Page 443 - Acquisition and Processing of Marine Seismic Data

434 9. VELOCITY ANALYSIS

in panel A) is assigned to the location indicated for two reflection hyperbolas in the input CDP
by A1 in the time-velocity panel, which is the gather, and can be used to flatten these two
point where 1500 m/s velocity and t(0) ¼ 0ms reflections during NMO correction. In this
zero-offset time intersect. Similarly, the other way, we hopefully get an individual enclosure
semblance values calculated for the rest of the in the semblance plot for each reflection hyper-
velocity values for t(0) ¼ 0 ms are placed at the bola in a recorded CDP, emanating exactly at
points marked from A2 to A6. Then, t(0) time the hyperbola’s t(0) time and RMS velocity.
is increased by a certain amount, e.g., 50 ms Selecting the velocity values from the horizon-
(panel B), and a new set of theoretical reflection tal axis against the zero-offset times from the
hyperbolas is obtained (dark blue curves) with vertical axis of these maximum enclosures
the same range of velocities. Six different sem- provides the RMS velocity function of the
blance values are calculated using the ampli- input CDP from zero to maximum trace time.
tudes coinciding with these hyperbolas on the This procedure of selecting time-velocity
recorded CDP, and are assigned to the relevant pairs from semblance plots is known as velocity
points (from B1 to B6) on the time-velocity panel. picking and is the standard procedure today in
The process goes on in this fashion: the sem- deriving the 2D and 3D velocity fields from mul-
blances are calculated for each hyperbola until tichannel seismic data. If this process is repeated
t(0) reaches the maximum trace time (t max )of for several CDPs with a certain CDP increment
the input CDP, and the calculated values are along the seismic lines in the survey, a 2D or
placed at the specific location in the time- 3D velocity field can be obtained for the whole
velocity panel determined by the velocity and survey. Fig. 9.8A shows an example semblance
t(0) time they pertain to. Thus, the time-velocity plot from a shallow 2D seismic line, and
panel is completed by the semblance values, Fig. 9.8B illustrates picked RMS velocity (dashed
each representing a different velocity-zero offset white line) and interval velocity functions (solid
time pair. black line) calculated by the Dix equation given
The key point here is that the semblance by Eq. (9.9). If the velocity functions picked from
value will be maximum only if one of these semblance plots of several CDPs along the seis-
theoretical reflection hyperbolas exactly coin- mic line are mapped, a velocity field can be con-
cides with an observed reflection hyperbola stituted, in which the variations in the RMS
in the recorded CDP gather. For instance, in velocity both in time and space directions
the schematic illustration in Fig. 9.7,two theo- are observed, as in Fig. 9.8C. This 2D or 3D
retical hyperbolas in panel B number 3 and RMS velocity field can be transformed into 2D
panel E number 5 (hyperbolas with dotted or 3D interval velocity using the Dix equation
curves in the upper panels) exactly coincide (Fig. 9.8D).
with two reflection hyperbolas of the input Furthermore, in practice, not only amplitudes
CDP. Therefore, summation (and hence, their that exactly coincide with the theoretical hyper-
semblance value) through these two specific bolas are involved in semblance calculations,
hyperbolas will be much higher than the sum- but also all the amplitudes within a narrow time
mations along the remaining theoretical hyper- gate along the theoretical hyperbola are included
bolas. If we prepare a contour map of the time- in the calculations (Fig. 9.9). This is because the
velocity panel of semblance values, we get the reflection signal is not a spike and consists of a
semblance plot displayed in the bottom right wavelet with a certain time length, or period.
of Fig. 9.7, which schematically shows two Therefore, the width of the time gate must be
enclosures at points B3 and E5. The velocities selected as close as possible to the dominant
associated with these two points (2500 and period of the seismic wavelet. Once the dominant
3500 m/s, respectively) are the RMS velocities frequency is calculated from the spectral analysis