Аннотация
Seismic interferometry involves the crosscorrelation of responses
at different receivers to obtain the Green's function between these
receivers. For the simple situation of an impulsive plane wave propagating
along the x-axis, the crosscorrelation of the responses at two receivers
along the x-axis gives the Green's function of the direct wave between
these receivers. When the source function of the plane wave is a
transient (as in exploration seismology) or a noise signal (as in
passive seismology), then the crosscorrelation gives the Green's
function, convolved with the autocorrelation of the source function.
Direct-wave interferometry also holds for 2D and 3D situations, assuming
the receivers are surrounded by a uniform distribution of sources.
In this case, the main contributions to the retrieved direct wave
between the receivers come from sources in Fresnel zones around stationary
points. The main application of direct-wave interferometry is the
retrieval of seismic surface-wave responses from ambient noise and
the subsequent tomographic determination of the surface-wave velocity
distribution of the subsurface. Seismic interferometry is not restricted
to retrieving direct waves between receivers. In a classic paper,
Claerbout shows that the autocorrelation of the transmission response
of a layered medium gives the plane-wave reflection response of that
medium. This is essentially 1D reflected-wave interferometry. Similarly,
the crosscorrelation of the transmission responses, observed at two
receivers, of an arbitrary inhomogeneous medium gives the 3D reflection
response of that medium. One of the main applications of reflected-wave
interferometry is retrieving the seismic reflection response from
ambient noise and imaging of the reflectors in the subsurface. A
common aspect of direct- and reflected-wave interferometry is that
virtual sources are created at positions where there are only receivers
without requiring knowledge of the subsurface medium parameters or
of the positions of the actual sources.
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