GPH 492/692 - Refraction Velocity Lab
John Louie, January 20, 2013
Copyright © 2012 John N. Louie, all rights reserved.
The Resource Geology Seismic Processing
System for Java (JRG) Home Page
Getting Java, JRG, Viewmat -
Preparing Your Data -
Running Viewmat -
Displaying Seismic Records -
Setting Survey Geometry -
Picking First Arrivals -
Velocity Estimation from First-Arrival Times
Objective and Due Dates --
This exercise allows you to pick first-arrival times from a refraction survey
and try two methods of estimating a velocity section.
First you will make a time-distance plot and use simple equations from the
notes to find refractor velocity, depth, and dip.
Then you will send your picked travel times to a local company, founded by
Mackay alumni, that has
developed the most advanced refraction-time inversion method available.
They have agreed to invert your data and send a velocity section back to each
of you so you can compare the two results.
This write-up mostly gives you detailed instructions on how to
run the Viewmat application. The actual assignment, telling you what to
do and what to turn in, is at the bottom.
- Third week of semester - each team completes and exhibits their results from the
data-labeling, viewing, and picking parts of
the exercise, and emails the pick files
to Optim Inc.
Make sure you save your results from questions 6, 8, 12, and 13 so you can complete question 14.
Points possible on Part I: 110.
- Fifth week of semester - having gotten Optim's result from your picks, each team answers the
last question on comparing the results, and the class compares and discusses team results.
Points possible on Part II: 20.
(Note that the Surface-Wave Lab is also being worked on that week.)
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 for free under license from the code's author.
The entire code you use here is Copyright 1998-2013 by John N. Louie.
When your team completes this lab exercise, you should have gained experience toward the following skills.
- Displaying seismic records and observing wave amplitude, frequency, and apparent velocity.
- Identifying various wave phases on seismic records: P waves; refractions; reflections; air waves;
S waves; surface waves; Rayleigh waves.
- Identifying and accurately picking P-wave first arrivals.
- Correctly interpreting source-receiver distances and survey geometry.
- Interpreting refraction velocities and intercept times, and their uncertainties.
- Computing quick estimates of refractor geometry in cross section.
- Interpreting the effects of uncertainties in time picks and velocities, on refractor geometry.
- Understanding the effects of possible blind zones on interpretations of refractor geometry.
- Understanding the effects of delays on velocity and depth interpretations.
- Understanding and comparing the quick estimates of refractor geometry against
SeisOpt® inversion solutions.
Getting Java, JRG, and Viewmat
For this exercise you will pick first-arrival times from seismic traces
using the instructor's RG Seismic Processing Package for Java (JRG).
To complete the exercise you will need three resources:
If you have trouble getting access to any of these resources, please see the instructor.
- A computer onto which you can install or use Java software: Windows; MacOS; Solaris; or Linux.
The computer lab downstairs in the DeLaMare Library is available for your use in this course.
You can use the lab any time it is not reserved for a class.
- An Internet connection so you can download the software and data, and email
your picks to Optim Inc.
- A spreadsheet application like MS Excel to plot your time-distance curves; not essential
but maybe easier than plotting them by hand.
Getting the Java Platform
You computer probably has Java installed already. Go to java.com
to find out, and install the latest free Java Runtime Environment (JRE). You will want to have
Java version 1.4 or later.
Click here for additional details on how to see if your computer
has Java installed, and where to get Java versions for older computers.
Getting JRG and Viewmat
Download and unzip the 492-refraction.zip
ZIP archive. Put it in a convenient place such as your desktop since you will be
using it constantly for this assignment.
If you will be using a UNR computer that you log into with
your NETID, you will need to copy this folder to a memory stick before you log off.
If you don't, you WILL lose your work!
The ZIP archive contains:
Click here for additional details which files are in the
ZIP archive, and how to download them separately.
- jrg.jar and
jrg500.jar, the JRG/Viewmat
application. Use jrg500.jar when you are working with a large data set, over 5 Mb.
- dixie-refr.jrg, a JRG Pack with the seismic data
you will pick, and the other information below, all ready to load into JRG/Viewmat.
- All these web pages for the assignment, for your convenience.
- dixie-refr.sgy, the seismic refraction is also contained
in a 1.0 Mbyte binary SEG-Y format file. SEG-Y is a standard defined by the
Society of Exploration Geophysicists for the interchange of seismic survey
data. These data were obtained in southern Dixie Valley, Nevada, by the Spring
1998 Geol 453/653 class. A journal paper describing our results is
R. E. Abbott, J. N. Louie,
S. J. Caskey, and S. Pullammanappallil, 2001, Geophysical confirmation of low-angle normal slip on
the historically active Dixie Valley fault, Nevada: Jour. Geophys. Res.,
(Note: the SEG-2 format is commonly written by engineering seismic recorders,
and the data has to be converted to SEG-Y).
- dixie-med-vp.txt, a text file of source and receiver
Preparing Your Data
Viewmat makes exporting your picks to Optim's
quick and automatic, especially if you follow good recording practice.
For this exercise your seismic traces are already
correctly labeled with source and receiver geometry.
If you were employed in contracting or supervising seismic refraction
surveys, you would always ask your recording observer to enter complete survey
geometry info into the instrument during the survey.
If the original field records have at least the correct shot numbers,
receiver numbers, and an in-line distance entered for each
channel, you have enough information for SeisOpt®@2DTM.
