Continuous series of tide records from 1905 to 1956 at San Francisco and Honolulu were reduced to smoothed values at 32-hour intervals. The cross spectrum of the resulting seri es was computed by the Tukey method for 750 narrow-frequency bands at intervals of 0.0005 cycles per day. The curves of spectral-energy density vary smoothly with frequency; the only significant peaks occur at tidal frequencies....
Groves, Gordon W., and Bernard D. Zetler. 1964. "The cross spectrum of sea level at San Francisco and Honolulu." Journal of Marine Research 22, (3). https://elischolar.library.yale.edu/journal_of_marine_research/1036