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NAME: __________________________________ LAB SECTION: __________________________________
LAB 8 :: SEISMIC REFLECTION
Define the term “normal moveout”. Be specific. To what type of reflectors does this term apply?
The moveout data below was collected during a seismic reflection survey using the fixed source method. Only the moveout data from the first reflector is given. Plot the data below in Excel (just copy and paste from the online version) and use the data to calculate various useful parameters as outlined below. Please show all calculation work in the space provided below and include your plot with your write-up.
Distance (m) Time (milliseconds) -10 76. -9 75. -8 75. -7 75. -6 75. -5 75. -4 75. -3 75. -2 75. -1 75. 0 75. 1 75. 2 75. 3 75. 4 75. 5 75. 6 75. 7 75. 8 75. 9 75. 10 76.
The following lab will introduce you to the basic concepts of seismic reflection as well as some data collected during seismic reflection surveys. You will use your knowledge of seismic reflection to calculate various parameters of interest. As always, you must show all of your work to get full credit. Your Excel graph should be printed out with your report.
Part I :: Moveout
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a) Determine the velocity (in m/s) of the layer making no assumptions about Δt being small. I.e. use equation 7. from your text. To determine the max error, use the data from x=0 m and x=10 m.
b) Now let’s assume that the receiver offsets are small compared to the layer thickness. Solve for the layer velocity (in m/s) using the simplified equation 7.3 in your text. Again, use the x=0 m and x=10 m data points.
c) Given your answers in a-b, do you think your assumption in part b is reasonable for this data set? To be quantitative about your decision, report the % error that results from your assumption.
d) Now that you know the layer’s velocity solve for the layer thickness, h1 (in meters).
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- Using the provided box, make a sketch of the subsurface interface geometry that would produce the moveout pattern plotted below. Note that the shot point was located at x=0 m.
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Define the term migration as it applies to seismic reflection. Why are many seismic reflection sections left unmigrated?
Given the unmigrated data below, sketch the likely geometry of the interface(s) after migration.
Part II :: Migration
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- Use the data below to make the requested calculations. Assume that all rays are traveling nearly vertically. Use units of grams, seconds, and meters.
a) Calculate the acoustic impedance for each layer.
b) Calculate the reflection coefficients for each interface.
c) Calculate the transmission coefficients for each interface.
Part III :: Acoustic Impedance, Reflection Coefficients and Transmission Coefficients
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d) What percentage of the amplitude of an incident ray originating at the surface will reach the lowest shale region? Assume no other attenuation.
e) Which interfaces produce reflected rays that will be recorded out of phase with respect to the original wave? How do you know?
f) Which interface will be easiest to detect in a seismic reflection survey? Why? Which will be easiest to detect? Why?
