144 



developed and several field experiments were made with this unit. No pressure- 

 actuated type instrument was developed at this time. 



A 25-foot spar buoy was made of 4-inch o. d. dural tubing, with the trans- 

 mitter mounted at one end and the sonic transducer at the other. The transduc- 

 er was mounted on a bracket at the bottom of the buoy, facing up toward the sur- 

 face of the water. Pulses of sound energy were radiated from the transducer to 

 obtain the echoes from the surface of the water and also the echoes from two 

 reflectors fastened 5 feet apart along the side of the buoy. The transmitted sig- 

 nal consisted of an initiating pulse, two echoes from reflector pads a known dis- 

 tance apart and an echo from the water surface. The receiver signal was am- 

 plified and applied to the unblanking circuit of a synchoronized oscillograph. The 

 oscillograph was then photographed with an oscillographic camera, which con- 

 verted the spot presentation of the oscilloscope into a continuous strip record. 



This system was used successfully on several occasions. Comparison 

 of these readings with moving pictures taken of the waves passing the spar buoy 

 showed the error to be less than 0.2 feet. Although comparatively satisfactory, 

 no further development of this sonic radio-link method of measuring ocean waves 

 has been made since these first tests. 



MEASUREMENT OF LONG PERIOD WAVES 



Waves in a storm area form a most irregular pattern, and have a wide 

 range of frequencies (Munk, 1951). Under storm conditions, most of the ener- 

 gy of the waves is concentrated, however, within a relatively narrow range of 

 periods, say from 5 to 9 seconds. After the waves leave the storm area, they 

 travel as swells. Selective attenuation of the shorter periods leads to a gradual 

 shift of the energy maximum toward the longer periods. Thus, due to disper- 

 sion and selective attenuation, the most prominent swell from a storm area sev- 

 eral thousand miles away has a period of 12 to 16 seconds. 



During the last few years, a number of automatic swell-recording instru- 

 ments have been installed by British and American organizations. These instru- 

 ments have demonstrated the existence of long forerunners to the swell, with 

 periods up to about 30 seconds. 



Four other types of waves have been noticed whose periods are between 

 the wind-generated swell and the tides. These are: 



(a) tsunamis, caused by submarine earthquakes or eruptions (popularly 

 known as tidal waves, although they have no relation to tide-producing 

 forces); 



(b) seiches, caused by atmospheric variations; 



(c) surf beats, related to the fluctuation in height of the incoming wind- 

 generated waves (Munk, 1949); and 



(d) harbor surging, the oscillation found in harbors (Knapp, 1951). 



Deep Water Mark III Wave Recorder - Hydrodynamic theory indicates that 

 pressure-type instruments installed in water about 600 feet deep will record the 

 long period wave but not the wind-generated waves of 4 to 20 seconds period. 

 The pressure recorder must be installed in depths less than one-half the wave 

 length of the shortest wave to be recorded to experience more than one percent 

 of the total pressure fluctuation generated by the wave at the surface of the wa- 

 ter. Thus, recorders installed in deep water will be able to feel pressure vari- 

 ations of only the long period waves and tides. 



In 1947, the Mark III, Model 3, deep water shore wave recorder, shortly 



