An acoustic modelling study of seismic airgun noise in Queen Charlotte Basin

This thesis presents a methodology for modelling underwater sound propagation from seismic airgun surveys. Subsequently, the method is applied to the case of predicting noise levels from offshore oil and gas exploration in the Queen Charlotte Basin region of the British Columbia offshore. Underwater noise levels are predicted using an integrated modelling approach that incorporates (i) an airgun array source model, (ii) a broadband 3-D transmission loss model, and (iii) environmental databases of high resolution bathymetry, historical CTD casts and geoacoustic properties of the seabed. As part of this thesis, an airgun array source signature model is developed and then used for predicting the acoustic source level of a seismic airgun array. The airgun model, based on the physics of the oscillation and radiation of airgun bubbles, is validated against real airgun data. Underwater sound propagation is modelled in three dimensions using a parabolic-equation (PE) based, " N x 2-D" transmission loss model. Broadband sound propagation is approximated by modelling acoustic transmission loss at third-octave band centre frequencies; noise levels are computed by subtracting transmission loss from the respective third-octave band airgun array source levels. High resolution bathymetry and geoacoustic data for Queen Charlotte Basin are extracted along centrolineal radial transects for input to the propagation model. Additionally, principal component analysis is applied to a large collection of historical CTD data to extract seasonal sound speed profile variations. Transmission loss is modelled using both winter and summer sound speed profiles to bracket seasonal variations in acoustic propagation conditions. Noise level estimates from the acoustic modelling may be used to determine impact zones around the survey vessel where marine mammals are expected to exhibit disturbance reactions to airgun noise. Results from this study will aid in evaluating the potential environmental impacts of seismic exploration activity on marine ecosystems in British Columbia's offshore region.