Annual Report 2000 - WIT

103
summation (Hanitzsch, 1997). We avoid stacking the migrated result over the offset
axis in order to gain a true-amplitude migrated data cube. This yields amplitudeversus-offset
information at every depth point in the migrated image. From imagegathers
at one surface location we picked the maximum amplitude at one reflector
point for every offset contained in the data in order to obtain the desired AVO curves.
0.4 0.8 1.2 1.6
Depth [km]
Receiver Offset [km]
0 0.4 0.8 1.2 1.6
|Rpp|
0.2
0.175
0.15
0.125
0.1
0.075
0.05
0.025
AVO for Dipping Layer
Analytically calculated
Migration result
Imagegather
0
0 0.5 1 1.5 2 2.5 3
Offset [km]
Figure 5: (a) Imagegather of the migrated result at 5.5 km. Residual moveout is caused
by the difference between the model used to generate seismograms and the smooth
model used for migration. (b) AVO curve from the imagegather to the left. The solid
line denotes the expected amplitudes derived by Zoeppritz' equations and circles denote
our migration result
Amplitude
3.5
3
2.5
2
1.5
1
0.5
Amplitude Comparison in Shotgather
x 10−3
4
Demigration Result
Input Seismogram
0
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2
Offset [km]
0.0003 0.0002 1e-04 0
Spectral Amplitude
Spectral Comparison
0 5 10 15 20 25 30 35 40 45
Frequency [Hertz]
Figure 6: (a) Comparison of amplitudes of one demigrated section (blue) with original
seismogram (red). (b) Comparison of frequency content of the demigrated section
(red) with the original seismogram (blue).
We tested our migration result by comparing picked amplitudes in the imagegather
of the migration output at a certain distance (Figure 5 a) with the analytical reflectivity