resistivity results

Team Resistivity Results
Sabina Kraushaar
Erik Novak
Tyler Seaman
Box 1
The arrays at Box 1 were laid out parallel and normal to the visible fault trace. As shown in the
inversion figures, the arrays at location 3 and the parallel array at location 1 have a shallower layer 1 of
1-1.5 m compared to the arrays at location 2, and the parallel array at location 2, which have
thicknesses of 2-3.2 m. We also see a shallower layer 2 in the array at location 2 normal to the fault.
This layer is 2.69-3.9 m while the other arrays have thicknesses 10-30 m. Additionally, the resistivity in
layer 1 at location 3 is significantly higher than at the other locations. This causes greater changes in
resistivity between the subsequent layers, which may prove helpful in identifying the fault trace at other
locations. All inversions followed an ‘H’ type model with the exception of Location 1 parallel to the
fault which was able to be modeled as both a ‘Q’ and an ‘H’ model with reasonable accuracy. This
results because while the ‘Q’ model had a slightly lower RMS value, it only included two layers. The
‘H’ model of this dataset allows for greater depth confidence, while only increasing the RMS value by
a factor of 0.13. We also see an anomaly in the array at Location 1 parallel to the fault. The resistivity
readings at A-spacing 10-31.6 m significantly drop at this location. This trend is best illustrated in
Figure 1. This array was located on the fault, parallel to the fault. The reason for this anomaly is due to
the higher conductivity of the gouge produced from movement of the fault in a strike-slip motion. This
will be our best indicator of the fault at locations that we cannot see the fault. The depths of confidence
for the arrays in Box 1 range from 5-35 m depending on the particular inversion model. The iR values,
which are the factor of error recorded when taking the measurement, are typically higher when the Aspacing is small. The iR values typically drop by a factor of 100 as the A-spacing rises. This drop
usually occurs around an A-spacing of 6.81m. This makes our data collected at the smaller A-spacing
less reliable. Therefore, anomalies seen at greater depths will be our best identification of the fault from
lateral resistivity anomalies.
Figure 1. Profile of apparent resistivity (ohm-m) vs a-spacing (m) for Box 1.
Box 2
The profiles of arrays taken parallel and normal to the proposed fault trace all exhibit similar
characteristics. Each array from Box 2 resembles a “Q” or “H” type model with a high resistivity value
at shallow depths and then a significantly lower resistivity value at greater depths. Overall, we expect
to see a major drop in resistivity along the trace of the fault because this could be a high conductivity
zone due to shearing of material. Unlike Box 1, location 1 parallel to the fault, which shows a major
drop in resistivity values, profiles from Box 2 do not show a similar drop. Most profiles have a slight
decrease in resistivity between an A-spacing of 1.00 and 1.50 m then a decrease in value between 1.50
and 7.0 m before leveling out at a constant value. The average thickness for the shallowest high
resistivity value layer ranges from 1.25 to 2.00 m. For arrays with 3 layer models, the average
thickness of the low resistivity layer is 4.50 to 40.00 m. Arrays with 4 layer models calculated an
average thickness of a second high resistivity layer value of 3.00 to 9.00 m. The average value for the
shallow high resistivity layer is 258 to 365 ohm-m. The average value for the infinite thickness layer is
50 to 144 ohm-m. The range of imaginary component values is a minimum of 0.01 to a maximum of
0.33 ohms. Finally, my depth of confidence for these arrays is approximately 20 meters based on the
low RMS values of my layered models.
Figure 2. Profile of apparent resistivity (ohm-m) vs a-spacing (m) for Box 2.
Box 3
All of the arrays in Box 3 were laid out parallel to the fault trace. In this area, there is no
apparent fault scarp, so it is unknown if one array was directly on the fault. Overall, these data did not
match up at depth, and varied internally compared to the other areas. However, all arrays (except for
location 8 and 10) showed a thin lower resistivity layer at the shallowest depth. The depth of the first
layer varied from ½ - 2 m, with one plot reaching 6.4 m. This is too shallow to see any fault trace. The
resistivities of these layers varied from 161 – 319 ohm-m. All arrays exhibited an H or K-type model.
The depth of confidence in this area generally varied from 1-3 m, which is considerably lower than the
other arrays, and could be because of the conductive playa clays at a shallow depth. This might explain
why these data do not correlate with each other at depth. At locations 13 and 14, the data do not fit into
models, and have imaginary components up to -1.95 ohms. This could be because of low resistivity
sands at a shallow depth, or could be bad data collection (collecting data with a line open).
Figure 3. Profile of apparent resistivity (ohm-m) vs a-spacing (m) for Box 3.
Figure 4. Profile of apparent resistivity (ohm-m) vs a-spacing (m) for Box 3, with imaginary
compenent error measurements, represented by scaled error bars.
Uncertainties and errors
There were certain errors associated with data collection. The spacings were accurate within
about 5 cm along the tape measure, and at times offset 30-60 cm perpendicular to the tape measure. At
some points, the line was open, meaning the current did not flow through the entire array. When this
happened, we had to move the rod out of soft sediments into harder packed ground. It is also possible
that data were recorded when the line was open, such as seen in Box 3. When recording data in the
playa, clays at the surface gave high conductivity values at a very shallow depth, which led to
inaccurate results at greater depths. This was observed in Locations 7 and 8 in Box 3. Other factors
that might influence errors in the data include layers of sand with high resisitivity, or underground
human-made metal objects with low resistivities. The GPS coordinates taken at each array also has an
accuracy of about 10 m, which might or might not be a source of error in interpretation.