2 Directional Gauge 



Gauge Location and Array Geometry 



As indicated in Figure 1, Harvest Platform is located about 20 km (10.8 n.m.) 

 west of Point Conception, California, in water with a mean depth of 202 m 

 (663 ft). Waves originating in the greater Pacific Ocean can reach the platform 

 via relatively unobstructed paths from the north, west, and south. The mean water 

 depth ensures deepwater wave conditions for waves with lengths shorter than 

 about 400 m (1,3 12 ft), or frequencies higher than about 0.06 Hz. Spectra repor- 

 ted herein are processed at frequencies between 0.04 and 0.16 Hz, so it is likely 

 that directional spectra for frequencies between 0.04 and 0.06 Hz are affected 

 somewhat by refraction. 



Directional wave detection is achieved with a spatial array of six subsurface 

 pressure gauges mounted on the Harvest Platform framework. Figure 2 shows a 

 plan view of relative gauge positions, and the array orientation in a geophysical 

 reference frame. Gauge spacing takes advantage of the maximum horizontal di- 

 mensions of Harvest Platform, and allows directional estimation for waves in the 

 frequency band noted in the previous paragraph. All gauges are mounted at a 

 depth of 1 5.72 m (5 1 .57 ft) below mean sea level, which ensures they will not 

 protrude through the sea surface under extreme wave conditions that have been 

 observed at this site. 1 To avoid aliasing in directional estimation, the lower reso- 

 lution wavelength limit is two times the shortest lag spacing of the array. In the 

 Harvest Platform array, this limit is 45.4 m (149.0 ft), which corresponds to a 

 wave frequency of about 0.18 Hz. Signal analysis used in this report was limited 

 further to 0.16 Hz to be conservatively clear of aliasing effects. 



Pressure Gauges and Data Path 



Individual sensors were Model TJE absolute pressure sensors manufactured by 

 Sensotec Transducer Company with operating ranges of to 100 psia (0 to 

 689.5 kPa), and a manufacturer's stated accuracy of ±0.1 percent of full scale. 

 The six gauges on Harvest Platform were sampled simultaneously at 1 Hz, 



Personal communication, 1991, Dr. R. J. Seymour, Coastal Data Information Program (CDIP), Scripps Institu- 

 tion of Oceanography (SIO). 



Chapter 2 Directional Gauge 



