Head waves (refractions) and effect of hidden layer
|Series||Geophysical References Series|
|Title||Problems in Exploration Seismology and their Solutions|
|Author||Lloyd P. Geldart and Robert E. Sheriff|
|Pages||79 - 140|
|Store||SEG Online Store|
Derive traveltime equations for head waves [equations (4.18a,b)]. What is the significance of intercept times and in these equations?
When a P-wave is refracted at a horizontal interface between two layers of different velocities, equation (3.1a) shows that . When , , so if increases sufficiently, will eventually reach its maximum value of , at which point the refracted wave will be traveling parallel to the interface as a head wave or a “refraction.” As the head wave progresses along the interface, it radiates energy upward into the first medium at the same angle as the angle of incidence on that interface. From equation (3.1a) we see that the angle of incidence at which the refraction is generated is (see Figure 4.18a); is called the critical angle. In addition to the head wave, a direct wave travels directly from the source to the receiver, that is, along the line in Figure 4.18a. Near the source the direct wave arrives before the refraction, but eventually the higher-velocity path of the refraction allows it to overtake the direct wave at point , the crossover point. The refraction exists at offsets greater than the critical distance , but between and it is often obscured by the earlier direct wave.
The traveltime curve of the direct wave is given by , where is the offset. For a two-layer situation the traveltime curve for the refraction from a horizontal refractor at
depth (Figure 4.18a) is (Sheriff and Geldart, 1995, 95)
The traveltime curve is a straight line of slope with a projected intercept on the -axis equal to . The slope of the direct-wave traveltime curve gives , while that of the refraction curve gives . Although the refraction curve does not extend to the -axis, we can project it to get the intercept . Thus we can find and get the depth from .
For a three-layer situation (Figure 4.18b) such that , two refractions exist. The shallow one has the critical angle , where , whereas the deeper refraction has angles of incidence given by . The traveltime curve for the shallow refraction is given by equation (4.18a) while that for the deeper refractor is (Sheriff and Geldart, 1995, 96)
Note that the first term in equation (4.18c) is not in equation (4.18a) because .
Equation (4.18c) can be generalized for layers in the form
A typical refraction path in Figure 4.18a is OMPR. Taking , the traveltime is
Equation (4.18b) can be derived in a similar way; from Figure 4.18b,
The traveltime curve is again a straight line with slope and intercept .
Show that the two geological sections illustrated in Figure 4.18c produce the same time-distance refraction curves.
Although no refraction will be generated at the intermediate interface because the velocity decreases at the interface, equation (4.18b) is still valid. Therefore, for the three-layer case in Figure 4.18c, Snell’s law gives
thus, , , and equation (4.18b) gives
For the two-layer case in Figure 4.18c, we use equation (4.18a); since ,
The effect of the 1.5 km/s hidden layer is to shorten the critical distance ( in Figure 4.18a), but this would probably not be detected and hence the depth to would probably be overestimated, for example,
What would be the depth to the top of the 6 km/s layer in the three-layer case of Figure 4.18c if km/s instead of 1.5 km/s?
When is 4.00 km/s, we have for the shallow headwave:
For the headwave from the 6.00 km/s layer,
Figure 4.18d shows the plot of x versus
While the 4.00 km/s layer does generate a refraction event, it might not be evident that it is a separate alignment. We might interpret the results as a two-layer problem missing the 4.00 km/s event, thus underestimating the average velocity to the 6 km/s layer and under-estimating its depth. If we should see the intermediate layer clearly enough, we could obtain the depths correctly.
What would be the apparent depth to the refractor in problem 4.18b if km/s instead of 6.00 km/s?
In the three-layer case, we have
If we do not know that the 1.50 Km/s layer exists, we would take , , , , km. For the two-layer case we would calculate the depth correctly since there is no hidden layer. However, we might not recognize the headwave since its slope (1/4.00) is not much different from that of the direct wave (1/3.00) (see Figure 4.18d).
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Also in this chapter
- Accuracy of normal-moveout calculations
- Dip, cross-dip, and angle of approach
- Relationship for a dipping bed
- Reflector dip in terms of traveltimes squared
- Second approximation for dip moveout
- Calculation of reflector depths and dips
- Plotting raypaths for primary and multiple reflections
- Effect of migration on plotted reflector locations
- Resolution of cross-dip
- Variation of reflection point with offset
- Functional fits for velocity-depth data
- Relation between average and rms velocities
- Vertical depth calculations using velocity functions
- Depth and dip calculations using velocity functions
- Weathering corrections and dip/depth calculations
- Using a velocity function linear with depth
- Head waves (refractions) and effect of hidden layer
- Interpretation of sonobuoy data
- Diving waves
- Linear increase in velocity above a refractor
- Time-distance curves for various situations
- Locating the bottom of a borehole
- Two-layer refraction problem