Borehole Seismic Air-Gun Sources: What's the Safe Distance from a Ship's Hull?

Abstract

Abstract When offshore borehole seismic surveys require the use of a supply ship or vessel to position an air-gun source, often the air-gun source is deployed over the side of the ship. For many years, this configuration has given concern to ship owners and captains. When conducting borehole seismic surveys, the air-gun source is fired several hundred times over the duration of the survey and there is a fear that the ship may be damaged. Now that more powerful gun clusters and arrays are being used, these larger air-gun sources must be positioned further away from the ship's side. An investigation was made of the excitation of the ship's hull caused by air-gun firing versus the excitation of the ship's hull under sailing conditions. This paper shows how a set of safe distances can be formulated, so that gun shocks to the ship's hull when conducting borehole seismic surveys are no worse than hull shocks experienced when the ship is sailing under moderate sea conditions. Recommendations for safe air-gun deployment methods are presented, so that the safe distance can be maintained throughout the borehole seismic survey.