Deadlines:

• Feb. 9 - complete and turn in the data-labeling, viewing, and picking parts of the exercise, and email the pick files to Optim LLC.
• Feb. 23 - having gotten Optim's result from your picks, answer the last question on comparing the results, and turn it in.

You are free to use the JRG package after the class is over (that's why the instructions here cover problems that may not occur with the assignment's data set). The package and its source code are available for use and open-source modification under the GNU Public License. The entire code you use here is Copyright 1998-2004 by John N. Louie.

If you have a Macintosh available, an alternative exercise that only uses out-of date methods is available from previous offerings of Geol 492/692. Consult with the instructor if you want to complete this alternative assignment.

1. A computer onto which you can install or use Java software: Windows 95, 98, NT, 2000, or XP; MacOS 7.6.1 or later; Solaris 2.5.1 or later; or Linux.
2. An Internet connection so you can download the software and data, and email your picks to Optim LLC.
3. A spreadsheet application to plot your time-distance curves; not essential but maybe easier than plotting them by hand.

Getting the Java Platform

• Your computer may already have Java installed. Any version above 1.1 will work for JRG.
  • On Windows 95 or 98, open a DOS Window from the Start menu, and type ``java -version'' and hit return. You probably don't have it.
  • On Windows NT, 2000, or XP, open a Command Prompt Window from Start->Programs->Accessories, and type ``java -version'' and hit return. NT and 2000 should have it, XP may not.
  • On MacOS 7.6.1 through 9.2, use a Finder or Sherlock search for JBindery. You probably don't have it.
  • On MacOS X, find the Terminal application in the Utilities subfolder of the Applications folder. Start that, and type into it ``java -version'' and hit return. You should have it already.
  • On a Sun, in Linux or another UNIX, open a Command Window or cmdtool and type ``java -version'' and hit return. You should have it already.

• If you have Windows, go to java.sun.com/getjava to download the Java 2 Standard Edition (J2SE) for your computer. The J2SE includes the Java Runtime Environment (JRE), which is the piece you want. The SDK is the Software Developers' Kit (a.k.a. JDK), which you won't need for this class.
  • The latest version downloadable from Sun Microsystems is fine if you have Windows NT, 2000, or XP, Linux, or Solaris - get it from http://java.sun.com/getjava/.
  • Download J2SE 1.2.2 from this link if you have Windows 95 or 98.

  MS-Windows 95, 98, and NT users can download the Windows Java Runtime Environment (JRE) 1.2 installation package from the instructor at this link (5.3 Mbytes).

• If you have a Mac with OS X, use Software Update to make sure you have the current Java runtime system from Apple (you should not need any of the Developer tools for these labs). The instructor has a download Java package for MacOS 8 and 9 available through this link (16.4 Mbytes).

• To run Java applications on Seismology Sun machines, please consult these brief instructions.

• For other computers, please go to http://java.sun.com/getjava/ to investigate the Java versions available. You need an environment for Java stand-alone applications (not applets) equivalent to Sun's JDK 1.1.3 or better.

Getting JRG and Viewmat

Create a new folder on your computer for this exercise. On a PC, you might call it ``c:\jrg''. Download the jrg.jar (<300 kbytes) into this folder. Make sure you are getting this binary file out of your web browser without it being converted to text. If you have put everything together in one folder and followed the commands below, and you still get java running but reporting ``class not found: Viewmat'', this could be the problem. In Netscape, point at the link here and hold down the right mouse button, then select ``Save Link As...'' and make sure it is type ``All Files''. If you have trouble, download the jrg+src.zip (<500 kb) zipped archive to the folder, then double-click on it to unzip it, and you should see the jrg.jar file. Or get the jar and data files on a cd from the instructor.

Getting the Seismic Data

Download the dixie-refr.sgy data file. This SEG-Y file is also a binary file that should not be saved as ``text.'' You can download the picklab.zip zipped file if you have trouble. You will also want to download the text file of source and receiver station locations dixie-med-vp.txt. These data were obtained in southern Dixie Valley, Nevada, by the Spring 1998 Geol 453/653 class.

