486 



TECHNIQUE OF WAKE MEASUREMENTS 



Figure 3. Fathometer record of echoes from ocean surface. 



training back the transducer, tiie effect is reversed, 

 thus causing a symmetrical pattern to appear in 

 Figure 2. 



30.1.2 



Fathometer Records 



Records of a wake, indicating its thickness and 

 transverse structure, are readily obtained with a 

 fathometer carried across the wake by a survey 

 vessel. Such records may be utilized also to compute 

 the vertical transmission loss, as long as a record from 

 a standard target observable through the wake — 

 for instance, the ocean bottom or surface — is also 

 available (see Section 32.3.3). 



Early experiments were carried out with the fathom- 

 eter mounted in the orthodox maimer on a surface 

 vessel. Records of the ocean bottom are then blanked 

 out in certain cases when the survey boat enters a 

 surface ship wake. This technique suffers from several 

 disadvantages. It does not give an accurate value for 

 the depth of the wake, since the duration of the wake 

 echo is affected by the beam width, the pulse length. 



and other factors as well as by the depth of the wake. 

 Also, the method is not very suitable for the measure- 

 ment of the transmission loss through the wake, be- 

 cause it requires the echo-ranging vessel to operate 

 in relatively shallow water in order to record the 

 bottom; furthermore, the depth and bottom char- 

 acter may vary considerably while this vessel is 

 moving. If, however, the fathometer is used in the 

 inverted manner, by mounting it on the deck of a 

 submerged submarine, those disadvantages are elimi- 

 nated; clear strong records are obtained both of the 

 highly reflecting ocean surface and of the surface 

 ship's wake, as illustrated by Figures 3, 4, 5, and 6. 

 Figure 3 shows a record obtained while the sub- 

 marine was diving from the surface. The depth scale 

 marked 5 to 50 applies to this dive, with the time 

 axis running from the right to the left. It can readily 

 be verified from the double record in the center of the 

 illustration that the weaker second reflection corre- 

 sponds to depths that are exactly twice the depth of 

 the stronger first reflection. Thus, the double record 



