142 



DEEP WATER MEASUREMENTS 



The problem of designing instruments to measure ocean waves in deep 

 water (to supply information about waves in the generating area and along the 

 decay path to the shore) never has been solved satisfactorily. The primary dif- 

 ficulty with this measurement problem is that of obtaining a fixed reference 

 against which the surface elevation of the water can be compared. Probably the 

 earliest attempt to measure waves in deep water was by nmeans of delicate aner- 

 oid barometers that measured the change in atmospheric pressure as a ship rode 

 over the swells (Gaillard, 1904). In this case an attempt was made to use the 

 atmospheric pressure as a reference. Measurements from submarines, using 

 pressure recorders, and from aircraft, using sensitive echo-type altimeters 

 have also been employed using the craft as a reference. 



A more direct approach to the problem of supplying a reference can be 

 found in the studies of floating spar buoy systems. The following pages describe 

 some of the experimental work completed toward the development of spar buoy 

 reference systems and wave recorders that have been used with the spar buoy. 



Long Line Damped Spar Buoy - This system employed a floating spar buoy at- 

 tached to a damping disk by a long wire cable. The cable was 600 feet long, 

 which enabled the damping disk to act in water that was relatively undisturbed by 

 wave action. The variation of buoyancy as a wave passed the buoy caused only 

 a negligible vertical motion of the systena. Thus, a reference was established 

 against the water surface elevation could be compared. 



Short Line Damped Spar Buoy (Folsom, 1945) - The short line system was de- 

 veloped because of the difficulty of installing and retrieving the long line system. 

 Since the damping disk was acted upon by water moving with the surface waves, 

 there was an appreciable vertical motion of the shore line system. The motion 

 of the buoy was assumed to be the sum of the vertical components of the water 

 particle orbits at the damping disk, and the motion due to the buoyancy varia- 

 tion as the wave passed the spar buoy. 



Correction for the Motion of the Spar Buoy (Rauch, 1945) - Equations have 

 been derived to calculate surface wave height when 



(a) the apparent wave height is known (as found by observing the time 

 history of the water surface on the spar buoy), or if 



(b) the sub-surface pressure time history is known (as found by a pres- 

 sure recorder attached to the bottom of the spar buoy). 



These equations account for 



(a) the orbital particle motion at the damping disk, 



(b) the motion of the disk relative to the particle motion, 



(c) the hydrodynamic attenuation of pressure, 



(d) the effect of waves passing over the spar buoy and 



(e) the phase lag of the spar buoy nnotion relative to the surface wave. 



Studies were also made to determine the tilt of the spar buoy for various 

 exposed lengths and for various wind velocities. A 36-foot spar buoy of alumi- 

 num tubing (3" od.) weighing 49.2 pounds was attached to a 3-foot damping disk 

 on a short line. Weights were added to the system to expose the buoy to vari- 

 ous lengths, and the tilt was measured. Calculations and experiments indicated 



Editor's Note: Since this paper was presented two significant advances 

 in open-sea wave studies have been developed at Scripps Institution. They are 

 Willard Bascom's deep-sea instrument station and James Snodgrass' wave- 

 sensing and telemetering unit. 



