Determining reflector location
The arrival time of a reflection at the source point is 1.200 s, near-surface corrections having been applied. Determine the reflector depth and horizontal location with respect to the source point, assuming zero dip and that the average velocity associated with a vertical traveltime is
After records have been picked, that is, after reflections have been identified and and measured, the next stage is to prepare a section displaying the reflection events in two-dimensions. Such a section can be prepared in several ways, one of which is by using a wavefront chart such as that shown in Figure 8.20a. A wavefront chart is a two-dimensional graph showing approximate wavefronts and raypaths for a given distribution of constant-velocity layers, Raypaths are found by starting with rays leaving the source at different angles and tracing them downward. Traveltimes to various points on the raypaths are calculated and contoured to show wavefronts. This assumes that waves started from, and returned to, the source, hence they must have been reflected by a bed perpendicular to the ray (parallel to a wavefront); thus the dip as well as the location of the reflector is determined. The reflection event denoted by the symbol -o- in Figure 8.20a corresponds to 110 ms/km.
We are given ; hence vertical depth, and the location is directly below the source.
Determine the reflector depth, dip, and horizontal location assuming that the dip moveout is 0.150 s/km and that the line is normal to strike.
As in (a) , but now it is slant depth;
Determine the depth, dip, and horizontal location assuming straight-line travel at the angle of approach and .
Because the path is a straight line, the velocity must be constant and we assume it is the starting velocity of 1830 m/s. Then,
Determine the reflector depth, dip, and horizontal location assuming straight-line travel at the local velocity above the reflector, 3840 m/s.
Assume that the migrated position is determined from the wavefront chart in Figure 8.20d.
Figure 8.20a gives 1200 m vertical depth, 170 m horizontal displacement, and dip.
Results are summarized in Table 8.20e, z being the vertical depth and the horizontal displacement.
|a) Vertical path||2630||1580||0|
|b) Dip moveout 150 ms/km||2630||1550||310|
|c) At approach angle||1830||1090||150|
|d) At local velocity||3840||2200||660|
|e) Curved raypath||1200||170|
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|Determining static corrections from first breaks||Blondeau weathering corrections|
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|Seismic equipment||Data processing|
Also in this chapter
- Effect of too many groups connected to the cable
- Reflection-point smear for dipping reflectors
- Stacking charts
- Attenuation of air waves
- Maximum array length for given apparent velocity
- Response of a linear array
- Directivities of linear arrays and linear sources
- Tapered arrays
- Directivity of marine arrays
- Response of a triangular array
- Noise tests
- Selecting optimum field methods
- Optimizing field layouts
- Determining vibroseis parameters
- Selecting survey parameters
- Effect of signal/noise ratio on event picking
- Interpreting uphole surveys
- Weathering and elevation (near-surface) corrections
- Determining static corrections from first breaks
- Determining reflector location
- Blondeau weathering corrections