making cmp’s from chapter 16 “elements of 3d seismology” by chris liner
TRANSCRIPT
Making CMP’s
From chapter 16 “Elements of 3D Seismology” by Chris Liner
Outline
•Normal Moveout
•Stacking
Normal Moveout
22 2
0 2
xT T
V
22
0 0 02( ) ( )
xT x T x T T T
V
x
T
Hyperbola:
Normal Moveoutx
T
“Overcorrected”
Normal Moveout is too large
Chosen velocity for NMO is too
(a) large (b) small
Normal Moveoutx
T
“Overcorrected”
Normal Moveout is too large
Chosen velocity for NMO is too
(a) large (b) smallsmall
Normal Moveoutx
T
“Under corrected”
Normal Moveout is too small
Chosen velocity for NMO is
(a) too large
(b) too small
Normal Moveoutx
T
“Under corrected”Normal Moveout is too small
Chosen velocity for NMO is
(a) too largetoo large
(b) too small
Vinterval from Vrms
122 2
1 1interval
1
n n n n
n n
V t V tV
t t
Dix, 1955
2i i
RMSi
V tV
t
Vrms
V1
V2
V3
Vrms < Vinterval
Vinterval from Vrms
Vrms T Vinterval from Vrms ViViT VRMS from V interval1500 0 01500 0.2 1500 450000 15002000 1 2106.537443 4000000 20003000 2 3741.657387 18000000 3000
SUM 3.2 22450000
Primary seismic eventsx
T
x
T
Primary seismic events
x
T
Primary seismic events
x
T
Primary seismic events
Multiples and Primariesx
TM1
M2
Conventional NMO before stackingx
TNMO correction
V=V(depth)
e.g., V=mz + B
M1
M2
“Properly corrected”
Normal Moveout is just right Chosen velocity for NMO is correct
Over-correction (e.g. 80% Vnmo)
x
TNMO correction
V=V(depth)
e.g., V=0.8(mz + B)
M1
M2
x
TM1
M2
f-k filtering before stacking (Ryu)
x
TNMO correction
V=V(depth)
e.g., V=0.8(mz + B)
M1
M2
x
T
M2
Correct back to 100% NMO
x
TNMO correction
V=V(depth)
e.g., V=(mz + B)
M1
M2
x
TM1
M2
Outline
•Convolution and Deconvolution
•Normal Moveout
•Stacking
NMO stretching
V1
V2
T0
“NMO Stretching”
NMO stretching
V1
V2
T0
“NMO Stretching”
V1<V2
NMO stretching
V1
V2
V1<V2
0 0T T0T 1T
1 1T TNMO “stretch” = “linear strain”
Linear strain (%) = final length-original length
original length
X 100 (%)
NMO stretching
V1
V2
V1<V2
0 0T T0T 1T
1 1T T
X 100 (%)
original length = 1T final length = 0T
NMO “stretch” = 0 1
1
T TT
X 100 (%)0
1
1TT
0T
X 100 (%)0
1
1TT
stretching for T=2s,V1=V2=1500 m/s
Green line assumes
V1=V2
Blue line is for general case,
where V1, V2 can be different
and delT0=0.1s (this case: V1=V2)
Matlab code
Stacking
+ + =
+ + =
Stacking improves S/N ratio
+ =
Semblance Analysis
22
1 1 2
22
1 1 2
22
1 1 2
“Semblance”
+
22
3 33
2 2 2
X
Tw
tt (
s)
+ =
Semblance Analysis
+
X
Tw
tt (
s)
V3
V1
V2
V
Peak energy