1. Geophysical Study of the Hot Creek Field Area near Mammoth Lakes, CA
Gel 110-Summerfield: Geophysics
Cody McMurdie
997436910
7/30/14
Group Partners: Matthew Wilson, Naoma McCall, Madeline Quasem, Thinh Tran,
Danielle Torres, Zenita Singh, Norleen Lowrans

3. Introduction
The purpose of this study is to gain an understanding of geophysical field
techniques and how they can be used to interpret the geologic properties that cannot be
directly observed from the surface. Properties such as porosity, density, pressure and
temperature can be measured efficiently using geophysical techniques rather than
expensive direct techniques like drilling. In our field site, there are faults near the
hydrothermal system that can channel the hydrothermal fluids near the surface which
would be a valuable geothermal energy resource to utilize. We can use geophysical
field techniques to confirm the locations of hydrothermally affected faults because we
are able to indirectly study the geophysical properties of the materials beneath the
surface. Our field site likely contains volcanics because of its location within the Long
Valley Caldera region. Using our geophysical field techniques we can locate boundaries
between volcanic bodies and see how deeply covered these units are covered in
alluvium. Boundaries between alluvium and a dense igneous unit would also be a good
place to search for evidence for a water table, which would likely show up in electrical
resistivity surveys. A water table in our site is not unlikely since we see a wet depressed
region near the Whitmore Hot Springs that could be a part of a water table from our site.
A water table located near a fault could be important because it could act as a conduit
which could be easily accessed from the surface. The different geophysical field
techniques that we learn throughout this study can be used together to confirm geologic
structures by looking at more than one geophysical property.
Regional and Local Geology

4. The field site we are interested in studying is located east of the Sierra Nevada
Mountain Range, in Long Valley, near Mammoth Lakes CA. Long Valley is undergoing
east west extension associated with normal faulting and Basin and Range extension.
This extension caused crustal thinning which resulted in volcanism and the eruption of
the Long Valley Caldera. Our field site is located along Doe Ridge, where 300,000-year-
old rhyolite flows from the Long Valley Caldera created the rolling hills we see in the
field (Farrar 2007). Doe Ridge is directly southeast of Hot Creek, a hydrothermal system
of groundwater heated by rising magma beneath Long Valley. En echelon normal
faulting of the eastern sierras is likely present in our field area, which could cause the
hydrothermal fluids of Hot Creek to migrate towards Doe Ridge. These fluids could
cause hydrothermal alteration, which would change the physical properties of the
subsurface layers. The majority of the Doe Ridge field site is also covered in alluvium,
which is likely deposited from the sierras and Quaternary in age. The relatively thin
alluvium layer is important because we have limited geophysics equipment, which
means we are limited on how deep we can gather subsurficial data.
Experimental Plan
Our goal was to conduct geophysical surveys near Hot Creek and obtain data
that would help us learn about the geological features of our site. We ran our data
measurements perpendicular to what appeared to be fault scarps in order to see if we
could find geophysical evidence that could confirm this structure. We were also
interested in seeing if we could observe differentiated layers beneath the surface. Since
the rhyolite from the Long Valley Caldera is covered in alluvium we can use seismic
refraction surveys to see how thick the alluvium cover is. Due to our field site’s proximity

