Exercises

1 - Examine the bandpass-filtered test records attached. What is the frequency range (the maximum and minimum frequency) of the surface-wave ground roll? What is its apparent velocity range? Is there any evidence that the apparent velocity changes with location on the reflection line? From the ranges in frequency and apparent velocity compute the total range of wavelength. What maximum receiver spacing g is needed to avoid spatial aliasing of these waves, given their maximum frequency and minimum apparent velocity?

Assuming that these surface waves are Rayleigh waves, what is the corresponding range of V_s? Assuming that all the materials in the section are Poisson solids, what range of V_p are the Rayleigh waves showing? How do these values compare with the refraction results? Does this have something to do with the wavelength of the waves?

Useful equations:

2 - Examine the bandpass-filtered test records for the frequency range of the refractions and reflections. What frequencies show the reflections best? Are there any other types of waves that share this frequency range? Assuming that the reflections arise at depths where V_p = 1.7 km/s, what is the range of reflection wavelengths implied by their range of frequencies?

What is the range of apparent velocity shown by the reflections? For the minimum reflection apparent velocity and maximum reflection frequency you find, compute the maximum g needed to avoid spatially aliasing the reflections. At this minimum apparent velocity, assuming a rock velocity at the surface of V_p = 0.5 km/s, at what angle from the vertical are the reflected waves hitting the receivers?

On a copy of a bandpass-filtered test record section, indicate the shallowest and the deepest reflections you think you can identify. What approximately are the two-way travel times t₀ of these reflections at zero offset? What are their dominate frequencies? For each of these reflections compute its vertical resolution z using the Widess criterion, and its Fresnel radius x_f. Assume V = 1.7 km/s. Would the upper or the lower reflection locate potential faults more accurately?

More useful equations:

3 - In the bandpass-filtered test records, observe the approximate frequency range of the air wave. What is its apparent velocity?

4 - Estimate stacking velocities for NMO correction from the attached suite of constant-velocity stacks from 0.9 to 1.9 km/s. First use proportional dividers or a scale to create yourself two scales at the edges of separate pieces of paper, one for measuring the midpoint coordinate, and the other for measuring time. Note that the midpoint coordinate increases to the north. Then identify at least six of the stronger reflections in the stacks. Where the reflections are strong you should be able to identify several separate reflections in a small area.

Organize the reflections you will pick into two or three columns. Each column is a range of stations for which you have some very shallow, some intermediate, and some deeper reflections. Turn in a copy of one of the CV stacks showing where each column is located, and indicating the reflections you will pick.

Now pick the stacking velocity of each of your selected reflections by flipping through the suite of CV stacks, concentrating on just one reflection at a time. Note the station, time, and velocity where each reflection is strongest and most continuous. Arrange your velocity picks by column in order of increasing time.

5 - Check your velocity picks by computing interval velocities for the intervals between each pick in each of your columns. For example, if one of your columns has three picks, at 0.02, 0.04, and 0.06 s, then you can use the Dix equation to find the rock velocity in two intervals, between 0.02 and 0.04 s, and between 0.04 and 0.06 s. Examine the interval velocities for correctness. Interval velocities above 4 km/s and negative velocities are unacceptable, but can easily result from reasonable picks.

6 - Compute rms velocities from your refraction results to check against your stacking velocity picks. For example, from your 3-layer model you might have V₁ = 0.6 km/s and h₁ = 8 m, V₂ = 1 km/s and h₂ = 15 m, and V₃ = 1.7 km/s. Compute the two-way travel time t_i and rms velocity to three separate reflectors: 1) the V₁-V₂ interface; 2) the V₂-V₃ interface; and 3) to a reflector within the V₃ layer 20 m below the V₂-V₃ interface.

7 - Now adjust your stacking-velocity picks so the interval velocities will come out more reasonably. Try making the velocity difference between adjacent picks in the same column as small as you can possibly justify given the reflection images in the CV stacks. Compute interval velocities for each column of your adjusted picks. If these velocities are still unreasonable, re-adjust your picks and try again.

If you give Louie a plain ascii text file with your refined picks in the following format by email or on a 3.5-inch diskette, he will produce a final stacked section for you from your picks.

	(Your Name)
	(Last 2 digits of your student number)
	(Number of columns)

	(Midpoint coordinate at center of 1st column)
	(Number of picks in this column)
	(Time, s) (tab or space) (Picked velocity, km/s)
 	. . . rest of picks in column

 	. . . rest of columns

8 - Suppose that at Washoe Lake you would like to design a seismic reflection survey to find a certain bedrock reflector. From magnetic and refraction data you believe this reflector sits 300 m directly below the gravity base station (station 0'N on line 0-EW) and dips 30 degrees north. Where on the north-south trending line 0-EW should you put your seismic reflection line, to make sure it will intercept the reflections you expect from this structure? At what time will the reflection arrive, and how long should you make your seismic records? If you want to end up with 10 m horizontal resolution of the location of this structure, what frequencies should you aim to record? What will the vertical resolution be? What angle will the seismic waves be traveling at when they hit the receivers, and given your desired frequency, what geophone and shot spacing should you use?