Abstract
Recently there has been much interest in the use of data from surface
arrays in conjunction with migration-based processing methods for
passive seismic monitoring. In this study we use an example of this
kind of data recorded whilst 18 perforation shots, with a variety
of positions and propellant amounts, were detonated in the subsurface.
As the perforation shots provide signals with known source positions
and origin times, the analysis of these data is an invaluable opportunity
to test the accuracy and ability of surface arrays to detect and
locate seismic sources in the subsurface. In all but one case the
signals from the perforation shots are not visible in the raw or
preprocessed data. However, clear source images are produced for
12 of the perforation shots showing that arrays of surface sensors
are capable of imaging microseismic events, even when the signals
are not visible in individual traces. We find that point source locations
are within typically 45 m (laterally) of the true shot location,
however the depths are less well constrained (\~150 m). We test
the sensitivity of our imaging method to the signal-to-noise ratio
in the data using signals embedded in realistic noise. We find that
the position of the imaged shot location is quite insensitive to
the level of added noise, the primary effect of increased noise being
to defocus the source image. Given the migration approach, the array
geometry and the nature of coherent noise during the experiment,
signals embedded in noise with ratios >=0.1 can be used to successfully
image events. Furthermore, comparison of results from data and synthetic
signals embedded in noise shows that, in this case, prestack corrections
of traveltimes to account for near-surface structure will not enhance
event detectability. Although, the perforation shots have a largely
isotropic radiation pattern the results presented here show the potential
for the use of surface sensors in microseismic monitoring as a viable
alternative to classical downhole methods.
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