South Lake Tahoe Resistivity Sounding - Echo 1
3/21/12
Methods
The Echo 1 resistivity sounding took place in South Lake Tahoe off Highway 50 on March
21, 2012. On the day of this field work students dug a trench that crossed the fault of interest
and then established the centers for each resistivity survey. The Wenner array centered on the
probable fault scarp and the line extended from East to West roughly. Conditions on this day
were sunny and warm. The ground at the top of the survey trench was covered in snow, wet in
some places and frozen in others. Trees and bushes made it difficult for exact measurements of
A-spacing. With the centers located, students used measuring tape to help determine our ten
different A-spacings for the potential and current electrodes in meters. We moved the first Aspacing measurement about 10 cm closer to center because there was not enough wire cable
to reach the expected A-spacing of 31.6 m.
We used the L&R Instrument MiniRes to collect data for the resistivity sounding surveys.
The distances between the electrodes are known as A-spacing’s. Students used 10 different Aspacing for each location of data collection. The two electrodes placed farthest away from the
MiniRes are known as the potential electrodes and the electrodes placed closer to the MiniRes
are known as the current electrodes. Each electrode is approximately 8 inches long and is
usually placed at a depth of 4-5 inches into the ground before data collection. As the a-spacing
starts begin to get smaller the electrodes closest to the MiniRes do not need to be placed as far
into the ground. The electrodes depth at each a-spacing did not have much variation. At each
set A-spacing students collected one resistivity sounding to calculate the apparent resistivity at
that particular A-spacing. Students did not attempt to collect any IP measurements for each
survey. The spacing per decade used for each resistivity sounding should be the value of 6. The
smallest and largest A-spacing used for each survey are at 1.00 and 31.6 meters.
South Lake Tahoe Resistivity Sounding - Echo 2
3/21/12
The Echo 2 resistivity sounding took place in South Lake Tahoe off Highway 50 on March
21, 2012. On the day of this field work Dr. Louie advised students to dig a trench perpendicular
to trench dug in Echo 1. The center determined for this survey was the same as the center for
Echo 1. The line centered on the probable fault scarp with the line extending roughly North to
South. We skipped spacings 9 and 10 due to pavement covering these locations which would
not allow us to place electrodes into the dirt.
South Lake Tahoe Resistivity Sounding – Echo 3, 3/23/2012
The Echo 3 resistivity sounding was taken in South Lake Tahoe off Highway 50 near
Lower Echo Lake on March 23, 2012. Students were advised to dig a trench through the snow
crossing the probable fault location and then establish the centers for each resistivity survey.
Conditions on this day were sunny and warm with thick snow cover. The ground at the bottom
of the survey trench was wet in places and frozen in others with some large plant life.
The Echo 3 sounding was centered on the probable fault scarp and extended roughly
east/west normal to the scarp. The electrodes were placed in dirt, but large boulders, ice, and
plants introduced the possibility of some poor electrode placements. Ten soundings were taken
using the Wenner array and the results were recorded. Recorded resistivity was extremely high.
This translated to even larger Apparent Resistivity values. Data was too large to be considered
useful.
South Lake Tahoe Resistivity Sounding – Echo 4, 3/23/2012
The Echo 4 resistivity sounding was taken in South Lake Tahoe off Highway 50 near
Lower Echo Lake on March 23, 2012. Students were advised to dig a trench through the snow
crossing the probable fault location and then establish the centers for each resistivity survey.
Conditions on this day were sunny and warm with thick snow cover. The ground at the bottom
of the survey trench was wet in places and frozen in others with some large plant life.
