Influence of port opening dynamics on the acoustic signature of pneumatic marine seismic sources

Abstract

Numerical modeling is a powerful tool for predicting the acoustic signature of pneumatic marine seismic sources to maximize low-frequency content and minimize excessively high-frequency content that may harm marine life. Idealized source models often overpredict the peak acoustic pressure, a key quantity in assessing source performance. The dynamics of the shuttle, the mechanical component that controls the early time gas flow rate from the source, contribute to this signal overprediction but often are neglected. We have computed the numerical solution of a quasi-1D model of a large volume source fired underwater. In our model, we couple the shuttle dynamics to the gas dynamics in multiple adjacent chambers in the source. The source is coupled to a spherical bubble through mass and energy conservation conditions. To capture the compressible effect of flow area change in the air exiting the source, we invoke a set of steady-state nozzle flow approximations. We compare model results against data from field tests, where acoustic pressure signals, local pressure within and just outside the source, and temperature inside the source were measured. The maximum value and slope of the initial acoustic pressure peak are overpredicted by the model, but internal pressure and temperature data agree with model predictions. We attribute early time discrepancy between the model and data to unmodeled pressure losses in the gas flow to the bubble and discrepancy over the bubble oscillation timescale to the bubble modeling error. We provide first-order corrections that reduce the model discrepancy in a manner consistent with internal thermodynamic data and quantify the effects of design parameters on the source dynamics.