AbstractThe oil and gas industry employs powerful, low-frequency impulsive sources for exploration during seismic surveys. These sources, known as seismic air gun arrays, have the ability to propagate long distances. Seismic surveys can last for weeks and happen simultaneously at multiple locations around the world. These operations have the potential to disturb and displace marine mammals and other forms of marine life.
There is a number of mitigation strategies that can be used to reduce the impact and exposure of marine animals to noise. For seismic surveys, the most cost-effective method consists in implementing a real-time monitoring plan around a predefined exclusion zone. If a marine mammal enters that area, the operations stop as requested by the Marine Mammal Observer (MMO) or Passive Acoustic Monitoring (PAM) technician who detected and identified the animal. Nowadays, these exclusion zones are calculated from sound field measurements or models, or a combination of both.
For the purpose of sound source verification (SSV), a model of the sound field is required before the survey starts. The pre-survey simulation of the sound field is obtained as a combination of an air gun model, which simulates the acoustic output of the seismic source, and a propagation model, which simulates the pressure attenuation with distance. The pre-survey simulations are used to calculate the exclusion zone that is to be monitored before the sound mapping survey can gather the data necessary to verify the model. Once the data has been collected, a report is issued within 24-72 h. The report summarises the results of the model validation and provides an updated exclusion zone.
In this context, there are three essential tools needed for noise impact assessment: a source model and propagation model for the pre-survey sound field simulations, and a software package that can generate a map of sound pressure levels from raw audio and position data. However, the tools currently available have some fundamental limitations: (1) commercial source models of air gun arrays are costly and are not designed for noise impact assessment, and (2) sound field processing is typically carried out with nonstandardised
software routines developed by the companies providing the SSV services.
In this thesis we present the design and implementation of two pieces of software: (1) a package for the computation of the sound field from raw audio and position data, and (2) a calibrated air gun array model that can compete with commercial software. The purpose of these tools is to provide efficient, accurate and free-access routines that can be used for the characterisation of the acoustic source, the sound field, and the propagation environment in the context of seismic surveys.
|Date of Award||22 Feb 2023|
|Supervisor||Philippe Blondel (Supervisor) & Chris Budd (Supervisor)|
- underwater acoustics
- seismic surveys
- sound source verification
- detection theory
- airgun modelling
- data processing