5. to the Hot Creek hydrothermal system, we may also have altered hydrothermal units
beneath the surface that could also be measured using our geophysical surveys. Since
we need multiple geophysical techniques to confirm any of the aforementioned features,
we ran all of our measurements in the same region, gathering data linearly across our
mapping region in roughly the same orientation each time. By studying the units
beneath the surface, we can better understand how much alluvium covers the lower
volcanic units and how this area is affected by the regional hydrothermal systems. Our
observations will also allow us to see how faulting affects the flow of hydrothermal
fluids. A change in geologic properties near a fault may be indicative of hydrothermal
fluids migrating through a fracture zone and causing alteration to the surrounding rock.
Individual Geophysical Experiments:
Seismic Refraction
A. Application of the technique to the problem
Seismic refraction is a technique that involves measuring wave velocities as the
waves travel through the earth. In our experiment we induced a compressional wave
with a sledgehammer and measured the amount of time it took the compressional wave
to reach the geophone monitors, which were oriented in a line. As the compressional
wave moves through the rock it may encounter layers of earth with different physical
properties that either speed up or slow down the wave. The time it takes the wave to
move through the earth is directly related to density, pressure, temperature, and
porosity of the layers beneath the surface. This information is useful because changes
in the wave velocity can indicate boundaries between rocks of different porosity or
lithology. This would be useful in determining if there is bedrock, hydrothermally altered

6. rock, or a water table beneath the alluvium in our field site. Geologic structures such as
faults can also be interpreted from seismic data if we see an offset in the layers that our
seismic model produces.
B. Data collection and analysis
In the field we measured seismic refraction by hitting an aluminum plate with a
sledgehammer that was connected to the Geometrics computer with a cable. When the
sledgehammer hit the plate, a signal was sent to the computer that tells the computer to
start recording data for a few milliseconds. The computer records data from geophones
that are sequentially arranged in a line. Each geophone contains a magnet and a
spring, which sends electrical signals back to the computer if it detects vertical motion in
the ground, caused by the hammer strike. Since the hammer strike radiates
compressional waves, the computer can measure the time it takes the wave to hit each
geophone. The geophones farther from the hammer strike will measure waves that
have traveled deeper in the earth, which means that the computer can then plot wave
velocity versus depth. We were able to reduce noise in our data by hitting the aluminum
plate with a hammer multiple times. The repetition increases the resolution of our data
because it allows the computer to differentiate induced compressional waves and
vibrational noise from the environment.
C. Results of modeling and interpretation
Using the SIQPC computer, we refined our data by making picks of where we
saw the compressional wave from the sledgehammer first arrive at each geophone. We
then plotted a seismic velocity versus depth plot, which showed three layers of different
seismic velocities. The top layer had the lowest seismic velocity and therefore a high

7. porosity, which is indicative of unconsolidated alluvium. The next layer was interpreted
as being consolidated alluvium because the compaction of alluvium would cause the
increase in seismic velocity in this layer. The bottom layer had the highest seismic
velocity. We interpreted this layer as being brecciated rhyolite because the seismic
velocity was not high enough for this layer to be non-brecciated rhyolite and faulting in
the region would allow us to have brecciated rhyolite in our field site. Faulting could also
result in the transport of hydrothermal fluids, which would also slow down the seismic
velocity in our survey. On our seismic velocity versus depth plot, we were able to
confirm a normal fault based on how layer 2 and 3 move upwards (Figure 3b). Since our
seismic survey can only measure to 120 feet, we do not have enough information to tell
if there is something beneath the brecciated rhyolite layer.
Gravity Surveys
A. Application of the technique to the problem
In the field we used a gravimeter to study changes in the downwards-
gravitational force on earth. The gravimeter is essentially a mass on a string in a
vacuum that measures how much downwards force gravity is exerting at that location.
The gravity at a location on Earth is dependent on the mass beneath the surface and
the elevation from Earth’s center of mass. The mass beneath the surface will change
with materials of different densities; rocks with high densities causing a stronger
gravitational pull. By measuring the slight gravitational changes as we walk along the
field area we can make inferences on how the densities of the rocks change throughout
the region. The gravimeter can provide evidence for faulting if we observe fault gouge,
which is less dense than the surrounding material. This sudden drop in gravitational pull

