Coherence-based approaches for estimating the composition of the seismic wavefield

TL;DR

This study investigates how array geometry and wavefield composition influence the precision of seismic wave-type measurements using ambient noise, with implications for geophysics and gravitational-wave detection.

Contribution

It quantifies the effects of array configuration and wavefield content on ambient seismic noise characterization using coherence and Wiener filters.

Findings

Seasonal variation in body-wave and surface-wave content.

Including underground stations improves noise waveform reproduction by about 4 times.

Reproduction residuals are less than 1%, limited by array geometry and seismic field variations.

Abstract

As new techniques exploiting the Earth's ambient seismic noise field are developed and applied, such as for the observation of temporal changes in seismic velocity structure, it is crucial to quantify the precision with which wave-type measurements can be made. This work uses array data at the Homestake mine in Lead, South Dakota and an array at Sweetwater, Texas to consider two aspects that control this precision: the types of seismic wave contributing to the ambient noise field at microseism frequencies and the effect of array geometry. Both are quantified using measurements of wavefield coherence between stations in combination with Wiener filters. We find a strong seasonal change between body-wave and surface-wave content. Regarding the inclusion of underground stations, we quantify the lower limit to which the ambient noise field can be characterized and reproduced; the applications of the Wiener filters are about 4 times more successful in reproducing ambient noise waveforms when underground stations are included in the array, resulting in predictions of seismic timeseries with less than a 1\% residual, and are ultimately limited by the geometry and aperture of the array, as well as by temporal variations in the seismic field. We discuss the implications of these results for the geophysics community performing ambient seismic noise studies, as well as for the cancellation of seismic Newtonian gravity noise in ground-based, sub-Hz, gravitational-wave detectors.