Of course, if you are asked to work with legacy seismic data that was recorded before your involvement with a project, you will often have to add geometry information yourself. If you have a data file without enough geometry included, do not worry. You can still display the records quickly, and enter the geometry info for the survey while you are using Viewmat.

You will need to export your seismic records as standard SEG-Y files for Viewmat. The instrument probably stores records internally in SEG-2 format. Every instrument manufacturer provides routines that run on their equipment and produce SEG-Y files. If possible, convert all the records from one survey into one large SEG-Y file, and transfer that to the computer where you will interpret them with Viewmat. The dixie-refr.sgy data file you are given for this exercise is in SEG-Y format, but does not contain correct geometry information.

If your data transfer involves using FTP over a network, make sure that you use the binary file-transfer mode. ASCII-mode transfers will corrupt any seismic data file. Any files of picks, plots, or parameters that you write out of Viewmat, however, are in plain text and will transfer fine in ASCII mode.

• On Windows, open a DOS or Command Prompt window, ``cd'' to the folder for this exercise, and enter: ``java -cp jrg.jar Viewmat''.

  	cd c:\jrg
          java -cp jrg.jar Viewmat
  

  On WindowsXP with Sun's J2SDK 1.4.1 installed, the commands would be:

  	cd c:\jrg
  	"C:\j2sdk1.4.1_01\jre\bin\java" -cp jrg.jar Viewmat

  Note Viewmat with a capital V. The java commands are case-sensitive.

  On Windows98 with Sun's JRE 1.2 installed, you have to run the commands:

  	cd c:\jrg
  	"c:\Program Files\JavaSoft\JRE\1.2\bin\java" -cp jrg.jar Viewmat
  

  (Copy them from here in the browser and paste it into the DOS window using the paste button at the top of the window.) This adds the jrg.jar file to the ``classpath'', allowing Viewmat to use all the .class files without unpacking them from the .jar file.

• On MacOS X, open the Terminal application, and use these commands to go to the folder you set up in your Home, and run JRG (also case-sensitive):

  	cd ~/jrg
  	java -cp jrg.jar Viewmat &
  

• On MacOS 8-9, first unpack the jrg.jar file with the jar application included in the MRJ JDK package. Then you can drag the Viewmat.class icon in the onto the JBindery icon. Use Find File or Sherlock to locate JBindery within your installed MRJ JDK folders. Then click the Run button. Note that you can save a double-clickable application out of JBindery for Viewmat.

1. Viewmat starts up with a default blank data set. Locate the menu bar at the top and select Read Binary File... from the File menu. A dialog box comes up asking you to navigate to the folder with your SEG-Y seismic data file. Select the ``dixie-refr.sgy'' file and click OK.

2. Another dialog pops up asking for a description of the data file. Leave Binary File Type set to SEG-Y. If you want to read in just a few records of a large multi-record file, you would enter the first and last field record numbers (field file IDs or FFIDs) into the fields below.
   • For the ``dixie-refr.sgy'' file you have to put some default values into the field record number fields, or Viewmat will put all the records side-by-side in one plane. Enter ``0'' and ``1000'' into the fields. With that, Viewmat will load all five records in the file as separate planes in a data volume.

     You may experience difficulties on older or smaller PCs if you try to load larger files, with more than 20 megabytes worth of records. (See the note below on increasing the memory available to Viewmat.)

   • Although the SEG-Y format does not officially support having records of different length together in one file, you may encounter them later. Enter the field record number of the first record that has the length you want, and Viewmat will pick out of the file all the records that are similar.

   • Some SEG-Y files written to disk from a tape, or from certain organizations, will not begin with the required 3600-byte tape reel header block. If you know this to be the case, uncheck that item in the dialog. The ``dixie-refr.sgy'' file does not have the reel header, so uncheck the box.

   • Although the standard binary format for SEG-Y trace data is 4-byte IBM Float, some conversion programs will write different formats. If you know the format and it is not IBM Float, select it under Trace Data Type. For our ``dixie-refr.sgy'' file, select IEEE/Java Float.

