A quantitative analysis of acoustic field directionality from a seismic airgun array

Marine seismic reflection surveys generate high-amplitude impulsive acoustic events using airgun arrays to study the geophysical characteristics of the seabed. These data can be used beyond seismic imaging, including for modeling short-range propagation, considering impacts on marine mammals, and extracting seabed properties and their effect on the acoustic field. Knowledge of the source characteristics is necessary to use these data, and proper modeling of sound propagation from these arrays requires characterization of the array beam pattern. Complex simulations of airgun arrays have been used in the past to model airgun array spectra, but beam patterns have not been thoroughly considered in the literature. Delay-and-sum combinations of these airgun signatures provide a simple beam pattern estimate, but this approach ignores variability in airgun position, timing, amplitude, interactions between airguns, and so on. The use of more complex notional airgun signatures can yield more accurate estimates, but these are more challenging to model and still ignore shot-to-shot variability. Experimentally determined beam patterns are evaluated here, and the variability in the results are considered, showing both similarities and notable differences from simulated results. The experimental results indicate that the source array depth impacts the ghost-free array beam pattern and that variability between shots is enough to significantly alter beam patterns. Overall, the observations suggest that accurate simulation of array beam patterns may require more complexity than is currently considered and that inclusion of uncertainty due to environmental and airgun shot variability is essential.