Spectrum of delay fluctuations in single-path underwater transmission over a range of 43 nmi

Two single-path transmission experiments were performed, one at 450 Hz and one at 400 Hz, to measure the spectrum of the fluctuations in acoustic travel-time difference between two receivers. In the first, a source on the ocean bottom 15 000 ft deep transmitted a pulsed cw signal to two bottomed receivers spaced 4300 ft in line with the source 19 nmi away. Large low-frequency fluctuations were found in the arrival-time difference between the two receivers. The energy in the spectrum of these fluctuations peaked around 0.2 to 0.7 cycles/h; it corresponded to a rms fluctuation in relative delay of about 1 ft. The second experiment used the same receivers to examine more rapid fluctuations, with a source suspended at 1200 ft depth 43 nmi away. In this case the results showed an additional peak in the fluctuation spectrum around 0.1 Hz. The energy in these fluctuations corresponded to a rms variation in acoustic path length difference of 0.11 ft. The faster fluctuations are ascribed to reflections from the undulating ocean surface. The slower ones are believed due to slow changes in the sound-speed profile.