3. When you have finished setting your SEG-Y options, click the Read Binary File button.
   • If you get an error message about the file not being in SEG-Y format, select Open Binary File from the plot window's File menu, and try changing your field record number selections, or the reel header checkbox.

   • If the data plot does not look like seismic waves, or appears badly clipped, try changing the selected Trace Data Type.

4. After a moment a new data display window will pop up with an image of the first record. You may only see the near-source traces, since the plot is of raw data without any amplitude balancing. You can close the old window of the zero data plot and proceed to the next section to effectively display your records.

1. To easily pick first arrivals, you will probably want to change how Viewmat initially shows your data. First, though, if you have loaded a multi-record file, press the Animate button. The first record of a survey is often the worst!

2. You may notice that your records are longer than they need to be for first-arrival picking. The ``dixie-refr.sgy'' file has just one seconds of data, so you will not need to change the time length. If you did want to change the length, from the Methods menu, and its On Each Vector sub-menu, you would select cutTime.

3. The cutTime dialog shows you the starting time of the traces, and their length in seconds. For many surveys you can probably enter a new Trace Length of a quarter of the old, or less, and press the cutTime button.
   • If you get an Out of Memory error in the DOS window running Viewmat, you have to close it down by selecting Exit Java from the File menu. Then start it up again, reading fewer records from your file. Or try running with more memory dedicated to Viewmat, with a command like jre -mx64000000 Viewmat typed into a DOS window (that will give 64 Mb to Viewmat; don't give a size larger than 3/4 of your PC's installed RAM).

4. If you have not cut away too much time, you can close the old plot by selecting Close Window from the old plot's File menu, to save memory.

5. Apply trace balancing by selecting tegain from the On Each Vector sub-menu of the Methods menu on the new display. Press the tegain button in the tegain dialog.

6. tegain changes the overall amplitude of the data, so it may appear gray or clipped. From the Edit menu, select Clip at 3*RMS for a better display. Press the Animate button to see all the records of a multi-record file.

7. You may want to change the aspect ratio of the display, or try a color image:
   • Select Plot Parameters from the Edit menu.

   • A Properties dialog appears that will allow you to change virtually all of the display options.

   • You might edit the Title text to be more than the file path.

   • After using cutTime, you might want to double the vertical exaggeration, or more.

   • For a color image, try setting the Color Table for Image to Seismic Red-White-Blue, a popular representation for reflection data. Time picks plot in black and may show up better on a color image. The default gray plot is called Seismic Variable Density.

   • If you have a survey with constant trace spacing and no gap, you can set the Vectors: Zero, Delta, and Units fields for the plot. This is not possible for the ``dixie-refr.sgy'' file. For example, with 48 reflection traces starting with a minimum offset of 50 feet at channel index 0, and 10 feet geophone group spacing, set Vectors: Zero to 50, Delta to 10, and Units to ``Offset, feet''.

   • If you have multiple records, and they were in order in the SEG-Y file, you can set the Planes: Zero, Delta, and Units fields for the plot. For example, with 23 records starting at record 1025, set Planes: Zero to 1025, Delta to 1, and Units to ``Field Record Number''. This is also not possible with our ``dixie-refr.sgy'' file.

   • Click the Apply Changes button in the dialog.

   • Go back to the display window and try Animate to check all the records.

   • You can make more changes in the Properties window, and click Apply Changes again.

   • After you have made a number of changes, save your parameters by selecting Save Parameter File from the File menu of the Properties window. You may have to navigate back to your data directory in the Save Viewmat Parameter File dialog that appears. Be sure to give the parameter file a name that will not over-write your data file! The next time you open this data file, just select Apply Parameter File from the display window's Edit menu, and your custom plot configuration will be restored.

8. To enlarge the data display, select 200% or 500% from the Zoom Image To: sub-menu of the View menu.

9. You may need to apply a bandpass filter to your traces to mitigate source-generated noise:
   • Choose In Place from the Methods menu to deselect it. Each filter attempt will then come up in a new window, which you can close if you don't like it. Go back to the source window to try again.

   • Select bpfilter from the On Each Vector sub-menu of the Methods menu. Leave the Pad factor at 2, and check that the Dt field shows the right time sampling.

