Assignment

1. Load a 3 Mb seismic reflection data volume into Viewmat. The data file is refl1001-1029.sgy. Make sure this binary data file does not download as ``text.'' If your browser gives you trouble with that, download the refl.zip zipped archive, and double click on it to find the refl1001-1029.sgy file. It is SEG-Y format, you want all 29 field records (shot gathers; so enter 1001 and 1029 into the record fields), it has the 3600-byte reel header, and has samples in IEEE float. Select Edit->Apply Parameter File and use the plreflpar parameter file.

   A geophysics senior at Victoria University of Wellington, New Zealand recorded this high-resolution survey along a beach in the Seatoun suburb of Wellington. Seatoun sits in a small fault-bounded basin, and recorded the highest earthquake-shaking amplifications of anywhere in the city. This survey complemented previous gravity work to confirm basin geometry and examine the stratigraphy for evidence of recent faulting.

   Note the first record is by far the noisiest. Try the Animate button. Answer this question: what type of wave has the most energy? This energy is stronger in the Seatoun records than in the Dixie Valley data you saw before because in Dixie Valley we used blasts in 3-m-deep holes, while in Seatoun the impact source was on the surface.

2. A Geometrics R-24 system stacked 10 sledgehammer hits per source point from 24 channels of single 12-Hz geophones spaced at 8 m. Offsets and the line progression was from west to east. The R-24 records line geometry along with the records, but incorrectly translates the coordinates to the SEG-Y format. To fix this up:
   1. Select Methods->On Each Vector->fixCoords. Note the offset range reported is way too small. In the Multiply Coordinates By field enter 100 and press the fixCoords button. Select fixCoords again to observe that offset now has the correct range of 8-192 m, then Cancel it.
   2. The records are 1 sec long; your computer may have an easier time if you just take 0 to 0.5 seconds of time. Select Methods->On Each Plane->cutTime and specify that Traces Start at 0 and Trace Length 0.5 sec, and press the cutTime button. After the new record display pops up you can select, on the old display, File->Close Window to save memory. Also select Edit->Clip at RMS on the new display. (Don't re-apply the parameter file; that would give the traces an incorrect starting time.)

3. Use a trace-equalization gain to see the reflections more clearly. Select Methods->On Each Vector->tegain. Press the tegain button; then select Edit->Clip at 3*RMS on the data window. Note that the slow waves have decreased amplitude but the noise before the first arrival is now equally strong. Try Animate again.

4. For filter testing, you will want to select Methods->In Place to turn off the substitution of a method's result over the original data. The next time you look at the Methods menu In Place should not be checked.

   Use the Methods->On Each Vector->bpfilter selection to make filter tests. For each test frequency band, leave Pad at 2 (takes longer but avoids ``wraparound''; you can see what happens with Pad=1), and verify that Dt is correct at 0.001 sec. The next 4 fields specify a trapezoidal frequency response. Nothing will be left below the frequency specified in the first field; response will ramp up linearly to the frequency in the second field; 100% response will be maintained between the frequencies of the second and third fields; and response will ramp down to nothing at the frequency of the fourth field.

   Each ramp at the ends of the pass band should have a ``20% taper''. So, if I wanted to pass the 50-100 Hz band, I would enter 40, 50, 100, and 120 Hz into the four fields, respectively. At 0 Hz and the Nyquist frequencies (the defaults shown when the bpfilter dialog pops up) you do not need any tapering.

   1. Try the 50-100 Hz bandpass filter with 20% taper suggested above. Since the filter has removed amplitude, there is less on the new display that pops up. Re-clip it at 3*RMS.
   2. Try the same filter with hardly any taper by entering 49, 50, 100, and 101 Hz into the 4 fields. If you compare, say, the two filters on Record 1010 at Plane Index 9, you will see that the reflection is less sharp, more ``ringy'' if you do not use a 20% taper. This is known as Gibbs's Phenomenon - if you try to filter too sharply in frequency, you end up with more energy at frequencies right where you were trying to cut them out. Close the window with the untapered filter results.
   3. Make yourself some notes of what phases you can and can't see in your 50-100 HZ tapered filter result. Now you can close that window. Going back to the original tegain'ed data display, use bpfilter to make several filter tests. Keep notes on each result so you can close the filtered data window before you try another one, to avoid memory problems. I suggest trying bands of 0-25, 25-50, 50-75, 75-100, 100-150, 150-200, 200-300, and 300-500 Hz. Use a 20% taper. Note: AC power in New Zealand cycles at 50 Hz. Since this survey was in an urban area, the seismic cables pick up that frequency inductively.
   4. 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 = 1800 m/s, what is the range of reflection wavelengths implied by their range of frequencies?
   5. Decide on a frequency band that will show the reflections best while mitigating other waves and noise. Use a 20% taper. Make a final filtered set of records, and then close your original records. You will have to re-apply tegain and also re-clip the display. Turn in one nice example record (by email if needed).

   Useful equations:

   

5. 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 = 1000 m/s, at what angle from the vertical are the reflected waves hitting the receivers?

