AVO versus AVA and effect of velocity gradient
How would you recalibrate the scale to change a plot showing amplitude variation with offset (AVO) into a plot of amplitude variation with angle (AVA)? What will be the effect if velocity increases with depth?
Assuming constant velocity above a horizontal reflector, the angle of incidence , where is the offset and the depth. We can calculate for a series of values and then stretch the scale to make it linear in . The amount of stretch increases as and increase.
If the velocity increases with depth, straight raypaths become curved as shown by the solid lines in Figure 3.11a. The effect is to increase as compared with the constant velocity case. Thus angles of incidence are often larger than assumed.
Calculate the angles of incidence on a reflector at a depth of 2.00 km at offsets of 2.00 and 3.00 km where the velocity increases linearly with depth from 2.20 km/s at the surface to 3.10 km/s at the reflector depth. Assume straightray travel at the average velocity.
The average velocity is , where is the depth and is the traveltime at the source. Since , . Then,
For km, the angle of incidence (2.00/4.00) . Similarly, for km,
Repeat part (b) for curved raypaths.
We are given , , (which is equivalent to ) and must find the values of for the different offsets. Solving equations to determine both and is not practical, so we assume values of , then use equation (4.17f) to obtain , and equation (4.17b) to find the offset , and then by trial we determine . The results of part (b) suggest that we start with . Then, , . From equation (4.17f) we have
The offset is
We next take , so .
For , we get
We could calculate offsets for intermediate values of , e.g., for , to get more accuracy. However, instead we shall interpolate between these pairs of values. We tabulate the values in Table 3.11a.
Interpretation gives, for the angles of incidence corresponding to and 3.00 km, the values 31.3 and 44.2 respectively. Comparing these with the results in part (b) of 26.6 and 36.9. We see that the angles of incidence for the curved raypaths are 18% and 20% greater than those for the straight-line paths, the difference increasing with offset.
|Variation of reflectivity with angle (AVA)
|Theory of Seismic Waves
|Geometry of seismic waves
Also in this chapter
- General form of Snell’s law
- Reflection/refraction at a solid/solid interface and displacement of a free surface
- Reflection/refraction at a liquid/solid interface
- Zoeppritz’s equations for incident SV- and SH-waves
- Reinforcement depth in marine recording
- Complex coefficient of reflection
- Reflection and transmission coefficients
- Amplitude/energy of reflections and multiples
- Reflection/transmission coefficients at small angles and magnitude
- Variation of reflectivity with angle (AVA)