In the course of preparing for a vertical seismic profile experiment on the JOIDES Resolution drilling ship at Hole 504B on Leg 148, measurements of air gun and water gun source signatures were made twenty days apart using a source monitor hydrophone suspended 142 and 102 m below the ship. Comparing the two data sets, it was noted that the source signatures changed. Several possible causes were considered, and the most significant change found was the emptying of one of the ship's large double-bottom fuel tanks. It is inferred that the sound impacting on the vertical port side of the ship is re-radiated by the bottom by means of the ship's framing. The amount re-radiated appears to be affected by the change in fluid level in the tank.

Date of initial receipt: 19 July 1994
Date of acceptance: 17 February 1995

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