This diagram shows three techniques of 

 studying the sea floor: 1. echo sounding 

 (ship at left) determines the depth of the 

 bottom by timing ultrasonic pulses echoed off 

 the sea floor; 2. seismic surveying reveals 

 the kinds of rocks composing the sea floor 

 as sound Vi/aves from an underwater explosion 

 are refracted along the bottom and their 

 travel times are recorded by a second ship 

 several miles away; 3. echo ranging gives 

 a general plan of sea floor features by (top) 

 "sweeping out" large areas with sound pulses. 



The existence of currents on the deep sea 

 floor is confirmed by underwater photographs. 

 This one shows ripple marks, ten inches 

 between crests, in sand on a seamount 

 at a depth of 1700 fathoms. 



its geological structures. For the purpose of charting the ocean 

 depths, sounding with lead and line has given way to precision 

 echo sounding, which traces on a moving sheet of paper a continuous 

 profile of the sea floor as the ship moves above it. An echo sounder 

 performs its task by sending to the sea bottom a succession of 

 ultrasonic signals, or "pings," which bounce oflF the sea floor and 

 return to the ship as an echo. The time interval between trans- 

 mitting each ping and receiving its echo gives a measure of the 

 depth, since the velocity of sound in sea water is known. Modern 

 echo sounders can make a record of the depth once every second 

 and to an accuracy of one fathom in three thousand. 



In recent years we have found another use for sound echoes. It 

 is called echo ranging. Instead of directing a series of pings straight 

 down to the ocean floor, we direct them nearly horizontally in a 

 narrow beam so they strike the sea bed obliquely. Echoes from 

 rock outcrops and even shipwrecks are recorded and form a picture 

 similar to that obtained by radar on the surface. With echo ranging 

 we can sweep out large areas of the sea floor and quickly obtain a 

 general plan of its features. 



To get a close look at the features of the bottom we have to 

 abandon sound echoes and turn to bathyscaphes, undersea photog- 

 raphy, or television. So far, it is from underwater photography 

 that we have learned the most. 



To collect samples of the rocks and sediments covering the sea 



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