Timing of the acoustic transmission is made difficult by the confusion of the directly transmitted ray with the ray reflected from the surface. The reflected ray travels a longer path than the direct ray, with a path-length difference varying with the water depth. The separation should be clearest at the highest tides, and for us this occured on December 30, 1996. This was just before the piling washed out, but our signal was still strong.
We have looked for evidence in the auto-correlation plot for two distinct maxima, corresponding to the two ray arrivals.