6. On a copy of a bandpass-filtered test record, indicate the shallowest and the deepest reflections you think you can identify (or describe the record numbers, offset, and time of a couple of examples). What approximately are the two-way travel times t₀ of these reflections at zero offset? What are their dominate frequencies? For each of the two reflections compute its vertical resolution z using the Widess criterion, and its Fresnel radius x_f. Assume V = 1800 m/s. Would the upper or the lower reflection locate potential faults more accurately?

7. Estimate stacking velocities for NMO correction from sixteen constant-velocity stacks between 1000 and 2500 m/s.
   1. Create the suite of 16 CV stacks from your best filtered set of records. Make sure you have fixed their coordinates. Select Methods->On Each Plane->cvstack. Note whether the correct offset range appears. Scroll through the text box at the top to read about the parameters required. Since the R-24 and the SEG-Y file in this case only provides source and receiver X-coordinates, appearing to be a purely east-directed line on a map, the cvstack dialog suggests the midpoint line run at N90E from the ``westernmost'' shot or receiver point. You will have to change more of the dialog's suggestions for this data set, like setting nm=37, dm=8, and arc=16. In addition: 1) make sure v0 is the correct minimum of your desired velocity range - so change it to 1000 m/s; 2) change nv to 16 because you want to test that many velocity values; and 3) change dv to 100 m/s, so v0 + (nv-1)*dv will now give the correct 2500 m/s velocity maximum you want to test.
   2. Click the cvstack button. As the computation proceeds, you can follow its progress from the output in the DOS window or Java Console. CV stacking can take several minutes.
   3. Although you cannot effectively save your processed shot records you can save and later restore your CV stack volume. From the CV stack display window select File->Write Binary File. Then select Edit->Plot Parameters to bring up the Parameter window, and from its File menu select Save Parameter File. Also write down the dimensions of the CV stack volume: ntau; nm; and nv. Later, when you want to work on the stacks further, start Viewmat and select Open Binary File. In the dialog that comes up you need to change Binary File Type to Raw Float, and enter: ntau Elements per Vector; nm Vectors per Plane; and nv Number of Planes. When you see the CV stacks displayed, select Edit->Apply Parameter File to load the saved plot parameters, and you can proceed with picking.
   4. With the volume of CV stacks, identify the stronger reflections. Where the reflections are strong you should be able to identify several separate reflections in a small area. Use the Animate button, or the Plane Index slider to page through the stacks at different velocities, focusing on small areas.
   5. You may notice there are few reflections stacking in below 0.3 sec. Use the cutTime method now on the CV stacks, and increase the plot's vertical exaggeration to 0.1, to get a nicer display.
   6. Organize the reflections you will pick into two or three columns. Each column is a range of midpoint distances 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.
   7. 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. Pick the distance, time, and velocity where each reflection is strongest and most continuous. Your picks appear in the same pick window, but these are NMO velocity picks, not first-arrival picks. So exporting for SeisOpt would not have any meaning. You can still, of course, copy and paste or Save your pick text to a spreadsheet.
      Just remember that you have to press the ``Show All'' button at the bottom of the Pick Window to see all the picks from all the planes (all the trial velocities) in one list. The Save button does write all the picks to a text file, which you can then open in a spreadsheet or Notepad.
   8. The pick text has the following column order: Amplitude; time index; time in seconds; trace index; trace distance from VP0 in meters; plane index; and NMO velocity in m/s; followed by location information for the midpoint. Arrange your velocity picks by column in order of increasing time.

8. 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 0.05, 0.1, and 0.3 s, then you can use the Dix equation to find the rock velocity in two intervals, between 0.05 and 0.1 s, and between 0.1 and 0.3 s. Examine the interval velocities for correctness. Interval velocities above 5 km/s and negative velocities are unacceptable, but can easily result from reasonable picks.

   Useful equations:

   
   

9. Now adjust your stacking-velocity picks so the interval velocities will come out more reasonably. (If you had gotten reasonable interval velocities the first time around, explain how much your picks could vary yet still produce reasonable interval velocities. You got good velocities, but what is their error?) 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. Turn in your picks and computed interval velocities for each column.

10. Now use Methods->On Each Plane->cmpstack to compute a final stacked section from your filtered shot records (not from the CV stacks). The main difference from the cvstack dialog is that instead of specifying a range of constant velocities you copy and paste in your final, adjusted velocity pick text. The default values for the other parameters are fine. Change your velocity values in notepad; you don't have to change the plane index values. Just make sure the pick text has the original column order. You don't have to paste in the columns after the velocity column. The dialog only looks at the first 7 columns, and only reads the time, distance, and velocity columns. Just make sure you have been working with all of the pick text, from all the CV-stack velocities.

    Using your final stacking velocity at the largest time you are displaying in the stack (say, 0.3 sec), adjust the plot's vertical exaggeration so it will have approximately no VE after accounting for this time-to-depth conversion. Label the plot with depth instead of time, and turn in a plot of this approximate depth-converted final stack.