1. With frequency response from zero to above the swell frequency; all 

 data are recorded and the swell data removed later. 



2. As in 1., but with swell frequencies filtered out between transducer 

 and recorder. 



3. That filters out the swell frequency and records all other data (rejec- 

 tion filter). 



4. That passes only signals with frequencies lower than that at the swell 

 (low pass filter). 



The best approaches would be 3. and 4. because the filtering is done at 

 the earliest possible stage and a roughly flat spectrum is recorded. The choice of 

 upper frequency cutoff is based on highest frequency of interest and highest fre- 

 quency recorded.^ A typical ratio is 2:1. There would be little difference in data 

 recorded in 3. and 4. because of the low energy content of the spectrum above the 

 swell frequency. Approach 1. is undesirable tecause of the small signal-to-noise 

 ratio and large amount of data to record and reduce. 



The discrete method can be imposed (either after recording or in real time) 

 or inherent. Inherent discreteness is found by timing particles traveling over 

 known distances. Care must be exercised in choosing the sampling interval so 

 that aliasing does not occur. 



REQUIREMENTS 



The characteristics and conditions for a successful flowmeter are as 

 follows: 



1. Outputs with orthogonal components, or a capacity to measure the 

 velocity directly with outputs for magnitude and angle; resultants in the horizontal 

 plane are sufficient, but three-dimensional types are more desirable. 



2. Measurement errors of the steady-state value less than 5 percent. 



3. A low-temperature coefficient needed because many of the signals of 

 interest are associated with large temperature changes — 5 to 10°C are typical. 



4. Meter mounts affixed to the sea bottom or stationary structures to pre- 

 vent motion relative to the earth; errors resulting from mounting to buoys are not 

 considered.^ 



5. Large dynamic velocity range. 



6. Bidirectional response, the angular response preferably a cosine 

 function. 



