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The topographic criterion is especially easy to apply to differentiate rock 

 or stony bottom from mud or sand bottom. As a general rule rocky bottom ap- 

 pears to show a rough trace on the echogram whereas sand and mud are smooth. 

 Care must be taken mentally to "filter out" the effect of water waves from the 

 trace. Where there are strong currents, sand bottom is sometimes formed into 

 a series of sand waves or "giant ripples" . Such waves are found, for example, 

 in the North Sea near England and along the Atlantic Shelf of the U.S. Here they 

 have an asymetrical wave form with a gentle slope facing toward the current and 

 a steep slope facing down current so that they are readily distinguished from the 

 irregular rough bottom typical of rock. The correlation of roughness of the 

 trace with stony bottom was nicely demonstrated to the writer on a cruise to the 

 Bering Sea where minor roughness characterized the trace. Sampling and 

 bottom photography showed the areas of rough trace consistently to be correlated 

 with stony bottom. 



In making a study of the bottom off Point Loma, California, two years 

 ago the writer noted that echo extension characterized the rocky areas. The 

 reason for this is not clear but it is suspected that it is due to irregular bottom 

 with rock facets normal to the incident sound beam and well out on the periphery 

 of the sound cone which reflect sound energy back to transducer at a sufficiently 

 high level to be recorded. If the bottom had been smooth the sound energy would 

 have been reflected away from the source rather than back to it. 



The bottom trace on fathograms frequently shows layering or some other 

 type of structure. This is most commonly related to topography which produces 

 side echoes but at other times is related to stratification beneath the bottom. 

 Good examples of this are seen in Murray's (1947) fathograms from the basins of 

 the Gulf of Maine, Hough's from Lake Michigan, from the shelf in the region of 

 the Congo River and from Lake Mead. Penetration of the bottom by sound at the 

 high frequencies used in most echo sounders seems to indicate the bottom is a 

 soft, non-compacted mud because it is likely that compact mud, sand, and rock 

 would permit little penetration. 



Regarding the strength of the echo off the bottom, it is known that rock, 

 sand, and mud reflect sound to different degrees. Although there is considera- 

 ble variation, sand is generally the best reflector of sound, rock is intermediate, 

 and mud is the poorest reflector of sound. Thus, the echo sound appears to of- 

 fer a useful method of determining bottom type by the criterion of echo strength 

 because the numerous pings tend to average out any variation in the echo strength 

 from a particular bottom. However, although recording papers show some in- 

 crease in darkening with echo strength, this is slight, so that the technique has 

 not proved very useful. Another technique of determining bottom type by echo 

 strength is to turn the gain high and then to count the number of bottom reflec- 

 tions. A strongly reflecting bottom commonly shows one or two more bottom re- 

 flections than others. 



VISUAL METHODS OF EXPLORING THE SEA FLOOR 



It is reported that 80% of our information about our environment is de- 

 rived through the sense of sight. It is also said that more is known about the 

 moon's earth-fixed face than is known about the ocean floor which is much more 

 accessible to all methods of exploration except that of "seeing". This points up 

 the fact that the sea floor will always remain a mysterious environment so long 

 as it cannot be seen. 



As a substance, water is a good medium of transmission of light, even 

 though it is a poor medium as compared to air. In the purest sea water one can 

 see as far as 200 feet. Fortunately the bottom water of the open sea almost 



