Field data are recorded in a multiplexed mode using a certain type of format. The data first are demultiplexed as described in Figure 1.5-2. Mathematically, demultiplexing is seen as transposing a big matrix so that the columns of the resulting matrix can be read as seismic traces recorded at different offsets with a common shot point. At this stage, the data are converted to a convenient format that is used throughout processing. This format is determined by the type of processing system and the individual company. A common format used in the seismic industry for data exchange is SEG-Y, established by the Society of Exploration Geophysicists.
|Shot interval in m||25|
|Group interval in m||25|
|Number of receiver groups||180|
|Minimum offset in m||175|
|Maximum offset in m||4,650|
|CMP interval in m||12.5|
|Fold of coverage||90|
|Number of CMPs||6,212|
|Line length in km||77.64|
|Sampling interval in ms||4|
|Maximum time in ms||8,000|
|Data volume in gigabytes||4.5|
Figure 1.5-3 shows selected shot records along the Caspian line under consideration. Note the strong amplitudes at the early part and the relatively weaker energy at the deeper part of the records. Such decay in amplitude primarily is caused by wavefront divergence. The dispersive nature of the guided waves resulting from normal-mode propagation within the water layer appears to vary from record to record. This results from a combination of varying water depth, depth of the source array, and water-bottom conditions (Analysis of Guided Waves).
Preprocessing also involves trace editing. Noisy traces, traces with transient glitches (see Figure 1.3-40), or monofrequency signals (see Figure 1.3-3) are deleted; polarity reversals (see Figure 1.3-2) are corrected. In case of very shallow marine data, guided waves are muted since they travel horizontally within the water layer and do not contain reflections from the substratum.
As seen in Figure 1.5-3, most marine data are contaminated by swell noise and cable noise. These types of noise carry very low-frequency energy but can be high in amplitudes. They can be recognized by their distinctive linear pattern and vertical streaks. The swell noise and cable noise are removed from shot records by a low-cut filtering as shown in Figure 1.5-4. Attenuation of coherent linear noise associated with side scatterers and ground roll may require techniques based on dip filtering (noise and multiple attenuation).
Following the trace editing and prefiltering, a gain recovery function is applied to the data to correct for the amplitude effects of spherical wavefront divergence. This amounts to applying a geometric spreading function, which depends on travel time (gain applications). Optionally, this amplitude correction is made dependent on a spatially averaged velocity function, which is associated with primary reflections in a particular survey area. Additionally, an exponential gain function may be used to compensate for attenuation losses.
The data in Figure 1.5-5 have been corrected for geometric spreading using a t2 scaling function. While primary reflection amplitudes are corrected for wavefront divergence, energy associated with multiple reflections, coherent linear noise generated by water-bottom point scatterers and the recording cable, and random noise also is inevitably boosted by geometric spreading correction.
Finally, field geometry is merged with the seismic data. This precedes any gain correction that is offset-dependent. Based on survey information for land data or navigation information for marine data, coordinates of shot and receiver locations for all traces are stored on trace headers. Changes in shot and receiver locations are handled properly based on the information available in the observer’s log. Many types of processing problems arise from setting up the field geometry, incorrectly. As a result, the quality of a stacked section can be degraded severely.
For land data, elevation statics are applied at this stage to reduce traveltimes to a common datum level. This level may be flat or vary (floating datum) along the line. Reduction of traveltimes to a datum usually requires correction for the near-surface weathering layer in addition to differences in elevation of source and receiver stations. Estimation and correction for the near-surface effects usually are performed using refracted arrivals associated with the base of the weathering layer (refraction statics corrections).
- CMP sorting
- Velocity analysis
- Normal-moveout correction
- Multiple attenuation
- Dip-moveout correction
- CMP stacking
- Poststack processing
- Residual statics corrections
- Quality control in processing
- Parsimony in processing