Transcript Time-reversal acoustics, seismic interferometry and virtual source

Time-reversal acoustics, virtual
source imaging, and seismic
interferometry
Haijiang Zhang
University of Science and Technology of
China
Time-reversal
acoustics
Fink, 1997
Time-reversal and super resolution
More heterogeneous and
scattered the medium
between the source and
the TRM, the more
focused the timereversed signal.
Time-reversed seismic location
– No picking required
– Automatic (realtime) location
– Using full
waveforms
Virtual source method
Sources Sk on the surface
Receivers in the borehole
The goal is to
construct seismic
record from virtual
source to receivers.
Bakulin and Calvert, 2006
Synthetic velocity
models
Virtual source gather (at 900 m)
Real source gather (at 900 m)
Constructing virtual source (VS) data
Ska (t)  wk (t) S˜ ka (t)
Bakulin and Calvert, 2006
Applications
Problem: Overburden+statics defocus VSP migration
Solution: VSP -> SWP Transform (Calvert, Bakulin)
VSP
VSP
SWP
Redatum sources below overburden
Local VSP migration
Shuster, SEG 09
VSP Geometry
1500
Reflection
wavefield
Depth
(m)
0
3500
0
Offset (m)
Time (s)
1000
(He , 2006)
3
VSP Geometry
1500
Reflection
wavefield
Depth
(m)
superresolution
0
3500
China
0
Offset (m)
Time (s)
1000
(He , 2006)
3
VSP Salt Flank Imaging
(Hornby & Yu, 2006)
120 shots
Overburden
?
98 geophones
Poor image of flank by standard migration
Interferometric Migration Result
Seismic Interferometry
• Interferometry is a family of techniques in
which waves, are superimposed in order to
extract information about the waves (from
wikipedia).
• Seismic Interferometry involves cross
correlating and stacking diffusive waves to
extract Green’s functions.
Cross-correlation and stacking
Gsa(t): Green’s function from source s to a.
e(t): Source time function.
Cross-correlation
Stacking: source averaging
Homogeneous medium
Circular sources
a) τ= -3T0
b) τ= -T0
c) Τ=10T0
Larose et al., 2005
Homogeneous medium
800 sources (partially
covered)
a) τ= -3T0
b) τ= -T0
c) Τ=10T0
Heterogeous medium
800 sources (partially covered)
a) first-part coda
b) late coda
Time-reversal interpretation of the
correlation process
Treat a as the virtual source
(Weaver and Lobkis, Ultrasonics, 40, 435-439, 2002)
20 00
1.1
1.0
noise
autocorrelation
15 00
10 00
0.9
0.8
50 0
0.7
0
0.6
0.5
-500
0.4
-1000
Pulse/echo
0.3
-1500
0.2
-2000
0.1
0.0
-2500
-0.1
-3000
-0.2
-3500
-0.3
-4000
-0.4
-0.5
-4500
20
30
40
50
60
70
80
90
10 0
11 0
time (microseconds)
(Weaver and Lobkis, Ultrasonics, 40, 435-439, 2002)