8. due to less dense material can help confirm the location of a suspected fault. The
gravimeter is also useful in locating ore deposits, which could show up in the gravity
data as a spike in the gravitational force due to a relatively dense ore located in the field
area.
B. Data collection and analysis
We measured gravitational anomalies in the field by traversing linearly while
collecting data every 50 meters with a Worden Gravimeter. At each location we
recorded the time of day, the UTM coordinates of our location, and the gravitation force
in milligals. Once we gathered our data, we plotted our measurement locations on a
1:24,000 scale map that was used to help calculate for error.
We accounted for error in the gravitational measurement with several techniques.
The first correction involved subtracting the base station measurement from each of the
measurements along the traverse. This allows us to get a relative measurement of the
gravitational force that is more reasonable and easier to understand. Next, we had to
account for instrument drift and changes in the lunar and solar gravitational pull that
occur throughout the day. To do this, we measured the base station at multiple times
during the day and graphed the gravity measurements with respect to time. This
allowed us to adjust the gravity measurements so that they would follow the same
gravitational fluctuations as the base stations.
We corrected for gravitational changes that occur from elevation change by using
the free-air correction equation, which multiplies the change in elevation from the base
station to a constant. The Simple Bouguer method is another correction we used that is
similar to the free-air correction, but uses an equation that takes into account the

9. assumed density of the layer of rock at that location. Finally, we used a method, which
removes error that would be caused by dramatic topographic changes in locations near
the measurement. This method involves using a grid overlay on our map to measure
changes in elevation near our measurement. The Complete Bouguer method combines
all three of the previous corrections, which gives us a set of final gravity measurements
that are corrected for errors. With our corrected set of gravitational measurements, we
were able to plot the gravitational changes along our traverse, which we then
interpreted with forward modeling using the GravMag program.
C. Results of modeling and interpretation
Once we plotted our corrected measurements in the GravMag program, we
noticed that on both plots we observed low gravity anomalies in the west and high
gravity anomalies in the east. We interpreted the low gravity anomalies on the west to
be areas that had higher amounts of alluvium at the surface relative to the east. This
data matched what we observed in the field because we saw rhyolite outcrops, with no
alluvium cover, in the east. We modeled our data to have layers similar to what we
observed in seismic to keep things consistent. We had a layer of unconsolidated
alluvium above consolidated alluvium, which was above rhyolite. We also modeled a
layer of basalt beneath the rhyolite because it helped us fit our model to the data. Since
we had a dip in gravity in the middle between our low and high gravity anomalies we
assumed that this location contained the fault. A fault at this location could be filled with
air, water, or clay minerals; all of which would give us our low gravity anomaly reading.
Since the two gravity transects both crossed the fault scarp and both models had this
low gravity anomaly we can be fairly certain that this zone is indeed faulted. Figures 5a

10. and 5b show the two gravitational models with the differentiated geologic layers and the
fault.
Magnetometer
A. Application of the technique to the problem
Magnetic fluctuations occur throughout the earth because magnetic fields can be
induced in earth materials containing iron. Since these materials have their own field
they will amplify and damper the earth’s natural magnetic field in predictable manner
which we can use to help us understand where bodies of high or low magnetic
susceptibility are located beneath the surface.
The magnetometer we used is called a proton precession magnetometer, which
is made of a coil wrapped around a fluid containing high amounts of hydrogen nuclei.
We then induce an electrical current through the wires with a battery, which then
induces a magnetic field in the fluid, aligning the hydrogen protons in the fluid. When the
current is removed, the protons in the fluid realign themselves with Earth’s magnetic
field. In doing so, the protons precess and induce a small amount of current back into
the wire which can then be measured by the multimeter in the computer. The amount of
precession by the protons is directly related to the amount of magnetic force the earth is
exerting at that location. We then use the magnetic field strength variations to explain
materials with high and low magnetic susceptibility beneath the surface. For example, a
fault filled with clay minerals may have a low magnetic susceptibility relative to the rest
of the background materials.
B. Data collection and analysis

