18. MICROTOPOGRAPHY 



A. S. Laughton 



1. Introduction 



The topographic forms of the sea floor that have so far been described are 

 ah large features dehneated by echo-sounding survey. It will have been ap- 

 parent, however, that there is a lower limit to the size of feature that can be 

 resolved by echo-sounding because of the limitations of wave-length, beam 

 angle and depth of water. Common wave-lengths are 3 cm (50 kc/s) in shallow 

 water (i.e. less than 200 m) and 15 cm (10 kc/s) in deep water. Objects with 

 dimensions less than several wave-lengths cannot be resolved. A more severe 

 limitation is, however, imposed by the finite width of the beam which allows 

 a point reflector to appear as a hyperbolic trace on the record as the ship 

 passes over it, the width of the hyperbola being determined by the beam angle. 

 Echo-sounders usually have an effective beam width of about 20°. In 200 m 

 the beam, therefore, covers an area on the sea floor 70 m wide and, in 5000 m, 

 an area nearly 1800 m wide. Although features somewhat smaller than these 

 can often be resolved by studying the form of the echo, it is clear that the beam 

 angle imposes a limitation on the lateral resolution. Vertical resolution, depend- 

 ing on time measurements, is generally much greater than this, being limited 

 ultimately by the existence of side echoes, and often by the vertical movement 

 of the ship on the surface. 



In order to study features that cannot be resolved by echo-sounding, it is 

 necessary to use either direct observation from a diving machine, such as a 

 bathyscaph, remote-controlled underwater television or automatic underwater 

 photography. The upper end of the size scale of features observed with these 

 techniques is limited chiefly by the scattering and attenuation of light in the 

 sea. In practice, in the clearest oceanic waters, the maximum range for the 

 visibility of objects is about 50 m, whereas in coastal areas it may be as little 

 as 20 cm. The lower limit depends on how close the observer is to the bottom 

 and the resolution of the film. 



The term microtopography is used here to describe those features of the sea 

 floor that can be observed by visual means in the scale range of 50 m to 1 mm, 

 which includes rock outcrops, sand waves and ripj)les, the benthic community 

 and disturbances of the sediment by faunal activity. It is clear therefore, 

 especially in deep water, that there is a gap in the size spectrum of observable 

 features where they are too small to be detected by conventional echo-sounding 

 but too large to be observed visually. 



The techniques of bathyscaph observation, underwater television and 

 photography are described in Volume 2, Chapter 23, and Volume 3, Chapter 19. 

 On account of the limited number of observations of the sea floor made by the 

 first two methods, most of the information discussed below has been derived 

 from a study of underwater photographs obtained from published papers and 

 from unpublished collections of photographs at the National Institute of 



[MS received August, 1960] 437 



