Figure F4. Even when weather conditions were excellent on site, a swell from the northwest produced ship heave that often exceeded 2 m at the shelf sites. We attempted to record the size of the heave by using the precision depth recorder (PDR) while fixed over the site. The recorder shows as much as 7.5 m change in water depth, mainly as a result of ship heave, although secondary effects of ship pitch and roll added to the PDR signal.