Water column sound speed variability can have significant effect on acoustic propagation. However, measurements of water column properties, either by ship based systems or moored instruments, are often neither spatially nor temporally colocated with acoustic measurements. Variability of the water column over the acoustic propagation path must be approximated by extrapolation. During SW06, a low-frequency sound source, broadcasting pure tones, was towed along radials from a vertical array comprised of both hydrophones and oceanographic sensors. Concurrently, and from the same ship, a towed CTD chain was used to measure the water column as a function of both time and space. Changes in sound speed as high as 15 m/s were recorded over a 5-km aperture. The CTD chain, in combination with environmental sensors on the VLA, allowed for continuous monitoring of the water column sound speed profile at both the source and receiver locations. It is shown that horizontal wavenumber predictions for the acoustic field are biased by the sound speed measured at the source compared to at the receiver. In addition, predictions of the acoustic field are improved when the range dependence of the sound speed profile is considered. [Work supported by NDSEG and ONR.]

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