of the length of the records, considerable table space is required to lay out 

 two or more records side-by-side to make comparisons. At CERC, specially built 

 tables approximately 30 feet long are used for laying out the profiles for 

 analysis and plotting. Also, to reduce the space problem and to provide a 

 working copy of the records, a Xerox 1860 machine is sometimes used to reduce 

 the profiles onto roll paper. The degree of reduction varies with each survey 

 and with the type of profile, but a 50-percent reduction is generally satis- 

 factory. The vertical detail may become too small or the fine character of 

 the stratigraphy lost if too much reduction is made. 



A difficulty arises when the survey tracklines are laid out in a box-grid 

 configuration — the parallel lines appear with opposing ship headings; therefore, 

 the subbottom features on the profiles are reversed. This condition makes 

 comparative interpretations between lines difficult. Until recently, this was 

 an unavoidable problem for the individual who interpreted the records, but now 

 the latest recorders have the facility to reverse the record in real time so the 

 recorder operator can insure all parallel line records print out in the same 

 direction. Individuals involved in profile interpretation may not have a dif- 

 ferent preference for profile orientation, but by convention east should be the 

 right for east-west tracklines, and south to the right for north-south tracklines. 



Additional problems may be encountered when operating seismic systems in 

 relatively shallow water, as is often the case for nearshore surveys. If water 

 depths are less than the depths of subbottom penetration, the multiple reflec- 

 tions from the sea floor and strong subbottom reflectors will be superimposed 

 on the profile and may partially mask the real data return. Masking by multiples 

 is not as great a problem for boomer systems as for other systems, but even so 

 the minimum water depth survey limits are about 15 feet (5 meters) . 



As part of the interpretation process, the important geological features and 

 acoustical reflectors are marked and identified on each of the profiles. These 

 are then placed on interpreted profiles or cross-sectional plots of the most 

 important seismic lines; information from sediment samples may also be included. 

 Along with the cross-sectional plots, the geological information is also dis- 

 played on different types of maps. The maps usually include bathymetric maps 

 containing detailed morphology of the study area by means of depth contours. 

 Isopach maps showing the thickness of unconsolidated sand and gravel resources, 

 as well as any fine-grained overburden present, can also be made from the seismic 

 data. Other maps of value are structure contour maps of important geological 

 reflectors or surfaces beneath isopached units; surficial sediment distribution 

 maps showing grain-size distributions and sedimentary contacts; and geological 

 feature maps that display a variety of information, such as buried stream valleys, 

 fault traces, pipeline routes, bedrock outcrops, sand wave and shoal areas, and 

 areas of the seabed where there is evidence of erosion .or deposition (Ploessel, 

 1978). 



IV. SIDE- SCAN SONAR EQUIPMENT 



1. Background and Principles of Operation . 



The development and the commercial availability of side-scan sonar equipment 

 are a major breakthrough for marine geology. The records or sonographs are some- 

 what analogous to a continuous series of oblique aerial photos. The principles of 

 how side-scan sonar equipment works are similar to those of echo sounders. For 

 echo sounders, as well as other acoustical seismic devices, the acoustical energy 



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