Effects of weathered layer (LVL) and permafrost
Assume that raypaths have angles of approach of , , , , and in the subweathering where the velocity is 2400 m/s. For a weathered layer 10 m thick with velocity 500 m/s, how do travel-times through the weathered layer compare with that for a vertically traveling ray? What are the horizontal components of the raypaths in the LVL?
Referring to Figure 5.10a,
|Incident||In low-velocity layer||In permafrost|
Substituting the values of , we get the results in Table 5.10a. The traveltimes in the LVL vary by only 0.5% over most of the range of , and, even for , change by only 1.5%.
For permafrost 100 m thick with a velocity of 3600 m/s, answer the questions in part (a).
We repeat the calculations of part (a) changing to 3.60 km/s and layer thickness to 100 m. The results are also shown in Table 5.10a. Because rays now have large horizontal components, the changes in and are considerable. This large ray bending makes corrections for permafrost very difficult. If , upcoming waves are totally reflected.
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|Effect of overpressure||Horizontal component of head waves|
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|Geometry of seismic waves||Characteristics of seismic events|
Also in this chapter
- Maximum porosity versus depth
- Relation between lithology and seismic velocities
- Porosities, velocities, and densities of rocks
- Velocities in limestone and sandstone
- Dependence of velocity-depth curves on geology
- Effect of burial history on velocity
- Determining lithology from well-velocity surveys
- Reflectivity versus water saturation
- Effect of overpressure
- Effects of weathered layer (LVL) and permafrost
- Horizontal component of head waves
- Stacking velocity versus rms and average velocities
- Quick-look velocity analysis and effects of errors
- Well-velocity survey
- Interval velocities
- Finding velocity
- Effect of timing errors on stacking velocity, depth, and dip
- Estimating lithology from stacking velocity
- Velocity versus depth from sonobuoy data
- Influence of direction on velocity analyses
- Effect of time picks, NMO stretch, and datum choice on stacking velocity