Purpose

All SeisOpt^®ReMi^TM software is highly confidential, and has great commercial value. While students are allowed to use this package here at UNR for work in this course, any other use not specifically authorized in writing by me or by Optim Inc. constitutes theft. Unauthorized copying of this software from our computers would subject you to academic disciplinary procedures, and perhaps civil and criminal penalties as well. That said, I encourage you to seek our permission to use SeisOpt^®ReMi^TM in any other course, project, or thesis work here at UNR.

The next part of the writeup lists your assignment, and contains the links to the data sets you will use.

The older version of this page describes how to run the beta version of SeisOpt^®ReMi^TM on Seismological Lab Sun workstations.

Accessing SeisOpt^®ReMi^TM

Assignment

1. Analysis of surface waves on refraction records: First we will identify and analyze surface waves recorded in Dixie Valley during the same survey we analyzed for refraction picks last week. We collected it as a seismic reflection survey; you can see the many different purposes a good data set can be used for.
   1. Download two separate records in binary SEG-Y format: DIXI0106.sgy and DIXI0114.sgy. Make sure these binary files do not get downloaded as ``text.'' If your browser gives you trouble with that, download the 2.9 Mb surf.zip zipped archive file, that has all the binary files used in this lab. Record 114 is from a playa in the center of Dixie Valley while record 106 is from a shallow granite pediment (the Piedmont) at the edge of the basin.
   2. Open each record separately in ReMi Vspect, which has a similar interface to the JRG/Viewmat open-source code you used in the refraction and reflection labs.
      1. In ReMi Vspect you can access all the needed tasks from the ``Vspect Process'' menu. Open SEG-Y files by selecting Step 1.b in the menu.
      2. A familiar but slightly different dialogue will appear after you have selected a SEG-Y data file. ``SEG-Y'' is already selected and the record numbers are already filled in.
      3. These SEG-Y files do have the 3600-byte reel header block present (so leave the box checked).
      4. These data have IEEE/Java float and not 32-bit integer trace sample data, so make the IEEE/Java Trace sample type selection.
      5. Change the range of traces to analyze to 1 through 48.
      6. click ``Read Binary File.''
      7. Note in the plot that the first 24 channels are out of order.
      8. In the Vspect Process menu select ``Step 2: Pre-Processing.''
      9. Apply the source-receiver geometry to each record, similar to how you applied geometries in the previous lab. Select ``Step 3: Apply or Erase Geometry...'' from the Vspect Process menu.
      10. In the Geometry window, for record 106 the single line of observer's report to use is ``0106 193 2 219 196 220 243''. For record 114, the observer's report is ``0114 53 2 76 53 77 100''. These observer's reports are not the same as what we used last week. They account for the trace ordering by defining two receiver patches, one reversed. Copy and paste the observer's report line into the Geometry window text area and click the Apply button below.
      11. For the station coordinates use the very same ``dixie-med-vp.txt'' file you used last week. Again click the Apply button below. Make sure the correct offset range is given. Applying geometry ensures correct analysis but will not fix the reversed appearance of the plotted records.
      12. Note these records have 4 seconds total time.
   3. In ReMi Vspect use the Pick Window (just like in Viewmat) to identify the following phases on record 114: P-wave refraction, direct S wave, Rayleigh wave. Report their approximate velocities. The criteria you used to identify these waves in the previous two labs will work here too. (For more information on surface waves see our textbook, or Telford et al. p. 153-154, 182.) Do you see an air wave? What measurement would confirm its existence? Is there an air wave on record 106?
   4. Identify some spatial aliasing effects on record 114. Turn in a plot with the artifacts circled, or describe them. Spots of the record showing a ``checkerboard'' pattern where you can't tell, just from the spot, which way the wave is going, are evidence of spatial aliasing.
   5. Now run the velocity-spectrum analysis on each record separately, for two different velocity-spectral plots. In the Vspect Process menu select ``Step 4: Compute p-f of Each Record.''
      1. When you run Step 4, a dialog box will pop up.
      2. Note that you should confirm the dialog box shows the correct source-receiver offset range for the record.
      3. Use the default Fmax, Vmin, and Np values, with the default ``Both Directions.'' Press the ``vspect'' button.
   6. Try several values for the ``Amplitude Clip'' levels of the p-f velocity-spectral plots, by selecting Edit->Plot Parameters in the plot's menu bar, like in JRG/Viewmat.
      1. Identify the Rayleigh-wave phase-velocity dispersion trend in each. Try to make a least a dozen picks along the lower edge of the trend in each. If you see more than one trend, pick the lower-velocity trend, since the fundamental-mode Rayleigh waves we want to interpret have lower phase velocities than the higher-mode overtones.
      2. What is the lowest frequency at which you can get a good phase-velocity estimate for each record?
      3. Turn in the two picked, grayscale velocity-spectrum plots (or email color plots).
   7. Discuss briefly how well your Rayleigh-wave phase-velocity picks from the p-f plot agree with the velocities of the waves you could observe directly in the record 114.
   8. Compare the phase velocities from the two records 114 and 106. How do they differ? Can you justify the difference from the information above about their recording locations?

