and load bearing properties of 

 an area can be approximated. 



Qualitative interpretations of acoustical data are made as a matter 

 of course by the experienced analyzer of SRP records and include such 

 parameters as: (1) echo intensity from the seafloor and subbottom inter- 

 faces, indicating hard (high reflectivity) and soft (low reflectivity) 

 layers, (2) penetrability of the seafloor (in unconsolidated sediments) 

 is generally inversely proportional to grain size, (3) point return, or 

 discrete hyperbolic echo returns, indicative of large irregularities com- 

 pared to the sound freauency recorded, such as boulder beds, or a weathered 

 bedrock surface. 



Quantitative interpretations of seismic reflection data include the 

 measurement of compressional wave velocities by underwav wide angle reflec- 

 tion techniques. Compressional sound velocities, however, do not vary 

 significantly within saturated marine sediments, so do not give unambi- 

 guous solutions to sediment types. Since the shear strength of the sedi- 

 ments is related to the shear wave velocities, its measurement would be 

 of immense value to the seafloor engineer. If the compressional and shear 

 wave velocities are known, the dynamic elastic properties of the material, 

 such as Poisson's ratio, can be calculated. Shear waves, though trans- 

 mitted by saturated marine sediments, are not propagated through the water 

 column and so are unavailable to surface, or even deep-towed acoustical 

 surveys . 



When both the compressional wave velocity and the reflectivity coeffi- 

 cient of a stratum are known, the bulk density can he calculated. The 



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