Stationary phase analysis of ambient noise cross-correlations: Focusing on non-ballistic arrivals
arxiv(2024)
Abstract
Stacked cross-correlation functions have become ubiquitous in the ambient
seismic imaging and monitoring community as approximations to the Green's
function between two receivers. While theoretical understanding of this
approximation to the ballistic arrivals is well established, the equivalent
analysis for the non-ballistic arrivals is alarmingly inadequate compared to
the exponential growth of its applications. To provide a fundamental
understanding of the cross-correlation functions beyond the ballistic arrivals,
we derive analytical stationary phase solutions for ambient noise
cross-correlations with a focus on non-ballistic arrivals. We establish the
mathematical and corresponding physical conditions that drastically
differentiate the non-ballistic arrivals in the stacked cross-correlation and
the actual Green's functions. In ambient noise environments, the coda waves due
to random medium scatterings of an impulsive source cannot be distinguished
from the cross-talk artifacts due to overlapping random noise sources.
Therefore, changes in the non-ballistic arrivals cannot be uniquely attributed
to changes in the medium or changes in the noise source environment without
additional constraints. The theoretical results demand that interpreting
large-elapse-time arrivals in the stacked cross-correlation functions as coda
waves for deterministic information about the propagation medium should be
conducted only after the source influence is sufficiently ruled out. Once the
source influence is eliminated, the stationary phase solutions for scattering
waves provide a solid basis for extracting reliable scattering information from
the noise correlation functions for higher-resolution imaging and monitoring.
MoreTranslated text
AI Read Science
Must-Reading Tree
Example
Generate MRT to find the research sequence of this paper
Chat Paper
Summary is being generated by the instructions you defined