Marine Vibrators: Synthetic Data Study

Abstract

The traditional airgun source has for a long time been the preferred type of source in marine seismic data acquisition due to its high impulsive pressure output. However, this impulsive energy has recently been of concern with regards to the health of marine life. Therefore, restrictions have been put in place such that airgun surveying is prohibited in areas where marine life may be sensitive to high pressure levels. An example of such an area is the Great Australian Bight where seismic survey applications have been rejected several times. Another example is the Lofoten area located on the coast in the northern part of Norway facing the Norwegian Sea. Seismic exploration is not allowed in this area because of the concern that the pressure created from the seismic airgun may harm the fisheries which is the main industry of the region. An alternative seismic source is the marine vibrator which aims to spread the pressure output over time thus creating a non-impulsive wavefield that is not as harmful to marine life. The output of such a source is more manageable than that of the airgun and the generation of low-frequencies without bubble pulse contamination is an additional benefit. These are the motivations behind the revival of the marine vibrator. This thesis work will first derive the fundamental equations describing such a non-impulsive wavefield generated by a marine vibrator. Furthermore, this thesis work will investigate what happens when motion is introduced into the non-impulsive source wavefield by deriving the aforementioned equations with respect to motion. These equations represents the source signature of a marine vibrator and will be used to generate synthetic data. Further processing and imaging will be employed in an attempt to better isolate the effects of motion on the data and the limits of resolution of the marine vibrator source will be investigated. Finally, controlled marine vibrator data will be compared with corresponding airgun data in order to establish the validity of the marine vibrator.