2. Comparison with last week's records:
   1. Load the data set used for the previous refraction lab into ReMi Vspect (using Vspect Process->Step 1.b) and apply geometry (Steps 2 and 3), just as you did before, using the observer's report lines from that lab.
   2. Run the velocity-spectrum analysis on all four records together, for one composite velocity-spectral plot, by executing on the data volume Steps 4, 5, and 6.
   3. Compare the result to the velocity-spectral result from record 106 you got above. Versions of record 106 were used in both these analyses, yet they look very different. Explain what happened. Hint: frequency resolution is proportional to the duration of time recorded. It is not dependent on the time sampling interval. How do the records we used to pick first arrivals differ in the amount of time recorded from the version of the records we use in this lab, question 1?

3. Analysis of microtremor noise. Here we look at a passive seismic record, taken without any active source like the small blasts used in the refraction survey you saw above. A 24-channel refraction layout was made along Mill St. in Reno, extending west from the corner with Rock Blvd. It was afternoon, and there was plenty of traffic.
   1. Download the rno-noise.sgy 1.1 Mbyte SEGY data file, setting Save As Type ``All Files'', and making sure a ``.sgy'' is at the end of the file name.
   2. Read the SEG-Y data into ReMi Vspect by selecting again Vspect Process->1.b. To see the ``rno-noise.sgy'' file you may have to switch the file-location window to type All Files. After you select the data file a familiar Open Binary File... dialog will pop up:
      1. This file has the default SEG-Y format.
      2. The First Field Record No. to Read is 0 (zero) and the last is 11.
      3. The 3600-byte Reel Header Block is present, so leave the box checked.
      4. The ``Trace Data Type'' is ``IEEE/Java Float''.
      5. The ``Traces to Analyze'' setting is now 1 through 24 for this data set.
      6. Click the Read Binary File button.
   3. Plot the record. The further instructions show how to apply an existing plot parameter file. Download the plrno-noise.par file (it is plain text). Now use the Edit menu on the data plot that came up, and select Apply Parameter File....
   4. Now you should be able to Animate and interpret the passive records. How many seconds long is each record? What is the aperture, the total distance spanned, by the 24-channel cable spread? Since the receivers were in a straight line and evenly spaced, the plot parameter file actually defines the geometry well enough for this analysis.
   5. Examine the records to decide where their energy comes from. You'll notice a thick, fuzzy group of high-energy waves running almost the length of each record; some have more than one group. Estimate the apparent velocity of such a group - it will be very low. Make a few picks in Viewmat along the center of the group, and you will be able to compute the velocity easily. This is not a seismic wave-propagation velocity. Convert the velocity to miles/hour and you will be able to say what is propagating along Mill St.
   6. Radiating from each group of fuzzy energy in these records are noticeable seismic waves that have reasonable velocities - 100 to several hundred meters/sec. Turn in a plot of one record (email is OK) where you have made a couple of picks of one such wave, and computed its velocity.
      Similar records are displayed on Louie's Sound of Seismic web site, which allows you to listen to the records sped up and converted to MP3.
   7. Now run the p-f velocity-spectrum analysis on all four records together, for one composite velocity-spectral plot. You can do this by executing Steps 2 through 6 from the Vspect Process menu.
      1. In Step 3, the Geometry window pops up. Push the ``Erase All'' button in the Geometry window, and then close the Geometry window and the warning dialog that comes up.
      2. When you run Step 4, note that you should confirm the correct dt=0.016 s and dx=15.24 m. Use the default Fmax, Vmin, and Np values, and Analyze Both Directions. Press the ``vspect'' button.
      3. When you run Step 5 (from the menu bar of the multiple p-f plot window), confirm the default ``Use all planes''.
      4. Step 6 (run from the menu of the single-plane p-f plot that pops up when you run Step 5) just shows you the Pick Window for the new combined p-f plot.
   8. On the combined p-f velocity-spectral plot resulting from Step 5, identify the surface-wave dispersion trend, and try to make a least a dozen picks along the trend. Make your picks close together at low frequencies and farther apart at high frequencies. Pick the lowest velocity reasonable, since a passive array records energy from all directions, and any energy not propagating in the array direction will have an artificially high apparent velocity.
      1. On the combined p-f velocity-spectral plot resulting from Step 5, select View->Zoom Image To:->200% to make picks more accurately.
      2. When you are done picking make sure you save your picks with the Save button in the Pick Window that Step 6 brings up. Use a simple name for the picks file like ``rkml-vspect.pck''.
      3. What is the lowest frequency you can get a good phase-velocity estimate for?
      4. Turn in a picked, grayscale plot (or email a color plot).
   9. Discuss briefly how well your Rayleigh-wave phase-velocity picks agree with the velocities of the waves you could observe directly in the noise record.
   10. According to the ``Quarter-wavelength'' rule of thumb, the wavelength of a Rayleigh wave that samples 0-30 m depth S-wave velocities best will be four times 30 m, or 120 m. Identify from your velocity-spectral picks a frequency and phase-velocity combination that matches that wavelength, and report your estimate for the 0-30 m depth average S-wave velocity at Rock and Mill.
   11. Examine the soil-type classifications for earthquake ground-shaking hazard mapping found at:
       • Seekins, Linda C. , Boatwright, Jack, and Fumal, Tom, 1999, Soil type and shaking hazard in the San Francisco Bay Area: U.S. Geological Survey web site at http://quake.wr.usgs.gov/hazprep/soil_type/ (the classification table is down the page a little).
       Using your 0-30 m velocity estimate, report the ground-shaking hazard classification for Rock and Mill (don't call any newspapers; we already know). Now you have boiled 1.1 megabytes of data down to one letter. Two professional journal papers have examined shear velocities at this location; Louie, 2001 and Scott et al., 2004.

