Geoscience Reference
In-Depth Information
45
°
phase
Minimum-phase
Zero-phase
90
°
phase
T=0
Fig. 2.22
Comparison of zero-phase and minimum-phase wavelets. Also shown are zero-phase
wavelets that have had their phases rotated by 45
◦
and 90
◦
; the latter develops an antisymmetric
form. The desired output from seismic data processing is usually a seismic section that represents
the earth reflectivity convolved with a zero-phase wavelet, because such a wavelet has the greatest
resolution for any given bandwidth. Seismic sources cannot be zero-phase since that would imply
output before time zero. Most air guns give signatures that are close to minimum-phase. This has
the maximum amount of front-loading of the wavelet possible for any given amplitude spectrum.
During processing the data are converted to zero-phase.
data since later processes such as velocity analysis will benefit from such data. However,
the earth attenuates higher frequencies in the wavelet as it passes through it, and this
alters the phase of the wavelet. Even if the wavelet going into the ground were to be
zero-phase, it would not remain so at depth. To combat this, if the zero-phase option is
chosen, then one needs to account for the phase distortion with depth by also applying
a deabsorption filter. If the minimum-phase option is chosen then the application of
the deabsorption filter is usually neglected, since absorption itself is a minimum-phase
process. Thus, the wavelet remains minimum-phase (but since it is not a constant phase
wavelet, its change with depth makes it impossible to zero-phase the entire section later
on).
Figure 2.23
shows a wavelet before and after designature.
Marine data are normally recorded with a 2 ms sampling interval. This is sufficient
to record frequencies up to 250 Hz, much higher than the frequencies that are actually
recorded from the earth, particularly at the deeper target levels where it is uncommon to
record frequencies higher than 30-50 Hz. The data are usually resampled to 4 ms, which
is sufficient for frequencies up to 125 Hz. An anti-alias filter is applied to ensure that any
higher frequencies in the near surface do not alias on to lower frequencies. This resam-
pling reduces the volume of data by 50% and speeds up all of the later processing stages.
