950 MISCELLANEOUS GEOPHYSICAL METHODS [Chap. 12 



water and Vg that in air, the formula gives a factor of 0.42 with which the 

 time difference in seconds is multiplied to obtain the distance in kilometers. 

 Capable of greater range is distance determination by the use of radio and 

 sound signals transmitted simultaneously from shore stations and light 

 vessels. In that case (by application of formula [12-20]) the distance in 

 kilometers is approximately equal to one and one-half times the observed 

 time difference in seconds. By receiving signals from two radio and under- 

 water sound transmitters of known position, the ship's position may be 

 found without difficulty. Conversely, a ship firing a depth charge and 

 transmitting a radio signal at the same time to two shore stations equipped 

 with hydrophones may be given its position by radio. This is of con- 

 siderable help to navigation in fog; a ship forty miles away from the 

 transmitters may thus locate itself within about 2000 feet. 



A similar procedure is applied in the "RAR" (radio-acoustic-ranging) 

 system of the U. S. Coast and Geodetic Survey to determine the position 

 of echo-sounding vessels. From the latter, a depth bomb is fired elec- 

 trically at the desired location, and the sound wave is picked up by two 

 Sono-radio-buoys anchored at known positions. The buoys are equipped 

 with short-wave transmitters which radio the instant of reception back 

 to the surveying vessel where the radio signals are recorded on a chrono- 

 graph. For this purpose, a hydrophone is suspended half way down the 

 anchor line of the buoy. The phone is connected to a tuned three-stage 

 transformer-coupled amphfier which, through a gas tube, trips the grid of 

 a 4 megacycle (2 to 5 watt) transmitter.^^^ This arrangement removes 

 the carrier between signals and makes for a longer life of the transmitter 

 batteries. The maximum hydrophone range to trip the transmitter is 80 

 to 100 km; the range of the radio transmitter in terms of signal required 

 to work the recording chronometer is about twice as great. The accuracy 

 of the RAR system depends, naturally, on how well the velocity of the 

 sound in sea water is known; and this in turn changes with refractions, 

 reflections, and variations in temperature and salinity (see page 944). For 

 a given area, velocities can be determined by RAR observations with 

 known vessel positions obtained from geodetic triangulation or astronomic 

 measurements. 



Marine sound-ranging methods are used for the location of mine ex- 

 plosions and depth charges, and for determining the range of a ship's 

 shell fire. The hydrophones are placed along a base line about twelve 

 miles long and are connected to a shore recorder similar to the type used 

 in atmospheric sound-ranging. If the secrecy of the installation is of no 



"1 A. M. Vincent, U. S. Coast and Geod. Surv., Field Eng. Bull. No. 11, 73, 

 (Dec, 1937). A slightly different circuit is described by H. G. Dorsey, op. cit., 

 p. 99. 



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