   • Enter your filter frequencies, in hertz, into the last four fields of the dialog box. Try to have a 20% taper on each side of the bandpass. For example, entering Low, Down=40; Low, Up=50; High, Up=100, and High, Down=120 will cut below 40 Hz and above 120 Hz, and preserve frequencies in the 50-100 Hz band.

   • Press the bpfilter button.

   • If filtering has removed much amplitude from your plot, select Clip at 3*RMS from the Edit menu.

   • Try Animate again to check the result on all the records. Or drag the Plane Index slider back and forth.

   • If you are happy with the filtered data for picking, you can close the un-filtered window.

1. For your first-arrival picks to export easily to SeisOpt@2D, and to easily make time-distance plots of your picks, you must have correct geometry information loaded from your SEG-Y file into Viewmat. Check this by selecting setHeader from the On Each Vector sub-menu of the Methods menu, and you will find essentially no geometry information included with our ``dixie-refr.sgy'' file:
   • The dialog pops up showing geometry info for the first trace in the data, at Header Index 0. You can enter any other index within the range given, and press the Check Header button to see the geometry for that trace. The traces and their headers are indexed in the order they are displayed - first to last record, left trace to right.

   • Sample a few headers at scattered indices and observe the consistency of your geometries. The record numbers and source point numbers should be correct as shown. Receiver point numbers are not well-determined from SEG-Y files, and it is all right if they are not correct.

   • Source and receiver coordinates must be correct. They must be in meters, and they must make sense according to your understanding of the survey's geometry. They may be two-dimensional: the X-coordinates or the Y-coordinates may be zero for every point. Elevations may be zero, but must be consistent.

     Our ``dixie-refr.sgy'' example was recorded on a 48-channel cable with 50 feet between takeouts. The low-index channels, on the left, are on the east end of the receiver spread. The high-index channels on the right are on the west end. The receivers remained stationary while the location of each record's source changed. The first record is from a shot 150 feet off the east end of the spread. The last record is from a shot at the west end. The other records are from shots in between.

   • Offsets should be in meters. They are not critical for exporting picks, and can be recomputed as long as the coordinates are correct.

   • Some SEG-Y files scale coordinates and/or elevations incorrectly, often by a power of ten. If this is the case, you can select fixCoords from the On Each Vector sub-menu of the Methods menu. The dialog that pops up will report the maximum and minimum offsets currently present in the trace headers, in meters. (These have nothing to do with any offset labeling you may have entered into the display Properties window.) The ``dixie-refr.sgy'' file has no offset or geometry information at all, so fixCoords is not needed. If the offsets are incorrect, you may have to scale the coordinates. Enter your factor to Multiply Coordinates By and click the fixCoords button to quickly scale all the headers and recompute offsets. Then bring up the fixCoords dialog again and check that the min. and max. offsets make sense.

2. Our ``dixie-refr.sgy'' file does not have enough geometry information included, and you must enter survey geometry information in the form of observer's reports and surveyor's reports.
   • Select Apply Geometry from the Edit menu of the data display.

   • The Geometry tool pops up. Basically, you create observer's and surveyor's reports in plain text in your favorite spreadsheet, Wordpad, or SimpleText, and copy and paste the text into the text area of the Geometry tool. After selecting the type of input geometry and unit system above the text area, you click the Apply button below.

   • To see how to create the text of the observer's and surveyor's reports, select a Type of Input Geometry above the text field, and click the Show Format button below. The text area will then give you a detailed explanation of the text the Geometry tool will need.

   • First, create an observer's report according to the format given in the tool. You will need one line of observer's report text per record, in the order the records were read from the SEG-Y file. Here is the observer's report for our ``dixie-refr.sgy'' file. It has five lines, in the order of the five 48-channel shot records:

     0106	193	1	196	243
     0078	207	1	196	243
     0084	219	1	196	243
     0090	231	1	196	243
     0095	243	1	196	243
     

     Copy it to the clipboard from your spreadsheet or from the web browser window here (using Control-C, Alt-C, or Command-C), then select the Geometry tool's text area and paste it in (using Control-V or Command-V). Make sure the Type of Input Geometry is set to Observer's Report, and click the Apply button below. Note that these five records were from a stationary array of 48 receiver channels extending from station 196-243, east to west. The first shot is from station 193, 3 stations east off the east end of the receiver line. The last is from station 243, on the west end; the other records are from shots within the receiver line. The shot at 219 is a center shot.

