Refraction Experiment at Manzanita Ln and
Downtown Reno, NV
Spring Break, 2010
Applied Geophysics 492, Dr. John Louie
Patricia Capistrant and Troy Christiansen
Background
Refraction survey methods are a relatively easy and cost effective geophysical survey method. It
is applicable in finding the depth of bedrock, the thickness of overlying alluvium, in estimating soil
mechanics, and in finding faults and other structures at depth.
To run a survey, a generated seismic wave must propagate through the ground. Once the wave
reaches a high velocity layer, a portion of the energy refracts back to the surface at a critical angle
(related to Snell’s Law). The signals from both compressional and shear waves reach the planted
geophones once refracted back. Following this, the recorded signals can be analyzed to find travel time
information useful in constructing a refraction profile.
Preparation
Texan refraction instruments were used in this field experiment. To prepare the instruments for
the field, the Texans had to be programmed. They required being connected to a computer, with which
they were synced with a GPS and programmed with an experiment title and data table. Additionally, the
sync process also programmed the Texans to turn on and off at a specified time. They were scheduled
to turn on the morning of each experiment at 9 am, and to shut off at 7pm on the evening of each
experiment date.
Once in the field, the Texan instruments, resembling Nalgene-sized water bottles, were hooked
to a geophone. To take a reading from the Boise State hammer truck, the geophone had to be placed in
the ground and leveled. Once removed from the ground, the geophone was unable to take readings.
Methods
A total of 3 refraction surveys were run during the Spring Break, 2010 experiment. The first ran
north-south from Windy Hill to the corner of Manzanita and Plumas. The second ran north-south from
Reno’s Auto Museum to the northern end of the University of Nevada. The third refraction experiment
ran east-west from the corner of Plumas and Manzanita, west to the corner of Manzanita and W.
McCarren.
N-S Manzanita Refraction Experiment
The first refraction experiment was executed Monday, March 14th. 14 geophones were placed
in a line approximately 0.9 mi apart, while a 15th geophone followed the Boise State hammer truck and
took readings at each stop in order to constrain time for the experiment. Five locations along the line
approximately equally spaced along the geophone line were then selected. The Boise State hammer
truck executed 8 hammer strikes at each location with the 15th geophone in close proximity to it. See
the attached excel file for exact time and location data.
Line 1: Windy Hill north to Manzanita
N-S Downtown to UNR Refraction Experiment
The second refraction experiment was completed Tuesday, March 16th. Again, using the same
methods as outlined above: 14 geophones were placed in a line with a north-south trend from Lake
Street, next to the auto-museum, north to the northern edge of the UNR campus. 8 hammer hits were
executed at each of the 5 selected locations equally spaced along the line with the time recorded. See
attached excel file for time and location data.
Line 2: Downtown Reno north to UNR campus
E-W Manzanita Refraction Experiment
The third refraction experiment was executed Thursday, March 18th along an east-west transect
from the corner of Manzanita and Plumas, west to the intersection of Manzanita and W. McCarren Blvd.
8 hammer hits were executed at each of the 5 selected locations equally spaced along the line, with the
time recorded. See attached excel file for time and location data.
*This experiment was unsuccessful, and no data was recovered from the Texans. Therefore, results and
analysis will not include data from this section.
Line 3: Manzanita and Plumas west to W. McCarren Blvd
Following the Experiment
Following each day’s experiment, the Texans had to be brought back to the lab, where they
were reconnected to the computer as well as the GPS. A data dump procedure was followed where the
data was put into a data table constructed during preparation. Once the data dump was confirmed, the
Texans were wiped clean and prepared for the next day’s experiment.
Analysis and Results
Methods of
Data analysis began with data organization. Excel sheets were constructed with waypoints,
times, and locations for the refraction experiments. Once this information was organized, an Observer’s
report was constructed as well as a Surveyor’s Report. The data dumped from the Texans was
transferred into files hosted on the class website. These files were opened using View Mat, where
appropriate geometry was applied using the Observer’s Report. From this frame, first arrival picks were
made and sent to Optim for analysis. Upon return from Optim, layer and structure analyses were made.
Time Errors
Using estimated thicknesses and velocities of layers, a time/distance plot was constructed. This
allowed us to estimate the time it would take a propagating wave to reach a geophone X meters away
from the source. These estimates helped in determining where to make picks on the first arrival
refraction plot.
Figure 1. Time-Distance Plot, used for estimation of wave arrival times
A small problem arises with time errors. The Texan refraction instruments are time synced
before they are used in each experiment; however, internal clocks on each may cause millisecond time
differences over the course of the experiment. These errors were calculated using .TSH files for each
experiment. If the errors were large (nearly as great as the ms arrival times of the propagating wave),
then this would likely cause the View Mat shot record to be un-resolvable. The errors are attached;
however, they are not significant enough to cause any major problems in the shot record plots.
Timing Errors
Refraction Exp.
Manzanita N-S Line
Texan
Error (msec)
1237
1240
1241
1242
1243
1244
1245
1246
1247
1248
1249
1250
1251
1252
1253
UNR Line
no changes
2
0
-4
-13
-4
-4
4
5
-5
7
8
16
-6
-3
-6
*add error to time
Figure 2. Time Errors for Manzanita N-S Line
Shot records for this experiment did not have distinctive first arrivals for any wave.
Consequently, it was essentially guesswork to make picks from the trace data. With the lack of data, it
was hopeless to identify or define any structures in the experiment areas.
See attached excel spreadsheets
1. N-S Manzanita Line
2. N-S Downtown Reno Line
Conclusions
Preliminary results (not shown here) from this experiment show very little resolvable data. This
indicates an error in either the execution of the experiment or an error in the instruments used for the
experiment. In any case, our refraction experiment did not yield any satisfying results for or against the
hypotheses and results of other groups.