11. In the field, we used a magnetometer that was attached to a rod and a battery
with a multimeter and computer that could convert the coil’s induced current
measurement into a magnetic field strength measurement (in nanoteslas). The
magnetometer was attached to a rod to avoid errors caused by any magnetic items in
the vicinity. We noticed that if we stood at different locations relative to the
magnetometer, we would get fluctuations in our measurement readings. This was likely
due to the fact that we were holding the battery and computer that has magnetic
material that could affect the reading. To minimize errors, we stood in the same
proximity to the magnetometer for each measurement and kept any other metal objects
far from the magnetometer.
We took measurements every 10 meters along three parallel transects which ran
approximately perpendicular to a presumed fault scarp. We took base station readings
every 2 hours in order to account for errors caused by periodic changes in Earth’s
magnetic field and by instrument drift. We took GPS locations of each reading so that
we could plot our data on the map and in the GravMag program.
C. Results of modeling and interpretation
After we acquired all of our magnetometer data, we plotted the data onto the
GravMag program to create a model to explain the magnetic field fluctuations that
occurred as we walked along our transect. The transect where we crossed the
suspected fault scarp we noticed a zone of low magnetic susceptibility (Figure 4a) which
is evidence for fault gouge with clay minerals accommodating the fault zone. The clay
minerals could be perhaps from hydrothermal fluids traveling along the fault, in which

12. case we would be interested in confirming the hydrothermal fluids so we could utilize
this location as a geothermal energy resource.
We also modeled our data to contain several igneous dikes that intruded
upwards through the alluvium (Figures 4a, b, and d). We put igneous dikes at these
locations because we noticed we had several high spikes in the magnetometer data that
could only be explained by a vertical feature with high magnetic susceptibility. We are
unsure about how deep the dikes go into the earth, but we expect them to be low
enough to be fed by an igneous unit. We also saw outcrops of igneous rock at the
surface in our field site, which could be locations where the dikes allowed igneous rock
to penetrate the surface.
Electrical resistivity
A. Application of the technique to the problem
Electrical resistivity is a measurement of how much a material slows down the
flow of electric current. In the field we can measure the resistivity of different layers of
earth by inducing an electric current and measuring the voltage drop as the electric
current travels through the earth. The measured voltage drop will allow us to calculate
the electrical resistivity of the different layers. Resistivity measurements let us to define
different geologic units because different units will have different physical properties that
alter the resistivity. For example, a decrease in resistivity is related to an increase in
fluid content, porosity, salinity, and/or clay content.
B. Data collection and analysis
Our resistivity survey involved two current inducing electrodes and two
voltmeters that were arranged in a Schlumberger Array. This orientation is defined by

13. having the two voltmeters in between, and five times closer together than, the two
electrodes. This method of resistivity sampling is considered to be active rather than
passive because we are inducing the electrical current in the earth with the electrodes.
The electrodes and voltmeters were placed in a line with a trend of 72 degrees because
we wanted to gather data that ran perpendicular to what we presumed to be a fault
scarp. We were able to sample resistivity at larger depths by increasing the spread of
the two electrodes and vice versa. We started with the electrodes 300 meters apart and
moved them in sequentially until we had five data measurements per station, which
would give us five resistivity measurements at different depths. We ran five different
transects which made up two parallel lines of stations that ran across the presumed
fault. We then used a MATLAB program called Zappo08 to create a model that fit our
resistivity measurements.
C. Results of modeling and interpretation
The resistivity data showed a general trend of decreasing resistivity with depth,
which is reasonable because geologic layers generally increase in density and therefore
conductivity with depth. At most stations there was a layer of unconsolidated alluvium
with a high resistivity followed by a layer of consolidated alluvium with a lower resistivity,
and finally a layer that is has a very low resistivity. The bottom layer doesn’t have a low
enough resistivity to be a solid body of rhyolite in most cases, so it can be interpreted as
brecciated rhyolite or hydrothermally altered units that contain clay or other conductive
materials. At station 2 and 3 we observed a layer of very low resistive material
sandwiched between two layers of highly resistive material (Figure 2c and 2e). The low
resistivity material could be interpreted as being a perched water table or a

