141 



A record obtained from a Rayleigh disk in the open sea is complicated not 

 only by multiple wave systems, or complex short-crested wave systems, and by 

 various ocean currents, but also by the irregularity of the waves in height and 

 period. Indeed, the analysis of such a record would seem to be impossible, 

 l3ut, being that ocean waves are variable in height, obtaining information about 

 wave direction from sub-surface current recorders is conceivable. 



For example, when the disk lies directly below the crest of a large wave 

 the direction indicated should be reasonably close to that of the crest. If the 

 crest were high in comparison to the average being recorded, the instruments 

 necessarily would be located near the center of a short-crested wave and the 

 orbital currents would lie in the vertical plane; also the magnitude of the hori- 

 zontal component of the orbital current at the wave crest would be large in com- 

 parison to normal ocean currents. The wave direction indicated should be 

 reasonably accurate. Selecting the correct time to read the record would re- 

 quire more information than could be obtained directly from the Rayleigh disk 

 record. The record obtained from a pressure type wave recorder near the 

 Rayleigh disk may supply the required information as it would indicate the pas- 

 sage of large waves. Perhaps recording the magnitude of the orbital velocity, 

 as well as the direction, would supply sufficient additional information to enable 

 a reasonable determination of wave direction. 



Regarding the instrument, the basic assumption has been made that the 

 Rayleigh disk, or whatever device is used, is capable of indicating the instan- 

 taneous direction of the currents without error. The designer of an instrument, 

 therefore, not only must consider the problem discussed above but also must 

 consider the problem of recording the direction of the sub-surface currents 

 without error. 



Tripartite Wave Recorders - The method of determining the direction of prop- 

 agation of microseismic vibrations by timing the arrival of the wave fronts at 

 three seismographs placed in a triangle has been used for a number of years 

 but generally has not proved successful. A technique of checking the reliability 

 of the measured bearing by also computing the velocity of the microseism from 

 the original data, was developed recently at the Naval Research Laboratories 

 (Kammer, 1951). This improved technique of determining the direction of 

 microseismic waves suggests a possible technique of determining ocean wave 

 direction. 



By placing three wave recorders in a triangle, with perhaps fifty feet 

 between recorders and timing the arrival of the wave at the three points, the 

 wave direction could be computed. The difficulty with this type of measurement 

 would be that the wave form recorded may not be identical at the three points 

 due to (1) short-crestedness of the waves and (2) transformation of the waves in 

 the direction of propagation. The variation of wave form at the three record- 

 ers would cause errors in the computed bearings. 



The validity of the bearings might be tested by computing the apparent 

 velocity with which the wave front crosses the three recorders. If variations 

 in the wave form introduce errors in the computed bearing they will also intro- 

 duce errors in the computed wave velocity. The computed velocity can be 

 checked against the velocity of the wave determined from the wave period and 

 hydrodynamic theory. By considering only the bearings associated with reason- 

 ably accurate wave velocities the direction of the waves might also be deter- 

 mined with reasonable accuracy. 



