You will notice on the display that each record (each plane) shows the seismograms from 48 receiver geophones recording the same seismic source, or shot. Each vertical strip of the color image shows the seismic amplitude with time after the shot increasing from zero seconds at the top to one second at the bottom. The time axis is reversed like this because structures that are deeper in the cross section below the survey line will show seismic waves that arrive later. So the signals from deeper structures will this way plot below the signals from shallow structure.

From left to right across the top of the color image, the receiver traces are arranged according to the geophone X coordinate. The X coordinate increases eastward, so left to right is east, making a north-looking section. Traces with earlier first arrivals, nearer the top of the color image, are closer to the seismic source. The first record has a source off the east end of the receiver line, the fifth record has its source at the left end, on the trace of the 1954 Dixie Valley magnitude 7.2 fault rupture, and the other records have sources on the line, each time only 3 m or so from one of the receiver geophones.

For more details on how to correctly apply geometry information when it does not accompany your data file, see this detailed explanation of how to load the Dixie Valley survey geometry.

1. Install Java and JRG.

2. Download the seismic and geometry data.

3. Run Viewmat, load the seismic data JRG Pack, and check the geometry.

4. Pick the first arrivals, correct them, and
   1. send the four SeisOpt^® files to satish@seismo.unr.edu.
   2. Also turn in a correctly labeled plot of the first record with the picks superimposed.

5. Save your picks, import them to a spreadsheet, and make a t-x plot of the off-end and reversed shots (the first and last records only). See the hints above on importing saved picks to Excel; or make a plot on graph paper by hand.
   1. Print out and turn in this plot, with all axes completely and correctly labeled, and indicating which picks are from the forward and downdip (source is downdip of the receivers) shot, and which are from the reverse and updip shot.

6. Make a dipping-layer-over half-space interpretation of the picks from the two end-shots, the first and fifth records. Draw straight-line fits of apparent velocity and intercept time on your plots by eye (only an experienced spreadsheet user will know how to have the fits done automatically, and hand-fits are just as good).
   1. First draw the V1 line from the source location at zero time (the (0,0) point) through the near-offset picks. Then draw another line at the higher velocity V2 through the rest of the points. (Always draw the shallowest layer's line first, then the next layer down, etc.)
   2. Now draw lines for the reversed shot's times. As you notice, the equations all assume you get just one V1. Most analysts, you should know, average the V1 slopes drawn on the forward and reverse. So you can do the same.
   3. But make sure your (0,0) constraint is really at zero source-receiver offset. Check the "sx" values in the spreadsheet for the x-coordinates of the shots.
   4. Constrain the higher-velocity fits with the reciprocal time, and mark that on the t-x plot.
   5. You will have to calculate the velocities of your fit lines by hand from your marked-up plot; this is easy if you have included many grid lines at fine x and t intervals.
   6. Use the appropriate equations from the text or lecture notes to compute layer velocities, refractor depth, and dip.
   7. Draw a cross-section showing refractor geometry.
   8. Turn in the section and the t-x plot with the fit slopes and intercepts. You can plot everything by hand on graph paper if you need to; just make sure the plots are neat and completely and correctly labeled.
   9. Report the dip. Which direction does the refractor dip? (Note: the higher station X-coordinates are to the east while the higher station numbers are to the west.)

7. Using another copy of your t-x plot, make the highest-velocity interpretation you can of the forward and reverse times, especially considering the error inherent in each pick (particularly the far-offset picks). Don't forget to fit a reciprocal time.
   1. Draw this high-apparent-velocity interpretation on the t-x plot and turn that in.
   2. Recompute the refractor velocity, dip, and depths, again for a single refractor.
   3. Report depths, velocities, and dip; drawing another section is not neccessary.

8. Now make the lowest-velocity interpretation you can.
   1. Turn in another copy of the t-x plot with the low-apparent-velocity fits.
   2. Report depths, velocities, and dip.
   3. How do these compare with the high-velocity model?
   4. What can you say about the accuracy of your original interpretation?

