Remote Sensing By Ocean Vessel 



Depth-sounders, side-scan sonar, and subbottom profilers are three major 

 ways to collect data on subaqueous environments from an ocean vessel. All 

 require the use of high resolution positioning systems. Fathometers are the 

 most common devices used for acoustic depth-sounding to conduct 

 bathymetric surveys. Side-scan sonar provides an image of the aereal distri- 

 bution of sediment, surface bedforms, and larger features such as shoals and 

 channels, and thus can be helpful in mapping directions of sediment motion. 

 Subbottom profilers and seismic techniques are used to examine the near- 

 surface stratigraphy below the seafloor. 



Bathymetric surveys are required for many studies of geomorphic 

 variability in coastal waters. Survey-quality fathometers include many devices 

 to improve accuracy, including correction for change in tide, change in draft 

 with vessel speed, and speed of sound. Even so, the maximum accuracy is 

 estimated to be + 0.2 m (Morton, Stewart, and Germano 1984); thus, such 

 errors should be considered in volume change measurements of features. 

 Survey lines are typically parallel and are run at an appropriate spacing 

 depending on the survey's purpose and the scale of features to be examined. 



Side-scan sonar permits the collection of surface characteristics data for the 

 seafloor. The resulting image of the bottom is similar to a continuous, 

 oblique aerial photograph, with lower frequency scanners providing less detail 

 but greater range than higher frequency scanners. Detailed information, 

 including the spacing and orientation of bedforms, and grain size differences 

 in seafloor sediments, can generally be distinguished on side-scan, as can 

 larger individual features. It is generally recommended that bathymetry be 

 run in conjunction with side-scan, because detailed information on the relief of 

 bottom features is valuable during the interpretation of side-scan sonographs. 

 The side-scan system is sensitive to vessel motion, making it suitable only for 

 work during calm conditions. 



The principles of subbottom seismic profiling are fundamentally the same 

 as in acoustic depth sounding. Subbottom seismics employ a lower frequency, 

 higher power signal to penetrate the seafloor. The signal is reflected from 

 interfaces between sediment layers of different acoustical impedance 

 (Figure 34). Coarse sand and gravel are often difficult to penetrate with 

 conventional subbottom profilers, resulting in poor records. New equipment 

 is helping to overcome such problems, although the data are of lower 

 resolution. Spacing and grid dimensions are usually the same as those used 

 for bathymetric and side-scan surveys, being dependent upon the nature of the 

 investigation. 



82 



Chapter 5 Investigation of Geomorphic Factors 



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