Geoscience Reference
In-Depth Information
Minimum
Zero
Minimum
Zero
Fig. A2.2
Two examples of zero- and minimum-phase wavelets. In each pair, the zero- and
minimum-phase wavelets have the same amplitude spectrum.
will be needed for the migration aperture. This might typically add a 2 km fringe on each edge of the
area that is to be well imaged. This makes small 3-D surveys relatively expensive; to image an area
9km
×
9 km (81 km
2
) would require acquisition over 13 km
×
13 km (169 km
2
).
Migration velocity
A velocity field used to migrate the seismic data to obtain a well-focussed and correctly located
image. This is usually closer to seismic velocities in the real earth than stacking velocities, particularly
where the velocities have been determined in the course of pre-stack depth migration which allows
for the complexities of ray-bending in the overburden. It may still be quite strongly affected by
anisotropy, however, and not be well suited to depth-converting picked horizons without further
adjustment to tie the well data; lateral resolution of the velocity field may also be an issue for detailed
well ties.
Minimum phase
There is an infinite number of seismic wavelets that share the same amplitude spectrum. One of these
is the minimum-phase wavelet, which is constructed so as to start at zero time and then have as much
energy as possible at the earliest times. In practice, the wavelet will have a maximum value in the first
or second loop
(fig. A2.2
)
. This type of wavelet is close to what is often generated by real physical
sources. Sometimes seismic data are processed to minimum-phase final output. In theory, seismic
reflections should then be picked at the zero-crossing corresponding to the start of the reflected signal
from the particular interface. In practice, they are usually picked at a maximum excursion on either
the first or second loop down from this zero-crossing pick, depending on where the most consistent
loop is found. Potentially, this can cause confusion about the polarity of the data if the picking
philosophy is not carefully documented. A further disadvantage of interpreting minimum-phase data
is that interactions between reflections from closely spaced interfaces are not as easy to visualise as
for the zero-phase case.
