Title: Mapping of noise sources areas and trialling of noise correlation methods in the marine environment, offshore Ireland
Researchers: Dr David Craig, Prof. Chris Bean
Ocean generated microseisms (OGM) are recorded globally with similar spectral features observed everywhere. Typically, OGM spectra display two peaks corresponding to the single frequency (0.05-0.1 Hz) microseisms (SFM) and double frequency (0.1-0.3 Hz) microseisms (DFM). SFM are generated in shallow water when ocean waves interact with the sea-floor [Hasselmann, 1963]. The resulting seismic waves have the same frequencies as the causative ocean waves. DFM are much more energetic and generated beneath counter-propagating ocean wave trains. According to existing theory, this type of OGM generation does not depend on water depth and results in seismic waves at twice the frequency of the causative ocean wave trains. Many modern seismological methods make use of OGM signals. For example, the Earth's crust and upper mantle can be imaged using ``ambient noise tomography`` [Shapiro et al., 2005]; analysis of the geometric dispersion of surface waves and spectral ratio techniques [Di Giulio et al., 2006] can provide information on velocity structure; and cross-correlation techniques can be used to monitor seismic velocity changes [Wapenaar et al., 2011; Brenguier et al., 2014]. For many of these methods an understanding of the source distribution is necessary to properly interpret the results [Harmon et al., 2010]. This project aims to identify microseism source areas and better understand the propagation of microseisms from deep ocean regions to terrestrial seismic stations.
Seismic arrays can be used to determine the direction of propagation and velocity of seismic waves incident on the array. Using two or more arrays allows the triangulation of source regions for these seismic waves. The velocity allows the identification of the phase (i.e. the type of seismic wave). In this project I use two three component arrays (vertical, east-west and north-south) situated in NW and SW Ireland to identify source regions and the type of seismic wave emanating from the source region. Discrepancies have been identified between the observed source locations and those predicted by microseism generation theory. Array observations show that for surface waves, regions at or near the continental shelf edge play an important role. However, body waves are observed from deeper water regions.
Off the NW coast of Ireland appears to be a particularly interesting area for microseism generation and from January to August 2016 a network of ocean bottom seismometers was deployed in this region. Significant differences were identified between deep water and shallow water regions. This led to the hypothesis that the shelf edge may play a significant role in the propagation of OGM from deep water to shallow water regions. This has been further investigated through a collaboration with Dr. Florian LePape (iCRAG marine spoke) who has produced a synthetic data set of seismic propagation through the region. Applying the same type of array analysis to this data-set showed that the propagation path of surface waves (the dominant portion of the microseism wavefield), for microseisms generated in deep water, are strongly modified by the continental shelf. This means array analysis of surface waves tend to point to secondary source regions at the shelf edge. However, microseisms also generate body waves which can pass beneath the shelf edge and hence be used to identify the primary source area. The body wave sources agree well with the source areas predicted by microseism generation theory.