   • Second, create a surveyor's report or station coordinates list according to the format given in the tool. You will need one line of station coordinate text per record, in any order. Note that there are two orders for the coordinate text accepted (X-Y-Z or Z-Y-X). Our ``dixie-med-vp.txt'' file is in Z-Y-X order. Copy it to the Windows clipboard from your spreadsheet (using Control-C, Alt-C, or Command-C), then select the Geometry tool's text area and paste it in (using Control-V or Command-V). Make sure the Type of Input Geometry is set to the appropriate Station Coordinates option, that the Unit System is set according to your input units, and click the Apply button below. Note the ``dixie-med-vp.txt'' file is in feet.

   • Use the fixCoords or setHeader tool from the On Each Vector sub-menu of the Methods menu to re-check your new geometry.

   • You can save your file, with its revised geometry, back into the SEG-Y file, or write a new SEG-Y or RG file. Use the File->Write Binary File... selection in the menu bar of the data plot.
     • From the Edit menu of the Plot Parameters window, also save a parameter file, to preserve your axis labeling.
     • When you read a SEG-Y file written by Viewmat, it will always have the 3600-byte reel header, so leave that option checked.
     • If you save a raw binary float file, you will lose all the geometry and other information in the trace headers.

     For examples of seismic data saved as sound files, check out my Sound of Seismic pages.

1. With the geometry verified, you can begin to pick your first arrivals. Select Pick Window from the View menu to open the picking tool.

   The Pick Window shows you the text of only the picks on the current plane, or record.

   • Try pressing the Auto-Pick button. This will erase any picks present that have not been saved, and attempt to make a first-arrival pick of each trace.

   • The picks on the record currently shown in the data plot will appear after a while in the pick window. In the data display window, move the Plane Index slider back and forth to observe the quality of the automatic picks on all your records. You will observe the pick text changing as it flips from the picks of one plane to those of the next. If you have exceptionally clean survey data, you may be satisfied with the automatic picks.

     Our ``dixie-refr.sgy'' data will auto-pick more than 90% correctly. Blank traces (that you can see have low amplitude before tegain) are not auto-picked, which is good. Some traces at very small offsets do not auto-pick well, since the pick time is too close to zero time. Some traces at farther offsets, where the first arrival is weak, auto-pick too late.

   • The picks are written out in the Pick window and marked on the data display. You can select pick text, copy it to the clipboard (with Control-C or Command-C), and paste it into another application such as Wordpad or Excel. You can also delete picks by removing lines, or add text annotations. Text in the window that does not represent a pick will be ignored. If you have added or deleted picks and want to see the resulting marks on the data display, refresh the display by animating, or by hiding the window and re-exposing it. The format of the pick text is: Amplitude; Time Sample Index; Time, seconds; Trace Index; Trace Label; Plane Index; Plane Label; Field Record Number; Source Point Number; Receiver Point Number; Source X Coord., meters; Source Y Coord., meters; Source Elevation, meters; Receiver X Coord., meters; Receiver Y Coord., meters; Receiver Elevation, meters; Offset, meters; and an allset field used by other routines.

   • In order to get the picks from all planes or records, you need to press the Show All button on the pick window. If you have a lot of planes, there may be too much pick text and it will not copy to the clipboard. In this case, press the Save button on the pick window, and all of the pick text will get saved into a file. You can import this file into spreadsheets such as Excel.

   • The automatic picks are based on where each trace exceeds a threshold amplitude factor relative to a small initial part of the trace. You can repeat your use of the Auto-Pick button as many times as you want to test different picking parameters, although you will lose all previous picks each time. You can adjust the threshold factor from the Auto-Pick Threshold sub-menu of the Edit menu on the Pick window, and the size of the initial trace sample from the Threshold Above sub-menu of the Edit menu.

