OBJECTIVE OF SEISMIC PROCESSING

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.

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.


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.


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.

BASIC SEISMIC PROCESSING SEQUENCE

Although seismic processing flows must be adapted according to the characteristics of the data, they typically include three major steps:

1.Preprocessing and Deconvolution: The objectives of these steps are to:
- Sort the data in the channel domain (demultiplexing)
- Delete defective traces (trace editing)
- Correct the amplitude of wavefront divergence (gain recovery), datum correction (elevation statics) and remove the seismic source effects (deconvolution).

2.Stacking and Velocity Analysis: During this step the data is:
- Sorted to CMP domain (CMP sorting)
- Moveout velocity is estimated (velocity analysis)
- The moveout is removed (NMO correction) and the reverberations are suppressed (multiple attenuation).

3. Migration: The goal of this step is to locate the reflections in the correct spatial location. This process is called seismic migration; this is a very important step because the locations of subsurface structures depend on correctly selecting the parameters.

Figure 3 shows a flow chart of a basic processing sequence. It is important to mention that static correction is applied to land data and multiple attenuation is a process mainly designed for marine seismic.

Figure 3