The Echo 4 sounding was centered on the probable fault scarp and extended roughly
north/south along the scarp. It shared the same center as Echo 3 but extended at right angles
to that sounding. The electrodes were placed in dirt, but large boulders, ice, and plants
introduced the possibility of some poor electrode placements. Ten soundings were taken using
the Wenner array and the results were recorded. Recorded resistivity was extremely high. This
translated to even larger Apparent Resistivity values. Data was too large to be considered
useful. P;
Results
There were a number of possible reasons for the high resistivity in this area. The ground
was frozen in places and saturated with fresh snowmelt in others. The ice may have had too
few ions to transmit current and therefore acting as an insulator. The presence of boulders,
plants, and frozen dirt made good electrode placement difficult. This may have been a fact, but
did not affect every sounding. As we moved towards the center our electrode placement
became more difficult due to cobblestone and dirt fill. Another possible reason for this high
resistivity is that there was a concrete layer located directly under the cobblestones and dirt
fills. This concrete layer acted as an insulator, which could have been an explanation for the
high resistivity values.
The Echo 4 sounding had readings, which were significantly lower than Echo 3. There
may have been three reasons for this. If a buried pipeline was parallel to the Echo 4 sounding, it
could transmit current at lower resistivity than Echo 3. Also, fault activity may have produced
conditions more favorable to electrical conductivity along the scarp. Finally, the poor conditions
of the survey may have produced completely unreliable data.
UNR North Parking Lot Resistivity Sounding
April 27th, 2012
The Lake Tahoe resistivity soundings of March 21st and 23rd (Echo 1, 2, 3, and 4) failed to
produce useful data. While we believed the faulty data was a result of the site conditions, we
wanted to eliminate the possibility of equipment error. In order to ensure the reliability of the
L&R Instruments MiniRes which we were using, we ran a resistivity sounding in the north
parking lot of UNR between McCarran Boulevard and the new medical building.
Conditions on April 27th, 2012 were sunny and warm with a slight breeze. The ground
was mostly dry, with some very slight soil moisture in the lowest topography. The surface
material had been disturbed by construction from both the University and the road
construction crews, but was compacted. It seemed unlikely that any deep disturbance had
taken place in that specific location. There was a steel culvert and a chain link fence about 3m
from the easternmost electrode.
We used a L&R Instruments MiniRes configured in a Wenner array parallel with
McCarran Boulevard. The distances between the electrodes are known as A-spacing’s. We used
ten different A-spacings. The two electrodes placed farthest away from the MiniRes are known
as the potential electrodes and the electrodes placed closer to the MiniRes are known as the
current electrodes. Each electrode is approximately 8 inches long and is usually placed at a
depth of 4-5 inches into the ground before data collection. As the A-spacing begins to get
smaller, the electrodes closest to the MiniRes do not need to be placed as far into the ground.
The electrode depth at each A-spacing did not have much variation. At each set A-spacing we
collected one resistivity sounding to calculate the apparent resistivity. We did not attempt to
collect any IP measurements for the survey. The spacing-per-decade was a value of 6. The
smallest and largest A-spacing used for each survey was 1.00 and 31.6 meters respectively. We
recorded apparent resistivity values ranging from 24.701 – 33.198 ohm-m.
We analyzed the data using the Resist V.1 fc7 program based on the original 1992
ERModel and ERSolve software by Douglas C. Burger and H. Robert Burger. The eight innermost
A-spacings gave a smooth curve, but the outer two A-spacings dropped in apparent resistivity
value by about 8.5 ohm-m.
We initially analyzed the data using all ten data points. This resulted in an eight layer
model with resistivities which cycled in value between layers. We were unable to fit the 9th data
point because the drop in apparent resistivity was too abrupt. The layer thicknesses and
resistivities as well as the plot can be seen in figure 1.
Figure 1
In order to obtain a simpler model, we eliminated the last two data points. We
determined that interference from the culvert and/or the chain link fence may have resulted in
artificially low resistivity values for the last two soundings. The new model had three layers, the
first two of which were thin (1.4m and 0.5m respectively). As before, there was a cycling of
resistivity between layers. The results of the second model can be seen in figure 2.
In conclusion, this resistivity sounding indicated that the equipment and methods used
to obtain subsurface geophysical data were not faulty either in the UNR parking lot or in the
Lake Tahoe Surveys (Echo 1, 2, 3, and 4). Instead, site conditions were the cause of poor data
collection in Lake Tahoe.