9. Make a three-layer, three-velocity interpretation of our data. The equations for multiple dipping interfaces, which you can get from Burger's text (on reserve in the DeLaMare Library with its software CD) on p. 89 and implemented in his RefractSolve program, are too complex to use here by hand.
   1. Just use the picks from the reverse shot (the last record) and assume no dip, unless you want to use RefractSolve as in previous offerings of Geol 492/692.
   2. What evidence do the reverse pick times show for an intermediate velocity? Do the forward pick times show any?
   3. Remember to start drawing V1 from (0,0), then V2, then V3.
   4. Report velocities and thicknesses from your best line fits. Of course refractor velocity interpreted from the reverse record assuming no dip will be lower than in the 2-layer dipping interpretation.
   5. Turn in another copy of the t-x plot with the 3-layer fits on the reverse times.

10. Supposing the intermediate layer is really a hidden thin layer,
    1. draw your 3-layer fits of the reverse picks on another copy of the t-x plot, but move your V2 lines back to just intersect the V1 to V3 crossover.
    2. Report velocities and thicknesses for this minimum amount of the intermediate thin layer, and
    3. turn in the fits on the t-x plot.
    4. How do the depths to the deepest V3 layer compare with those from your best-fit 3-layer interpretation above?
    5. What constraints do you have on the depth of the deepest refractor? Your inferred hidden-layer errors may be applicable to the dipping-refractor interpretations as well.

11. Evaluate the effect of the time picks not landing exactly on your fit lines.
    1. Do the data show any delays that seem consistent on both forward and reverse shots?
    2. Starting with your 2-layer interpretation, measure the maximum delay by fitting a line to the slope of the longer-offset refraction arrivals, and then moving it (while keeping it at the same apparent velocity or slope) to a) just graze the earliest times from that layer, and b) just graze the latest times.
    3. Turn in a copy of the t-x plot with the maximum and minimum times.
    4. Subtracting the two intercept times should give you an estimate of the maximum .

12. Assume all your delay results from structural deflections h in the refractor.
    1. Using the equations in the notes, compute this maximum deflection. Ignore the dip, or intermediate layers.
    2. What proportion of the average refractor depth is this deflection?
    Later we can look into assuming the refractor is also a density difference of, say, 0.4 g/cc, and use the simple infinite-plate formula from gravity to estimate what is the maximum number of milligals of gravity anomaly these structural deflections might produce; and whether we could observe such structure with our gravity instruments.

13. Next assume all your delay results from lateral velocity changes in the shallowest layer (again ignoring dip and intermediate layers).
    1. Compute the new surface velocity V0.
    2. What proportion of V1 is the velocity change V1 - V0?

    Assuming that resistivity in this layer changes in proportion to changes in velocity due to changes in porosity, it is possible to estimate the resistivity changes if you have some locations where resistivity and seismic data overlap in the same environment. We may look at this later.

14. (Turn in Feb. 23) When you have the results of Optim's SeisOpt^®@2D^TM on your picks of all 5 records,
    1. compare them against the information on dipping structure and errors you derived from the questions above.
    2. Turn in both the SeisOpt^® velocity section and the hitcount section (black and white printouts of color plots are OK).
    3. Write a paragraph comparing SeisOpt^® results against the simple layer calculations. Address the following:
       1. Based on the velocities above and below your dipping refractor, draw the location of the refracting ``interface'' through the SeisOpt^®@2D^TM velocity section, on the copy you turn in.
       2. Compare the depth and dip of the ``interface'' between your layer calculations and the SeisOpt^® interpretation.
       3. Compare your intermediate-layer calculations against the velocity gradients you see near the ``refractor'' in the SeisOpt^® velocity section.
       4. Compare the depth deflections in the ``refractor'' as seen in the SeisOpt^® section against the estimates of h you computed from for question 12.
       5. Compare the velocity variations above the ``refractor'' as seen in the SeisOpt^® section against the estimates of V₀ you computed from for question 13.