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Ray Paths - Seismology - Lab Notes, Study notes of Geology

AMET UniversityGeology

Fundamentals of these Lab Notes are as follows : Ray Paths, Geophone, Direct Ray, Reflected Ray, Crossover Distance, Critical Angle, First Arrival, Geophone, Layer Thickness, Algebra Skills

Typology: Study notes

2012/2013

Uploaded on 07/19/2013

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Lab 6 :: Seismic Refraction Lab GLY3160 / PHY3160
Page 1 of 7
NAME: __________________________________ LAB SECTION: __________________________________
LAB 6 :: SEISMIC REFRACTION
1) On the picture below: Draw and label the paths of the direct ray, the reflected ray, and the refracted ray to both
geophones. Note that geophone 1 is at the critical distance and geophone 2 is at the crossover distance . You
donโ€™t have to make your drawing perfectly to scale, but use a straight edge and label the critical angle, ic ,
everywhere that it occurs.
2) Which ray is the first arrival at distances less than geophone 1โ€™s location? Why?
3) What ray arrives first between geophone 1 & 2? Why? What ray arrives first after geophone 2? Why?
4) Which ray is never the first arrival? Why?
The following lab will introduce you to the basic concepts of seismic refraction as well as some actual data collected during
seismic refraction surveys. You will use your knowledge of seismic refraction to calculate various parameters of interest.
Part I :: Ray Paths
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Page 1 of 7

NAME: __________________________________ LAB SECTION: __________________________________

LAB 6 :: SEISMIC REFRACTION

  1. On the picture below: Draw and label the paths of the direct ray, the reflected ray, and the refracted ray to both geophones. Note that geophone 1 is at the critical distance and geophone 2 is at the crossover distance. You donโ€™t have to make your drawing perfectly to scale, but use a straight edge and label the critical angle, ic , everywhere that it occurs.

  2. Which ray is the first arrival at distances less than geophone 1โ€™s location? Why?

  3. What ray arrives first between geophone 1 & 2? Why? What ray arrives first after geophone 2? Why?

  4. Which ray is never the first arrival? Why?

The following lab will introduce you to the basic concepts of seismic refraction as well as some actual data collected during seismic refraction surveys. You will use your knowledge of seismic refraction to calculate various parameters of interest.

Part I :: Ray Paths

Page 2 of 7

  1. Using your trigonometry and algebra skills, show that the equation for the critical distance is:

๐‘‹๐‘ = 2โ„Ž 1 ๐‘ก๐‘Ž๐‘› [๐‘Ž๐‘Ÿ๐‘๐‘ ๐‘–๐‘› (

)]

Where h 1 , is the upper layer thickness and v 1 and v 2 are the velocities of the two layers_._ Make sure to include a sketch of your setup and describe what you are doing. Please be neat and explain your steps. Hint: recall that the critical angle, ic , depends on the two velocities, v 1 and v 2.

Any student that can show that ๐‘‹๐‘ = 2โ„Ž 1 ๐‘ก๐‘Ž๐‘› [๐‘Ž๐‘Ÿ๐‘๐‘ ๐‘–๐‘› ( ๐‘ฃ 1 ๐‘ฃ 2 )] =^

2โ„Ž 1 [(๐‘ฃ2๐‘ฃ1) 2 โˆ’1]

1/2 will receive bonus points.

  1. What are the equations for the arrival times of the direct ray and the first refracted ray given the layer thicknesses and the layer velocities? Describe each parameter in these equations.

Page 4 of 7

  1. How many layers did you detect? How did you distinguish where a new layer started on the t-x diagram?

  2. What are the velocities of each layer in km/s? Show/Explain your work (as always!).

  3. How many layer thicknesses can you determine from this data? What are the thicknesses of each layer in meters? Excluding the first layer, what are the depths to the top of each layer in meters?

Page 5 of 7

  1. Construct a t-x diagram (using Excel) from the data below. Plot both data sets on the same plot. Note that both surveys had the same geophone locations but the reverse was shot backwards. Hint: Instead of plotting them like the forward lines and reverse lines in your textbook, I recommend plotting them both as forward lines and seeing if each velocity and thickness calculations yield similar values.

DISTANCE TRAVEL TIMES (MILLISECONDS)

(M) FORWARD REVERSE

  1. What does this data tell you about the interface(s) geometry at depth?

  2. Fill in the box below with a simplified sketch of the subsurface interface geometry suggested by this data.

Page 7 of 7

  1. Using your existing Maple solution, determine the maximum thickness of layer 2 that would remain mathematically impossible to detect using seismic refraction. Use the parameters in the figure below. Be sure to put text comments into your Maple file so that I know what you did and why. To double check your answer, plot the predicted linear functions for the direct ray (red), the 1st^ refracted ray (blue) and the second refracted ray (green) on the same plot. Explain your graph in the Maple file using text comments. Do not use guess and check. You can directly solve for the maximum undetectable thickness.

  2. In practice, how much thicker than this maximum thickness would the layer need to be in order to be detectable in a reasonable seismic refraction survey? Why? Hint: Most seismic refraction surveys are limited to 12 or 24 geophones.

  3. The previous question highlighted one of the known caveats of seismic refraction. What is the other known limitation of seismic refraction and why does this happen?