   • Once you are satisfied that the automatic picker has done the best job it can, you can go on to correct the picks.

2. Hand-pick traces that the auto-picker could not pick well by simply clicking the mouse button while pointing at the trace and time you want to pick.
   • The text for the new pick will appear at the end of the list in the Pick window, in the order you hand-pick; you may have to scroll down to see it.

   • Only the traces you either auto-pick or hand-pick will get picks; Viewmat does no interpolation or phase correlation between picks.

   • Re-picking a trace will not cause previous picks of that trace to disappear - you would have to delete those from the Pick window by hand. Deleting superseded picks is not necessary, because only the last-made pick on any trace will be exported to SeisOpt. All picks in the pick window will still appear if you save the pick text, however, or save a plot of the record display.

   • Use the Plane Index slider on the data display to cycle through and pick traces on each record.

   • You do not have to pick every trace. You do have to re-pick each trace incorrectly picked by Auto-Pick. While missing picks may decrease resolution and depth penetration in your SeisOpt results, globally optimized discovery will still produce the best result possible from even sparsely and irregularly sampled pick data.

   • During a long session of hand-picking, periodically click on the Save button in the Pick window. If your PC freezes, you can read in your SEG-Y file again (after it is saved with correct geometry), open the saved pick text in Wordpad to paste each plane's text back into the Pick window (using Control-V or Command-V), and pick up where you left off.

   • Click the Save button in the Pick Window when you are done re-picking. This saves the pick data for all planes into one file. The saved pick file is plain text, so you can import it into your favorite spreadsheet (or paste it in and then use Edit->Text to Columns) to make a time-distance plot of the picks from the two end records.
     • For instance, when you import the pick text into Excel, under Delimiters you can check Space and Other, and enter the equals sign ``='' as a delimiter.
     • After importing the picks into a spreadsheet, make an X-Y scatter plot using column U (after "gx" in Excel) ``Receiver X Coord., meters'' as the X-axis and column C ``Time, seconds'' as the Y-axis.
     • Just plot the picks from the first and last records by deleting all but the first 48 and last 48 rows in the spreadsheet. Check the ``rec'' numbers to be sure. You may want to choose Sort... from the Data menu in Excel and sort by ``svp'' (column K) and secondarily by ``gvp'' (column M). Then you should have an easy time deleting the rows from the middle shots. If you remember that the erroneous automatic picks were too late, you can also look at column C to remove the rows from the superceded automatic picks, that you re-did by hand.
     • Plotting is easiest in Excel if you select column C, copy it, and then select column V and paste it in. Now select columns U and V, containing the receiver X coordinate and the pick time, respectively.
     • Go to the ChartWizard and select an XY Scatter plot with grid lines. Make sure to properly label the axes. Always use only approved SI units and their abbreviations.
     • When you are looking at the plot, you can double-click on the time axis to set it to a more useful scale, by unchecking automatic and using a maximum of 0.4 s. You can also set finer grid-line intervals, for easier calculation of slopes you draw on the printout of the plot by hand. You can plot the time axis increasing up or down as suits you.
     • Make a printout of this plot (just print the page the plot is on). Make several copies of this plot for you to draw on.

3. Once you are satisfied with the quality of your picks, export them ready for SeisOpt by simply pressing the SeisOpt button. A dialog will appear asking you where to save the files, and what base name to use. Viewmat will save four files, and quietly over-write any existing files in the directory that have the same names: ``riotsinput'', that provides parameters for executing SeisOpt; the base name appended with ``.src'', with source coordinates and the number of receivers picked per source; the base name appended with ``.rec'', giving receiver coordinates; and the base name appended with ``.obs'', giving the pick times. All you have to do then is email the four files to Optim LLC at satish@seismo.unr.edu, and Sathish Paullammanappallil will run SeisOpt from your picks.

4. To save a plot of the picks on a representative record, go to the data display window and select Write PostScript File from the File menu. This will save a plot of the record currently shown, with all the labels and picks.
   • If you want to save a plot of more than one record, you have to move the Plane Index slider to show another record, and then select Write PostScript File again. Viewmat will suggest a file name that includes the unique index of that plane.

