The main goal of seismic processing is to obtain the best
image of the subsurface. To achieve this goal the seismic processing should
improve the signal-to-noise ratio and locate the reflections in their
real spatial position.
image of the subsurface. To achieve this goal the seismic processing should
improve the signal-to-noise ratio and locate the reflections in their
real spatial position.
We refer to as noise any energy that is
recorded and does not come from the primary reflections. There are two types of
noise:
- Coherent noise: Seismic energy that is consistent from trace to
trace. The most common sources of coherent noise are interbed multiples, ground
roll, power lines and surface vibrations. - Random or ambient noise: Energy that lacks any relationship between
traces. Usually, the random noise is caused by instrumental noise, winds and
geophone coupling problems.
The most effective noise attenuation method (especially for
random noise) is CMP stacking. Coherent noise is usually more difficult to
suppress, and needs more specialized processes as: radon filters (multiple
suppression), notch filters (power line noise), f-k filter (wind noise),
etc.
The second task of seismic processing is to locate the
reflections in their real spatial position; this task is known as
imaging. The method used to archive this task depends on the
acquisition geometry. Today the most used acquisition geometry is known as
multifold acquisition geometry. Figure 1
shows a schematic representation of the multifold geometry, this geometry
consists on a number of receiver stations that are separated the same distance
(station distance). Each receiver station records the wavefront produced by the
seismic source; after the wavefront is recorded (during a fix time interval
known as record length), the receivers are move for the next seismic shot
location.
d('GP3102F01','Figure 1')<
After the data is loaded, and some initial processes are
applied, it is sorted from the acquisition domain (shot gather domain) to the
common-mid-point domain (CMP). The CMP domain is explained in the Figure 2;
the data is sorted in groups (gathers) of traces that have the same
source-receiver mid point. In this domain is where the most important imaging
processes are applied.
d('GP3102F02','Figure 2')
After the stacking, the seismic section usually does not
represent accurately the location of the reflector. This is because of the
normal incidence travel path is only valid for horizontal seismic interfaces.
The process used to correct this effect is called seismic migration. Seismic
migration improves the seismic image because the locations of subsurface
structures (especially faults) are correct in migrated seismic data. Migration
collapses diffractions from discontinuities and corrects bow ties to form
synclines.
The most important decision to be taken during a seismic
processing project is the processing flow. The processing flow should be adapted
to the seismic data characteristics. The ability of the processor to find the
best combination of process is critical for the quality of the final
section.
