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 in order to maximize low frequency content and minimize high frequency content. 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 are often neglected. We perform numerical solution of a quasi-one-dimensional model of a pneumatic source suitable for large volume sources fired underwater. In our model, we couple shuttle dynamics to the gas dynamics in multiple, adjacent chambers in the source. 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 source is coupled to a spherical bubble through mass and energy conservation conditions. The air exiting the source experiences a change in flow area as it expands or contracts during flow from a constant cross-sectional area firing chamber to a port area dependent on shuttle position. To capture the compressible effect of flow area change, we invoke a set of steady-state nozzle flow approximations.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 model and data to unmodeled pressure losses in the gas flow to the bubble and discrepancy over the bubble oscillation timescale to bubble modeling error. We quantify the effects of design parameters on the mass flow rate to the bubble, and find that shuttle oscillations can lead to briefly reduced mass flow rates as the shuttle partially obstructs the port.