with transmitted signals. Additionally, as indicated by Bigelow 42 and by 

 SEISCOR, 43 instrument errors, signal propagation errors, and operator errors 

 also affect the repeatability of readings. In general, most of the indicated 

 electronic systems are not satisfactory for engineering surveys. However, 

 the systems utilizing land-based triangulation survey techniques may be 

 satisfactory for structures of minor importance. These systems are limited 

 to line-of-sight areas. 



The use of a satellite system appears to be very promising for 

 navigation in open ocean areas. It can be used for navigating to some pre- 

 selected general site, which can then be precisely surveyed utilizing a locally 

 more accurate and relative means of positioning. 



The use of bottom acoustic systems for positioning, such as those 

 offered by Alpine Geophysical, Edo Western, and Bendix, may be sufficiently 

 accurate for the requirements of many structures. These systems coupled 

 with a dynamic ship-positioning system, such as is installed on the A. C. 

 Electronics vessel, the Swan, may fulfill the survey positioning requirements 

 of many seafloor projects. It is believed, however, that the best accuracies 

 for positions would be obtained by using submersibles together with these 

 acoustic systems which are placed at known absolute coordinates with respect 

 to the reference system used by the U.S. Geodetic Survey. 



The use of lasers for precise locations has been attempted by several 

 organizations. As indicated by Thomas, 44 experiments are being conducted 

 with lasers to replace the light sources in Geodimeters. For underwater 

 applications, TRW Systems Group 45 has developed a laser surveying system 

 for diver operations at water depths to 60 feet. The system consists of a unit 

 analogous'to a surveying transit, a stadia rod, and a tape. The design objective 

 accuracies for distance and angles of the system are on the order of ±0.5 foot 

 and ± 1 minutes, respectively, for locating corners of a rectangle 30 x 90 feet. 

 Lasers also have been used on various terrestrial construction projects. The 

 major limitation of laser techniques is that they are applicable only to line-of- 

 sight surveys. In underwater applications of lasers, the light energy is attenuated 

 by the many particles in the water column such as plankton and organic debris. 



The visual survey systems include the use of transits or theodolites in 

 triangulation techniques. The base line for the triangulation survey is usually 

 on land, but part of this line could span bodies of water. Since these techniques 

 are routinely used in terrestrial surveys, they will not be discussed here. 



Subbottom Profilers 



This discussion of subbottom profilers is based principally on the 

 survey made by Schlank 46 on currently available profiling equipment. A 

 summary of the manufacturers of subbottom equipment and the various 



39 



