Making CMP’s From chapter 16 “Elements of 3D Seismology” by Chris Liner

Outline Normal Moveout Stacking

Normal Moveout x T Hyperbola:

Normal Moveout x T “Overcorrected” Normal Moveout is too large Chosen velocity for NMO is too (a) large (b) small

Normal Moveout x T “Overcorrected” Normal Moveout is too large Chosen velocity for NMO is too small (a) large (b) small

Normal Moveout x T “Under corrected” Normal Moveout is too small Chosen velocity for NMO is (a) too large (b) too small

Normal Moveout x T “Under corrected” Normal Moveout is too small Chosen velocity for NMO is too large (a) too large (b) too small

Vinterval from Vrms Dix, 1955

Vrms V1 V2 V3 Vrms < Vinterval

Vinterval from Vrms

Primary seismic events x T

x T

x T

x T

Multiples and Primaries x T M1 M2

Conventional NMO before stacking x T NMO 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 T NMO correction V=V(depth) e.g., V=0.8(mz + B) M1 M2 x T M1 M2

f-k filtering before stacking (Ryu) x T NMO correction V=V(depth) e.g., V=0.8(mz + B) M1 M2 x T

Correct back to 100% NMO x T NMO correction V=V(depth) e.g., V=(mz + B) M1 M2 x T M1 M2

Outline Convolution and Deconvolution Normal Moveout Stacking

NMO stretching V1 V2 T0T0 “NMO Stretching”

NMO stretching V1V1 V2V2 T0T0 “NMO Stretching” V 1 <V 2

NMO stretching V1V1 V2V2 V 1 <V 2 NMO “stretch” = “linear strain” Linear strain (%) = final length-original length original length X 100 (%)

NMO stretching V1V1 V2V2 V 1 <V 2 X 100 (%) original length = final length = NMO “stretch” = X 100 (%)

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 “Semblance” + X Twtt (s)

+ = Semblance Analysis + X Twtt (s) V3V3 V1V1 V2V2 V Peak energy