648 



[chap. 18 



problem but can be avoided by placing the transducer into a barrel of sand 

 (first suggested by Judith Munk) or burying it in the sea bottom. Off-shore 

 recording of long-period waves has been accomplished by means of an absolute 

 pressure transducer on the sea bottom with a dynamic range of 10^ : 1 (Snod- 

 grass et al., 1958). Here the output has been recorded in digital form, and all 

 filtering is accomplished by numerical methods. 



To recorder 



Air 

 capillary 



Float 



Water 

 capillary 



-/^ Pressure 

 ~y-y gauge 



Orifice 



Fig. 1. Schematics of tide-gauge (left) and long-wave recorder (right). The tide-gauge 

 records the water level in a well which is shielded from (high-frequency) ocean waves 

 by an orifice. In the long-wave recorder the top of the well is capped, and the pressure 

 differential between the entrapped air and atmosphere is recorded. Tides are removed 

 by a tiny capillary leak. 



3. The Spectrum 



Fig. 2 shows a typical background record from a Snodgrass long-period 

 recorder peaked at 1 c/ks (period 1000^), with half-power points at 0.1 c/ks 



1500 



1530 



1600 



1630 



1700 



Fig. 2. Sample wave record, taken on 23 February, 1956, off Camp Pendleton, California, 

 at a depth of 20 ft. The vertical scale is in chart inches. (After Snodgrass, 1958, Fig. 5.) 



(10,000^) and 10 c/ks (100^) respectively. The effect of swell is barely discernible 

 as a thickening of the record trace (ordinarily a fine line) to about 0.1 chart 

 inches. The high frequency wiggles of the thickened trace are associated with 



