figure presentations. Both types of seismic reflection and side-scan sonar 

 equipment are particularly useful in conducting surveys to locate and quantify 

 sand and gravel resources, but they can also be used for many other purposes in 

 any of several subaqueous environments. 



II. APPLICATIONS OF SEISMIC REFLECTION AND SIDE-SCAN SONAR EQUIPMENT 



CERC experience has shown that the major cost of any survey is leasing or 

 purchasing a survey vessel and a precision navigation system. Therefore, it is 

 cost effective to simultaneously use as many pieces of acoustical survey equip- 

 ment as possible to maximize the geological and geophysical data available for 

 interpretation. Echo sounders, side-scan sonar, and the various seismic sub- 

 bottom profiling systems each provide slightly different but complementary data. 

 When interpreted together these data yield an integrated, more meaningful and 

 accurate picture of the geophysical and geological character of a survey site 

 than if fewer data were used (Randall, 1980) . 



When deciding which types of survey equipment to use, it is important dur- 

 ing the project planning phase to identify exactly what the study objectives are, 

 and what features of the sea floor and subbottom are to be emphasized. As a 

 general rule with seismic reflection and side-scan sonar equipment, good reso- 

 lution of geological details is incompatible with deep penetration on seismic 

 profiles, or wide lateral coverage with side-scan sonar. Thus, it is important 

 to use as many pieces of equipment with overlapping capabilities as can possibly 

 be operated from the survey vessel or are affordable. It is just as important 

 to have highly trained and experienced personnel involved in all phases of the 

 survey to insure proper interpretation of the geophysical data. Since the raw 

 information on geophysical records does not often yield obvious conclusions, 

 and therefore several interpretations are possible, highly trained and experi- 

 enced personnel skilled in record interpretation are necessary to accurately 

 reduce the data and derive synoptic descriptions of the survey area. 



Navigation for Accurate Position Control. 



The use of a navigational system that is capable of accurately determining 

 the horizontal position of bottom and subbottom features recorded is important 

 for any geophysical survey. For most nearshore surveys, the various electronic 

 microwave range-range systems have been used most successfully. They are rela- 

 tively trouble free, easy to operate, and are accurate to about 10 feet (3 

 meters). However, they are limited to line-of-sight range of about 100 miles 

 (160 kilometers) . LORAN-C is another system of navigation that is widely avail- 

 able and may be used; however, its accuracy is on the order of 600 to 1,500 feet 

 (183 to 457 meters) depending on the geographical location. LORAN-C should be 

 used only in broad reconnaissance surveys or when microwave systems are not 

 available. 



With any geophysical system, event marks should be put on the records at 

 timed intervals corresponding to map longitude and latitude positions. Time 

 spacing for the event marks will vary depending on the nature of the survey. 

 For very accurate surveys with many geological features, event marks should be 

 about 2 to 5 minutes apart; for straight tracklines, in relatively featureless 

 areas, longer time intervals may be used. It is also important to keep in mind 

 that the navigational system and the event marks on the records note the posi- 

 tion of the vor-;r,el and not the position of seismic and side-scan equipment 