Keep in mind that geometry information is best entered and checked in the field,
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.
In this course, after we have recorded seismic data during our Spring Break field
exercise, we will need to export our seismic records as standard SEG-Y files for
Viewmat. Most recorders store 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.jrg JRG Pack you are given for this
exercise contains all the correct geometry information, but the original
dixie-refr.sgy data file in SEG-Y format does not.
Double-click on the 492-refraction.zip archive to unpack it and
open the folder. Start the JRG/Viewmat application by double-clicking on the
jrg.jar file. If you have trouble with this, try these
detailed instructions for older computers.
If while working with a large data set Viewmat stops running, or you can still make
menu selections but it doesn't seem to be actually working on the data,
then the application is probably in an Out of Memory condition. Quit Viewmat
and start it again by double-clicking on the jrg500.jar file, which will
allocate most of your computer's RAM to the application.
Viewmat starts up with a default blank data set. Locate the menu bar at the
top and select Load JRG Pack... from the File menu.
A dialog box comes up asking you to navigate to your JRG Pack folder
dixie-refr.jrg. If you are not able to
select the ``dixie-refr.jrg'' folder directly, select any of the files inside it and click OK
For reading SEG-Y data files, see these detailed instructions.
Once you have opened and worked with a data set in Viewmat, you can save your
work at any time as a JRG Pack from the File menu, so later you can
load it in easily to continue working. It is always a good idea
to save your work every 15 minutes or so as a JRG Pack, onto your memory stick if
you are on a public or lab computer.
Displaying Seismic Records
To easily pick first arrivals on some data sets, you will probably want to change how Viewmat
initially shows your data. The dixie-refr.jrg JRG Pack has everything set up
for easy first-arrival picking, but see these detailed instructions
if you want to learn how to edit the display parameters.
- First, if you have loaded a multi-record file,
press the Animate button. The first record of a survey is often the worst!
- 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.
Trace balancing changes the actual data amplitude values in a way that can't be
undone, so make sure you save your results as a new file or JRG Pack, and don't over-write the
- tegain changes the overall amplitude of the data, so it may appear gray or
clipped. To pick first arrivals, you want to set a low clip value that emphasizes when the
trace amplitudes go from positive to negative. This will oversaturate the large-amplitude
waves, but we are not looking at those in this lab.
From the Edit menu, select Clip at RMS for a better display. Changing the
clip does not alter the data, only how it is displayed. Press
the Animate button to see all the records of a multi-record file.
- To enlarge the data display, select 200% or 500% from the Zoom Image To:
sub-menu of the View menu. Changing the
zoom or vertical exaggeration do not alter the data, only how it is displayed.
- This exercise does not benefit from filtering the data, but with other data sets
you may need to apply a bandpass filter to your traces to mitigate source-generated
noise. Check these detailed filtering instructions.
- Save another JRG Pack with a new name after making your adjustments (again, to
your memory stick if you are on a public computer).
Setting Survey Geometry
For your first-arrival picks to export easily to
SeisOpt®@2DTM, and to easily make
time-distance plots of your picks, you must have correct
geometry information loaded from your data file into Viewmat. The geometry describes
the map locations of the seismic sources and receivers for each record. This information
usually comes with the SEG-Y file from your acquisition contractor. Our Dixie Valley
refraction data geometry is correct when you load the dixie-refr.jrg JRG Pack.
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
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.
Picking First Arrivals
The heart of this exercise is picking the times of the first-arriving waves on
each trace on each record. See the detailed
picking instructions to find out how to make your time picks, edit them,
save and import them into Excel, export them for Optim, and see a data
You should complete all but the last question by the thrid week of the semester, and send your
team's picks to Sathish Pullammanappallil by then. He will send the optimized
velocity result back to you in several days, and you should answer the
last question and turn in all the answers by the fifth week of the semester.
- Install Java and JRG.
- Download the seismic and geometry data.
- Run Viewmat, load the seismic data JRG Pack, and check the geometry.
- (10 points) Pick the first arrivals, correct them, and
- send the four SeisOpt® files to
- Also turn in a correctly labeled plot of the first record with the
picks superimposed. Email the ''.ps'' file out of your team's saved JRG pack to the instructor
for comparison against the other teams' results.
- (10 points) 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 your team's plot and give it to the instructor, 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.
- (20 points) 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).
- 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.)
- 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.
- 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.)
- (10 points) Using another copy of your t-x plot, make the highest-velocity
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
- Recompute the refractor velocity, dip, and depths, again for a single refractor.
- Report depths, velocities, and dip; drawing another section is not neccessary.
- (10 points) 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?
- (10 points) 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.
- Just use the picks from the reverse shot (the last record) and assume no dip,
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
- (10 points) 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.
- (10 points) 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
- 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
- (10 points) Assume all your delay results from
structural deflections h in the refractor.
Later we can look into assuming the refractor is also a density difference
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.
- 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?
- (10 points) Next assume all your delay results from lateral
velocity changes in the shallowest layer (again ignoring dip and intermediate
- 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.
- (20 points) (Turn in Feb. 24) When you have the results of Optim's
SeisOpt®@2DTM 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
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.
Getting Java, JRG, Viewmat -
Preparing Your Data -
Running Viewmat -
Displaying Seismic Records -
Setting Survey Geometry -
Picking First Arrivals -