Experimental study of viscosity effects on air‐gun signatures

Abstract

In computer modeling of air guns the theory is often based on the assumption of an ideal freely oscillating spherical air bubble in an ideal fluid. Theoretical and experimental air‐gun signatures do not match perfectly. Synthetic signatures are under‐damped compared to real signatures. Several physical effects have been proposed as an explanation of this mismatch, among them viscosity effects. The viscosity of the surrounding liquid may contribute to the damping of the bubble oscillation. The numerical value of the viscosity of the surrounding liquid has to be increased considerably beyond the actual value of water to obtain sufficient damping of the synthetic signatures. We therefore performed an experiment to study the effect upon the pressure signature from an air gun when changing the viscosity of the surrounding medium. The motivation for the experiment was to quantify the influence of viscous terms on the output pressure waveform from an air gun. The experiment was carried out in an [Formula: see text] tank. The source was a BOLT 600 B air gun with a firing chamber of [Formula: see text] [Formula: see text]. The gun was placed at 0.5 m depth, and the hydrophone was placed 0.22 m from the gun ports. This configuration was kept constant during the experiment. We observed changes in the output pressure waveform generated by the gun at different liquid viscosities. Sets of five signatures recorded at 12 viscosity values in the range 6–723 centipoise, were analyzed. The effects on the pressure signature when increasing the viscosity of the liquid surrounding the gun are to decrease the primary to bubble ratio and the bubble period; thus, the opposite of what one should expect. We therefore conclude that viscosity is not the main physical effect that explains the damping of an air gun signature.