Rapid Global Finite-Frequency Ambient Noise Source Inversion

With the development of full waveform ambient noise tomography methods, the importance of knowing the heterogeneous noise source distribution has increased to circumvent common assumptions in ambient noise seismology, e.g. Green's function retrieval, and simplify tasks like near real-time monitoring of the sub-surface. Furthermore, since the mechanism creating secondary microseisms is well understood (Ardhuin et al., 2011) noise correlations which contain information about the source distribution also act as a new observable of the ocean state.

Simulating ambient noise cross-correlations on a global scale requires a computationally efficient approach. Pre-computed high-frequency wavefields, e.g. from Axisem (Nissen-Meyer et al., 2014), and spatially variable grids enable us to forward model cross-correlations on a global scale for any source distribution even in the microseismic frequency range (up to 0.2 Hz) within a minute for 5,000 station pairs on about 600 cores.

To invert for the source distribution, we use a gradient-based iterative method employing finite-frequency sensitivity kernels. The measurement used is the logarithmic energy ratio of the causal and acausal branch of the cross-correlations making it insensitive to unknown 3D Earth structures. The inversion process is automatised on a cluster, which allows for short run-times and minimal required user interaction.

Synthetic inversions for noise sources in the North Atlantic show promising results. Structure and spatial distribution of microseismic noise sources are well resolved at scales of few hundred kilometres. Additionally, we invert real data in the microseismic frequency range which can then be compared to ocean bottom pressure maps from Ardhuin et al. (2011), i.e. secondary microseism distribution inferred from wave height observations. Moreover, implementation with daily data downloads paves the way to daily noise source maps.

References

Ardhuin, F., Stutzmann, E., Schimmel, M., and Mangeney, A. (2011). Ocean wave sources of seismic noise. Journal of Geophysical Research: Oceans, 116(C9).

Nissen-Meyer, T., van Driel, M., Stähler, S., Hosseini, K., Hempel, S., Auer, L., Colombi, A., and Fournier, A. (2014). AxiSEM: broadband 3-D seismic wavefields in axisymmetric media. SolidEarth, (1):425-445.