Abstract
Structural imaging using body-wave energy present in ambient seismic
data remains a challenging task, largely because these wave modes are
commonly much weaker than surface wave energy. In a number of situations
body-wave energy has been extracted successfully; however, (nearly) all
successful body-wave extraction and imaging approaches have focused on
cross-correlation processing. While this is useful for interferometric
purposes, it can also lead to the inclusion of unwanted noise events
that dominate the resulting stack, leaving body-wave energy overpowered
by the coherent noise. Conversely, wave-equation imaging can be applied
directly on non-correlated ambient data that has been preprocessed to
mitigate unwanted energy (i.e., surface waves, burst-like and
electromechanical noise) to enhance body-wave arrivals. Following this
approach, though, requires a significant preprocessing effort on often
Terabytes of ambient seismic data, which is expensive and requires
automation to be a feasible approach. In this work we outline an
automated processing workflow designed to optimize body wave energy from
an ambient seismic data set acquired on a large-N array at a mine site
near Lalor Lake, Manitoba, Canada. We show that processing ambient
seismic data in the recording domain, rather than the cross-correlation
domain, allows us to mitigate energy that is inappropriate for body-wave
imaging. We first develop a method for window selection that
automatically identifies and removes data contaminated by coherent
high-energy bursts. We then apply time- and frequency-domain debursting
techniques to mitigate the effects of remaining strong amplitude and/or
monochromatic energy without severely degrading the overall waveforms.
After each processing step we implement a QC check to investigate
improvements in the convergence rates - and the emergence of reflection
events - in the cross-correlation plus stack waveforms over hour-long
windows. Overall, the QC analyses suggest that automated preprocessing
of ambient seismic recordings in the recording domain successfully
mitigates unwanted coherent noise events in both the time and frequency
domain. Accordingly, we assert that this method is beneficial for direct
wave-equation imaging with ambient seismic recordings.