Page 435 - Acquisition and Processing of Marine Seismic Data

426 9. VELOCITY ANALYSIS

FIG. 9.2 Schematic display of difference between the hyperbolas used to calculate NMO and stacking velocities. NMO
velocity is obtained from the reflections of small spread hyperbola (O-A), whereas stacking velocity is derived from the
best-fit hyperbola to full spread (O-B). Modified from Hubral, P., Krey, T., 1980. Interval velocities from reflection time measurements,
SEG Monograph.

to convert RMS velocity into interval velocity, the velocity used in seismic processing is that the
which is used by some of the migration algo- velocity required for stacking and migration is
rithms such as prestack Kirchhoff depth migra- not necessarily the same. The stacking velocity
tion. If RMS velocities of two reflections with for dipping layers is given by the Levin equation
arrival times of t 2 and t 1 are V 2 and V 1 , the Dix in which the stacking velocity is obtained by
interval velocity can be expressed as (Dix, 1955) dividing the velocity of the upperlying medium
by the cosine of inclination angle of the interface,
2 2
1
2
V 2 ¼ V t 2 V t 1 (9.9) whereas the velocity of the upperlying medium
DIX
t 2 t 1 itself is used in the migration process. That is,
stacking velocities are dip dependent, while the
migration velocities are not.
9.2 VELOCITY DETERMINATION The basis for the velocity estimation from seis-
FROM SEISMIC DATA mic data depends on the NMOs of the reflection
hyperbolas. For a given hyperbola, the higher the
Basically, seismic velocity is obtained from sonic maximum NMO time, the higher the accuracy of
logs of the nearby wells and from the velocity the velocity estimated from the data for that spe-
analysis of multichannel seismic data. Velocity cific reflection. The point here is that the NMO
functions from sonic logs are much more time decreases as the depth of reflections
detailed, and only a smoothed version of sonic increases (Section 1.3.4). The same issue also
log velocity can be derived from seismic surveys. arises for short cable lengths: maximum NMO
Obtaining velocity from seismic data is a regular time is relatively small for the data collected
process, because the velocity distribution in 2D with short streamers. Therefore, seismic velocity
and 3D is required for subsequent NMO correc- of the deeper reflections generally has low
tion, migration, and some of the demultiple resolution as compared to that in the shallow
methods. Oneofthe mostimportant points about parts. Similarly, the velocity obtained from