High resolution seismic data were collected using a Deep-Tow Acoustics/Geophysics System (DTAGS) near ODP site 889/890, offshore Vancouver Island for deep sea gas hydrate study. The near bottom configuration and wide high-frequency bandwidth allow much better resolution of the structures in the upper ~400 m. However, the short 2--6 m wavelengths imply accurate positioning of the source and receivers to properly determine moveout velocities and to stack the data. Consequently, a method of estimating the depths of the source and hydrophones using sea-surface reflection times was developed. Travel time corrections based on these sea surface measurements were applied to the data by marching a time window along the seismic trace to account for the angular dependency of the reflected raypath. The need for these corrections is clearly illustrated by the results, which show nonlinear depth variations of up to 50--100 m for both the source and the hydrophone array. ¿ 1999 American Geophysical Union