profiling systems, energy sources, detectors, and recorders is shown in 

 Table B-3. Some of these systems shown are high-resolution, shallow- 

 penetration profilers that are applicable to surveys for seafloor structures. 



A subbottom profiling system consists of four components: energy 

 source, detector system, amplifier and filter, and recorder. There are many 

 types of energy sources, the common names of which include the following: 

 transducers, sparkers, boomers, line sources, pneumatic sound sources, and 

 thermodynamic sources. The actual sound is created by electromechanical, 

 electric spark, pneumatic, gas explosion or other means. The detector 

 system is usually an array of hydrophones that complement the type of 

 sound source being used. 



The objective of a subbottom system for use in surveying for 

 ocean-floor structures is to be able to delineate structural characteristics 

 of the seafloor which may influence the stability of the structure. These 

 characteristics may include a very thin layer (<6 inches) of weak sediments, 

 a similar layer which could be the path for a failure plane, tension cracks, 

 discontinuities, and faults. To detect these features, it is necessary to utilize 

 high resolution profilers. 



The limitations of the profiling technique in general are significant, 

 although efforts are being made to minimize and understand these effects. 

 These limitations include the inability to accurately establish the velocity 

 of sound through the various sediment layers in a given profile, to differen- 

 tiate extraneous signals and noise, and to accurately interpret records that 

 include side echoes and multiple reflections. Therefore further research in 

 subbottom profiling systems is required to develop one suitable for site 

 surveying for seafloor structures. 



These subbottom profilers, various bottom profilers, and side-scan 

 devices are summarized in Table B-4. 



Bottom Mapping Systems 



There are basically two ways to develop topographic charts of the 

 seafloor. The first method is by the use of a bottom profiler such as a 

 Precision Depth Recorder (PDR), Precision Graphic Recorder (PGR), or 

 similar depth-sounding system together with a surface positioning system. 

 This technique is based on the velocity of sound through the water column. 

 Since the velocity of sound is a function of the salinity, temperature, and 

 pressure of the water, a correction for these factors is required. The record- 

 ing of the depth and respective positions during the survey on magnetic tape 

 in computer-processable format, as is being done by NCEL, facilitates data 

 reduction and application of corrections. Maps developed by the use of these 



40 



