OCEAN ACOUSTIC TOMOGRAPHY AT 1000-KM RANGE USING WAVE-FRONTS MEASUREDWITH A LARGE-APERTURE VERTICAL ARRAY

Broadband acoustic signals transmitted from a moored 250-Hz source to a 3-km-long vertical line array of hydrophones 1000 km distant in the north central Pacific Ocean were used to determine the amount of infor mation available from tomographic techniques used in the vertical plan e connecting a source-receiver pair. A range-independent, pure acousti c inverse to obtain the sound speed field using travel time data from the array is shown to be possible by iterating from climatological dat a without using any information from concurrent environmental measurem ents. Range-dependent inversions indicate resolution of components of oceanic variability with horizontal wavelengths shorter than 50 km, al though the limited spatial resolution of concurrent direct measurement s does not provide a strong cross-validation, since the typical cast s pacing of 20-25 km gives a Nyquist wavelength of 40-50 km. The small t ravel time signals associated with high-wavenumber ocean variability p lace stringent but achievable requirements on travel time measurement precision. The forward problem for the high-wavenumber components of t he model is found to be subject to relatively large linearization erro rs, however, unless the sound speed field at wavelengths greater than about 50 km is known from other measurements or from a two-dimensional tomographic array. The high-ocean-wavenumber resolution that is in pr inciple available from tomographic measurements is therefore achievabl e only under restricted conditions.