Michael Wickes
Devin Katzenstein
Danielle Molisee
Jessica Pence
Methods Paragraph
GPH 492/692
I.
Methods
Seismic reflection and refraction data was collected during UNR’s spring break, 2013:
March 16 through 24, in Schurz, Nevada. Data was obtained south of Schurz along the three
lines shown in figure 1. The first day of surveying, March 18 2013, was performed to the west of
Highway 95. 127 records were obtained, not including the error records. The error records
were eventually recorded a second time, in order to complete the data set. Two 48-channel
geophone layouts were recorded as stationary arrays with hammer points progressing from SW
to NE progressing through each array. Both arrays were linear and had a perpendicular trend to
the suspected fault line. The azimuth of the lines was approximately N65E. The Bison was
placed on the fault for the first layout, and east of the fault for the second layout. On March 20,
2013, the second set of reflection/refraction data was collected. On this day, 135 records were
obtained utilizing two similar layouts. The data for this line was collected to the east of highway
95, south of the first day’s data collection (fig. 1) location. The final day of data collection
occurred on March 22, 2013. The line was located further south-east of Highway 95. 136
records were recorded on this day. Bison placement was always east of the array line. Flags 148
and 149 were on opposite blocks of the suspected fault.
Equipment utilized in data collection: 48 geophone groups with 6 100-Hz geophones per group,
one trigger cable which connected the sledge hammer to the Bison seismograph, two 24-channel
geophone cables which connected to the geophones, hundreds of marker flags which were numbered to
indicate hammer and geophone locations, a 16 pound sledge hammer, a steel plate, hand-held radios
for ready communication, the Bison seismograph, personal protective equipment (ppe), one deep-cycle
12V battery, the Bison power cord and adapter, a field notebook, and a rock hammer to pick up the
steel plate.
For each data collection day, three flags were laid out with one flag at the perpendicular
crossing, two guide flags approximately 100 meters east of the middle flag, and one flag approximately
100 meters west of the middle flag. After the guide flags were in position and a plan set, the equipment
was unloaded from the UNR Seismo trucks. The Bison was positioned at the perpendicular intersection
of the array and the fault, first. From this point, marker flags were laid out at 2-meter intervals, using a
measuring wheel or by chaining. The two meter measurements were critical for data collection and
accuracy during processing. These flags also provided spacing for consistent geophone bundle
placement and marked the location of head cable takeouts. These locations are shown in the observers
report. Two colors of flags were used to differentiate between channels 1-24 and 25-48. From these
cable takeout locations, the steel plate was placed two meters to the south-west. After accurate
flagging was placed, the two geophone cables were laid out. After the cables were placed and inspected
for linear accuracy, the geophone groups were placed alongside the cables. The geophones were then
placed into the ground in order to form in-line, 2-m-long geophone arrays, then stepped on to ensure
good coupling with the ground. The groups were then plugged into the geophone cables. After the
cables were plugged in, each bundle was then checked multiple times by multiple students for waving
wires and other sources of possible error. Noise from error was corrected during processing.
During recording, the trigger cable was unrolled to the west of each line, and the sledge
hammer and steel plate was moved to the first hammer position. Hit locations were decided upon by
the bison operators. Hand held radios were used to relay information for hit locations, hit readiness,
and erroneous recordings. The observers report shows hit locations for both lines. All cables and
adapters were then plugged and secured into the Bison. The battery was then connected to supply
power. The Bison was then calibrated to each day’s readings. After the Bison was set up, hammer hits
commenced at each specified location. Erroneous hammer hit records were cleared upon detection.
After ten hammer hits were applied at each specified location, the line was cleaned and replaced for a
second row of data collection. After each day’s data collection, the records were moved from the Bison
to the tough book computer.
After data was uploaded from the Bison, JRG packs were created for each line. Geometry was
processed in excel and applied to each line. This can be seen in the observers report. After geometry
was applied, first arrivals were chosen, if arrivals were not seen clearly the amplitude clip was adjusted
in the plot parameters window. After first arrivals were chosen, reflection processing commenced.
Frequency was determined from arrival times; this information was used to determine proper filters.
Filtering was then done on each vector. A bp filter was applied based on theoretical velocities of the
ground. The filter parameters for lines one and two were: low down 40, low up 60, high up 200, and
low down 250 (Hz). The filter parameters for line one are: Low down 120, low up 170, high up 350, and
high down 300 (Hz). A te gain was used for each line. After the te gain was applied, each plane was
edited with cut time. The cut time deleted all of the superfluous data, where reflections could not be
seen. A dip fill was applied to lines one and two, line three did not require it. Stacking velocities were
then chosen based on geometry and velocity values. Lines one and two required lower velocity values,
while line three required higher velocities. Lines one and two used velocities from 500 to 2500 m/s,
while line three used velocities from 1000 to 3000 m/s. After these processes were complete, clearer
reflections could be seen and picked more accurately. After accurate cv filters were made, a cmp stack
was constructed for each line. Cmp stacks are constructed by taking the picks from the cv stack and
applying them to the original filtered record, in the make vels section. After parameters are correctly
applied dix interval velocities are applied with cv picks. Velocities are taken from the product, and
applied to the original filter to produce a cmp stack.
II.
Results
Processing and analysis of the Schurz reflection/refraction data provided
evidence of a newly discovered fault. The fault appears to pass under each of the
reflection/refraction lines at the following listed coordinates: Line 1) latitude
38*55’4.89”N longitude 118*48’28.11”W, Line 2) latitude38*54’48.02”N , longitude
118*48’7.00”W, Line 3) latitude 38*54’19.66”N, longitude 118*47’37.84”W. Figure 1
illustrates probable fault locations and fault lineament. Interpretation of these fault
locations were taken from cv stacks. The cv stacks also provided the depth and velocity
of deepest reflections. Reflection line 1 shows clearest and deepest reflections at 1900
m/s at 123.5 meters depth. Line 2 shows reflections at 1800 m/s at 108 meters depth.
Line 3 shows reflections at 1800 m/s at 261 meters depth. Reflections can be seen up to
3000 m/s, but clarity begins to diminish at an average of 1800 m/s. Cmp stack velocity
corrections are provided in table 3-6. Line 1 asymptotic velocity is 1024 m/s with a
frequency of 87 Hz. Line 2 has an asymptotic velocity is 999.8 m/s, with a frequency of
167 Hz. Line 3 has an asymptotic velocity of 1999.8 m/s with a frequency of 153.8 Hz.
Results seem accurate, but cannot be exact and errors were found. Day two
had error due to human failures. Some of the geophone bundles were not connected,
or connected poorly for the first fifteen hit locations. Other errors on day two were due
to wind. Cables and blowing parts were secured, but noise was still a possibility. Other
sources of error were due to poor hits, multiple records were rerecorded. Processing
errors were a possibility, but filters were designed with the closest accuracy. Much of
the data was up to interpretation after processing.
Figure 1: Locations of Reflection/Refraction Lines in Schurz, and probable fault location.
Figure 2: Layout Specifications for the Reflection/Refraction Lines
Figure 3: Line Spacing and Hit Information
Figure 4: Line 1 cmp
Figure 5: Line 2 cmp
Figure 6: Line 3 cmp