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|Title: ||Non-physical energy in seismic interferometry|
|Authors: ||King, Simon James|
|Supervisor(s): ||Curtis, Andrew|
|Issue Date: ||25-Jun-2012|
|Publisher: ||The University of Edinburgh|
|Abstract: ||Non-physical arrivals produced by seismic interferometry, the process whereby
Green’s functions are synthesized between two points by cross-correlation, crossconvolution
or deconvolution, are often considered to provide little information
about the Earth’s subsurface. Their contributions are usually suppressed in
interferometric Green’s function estimates to suit existing methods of seismic
velocity estimation which favour the more familiar physical arrivals. In this thesis
we show that the non-physical arrivals retrieved in exploration-type settings are
useful for determining the long-wavelength seismic velocity structure and can be
used to obtain improved Green’s function estimates.
First, we estimate the seismic velocity and layer thickness by measuring the
signal coherency along traveltime curves between two receivers in a collection of
traces consisting of cross-correlated wavefields, known as the correlation gather.
The traveltime curves represent the traveltime differences between wavefields
recorded at the two receivers. When the procedure is used to find the velocity and
thickness of the uppermost layer, the traveltime curves implicitly incorporate the
physical and non-physical wavefields in the Green’s function estimates. When the
procedure is applied to a model with more than one layer, the traveltime curves
correspond to non-physical wavefields only in the Green’s function estimates.
Instead of suppressing multiple reflections as in conventional methods, the
procedure incorporates the traveltimes of multiple reflections to constrain velocity
and thickness estimates.
The procedure above is most suitable for recovering the first-layer seismic
velocity. We propose a simpler method to estimate the seismic velocities
corresponding to deeper layers. We find that the Green’s functions contain very
weak reflections, but are dominated by non-physical refractions if retrieved using a
limited source aperture. The seismic velocities are easily identifiable as repeating
bright spots after transforming the refraction-dominated Green’s functions to the
− p domain.
We show that non-physical reflections can be used constructively to provide
physical reflections, and therefore improved Green’s function estimates, by using a cross-convolution operation in a new variant of seismic interferometry,
called source-receiver interferometry. We also show that non-physical reflections
associated with the cross-correlation of reflections from different interfaces allow
for the direct estimation of interval velocities and layer thicknesses. This method
removes the necessity to first find the root-mean-square velocities and two-way
traveltimes required to compute the interval velocities by Dix inversion.
Overall, this thesis significantly improves our understanding of how nonphysical
energy in seismic interferometry both provides useful information
about the Earth’s subsurface and contributes to physical energy in particular
|Sponsor(s): ||Natural Environment Research Council (NERC)|
Schlumberger Cambridge Research
|Keywords: ||seismic interferometry|
|Appears in Collections:||Earth and Planetary Science Research Institute thesis and dissertation collection|
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