On localising North Atlantic’s microseism sources using time reversal imaging

The ambient seismic noise is dominated by surface waves associated with ocean microseisms that are defined by a strong acoustic/seismic coupling at the seafloor. The understanding of microseism sources not only benefits seismic passive imaging and monitoring, but can also help track ocean storms using seismic signals recorded on land. Numerical simulations show that heterogeneous seafloor morphologies and structures can significantly affect the propagation of microseism’s surface waves and therefore the accuracy in locating their origin using traditional methods, such as array beamforming. Here, we aim to investigate how the use of time-reversal imaging can help overcome those limitations. The technique has mainly been developed for acoustics but has been applied successfully in seismology for earthquake localisation. Time-reversal imaging consists on back-propagating, through a realistic model, the signal measured at a network of receivers so that it eventually refocuses back at its origin. For this study, simulations are performed using the code SPECFEM3D and a regional 3D acoustic/elastic model of the Irish offshore, with seismic receivers homogeneously distributed along the coast of Ireland. First, methodologies and stations layout are tested with synthetic data generated from forward modelling using different source distributions. Processing approaches for the time-reversed simulation results are investigated in order to optimize the recovery of the original sources. Following those tests, real time windows of seismic noise “events” are then back-propagated into the model with the aim to map the microseism sources associated with local storms in the model area. The results are compared with microseism sources derived from global ocean wave models. Overall, the use of time-reversal imaging for microseism sources localisation looks promising. Although challenging due to the diffuse distribution of sources, there is good potential for developing further our understanding of microseism sources and monitor dominant microseism generation areas in the North Atlantic region with the implementation of a larger model.