on the sonograph. In addition, the height of objects above the sea floor can 

 be calculated from side-scan records if the length of the shadow is known, 

 along with the height of the towfish above the seabed (D'Olier, 1979) . 



An important aspect of side-scan sonar interpretations is being able to 

 distinguish between "real" or "meaningful" reflection returns and the inter- 

 ference from various sources. There are three major sources of interference 

 that may be expressed on the records: 



(a) Continuous intevfevence results from operating other seismic 

 equipment simultaneously with the side-scan. The benefits of running 

 several instruments at the same time during surveys far outweigh any 

 interference problems, and often the problems can be minimized by 

 towing the instruments farther apart and making adjustments to fine 

 tune the recorders. 



(b) Discontinuous interference is usually very obvious v/hen it 

 appears and is usually associated with wake waves from passing ships. " 



(c) A third type is caused by schools of fish or dense concen- 

 trations of suspended mud in the water column. The acoustical sig- 

 nal is partially dispersed or reflected before it reaches the sea 

 floor. This results in light areas across all the ranges; whereas 

 dark areas appear in the depth profile. 



Whenever interference artifacts are recorded during a survey, the technician 

 operating the recorder should note their occurrence on the records. This 

 will insure that the person interpreting the data later is not confused. 



V. SUMMARY 



In the past three decades, great technological advances have been made in 

 the development of continuous seismic reflection and side-scan sonar equipment 

 for use in geological and engineering studies of marine, estuarine, and lake 

 environments. Various types of acoustical equipment are available that vary 

 in frequency and power and must be carefully matched to the objectives of any 

 survey. For the past two decades CERC has conducted 18 surveys and collected 

 about 10,000 track miles (16 093 kilometers) of seismic profile data over the 

 Atlantic, Gulf of Mexico, and Pacific Continental Shelf areas, as well as in 

 Lake Erie and Lake Michigan to locate and inventory resources of sand and gravel 

 and describe the geological character of each region. Experience from these 

 surveys regarding use of geophysical equipment is summarized in this report. 



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