Assignment Task
Velocity analysis and imaging
i) What kind of map/plot is displayed in the left panel?
ii) What do the white and black curves in the left window represent?
iii) What sort of gather is shown in the middle panel?
iv) What kind of correction is applied to the middle panel? Show a formula for it.
v) Finally what does the “Stack” in the right panel mean, and how is it com- puted?
vi) Shows a CMP (or CDP) gather and an associated Semblance map. CMP (CDP) gather
vii) Explain shortly what happens in seismic migration. Make a simple figure show- ing a dipping interface in a unmigrated seismic section and the corresponding interface after migration. Use the figure to find a simple mathematical relation between the dip angles of the interfaces in the unmigrated and migrated sections. We assume that the velocities are homogeneous through the section.
2. Refraction seismic
1) Draw a figure with two horizontal layers. Assume that we have a seismic source and receivers on the surface of the top layer, and draw the direct wave and the refracted wave associated with the interface between the two layers. What are the necessary conditions (mention 2) for us to be able to measure this refracted wave?
Snell’s law can be written as
sin θ1/ V1 = sin θ2/ V2
i) Make a sketch showing what the symbols in the equation represent and use the equation to derive an expression for the critical angle (assume that the velocity in layer 2 is greater than the velocity in layer 1).
ii) Determine the critical angle for an interface between two layers with veloci- ties of 2200 m/s and 3000 m/s.