USE OF HIGH RESOLUTION SEISMIC REFLECTION AND 

 SIDE-SCAN SONAR EQUIPMENT FOR OFFSHORE SURVEYS 



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S. Jeffress Williams 



I . INTRODUCTION 



In earlier times, when men were mapping the depth and morphology of the 

 sea floor for navigational purposes or scientific interests, a simple line and 

 weight were used. This method was often inaccurate and time consuming in deep 

 water and areas of high current velocities. A major breakthrough came when 

 acoustical echo sounders were developed during the 1920 's. Echo sounders or 

 fathometers measure the two-way traveltime of a high frequency acoustical 

 pulse emitted from a survey vessel to the sea floor and its return to the sur- 

 face. The traveltime is then converted, using the determined sound velocity, 

 to precise water depths. During World War II, graphic paper recorders were 

 added to sounders to yield a continuous profile of the bottom relief along a 

 charted path of a survey vessel. Soon, improved sounders and recorders were 

 being widely used, proving that the sea floor regions were far more varied and 

 dynamic than had been previously supposed, and contained mountains, deep 

 trenches, and broad continental shelves, along with features of smaller relief 

 such as dunes and ridges. 



Echo sounders operate at high frequencies (most commonly 30 to 200 kilo- 

 hertz) and are effective in penetrating the water column, but in unconsolidated 

 sediments the signal is quickly absorbed and does not penetrate much of the 

 sediment. New technology following World War II produced higher power and 

 lower frequency systems that penetrate the bottom sediments and yield a pic- 

 ture of their internal structure. These systems are termed seismic vefleo- 

 tion subbottom profilers; a large variety of these have been widely used in 

 the past three decades for oceanographic research, oil and gas exploration, 

 and engineering site investigations. 



Side-scan sonar is another line of equipment using a mcfdified echo sounder 

 that was developed during World War II to detect submerged enemy submarines. 

 Side-scan sonar is an acoustical instrument that is normally towed behind a 

 vessel and emits acoustical signals to both sides. The acoustical pulses are 

 reflected from features on the sea floor and returned to the recorder to show 

 even small-scale positive and negative relief of the sea floor. The differences 

 in signal intensity are also found to be useful in identifying sea floor areas 

 of different sediment composition. 



Echo sounders, seismic reflection profilers, and side-scan sonar equipment 

 are now widely used for many survey purposes in marine, estuarine, and lake 

 environments. The geophysical equipment is vital to the exploration and geo- 

 logical analysis necessary for performing sand and gravel assessment studies; 

 however, these studies are just one part of any survey effort. In general, 

 the surveys conducted by the Coastal Engineering Research Center (CERC) are 

 divided into five phases: (a) Planning and survey design; (b) geophysical data 

 collection; (c) sea floor and subbottom sediment sampling; (d) laboratory anal- 

 ysis, data reduction, and interpretation; and (e) final report preparation and 