14. hydrothermally altered layer made of clay. The deep layer with a high resistivity is
interpreted as being a basalt flow containing volcanic glass, which would not be
conductive. Station 5 has a layer of low resistivity that is close to the surface of the
earth, which is indicative of it being a vadose zone from the recent rainfall (Figure 2i).
The overall trend of the resistivity is consistent in that resistivity tends to decrease with
depth with the exception of some stations that show clay layers or layers enriched with
water.
Magnetotelluric
A. Application of the technique to the problem
We can also measure the resistivity of the earth using the earth’s naturally
induced electric currents. The induced currents, called telluric currents, are caused by
variations in solar wind hitting the magnetosphere, which causes magnetic field
fluctuations. The telluric currents are considered alternating currents rather than direct
currents because they alternate back and forth as the magnetic field fluctuates. We can
measure the telluric currents in earth using electrodes and measure the magnetic field
that causes those currents using a magnetometer. By comparing these two values
using a ratio, we can observe the resistivity of earth by measuring how much current is
lost as it travels through the earth. Since different frequencies of electromagnetic waves
will penetrate the Earth at different depths, we can repeat the experimental observations
at different frequencies and see how resistivity changes with depth.
The resistivity of a the earth will change with lithology, porosity, and pore filling
fluids. We can use the resistivity to see boundaries between geologic layers that differ in
these properties. For example, we may be interested in finding the boundary between

15. alluvium and bedrock, in which case we would expect to see an increase in electrical
resistivity at the bedrock layer. Similarly, we could interpret a change in resistivity as a
water table, compacted alluvium, or fluid filled porous rock depending on how much the
resistivity changes with depth.
Since we use the magnetotelluric set up in two dimensions, we can also better
understand the geometry of the geologic features of our site. An example would be
seeing a decrease in resistivity in the north-south direction, which could be indicative of
a north-south oriented fault containing conductive clay minerals. We call this type of
data, high resistivity in one direction relative to another, anisotropic. Anisotropic data
may trend towards high or low resistivity, which would help indicate whether the fracture
zone is accommodated by clay minerals or is left unaccommodated and filled mostly
with air.
B. Data collection and analysis
The magnetotelluric technique uses four electrodes and two magnetometers.
Two electrodes and a magnetometer are oriented north-south and the other two
electrodes and magnetometer are oriented east-west. This ensures that we can gather
electric and magnetic field information from waves with any orientation. The electrodes
measures the voltage created from the telluric currents and the magnetometer uses a
coiled wire to measure the fluctuating magnetic field that induces the telluric currents.
We ran wires back to the central Geometrics computer that measured the voltages at
different frequencies so we could see how electrical resistivity of earth changes at
different depths. With this data, the computer plots north-south resistivity versus depth
and east-west resistivity versus depth. Having two both dimensions plotted on the same

16. graph allows us to interpret any anisotropic wave data that would be caused by geologic
structures.
The method our group used for magnetotellurics is called passive, because we
did not induce the telluric currents that we measured. Generally we would want to use a
transmitter that amplifies high frequency waves so we could have better resolution in
our shallow depth readings, but we did not have a sufficient battery supply to allow our
group to use the transmitter device. Magnetotellurics is unique from the other resistivity
measurements because it allows us to measure deeper without having to induce a
current ourselves. It is also unique in that magnetotellurics can give us a three-
dimensional model of the field area since we measure resistivity versus depth in the
north-south and east-west direction.
C. Results of modeling and interpretation
In this experiment, we were mainly interested in confirming locations of a
presumed fault. We conducted magnetotelluric surveys at 7 stations that were located
on and around the presumed fault (Figure 1). Our resistivity versus depth plots showed
a general decrease in resistivity with depth with the exception of some data points that
had low resistivity at low depth which we interpreted as a vadose zone that was located
near the surface because of recent rainfall.
Stations 1,4, and 7 had data that was interesting because it had noticeable offset
between the north-south and east-west resistivity plots (Figures 1a, 1d, and 1g). In
these cases, the east-west measurements were more resistive than the north-south
measurements. We were fairly certain that station 4 and 7 were located along a fault
scarp, so we interpreted the offset in resistivity measurements as confirmation of fault

