A COMPARISON OF MEASURED AND PREDICTED BROAD-BAND ACOUSTIC ARRIVAL PATTERNS IN TRAVEL TIME-DEPTH COORDINATES AT 1000-KM RANGE

Broadband acoustic signals were transmitted from a moored 250-Hz sourc e to a 3-km-long vertical line array of hydrophones 1000 km distant in the eastern North Pacific Ocean during July 1989. The sound-speed fie ld along the great circle path connecting the source and receiver was measured directly by nearly 300 expendable bathythermograph (XBT), con ductivity-temperature-depth (CTD), and air-launched expendable bathyth ermograph (AXBT) casts while the transmissions were in progress. This experiment is unique in combining a vertical receiving array that exte nds over much of the water column, extensive concurrent environmental measurements, and broadband signals designed to measure acoustic trave l times with 1-ms precision. The time-mean travel times of the early r aylike arrivals, which are evident as wave fronts sweeping across the receiving array, and the time-mean of the times at which the acoustic reception ends (the final cutoffs) for hydrophones near the sound chan nel axis, are consistent with ray predictions based on the direct meas urements of temperature and salinity, within measurement uncertainty. The comparisons show that subinertial oceanic variability with horizon tal wavelengths shorter than 50 km, which is not resolved by the direc t measurements, significantly (25 ms peak-to-peak) affects the time-me an ray travel times. The final cutoffs occur significantly later than predicted using ray theory for hydrophones more than 100-200 m off the sound channel axis. Nongeometric effects, such as diffraction at caus tics, partially account for this observation.