   • You can spool or download the PostScript file to a PostScript-capable printer.

   • This file is Generic PostScript code, not EPS, and can't be imported to older word-processing programs. Versions less than four years old of applications such as Corel, Canvas, Word, and Illustrator will import the PostScript.

   • You can obtain the free Ghostview application from the Univ. of Wisconsin at www.cs.wisc.edu/~ghost/, which will display and print the PostScript on your PC and non-PS printer.

   • You can save a color plot as a PostScript file, and then edit the PostScript code to print an effective plot in black and white:
     1. Open the PostScript file in Wordpad or SimpleText.
     2. Search for the text ``/colorplot true def'', not far from the top of the file.
     3. Change that text to ``/colorplot false def''. It will now effectively use the Seismic Variable Density gray scale.
     4. Save the file.
     5. Spool or download it to a monochrome printer.
     6. The color information is preserved; you can always re-edit the PostScript file before sending it to a color printer.
     This procedure is better than trying to change the colors in a markup program.

1. Install Java and JRG.

2. Download the seismic and geometry data.

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

4. Pick the first arrivals, correct them, and send the four SeisOpt files to satish@seismo.unr.edu. Also turn in a correctly labeled printout 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. Print out and turn in this plot, with all axes completely and correctly labeled, and indicating which picks are from the forward and updip (``shooting updip'') shot, and which are from the reverse and downdip shot.

6. Make a dipping-layer-over half-space interpretation of the picks from the two end-shots. 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). First draw the V1 line from (0,0) 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.) Now draw lines for the reversed shot's times. As you notice, the equations all assume you get just one V1. Burger's software, you should know, averages the V1 slopes drawn on the forward and reverse. So you can do the same. 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. Constrain the higher-velocity fits with the reciprocal time, and mark that on the t-x plot. 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. Use the appropriate equations from the text or lecture notes to compute layer velocities, refractor depth, and dip. Draw a cross-section showing refractor geometry. 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. 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. Draw this high-apparent-velocity interpretation on the t-x plot and turn that in. Recompute the refractor velocity, dip, and depths, again for a single refractor. Report depths, velocities, and dip; drawing another section is not neccessary.

8. Now make the lowest-velocity interpretation you can. Turn in another copy of the t-x plot with the low-apparent-velocity fits. Report depths, velocities, and dip. How do these compare with the high-velocity model? 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 p. 89 and implemented in his RefractSolve Mac program, are too complex to use here by hand. 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. What evidence do the reverse pick times show for an intermediate velocity? Do the forward pick times show any? Remember to start drawing V1 from (0,0), then V2, then V3. 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. 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, 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. Report velocities and thicknesses for this minimum amount of the intermediate thin layer, and turn in the fits on the t-x plot. How do the depths to the deepest V3 layer compare with those from your best-fit 3-layer interpretation above? 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. Do the data show any delays that seem consistent on both forward and reverse shots? 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. Turn in a copy of the t-x plot with the maximum and minimum times. 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. Using the equations in the notes, compute this maximum deflection. Ignore the dip, or intermediate layers. 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). Compute the new surface velocity V0. 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. When you have the results of Optim's SeisOpt on your picks of all 5 records, compare them against the information on dipping structure and errors you derived from the questions above. Turn in both the SeisOpt velocity section and the hitcount section (black and white printouts of color plots are OK). Write a paragraph comparing SeisOpt results against the simple layer calculations. Address the following:
    • Based on the velocities above and below your dipping refractor, draw the location of the refracting ``interface'' through the SeisOpt velocity section, on the copy you turn in.
    • Compare the depth and dip of the ``interface'' between your layer calculations and the SeisOpt interpretation.
    • Compare your intermediate-layer calculations against the velocity gradients you see near the ``refractor'' in the SeisOpt velocity section.
    • Compare the depth deflections in the ``refractor'' as seen in the SeisOpt section against the estimates of h you computed from for question 12.
    • Compare the velocity variations above the ``refractor'' as seen in the SeisOpt section against the estimates of V₀ you computed from for question 13.