Matched Field Processing of Three‐Component Seismic Array Data Applied to Rayleigh and Love Microseisms

Abstract

We extend three‐component plane wave beamforming to a more general form and devise a framework, which incorporates velocity heterogeneities of the seismic propagation medium and allows us to estimate accurately sources that do not follow the simple plane wave assumption. This is achieved by utilizing fast marching to track seismic wave fronts for given surface wave phase velocity maps. The resulting matched field processing approach is used to study the surface wave locations of Rayleigh and Love waves at 8 and 16 s based on data from four seismic arrays in the western United States. By accurately accounting for the path propagation effects, we are able to map microseism surface wave source locations more accurately than conventional plane wave beamforming. In the primary microseisms frequency range, Love waves are dominant over Rayleigh waves and display a directional radiation pattern. In the secondary microseisms range, we find the general source regions for both wave types to be similar, but on smaller scales differences are observed. Love waves are found to originate from a larger area than Rayleigh waves and their energy is equal or slightly weaker than Rayleigh waves. The energy ratios are additionally found to be source location dependent. Potential excitation mechanisms are discussed which favor scattering from Rayleigh‐to‐Love waves.

Global‐Scale Full‐Waveform Ambient Noise Inversion

“…Although the nature of the ambient noise sources is interesting in itself, its investigation gained importance through the peculiarity of these assumptions. In fact, an overwhelming number of studies consistently show that they are not generally met and that the ambient noise field at any frequency is excited by a heterogeneous and nonstationary distribution of sources (Ardhuin et al, , ; Ermert et al, , ; Gal et al, , ; Juretzek & Hadziioannou, ; Stehly et al, ). The reconstructed Green's functions are thus deteriorated, and information should only be extracted with caution (Delaney et al, ; Halliday & Curtis, ; Fichtner, , ; Kimman & Trampert, ; Snieder et al, ; Stehly & Boué, ; Tsai, , ).…”

Short Timescale Analysis of Microseisms and Application to Array Calibration

“…These two approaches are identical for a farfield source and homogeneous velocity, but for sources in the nearfield or inhomogeneous velocities, which result in a nonplanar wave propagation over the array, the use of the traveltime (under the assumption of a known phase velocity) is advantageous. This modification generalizes beamforming into the class of MFP (e.g., Baggeroer et al, ) and has been previously used for exploration purposes (e.g., Corciulo et al, ) and the analysis of microseisms (e.g., Gal et al, ). Hence, the underlying base equation which we will adapt in the following for the analysis of short‐time microseisms is defined as where x is the spatial location where the beam is focused and t n ( x , f ) is the corresponding traveltime for a given phase velocity model.…”

“…For the traveltimes t n ( x ) of surface waves, which we require in equation as the input, we will follow the approach of Gal et al (), where fast marching (Rawlinson & Sambridge, ) in combination with a phase velocity map is used to generate traveltimes and accounts for the deviation of raypaths from the great circle paths. In the underlying phase velocity map we use the updated maps USANT15 of Ekström () at 10 and 20 s for Rayleigh and Love waves.…”

“…Microseism sources have been extensively investigated with single station (e.g., Bromirski & Duennebier, ; Koper & Burlacu, ; Schimmel et al, ; Schulte‐Pelkum et al, ; Stutzmann et al, , , ) and array methods (e.g., Euler et al, ; Farra et al, ; Friedrich et al, ; Gal et al, , ; Gerstoft et al, ; Koper et al, ; Landès et al, ; Liu et al, ; Reading et al, ) to estimate their spatial generation location and temporal variability and compare their observed power to hindcast models (Farra et al, ). One of the most significant findings in the last two decades in the field of microseisms was that one can estimate the approximate interstation Green's function between two receivers by cross correlation of their respective microseism wavefields (Campillo & Paul, ; Shapiro & Campillo, ; Snieder & Hagerty, ; Roux et al, ) which makes it possible to extract information on the medium between the stations.…”

“…In this work, we will analyze the microseism wavefield on short timescales (order of seconds) with seismic array techniques (Rost & Thomas, , ; Gal & Reading, ). We have designed a novel framework which makes use of three main components: (1) The analytical signal representation obtained via the Hilbert transform to estimate the time‐frequency representation of the wavefield, (2) fast marching (Rawlinson & Sambridge, ) in combination with a phase velocity map used to generate source traveltimes, and (3) used as input into a matched field processing (MFP) beamformer (Gal et al, ). The novel approach is first tested on an earthquake example and is then applied to the analysis of the primary and secondary microseism wavefields to infer its properties on short timescales.…”