Lab 5
Active Source Seismology: Refraction Survey
Read chapter 4 out of Fowler
Many advances in seismology have been motivated by exploration for natural resources.
Today we will do an active source refraction survey similar to what is done when
looking for oil, but on a much smaller scale of course. The source is a shotgun blank,
which creates an explosive source. Explosive sources only generate compressional
energy, i.e. only P waves leave the source. However there can be and usually is some
energy that gets converted to S waves when the P waves hit an interface (a different kind
of rock, the surface etc...). We will be recording the energy from the shot using a linear
array of 36 geophones, which are low cost seismometers measuring ground motion in the
vertical direction. This equipment is good enough for finding water table depths, and soil
or rock interfaces. For refraction surveys a general rule of thumb is that your array
should be ~3-5 times longer than the depth of an interface that you want to image (hint).
Record sections 1 and 2 were arrays 18m long. Each record section has 36 wiggly lines,
each of which represents ground motion at one location as a function of time. The source
is at time = 0ms. Note that at 0ms there is lots of 'noise' then all of a sudden things get
quiet and bang, there is a coherent signal that propagates along the array. You may ask
why noise caused by a little wind blowing grass shows up as large an amplitude as a
shotgun blast, and the answer is that the machine uses something called Automatic Gain
Control (AGC) which normalizes amplitudes within a certain time frame. If this was not
applied, then the wiggles from the shotgun would be so large that they would go off of
the screen. AGC is used only to make viewing the phases we are interested in more clear
and easy to see.
First arrivals:
New record sections 1-2
What you should see from these two record sections is that a line connecting the first
arrivals changes slope in the first half of the array (this does not show up well on new
record section 2). Where it changes slope is the crossover point. With your eyeball and a
ruler, draw 2 lines that fit the first arrivals for new record section 1. They should
intersect at the crossover point. The slope of the first segment closest to the source will
be 1/v1, the second line will have a slope 1/v2. The only difference between the 2 record
sections is that we moved the source from one end of the array to the other to see if the
lower layer was dipping or not. Are your v2’s noticeably different for record sections 1
and 2? The book gives a good explanation of the effects of a dipping layer.
Record section 3
Measuring the refracted head wave's slope is easy, but the direct wave's slope and
pinpointing the crossover point is difficult isn’t it? That is why we did a survey that
produced record section 3. This was in the same direction as the one that produced new
record section 2, but a little shorter. As you can see, measuring the direct wave's slope is
much easier now, as is finding the crossover point. Measure the period and use v1 to
calculate the wavelength of the direct P wave.
Since the phases are very difficult to consistently draw a line through, I will give you two
points you should be drawing lines through.
New record section 1:
geophone #15, 19ms
geophone #35, 33ms (head wave)
New record section 2:
geophone #14, 17.5ms geophone #31, 27ms (head wave)
(just draw in a line that fits the direct P wave. You wont
use it any calculations, but its good to see this)
Record section 3:
geophone #1, 5ms
geophone #30, 27ms
(direct P wave)
Remember, new record sections 1 and 2 have 0.5m spacing between geophones, and 3
has 0.3889m spacing.
Technical note: if you do a connect the dots you will notice that the line wont be straight
and doesn’t go to 0m at time = 0. The source is actually about 1-2 feet deep so for your
calculations and modeling, assume we did a perfect job in collecting these data and
ignore those discrepancies. Record section 1 and 2: Ignore the first 3 seismograms.
Record section 3: Ignore the first 5 seismograms. These seismogram’s first arrivals
don’t fall on a nice line that go through most of the data.
One man's junk is another man's treasure:
What is all that confusing stuff after the first arrivals?!! It’s kind of linear and coherent
sometimes, but not all. Welcome to the real world that geophysicists and engineers face
all the time. Though you may stare at these record sections for long periods of time and
not be able to think of a situation that could produce this, that doesn’t mean that all hope
is lost for learning something about the ground in this area. Look at new record section
2, and find the phase that goes through these two points
geophone #30 102 ms
geophone #6 45 ms
This is a surface wave, the Rayleigh wave. In refraction surveys it is sometimes called
ground roll. Draw a line through the Rayleigh wave and calculate its velocity. The
Rayleigh wave velocity for the idealized case of a poisson solid (when Vp=sqrt(3)*Vs) is
0.92Vs. Calculate Vs1 (the S wave velocity for the upper layer). Notice that the
Rayleigh wave has a much slower velocity (the slope is greater) than either of the first
arrivals. It is also a lower frequency phase/the wavelength is much longer. Measure the
period of the Rayleigh wave, calculate its wavelength, and compare it to the direct P's
wavelength.
Modeling:
Now that we have these measured values we can make a travel time curve. In Excell
make a column of 36 distances from the source in meters setting the first one at 0.5 m and
with a spacing of 0.5m so that the last one will be at 18m. In another column calculate
the travel time of the direct P wave using v1. Do the same for the Rayleigh wave. In two
more columns, calculate the travel times for the head wave and a reflected wave. Plot
them all on one big page sized plot using different symbols. Make the x axis distance
from source in meters and the y axis time in seconds. Assume a source at the surface.
What to turn in:
-Derivations of equations used to calculate travel times for the different phases (its in the
book):
1. direct P
2. head wave
3. reflected wave
4. Rayleigh wave (=0.92Vs will be fine)
This reflected and head wave will involve a bit of geometry.
-A big cartoon with your own version of Figure 1 to show what is going on, it doesn’t
have to be elaborate and contain 36 receivers. Make it such that someone looking at your
equations can easily see what each variable is. Put both the variable name and its value
e.g. z=10m
-Calculations for
1. v1
for example:
v1=P wave velocity in upper layer = 1/slope = delta distance/delta time = 10m/1s =10m/s
2. v2
3. the Rayleigh wave velocity
4. Vs1
5. direct P wave wavelength
6. Rayleigh wave wavelength
7. depth to second layer
8. critical angle
9. distance where the head wave first shows up.
(1-6 should be short)
-Plot of travel time curve with each of these labeled:
1. direct wave
2. head wave
3. reflected wave
4. Rayleigh wave
5. crossover distance (X_crossover)
6. the distance where the head wave first shows up (X_critical)
VERY IMPORTANT
You will find that the head wave velocity for new record sections 1 and 2 differ, which
means that the lower layer is dipping. Do you have to take that into account when doing
your calculations, derivations, and plotting? No (your welcome). Just use the average of
the two head wave velocities and assume the layers are flat. Use whatever crossover point
you like best, but be sure to say which one it is.
Abstract:
Don’t worry too much about procedure and method or setup. A brief explanation of what
a refraction survey is and does will be fine. Try to make this abstract focus on the
geologic interpretations. Pretend MRC Greenwood wants to build yet another building
on this site and needs to know what is down there to keep the costs down. Why she is
doing this is up to you, have fun with it if you want, but don’t spend more than 1 sentence
on this. Think about the velocity differences between the two layers. Could layer 2 be
solid rock or not? Give your geologic interpretation. (consult your problem set 2 for
typical P wave velocities). Be sure to include something like:
“We found that the head wave velocity for the West shooting array to be faster than that
of the East shooting array indicating that it is going updip/downdip (you have to figure
this out). For simplicity in calculations we averaged the two apparent head wave
velocities.”
Note: When you see v1 and v2 that refers to the P wave velocity of layer 1 (upper) and
layer 2 (lower), respectively. Vs1 refers to the S wave speed of layer 1.
The labels on the figures are correct, but be aware that distance decreases to the right on
new record section 2.