ACOUSTIC TOMOGRAPHY AT BASIN SCALES AND CLOCK ERRORS

A basin-scale acoustic tomography simulation is carried out for the no rtheast Pacific ocean to determine the accuracy with which time must b e kept at the sources when clocks at the receivers are accurate, A seq uential Kalman filter is used to estimate sound-speed fluctuations and clock errors, Sound-speed fluctuations in the simulated ocean are est imated from an eddy-resolving hydrodynamic model of the Pacific forced by realistic wind fields at daily resolution from 1981-1993, The mode l output resembles features associated with El Nino and the Southern O scillation, as well as many other features of the ocean's circulation, Using a Rossby-wave resolving acoustic array of four fixed sources an d twenty drifting receivers, we find that the percentage of the modele d ocean's sound-speed variance accounted for with tomography is 92% at 400-km resolution, regardless of the accuracy of the clocks, Clocks w hich drift up to hundreds of seconds of error or more for a year do no t degrade tomographic images of the model ocean, Tomographic reconstru ctions of the sound-speed field are insensitive to clock error primari ly because of the wide variety of distances between the receivers from each source, Every receiver ''sees'' the same clock error from each s ource, regardless of section length, but the sound-speed fluctuations in the modeled ocean cannot yield travel times which lead to systemati c changes in travel time that are independent of section length, The K alman filter is thus able to map the sound-speed field accurately in t he presence of large errors at the source's clocks.