Lab 7: Seismic Refraction in the Field
GLY3160 / PHY3160
NAME: ________________________________________
LAB SECTION: __________________________
LAB 7: SEISMIC REFRACTION IN THE FIELD
Now that you have experience with seismic refraction theory, we will collect field data using a 24 channel seismic
refraction setup and then interpret the results. The goal is to determine the velocity structure and depth to bedrock.
Part I: Travel Times
While the theory behind seismic refraction is all relatively straightforward, interpreting actual field data can be
challenging. The main obstacles are recognizing signal from noise and discriminating the actual first arrivals of the
direct and refracted rays from the air wave, ground roll, and reflected rays. Below is a theoretical travel time plot
(from Burger et al. 2006) for the main ray arrivals observed in a field seismogram. We have talked about all of
these rays with the exception of the air wave and the ground roll. The air wave arises due to the sound of the
hammer hitting the metal plate and travels as a sound wave. The ground roll is the surface wave(s) from the
hammer hit. Answer the questions below using 3 or less sentences.
Figure 1. Theoretical
travel time plot for
seismic rays in a
typical seismic
refraction survey.
Distance (m)
1) Using the proper earthquake wave types (P, S, Rayleigh, Love), which type of wave is the first to arrive at any/all
geophones? Why?
2) Using the plot above, which wave has the slowest velocity? To the nearest 10 m/s give the velocity of this wave.
(hint: Do not attempt to calculate this velocity from the plot. Instead consult the almighty Google).
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Lab 7: Seismic Refraction in the Field
GLY3160 / PHY3160
3) What kind of wave (compressional or shear) is the air wave? What physical property requires this to be true?
Why?
4) Which wave is faster, the ground roll or the direct ray? Use your qualitative knowledge about earthquake waves
to briefly describe why this makes sense.
5) On the figure 1 above, estimate the critical distance and the crossover distance.
Critical Distance = __________ meters
Crossover Distance = __________ meters
Part II: Field Data and Interpretation
For reasons unknown to me, field seismic software often plots distance on the vertical axis and time on the
horizontal axis. The figure below shows a computer-generated synthetic seismogram showing the arrivals of the
various waves shown in the figure from part I. Note that this is a synthetic seismogram, so there is no noise. The
only wiggles on this figure are from arrivals of the waves in the figure in part I. In practice, field data is MUCH
noisier than this, and the ground roll often is tough to differentiate from the direct ray and the air wave. The good
news is that seismic refraction is only concerned with picking the first arrivals, which are always the direct and
refracted (or head) waves, so most of the mess from the other wave arrivals does not matter.
Figure 2. Synthetic
seismograms for a typical
seismic refraction survey.
Note that the vertical axis
is geophone number (or
distance) and the
horizontal axis is time.
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Lab 7: Seismic Refraction in the Field
GLY3160 / PHY3160
6) Using the computer program, Pickwin, load in each of the 2 shot points and pick the first arrivals as shown in
class. For the data, assume that only two layers were detected. Once you are satisfied with your picks, save your
picks as a .vs file.
7) Load your .vs files into Microsoft Excel. Note that the top three lines are not travel time data. They are just
header information. The third line does have the shot location and number of geophones as the first two
columns, so this may be useful. The third column of data can be deleted for all rows. Load all of these data into
a single Excel sheet.
8) Make a plot of all of your first arrivals for the two different shot points. This data should look something like the
plot below, although your data will be different. Feel free to use whatever colors you want, so long as they are
easy to see and professional-looking. Label each shot location in meters in the legend as I did below.
25
Time (ms)
20
15
10
5
0
0
5
10
15
20
25
Distance (m)
Shot 1 (25 m)
Shot 2 (0 m)
9) Using Excel, translate each dataset so that each shot point is at x=0 m with positive distances. This way you can
mathematically calculate the tint values, layer thicknesses, and layer velocities just like you have been doing in
class and lab. If you are confused about how to do this, just think about how you have been translating
functions/data in your past labs, and apply similar mathematical corrections to your distance data.
a.
Make a plot of each translated data set as a standard t-x diagram, and apply a linear regression to each ray’s
arrivals just like you did in the last lab. Include your linear equations on the plot and be sure to make clear
which arrivals are the direct ray and which are the refracted ray arrivals. You will want to be clever about
how you do this in Excel because if you plan this out in a smart way, you can save yourself a lot of time.
Remember that you can copy entire sheets, which will include formatted plots.
b.
Paste the plot into a Microsoft Word document and give your figure a brief caption below each plot (2-3
sentences will do).
c.
Below your figure and caption, use your data from your t-x diagram to calculate each layer’s velocity, and
the depth to bedrock (you can do this by hand if you want). Make sure that your plot, caption and
mathematical calculations are legible and all fit onto a single page. If they don’t, consider rescaling your
plot so there is more space. Just make sure that your plot is presented in high quality and is easily readable.
d.
Repeat this process for the second shot location and fill in the data on the next page for each shot point.
Include your 3 plots at the end of this lab. Use one plot per page including your caption and calculations.
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Lab 7: Seismic Refraction in the Field
GLY3160 / PHY3160
Part III: Final Results
10) Now that you have processed all of your field data, fill in the table below with your results. Because the velocities
calculated are “apparent velocities,” calculate an average velocity and depth to get reasonable estimates for
the actual layer velocities and depth to bedrock.
Shot Number
Shot Location (m)
V1 (m/s)
V2 (m/s)
Depth to Bedrock (m)
1 (forward)
2 (reverse)
Averages
N/A
11) Seismic refraction data does not directly provide a picture of the subsurface, but it does supply the data required
to visualize the subsurface geology. Use your data from above to make a sketch of the subsurface geology on
the quad. Which way is the bedrock interface dipping? Towards geophone 1 (towards the octagon) or towards
geophone 24 (towards the rock garden)? It is wavy or planar? How do you know? Be sure to give a clear
explanation of your reasoning below. Draw a simple sketch of the bedrock interface below with geophone 1 and
24 labeled. This sketch does not need to be to scale, but it must be labeled and clear.
Geophone
#1
Geophone
#24
Subsurface Interface Description (2-3 sentences)
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