952 MISCELLANEOUS GEOPHYSICAL METHODS [Chap. 12 



in the location of shipwrecks and submarines on the bottom, the determina- 

 tion of the character of the ocean floor, and the detection of fish shoals. 

 Echo-sounding was tried at an early date on icebergs; Fessenden found at 

 the time that (on account of the irregular surface of the berg under water) 

 the ice echoes were much more feeble than the sea-bottom echoes. This 

 may possibly be overcome by the use of high-frequency sounds and hori- 

 zontally directed beams. In this manner ranges of at least several hundred 

 yards may be obtained. It has been reported^^^ that greater ranges (up 

 to three miles) are obtainable by listening to the bursts which apparently 

 develop in the iceberg from its cracking under water. 



Marine echo-sounding methods involve the principle of distance deter- 

 mination by measuring (1) the direction of the return ray, and (2) the 

 time interval that elapses between the initiation of a sound impulse and 

 the arrival of the echo. The first system requires that the depth be 

 comparable with the length of the triangulation base, that is, the length 

 of the ship. Therefore; this method is appUcable only at shallow depths, 

 to about 100 fathoms. As originally applied, this method utilized the 

 propeller noise as sound. A group of submarine detectors was mounted 

 forward on the ship and connected to a compensator, whereby the direc- 

 tion of the incoming sound could be determined. If <p is the angle which 

 the sound, reflected from the sea bottom, makes with the horizontal, and 

 if 2a is the distance of the detector group from the propellers, the depth 

 to bottom is given by rf = a tan (p. This method is not particularly fast 

 nor is it very accurate. 



In all other echo-sounding procedures, the time interval that elapses 

 between the initiation of a sound impulse and the arrival of the echo is 

 measured. The reflection of a sound impulse generated by striking a bell 

 may be perceived by the human ear. However, the intensity of puch an 

 impulse would not be suflBcient to actuate an automatic indicating device, 

 nor would this timing method be accurate enough. Large intensities may 

 be generated by crowding the available energy into a short space of time, 

 for example, by the detonation of an explosive, or by a condenser discharge 

 into an electromagnetic or magnetostriction oscillator. A wide frequency 

 range has been utilized. High frequencies, although subject to great 

 absorption, offer definite advantages in regard to directional selectivity. 

 In shallow water they are the only ones applicable. Since, for a depth 

 accuracy of five feet, a time interval of 2 milliseconds must be measured, 

 the length of the initial impulse cannot be more than 1/10 of this interval. 

 Inasmuch as, for moderately damped transmitters, the impulse dies out 

 after about 10 oscillations,*'* frequencies ranging from 10 to 50 kc. are 



"2H. T. Barnes, Nature, 124, 337 (Aug. 31, 1929). 



"3 H. Hecht and F. A. Fischer, Handb. Exp. Phys., 17(2), 433-439 (1934). 