17. gouge located along the fault zone. The north-south data was less resistive because
clay minerals in the fault gouge would allow for electric currents to run more freely along
the fault zone. Station 1 had similar data to stations 4 and 7, so we interpreted this as
an eroded section of a north trending fault that is parallel to the fault scarp we observed
at stations 4 and 7. The other stations’ data measurements showed some gaps
between the north-south and east-west data, but they were not significant enough to be
interpreted as any type of geologic structure. Further geophysical analysis at these
stations would be needed to confirm geologic structure because the data contained a
fair amount of noise.
Synthesis and Conclusion
Based on all of our geophysical surveys, we can conclude that the field site
contained at least 3 different lithologic units: unconsolidated alluvium, consolidated
alluvium, and a fractured rhyolite unit. The seismic data revealed these three layers
when we plotted seismic velocity with depth. As we surveyed deeper, the seismic
velocity increased which is indicative of density increasing with depth. Resistivity and
magnetotelluric surveys also confirmed the three-layered model because we saw that
electric resistivity increased with depth in most places with few exceptions. Gravity
survey models included a basalt layer beneath the rhyolite unit, which is not
unreasonable because there are basalt flows directly west of our field unit. This basalt
flow was also interpreted to be present from the resistivity data. We saw that at very
deep locations, the resistivity measurements had very high values, which we interpreted
to be the basalt, since a basalt would have volcanic glass that would act as an insulator
making it a very resistive material.

18. A fault was also confirmed in our field site by nearly all of our geophysical
surveys. The Seismic refraction data contained an offset in the three layers which
helped us interpret the orientation of the fault (Figure 3b). The gravity survey also
confirmed this fault by having a low density clay unit be responsible for the low gravity
anomaly we see at the fault location (Figures 5a and 5b). Finally, our resistivity
measurements from magnetotellurics confirmed that the north trending fault scarp has
clay minerals accommodating the fault zone since we see anisotropic data with less
resistive materials along our fault zone (Figures 1a, 1d, and 1g).
The water table in our field site was only interpreted by our resistivity model, so it
is less certain than the presence of the fault. The seismic data interpretation of
consolidated alluvium could be reinterpreted as the water table, but we would need
more resistivity and seismic data to be certain. Also we are unsure if the clay minerals
located in the fault gouge are currently being altered by hydrothermal fluids or if the fault
was previously altered and now dry and inactive. Confirming the presence of the water
table would make it more likely that the fault is actively acting as a conduit for
hydrothermal fluids. This would be important to know if we wanted to use the field site
as a geothermal energy resource.
There are many sources of error that could have been introduced while we were
making our geophysical surveys. Without more data collection, we cannot make any
certainties about the presence hydrothermal fluids or a water table. Some sources of
error could be caused by subjectivity with the data modeling for our seismic, gravity,
magnetometer, and resistivity surveys. Some of our data may have shown incorrect
measurements because there is instrument errors and noise that is picked up with each

19. measurements. Repetition of surveys at our field site and higher quality survey
equipment could help improve the resolution of our data by minimizing the amount of
skewed data. The importance of this study was that it gave us a general idea about the
subsurficial geology of our field site. We were able to identify a fault zone and identify
general trends in density and lithology of the subsurface rock. The data we gathered
can be used to help narrow down locations where we could further explore for natural
energy resources such as hydrothermal conduits located in our field site.
References
Farrar C. et al, Boiling Water at Hot Creek—The Dangerous and Dynamic Thermal
Springs in California’s Long Valley Caldera: U.S. Geological Survey Fact Sheet:
3045 version 1.0, 2007.