ACOUSTIC MEASUREMENTS OF SURFACE GRAVITY-WAVE SPECTRA IN MONTEREY BAYUSING MODE TRAVEL-TIME FLUCTUATIONS

Earlier work by Miller et al. [J. Acoust. Soc. Am. 86, 326-345 (1989)] suggested that the surface gravity wave frequency-direction spectrum could be measured using acoustic tomography signals. An experiment was performed in Monterey Bay, California, from 12-16 December 1988 to va lidate this earlier work. During that period, an acoustic tomography s ource with a center frequency of 224 Hz and bandwidth of 16 Hz was dep loyed, continuously repeating its coded transmissions every 1.9375 s. The transmissions were received on a number of modified sonobuoys plac ed on the continental shelf at a range of approximately 55 km. Althoug h the acoustic propagation was complicated by the presence of the Mont erey Submarine Canyon, a hybrid ray and mode interpretation explains t he observations. The main effect of the surface waves on the acoustic signals is determined to occur in the shallow continental shelf portio n of the path. On the shelf, it is shown that the resolved acoustic no rmal modes exhibit the fluctuations in travel time described in our ea rlier work. The acoustic measurements of the surface wave spectrum com pared well with measurements made by a pitch-roll wave buoy maintained in the area by the National Data Buoy Center (NDBC). The acoustic and buoy spectra agreed well on the location of the peak swell frequency; the root-mean-square (rms) wave amplitudes from the two measures agre ed to within 7%.