Michael Wickes
Devin Katzenstein
Danielle Molisee
Jessica Pence
Methods Paragraph
GPH 492/692
I.
Methods
Seismic reflection and refraction data were collected during UNR’s spring break, 2013:
March 16 through 24, in Schurz, Nevada. Data were obtained south of Schurz along the Line 1
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 included: 48 geophone groups with six 100-Hz geophones
per group, one trigger cable that connected the sledge hammer to the Bison seismograph, two 24channel geophone cables that connected to the geophones, hundreds of marker flags that were
numbered to indicate hammer and geophone locations, a 16 pound sledge hammer, a steel plate, handheld 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, Line 1 flags were laid out with one flag at the perpendicular
crossing, two guide flags approximately 100 meters northeast of the middle flag, and one flag
approximately 100 meters southwest 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, starting with flag 148 and counting down to flag 064 to the southwest, 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. 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 southeast. After accurate flagging was
placed, the two geophone cables were laid out, first covering flags 101 through 148, and then later in
the day from flags 125 through 172. 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 soft
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
noise. Noise was mitigated during processing as far as practical.
During recording, the trigger cable was unrolled to the southwest end of each line, and the
sledgehammer and steel plate was moved to the first hammer position, flag 064 at the beginning of the
day. Hit locations were decided upon by the bison operators. Handheld radios were used to relay
information for hit locations, hit readiness, and erroneous recordings. The observer’s 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. After the Bison was set up, hammer hits commenced at
each specified location. To build CMP fold at the ends of the stationary recording patches, SW off-end
shots were placed at flags 64, 68, 72, 76, 80, 84, 88, 92, 94, 96, 98, and 100; then roll-on and -off shots at
every flag between 101 through 148; followed by shots off the NE end at 150, 154, 158, 162, 166, 170.
Erroneous hammer hit records were cleared upon detection. After ten hammer hits were applied at
each specified location, the geophone groups at flags 101-124 picked up and moved to flags 149-172.
For this second stationary array, SW off-end shots were placed at flags 101, 105, 109, 113, 117, and 121;
then roll-on and -off shots at every flag between 125 through 172; followed by shots off the NE end at
flags 174, 176, 178, 180, 182, 186, 190, 194, 198, 202, 206, 210, 214, and 218. Figure two shows flag
placement. After each day’s data collection, the records were moved from the Bison to the ToughBook
computer.
After data were uploaded from the Bison, JRG packs were created for each line. A simple
straight-line relative geometry was created in Excel and applied to each line. This can be seen in the
observer’s 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-arrival times were
picked and sent to Optim for SeisOpt® @2D™ analysis, reflection processing commenced. Filtering was
then done on each seismogram. A bandpass (bp) filter was applied based on picks of raw reflection
period apparent in the records. The filter parameters for Lines 1 and 2 were: low down 40, low up 60,
high up 200, and low down 250 (all in Hz). The filter parameters for Line 3 are: Low down 100, low up
170, high up 350, and high down 300 (Hz). A trace-equalization (te) gain was used for each line. After the
tegain was applied, each plane was edited with cut time. The cut time deleted all of the superfluous data
after 0.5 seconds two-way travel time, where reflections could not be seen. A Hale dip filter was applied
to Lines 1 and 2 to cut surface-wave noise; Line 3 did not require it. Stacking velocities were then picked
from constant-velocity (cv) stacks. Lines 1 and 2 required lower velocity values, while Line 3 required
higher velocities. Lines 1 and 2, in the unsaturated alluvium, examined velocities from 500 to 2500 m/s,
while Line 3, in the saturated playa, used velocities from 1000 to 3000 m/s. After cvstack velocities were
picked and checked for their Dix interval velocities, a common-midpoint (CMP) stack was constructed
for each line.
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, interpreted from the CMP stacks. The cv-stack picks
provided the depth and velocity of the deepest reflections. Reflection line 1 shows
clearest and deepest reflections at a normal-moveout (NMO) velocity of 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 as deep as 261 meters depth. Reflections can be seen up
to 3000 m/s NMO velocity, but clarity begins to diminish at an average of 1800 m/s. CV
stack velocity corrections are provided in table 3-6 and were used to create the cmp
stacks. Checking against the raw records, line 1 shows an reflection-hypoerbola
asymptotic velocity of 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.
Using the corrected deep velocities from the cv stacks, a CMP stack and then a
depth-converted CMP stack for each line was then created. Data clarity for Line 1 is
moderate and is shown in Figure 4 below. The vertical exaggeration for the Line 1 stack
is approximately 0.2 and shows several strong reflections (represented by the yellow
lines) with the strongest reflections at depths of 57 m, 77.9 m, and 106.4 m. A
disturbance causing a fall-out in data clarity can be seen to the right of the stronger
reflection (represented by the area circled in red); this disturbance does not appear to
follow a linear trend, however, the area that the disturbance covers ranges from 50 m150 m in the vertical direction and 160 m-168 m in the horizontal direction. As discussed
above the clarity of the data in Line 2 is the least out of the three lines and therefore
fewer reflections can be seen in the stack as shown in Figure 5 with a vertical
exaggeration of 0.22. The strongest reflections that are visible are at depths of 34 m,
54.9 m, and 167.4 m from the surface. A disturbance in the middle of the section
(represented by the red line) appears to be linear and trends at approximately 21.3°
from vertical. The clarity of data in Line 3 is the best out of the three lines and shows
many strong reflections including the deepest reflection found in the reflection study as
shown in Figure 6 with a vertical exaggeration of 0.44. The strongest reflections shown
in Line 3 are depths of 13 m, 64.8 m, 91.8 m, 124.6 m, and 256.9 m below the surface.
Another linear disturbance is seen in the middle of this line with a trend of
approximately 83° from horizontal. These disturbances are also shown for all three
lines by the shallow depths of accurate refraction velocities in the velocity gradient plots
as seen in Figures 8, 9, and 10.
The gravity team found a possible graben near line 3. CMP stacks with
refraction velocities may support this theory. The fault scarp does not seem to be the
exact location of the fault, due to erosion as depicted in Figure 7. The fault is perceived
to be to the west of the visible fault scarp. A possible splay was detected over line
three. The fault goes near the middle flag on line three, but also seems to follow the
vegetation lineament. Further data collection to the west of line three could support
this theory. (Figure 1)
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. Processing
velocity data is found to be within ten percent accuracy. Found velocities were very
close to found cmp refraction velocities. Much of the data was up to interpretation
after processing.
Figure 1: Locations of Reflection/Refraction Lines in Schurz, probable fault location and possible splay.
Figure 2: Layout Specifications for the Reflection/Refraction Lines
Figure 3: Line Spacing and Hit Information
Figure 4: Line 1 depth converted cmp stack
Figure 5: Line 2 depth converted cmp stack
Figure 6: Line 3 depth converted cmp stack
Figure 7: Scarp Erosion Hypothesis
Figure 8: Line 1 cmp stack and refraction velocities
Figure 9: Line 2 refraction velocity and cmp stack
Figure 10: Line 3 cmp stack and refraction velocity