Abstract
Cross-correlations of ambient seismic noise are widely used for seismic
velocity imaging, monitoring, and ground motion analyses. A typical step
in analyzing Noise Cross-correlation Functions (NCFs) is stacking
short-term NCFs over longer time periods to increase the signal quality.
Spurious NCFs could contaminate the stack, degrade its quality, and
limit its use. Many methods have been developed to improve the stacking
of coherent waveforms, including earthquake waveforms, receiver
functions, and NCFs. This study systematically evaluates and compares
the performance of eight stacking methods, including arithmetic mean or
linear stacking, robust stacking, selective stacking, cluster stacking,
phase-weighted stacking, time-frequency phase-weighted stacking,
$N^{th}$-root stacking, and averaging after applying an adaptive
covariance filter. Our results demonstrate that, in most cases, all
methods can retrieve clear ballistic or first arrivals. However, they
yield significant differences in preserving the phase and amplitude
information. This study provides a practical guide for choosing the
optimal stacking method for specific research applications in ambient
noise seismology. We evaluate the performance using multiple onshore and
offshore seismic arrays in the Pacific Northwest region. We compare
these stacking methods for NCFs calculated from raw ambient noise
(referred to as Raw NCFs) and from ambient noise normalized using a
one-bit clipping time normalization method (referred to as One-bit
NCFs). We evaluate six metrics, including signal-to-noise ratios, phase
dispersion images, convergence rate, temporal changes in the ballistic
and coda waves, relative amplitude decays with distance, and
computational time. We show that robust stacking is the best choice for
all applications (velocity tomography, monitoring, and attenuation
studies) using Raw NCFs. For applications using One-bit NCFs, all
methods but phase-weighted, time-frequency phase-weighted, and
$N^{th}$-root stacking are good choices for seismic velocity
tomography. Linear, robust, and selective stacking methods are all
equally appropriate choices when using One-bit NCFs for monitoring
applications. For applications relying on accurate relative amplitudes,
both the robust and cluster stacking methods perform well with One-bit
NCFs. The evaluations in this study can be generalized to a broad range
of time-series analysis that utilizes data coherence to perform ensemble
stacking. Another contribution of this study is the accompanying
open-source software, which can be used for general purposes in
time-series stacking.