4. Modeling S-velocity structure.
   Modeling will allow you to estimate a shear-velocity versus depth profile for the Rock and Mill St. site from the Rayleigh-wave dispersion picks you made from the p-f plot. Use your saved pick file for the Rock and Mill refraction microtremor survey. You can look at these instructions for an older platform to do the modeling with the ReMi Disper application on an MMV PC. The interface has changed but the principles are the same.
   1. Double-click on the ReMi Disper icon on the PC's desktop. Expand the Disper window to full screen.
   2. From ReMi Disper's menu bar select File->Load Picks... to open your saved ReMi Vspect pick file. You will load picks from a ``Dispersion File (Vspect compatible)'' type file.
      1. Note the picks are not fit at all by the default class B-C boundary model profile that Disper starts with. You will have to lower the model velocities significantly to fit. Go ahead on make each of the three layers' velocities a third to a half of the velocities Disper started with. Just click within the red area of each layer to set its velocity.
      2. At the same time, you want to narrow the velocity range of the period-versus-phase velocity plot at the bottom, just to be able to better compare the model dispersion curve with your picks. Enter near the top a minimum velocity of 200 m/s and a maximum of 700 m/s, hitting the Tab key after each entry. You should be able to see the variation in the picks much more clearly now.
   3. If your picks are at phase velocities less than 200 m/s or more than 800 m/s, go back to question 3.h. and reconsider.
   4. You should see one or two ramps in phase velocity as period increases. Short periods are from short wavelengths that sample low-velocity materials near the surface; longer-period phase velocities sample deeper, faster materials with longer wavelengths.
      1. Start at the top by adjusting the shallowest model velocity to match the shortest-period picks.
      2. On the model drag the shallowest interface, the bottom of the shallowest velocity, up to make the layer thinner. As you do this, the model dispersion ramp will move to the left, to shorter periods, and should better match your Rock and Mill dispersion picks. (Wait to see the up-down arrow cursor before dragging the interface depth, to avoid changing the velocity by mistake.)
      3. Now you can raise the second layer's velocity to better match the intermediate-period picks, to the right of the first ramp in your picks. Note that the ramp in the model dispersion will also steepen, and you will have to go back and adjust the thickness of the top layer.
      4. If a ramp from an interface is not very clear in your picks, you will probably find that you can fit your picks by either raising the velocity below the interface, or by making the interface shallower. This is a velocity-depth tradeoff. Avoid making velocities too high, or (for this data set) introducing any velocity decreases with depth.
      5. Now move down to the next interface, fitting the longer-period picks.
   5. Use File->Save As to save your model velocity profile and other results to disk (or your flash card).
   6. Report the maximum depth your dispersion data constrain velocities to. Evaluate maximum depth by increasing the maximum velocity back to 2500 m/s, and click to check ``Layer Add'' under the velocity profile. Add a deep layer, at 150 m depth or more, and uncheck Layer Add. Now give this new layer a much higher velocity, like twice as much as the formerly deepest layer. Then drag the top of the new high-velocity layer up, and watch to see when it starts to affect the dispersion curve's fit to the picks. Where it starts making a difference to picks you believe, that will be you maximum depth of constraint.
   7. Report any attempts to evaluate velocity-depth trade-offs. You can do this evaluation by fitting a ramp in the picks with high velocities, and saving the model; and then fit it again with shallow depths, saving the alternative model under another name. What ranges of velocity and depth allow fits to the data?
   8. Compare the Vs30 and IBC (International Building Code) Site Class letter reported by ReMi Disper at the bottom against your quarter-wavelength Vs30 and Seekins et al. site class letter from questions 3.j. and 3.k.