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To demonstrate effect of using a velocity model differing
from the actual medium velocity I constructed a
synthetic dataset that is representative of the drill-bit
problem:
240 channels,
one source at a depth of 1000 feet, and a velocity 9000 feet/sec.
Random noise was added to seismic traces; the power of the noise
equals the power of the signal. The data were filtered with a
70-100 Hz bandpass filter. The semblance coefficient was computed for two
velocity models: (1) a velocity 9000 feet/sec (corresponding to the
``true'' velocity), and (2) a velocity of 10000 feet/sec.
Figure and show the maximum value of
semblance as a function of depth for these two cases.
It can be seen that using wrong velocity, we still are able
to locate the source, though the depth of maximum is shifted
and the maximum value of the semblance has been decreased.
**v9000
**

Figure 3 v9000

Semblance as function of depth using the true velocity model.

*V*_{true}=*V*_{model}=9000 *feet*/*sec*.

**v10000
**

Figure 4 v10000

Semblance as function of depth using an incorrect velocity model.

*V*_{true}=9000 *feet*/*sec*

*V*_{model}=10000 *feet*/*sec*

** Next:** Small scale velocity fluctuations
** Up:** VELOCITY VARIATIONS
** Previous:** VELOCITY VARIATIONS
Stanford Exploration Project

12/18/1997