Prepared by: Daniel Shaltry, Geoscience 493: Internship
Submitted to: Kasper van Wijk, Supervisor
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Abstract
Neal Hot Springs (NHS) is an active geothermal site and home to a new binary power plant built by U.S. Geothermal and funded through the Department of Energy. Power production is scheduled to begin in late 2012 and is proposed to generate 25 mega-watts of power to its customer Idaho Power.
The project has also served Boise State University as an ideal location for geophysical exploration and research. Research began in spring of 2011 during BSU’s annual geophysics field camp. Students and faculty conducted various geophysical surveys to gain insight into the controlling geological structure of the area. Studies of the site continued into 2012 with the monitoring of a passive seismic network, a
Controlled-Source Audio-Frequency Magneto-Telluric (CSAMT) survey conducted for the National Student
Geothermal Competition, as a research site for graduate students at BSU, and as a chance for me to earn my undergraduate geophysics field credits. As part of the field experience I participated in CSAMT acquisition, conducted a magnetic survey, analyzed vibroseismic data obtained by the 2011 field camp, and continue to monitor and review data from the passive seismic network.
It is concluded in this report that magnetic, and vibroseismic data indicate that a large normal fault 5 kilometers east of the power plant and trending from northwest to southeast is the Cottonwood
Mountain Fault (CMF). CMF was previously mapped trending in the same direction, but our data suggests that its actual location is approximately 3 kilometers east of its previously mapped location.
Magnetic and gravity data confirm the location and trend of the previously mapped Fault A. The CSAMT survey concludes that a portion of the production fault immediately south of the power plant was imaged to a depth of 500 meters and exhibits a volume of low resistivity. This volume is interpreted as a geothermal reservoir within the hanging wall of the production fault. Passive seismic data indicates that the area surrounding NHS exhibits very little natural seismic activity. So far, with over one year’s worth of data we have identified two small earthquakes within the vicinity of NHS. It is not confirmed that these events are related to NHS, but both events took place within just kilometers of the seismic network and have not been catalogued by any other network. Furthermore, we have not confirmed any seismic activity induced by the power plant thus far.
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1.0
– Introduction
1.1
– Background………………………………………………………………………………………………………………… 4
1.2
– Geologic Setting…………………………………………………………………………………………………………. 5
1.3
– Geologic Conclusions…………………………………………………………………………………………………. 6
2.0
– Methods
2.1
– Summary………………………………………………………………………………………………………………… 7
2.2
– Passive Seismic…………………………………………………………………………………………………………. 7
2.3
– Active Seismic………………………………………………………………………………………………………… 16
2.4
– Magnetic……………………………………………………………………………………………………………..... 19
2.5
– CSAMT…………………………………………………………………………………………………………………… 22
3.0
– Conclusions
3.1
– Integrated Magnetic and Active Seismic……………………………………………………………….. 25
3.2
– Integrated Gravity and Magnetic…………………………………………………………………………. 27
3.3
– CSAMT…………………………………………………………………………………………………………………. 28
3.4
– Passive Seismic……………………………………………………………………………………………………. 28
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1.0
- Introduction
1.1 - Background
NHS is the result of deep and complex intersecting faults and fault blocks in an area of anomalously thin continental crust. Cold meteoric waters entering the subsurface by regional drainage saturate the fault zones at depths of 1000 to 1500 meters, then rise back to the surface at near boiling temperatures [NHS Field Camp Report, 2011]. This geothermal system covers a relatively large area in eastern Oregon including the city of Vale where the resource has been utilized since the early 1900’s, primarily for residential and commercial space heating [Colorado School of Mines Field Camp
Report, 2011]. Interest in using NHS for a geothermal power plant was first taken up by
Chevron Minerals in 1979 during a time of high oil prices. As oil prices stabilized shortly thereafter interest in NHS as an alternative energy resource diminished. Then, in 2006 U.S. Geothermal took
Figure 1: Location of NHS
interest in NHS as a potential site for a binary power plant. They currently hold a series of long-term leases to the geothermal rights at NHS covering an area of 9.6 square miles [Colorado School of Mines
Field Camp Report, 2011]. In May 2008 U.S. Geothermal successfully drilled two production wells; one with geothermal fluid temperatures of 141.6°C at a depth of 702.6 meters, and another in October 2009 with temperatures of 141.1°C at 882.7 meters [Clutter, T., 2010]. As of August 2012, construction of the binary power plant has been completed and power production is scheduled to begin in October 2012.
The NHS geothermal project has served as an excellent opportunity for geologic and geophysical exploration for students at Boise State University. Student research at the site began in May 2011 with BSU’s annual geophysics field camp. During the field camp many geophysical methods were utilized including, but not limited to, the installation of a passive seismic network, vibroseismic sweeps, gravity and magnetic surveys, and electrical surveys. The data collected have provided geophysics students with experience in many geophysical applications. It has also provided me with the chance to gain field, processing, and interpretation experience as well as earn my required field credits for graduation.
As the only geophysics student who needed to earn field credits during summer 2012 I joined faculty, graduate
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Figure 2: Map of the Neal area with well locations. The transect line crosses the half-graben formation shown in Figure 4.

students, and volunteers for data acquisition at Neal. During the week long camp-out we collected data from the passive seismic network, and conducted magnetic and CSAMT surveys. The magnetic survey and passive seismic data acquisition went well in the field. We obtained magnetic data along a 5.5 mile line and added another 45 days’ worth of seismic data to our database. The CSAMT survey proved to be challenging, and consumed most of the time spent in the field. After much troubleshooting due to equipment malfunctions and logistical hurdles we ended up with CSAMT data along three transect lines.
After the camp, time was spent on campus processing and interpreting the data. In addition to the passive seismic, CSAMT, and magnetic data that we collected I also reviewed data from the 2011
Field Camp. Namely, data from the vibroseismic sweeps was processed and interpreted to correlate reflectors with the geologic map of the area. Gravity data obtained during the 2011 Field Camp were also reviewed and compared with the newly acquired magnetic data. The ultimate goal of my field experience is to integrate the methods described above and to produce and explain figures that illustrate the results of those methods.
1.2 - Geologic Setting
The project at NHS is located due west of Vale, Oregon and lies at the intersection of three regional geologic features of the western U.S. These are the Columbia River Basalts (CRB), the Oregon-
Idaho graben, and the northwestern boundary of the
Western Snake River Plain (WSRP) [NHS Field Camp Report, 2011]. The large Cenozoic flood basalts of the CRB are the oldest of the three and were formed by extensional faulting related to the formation of the Basin and Range Province due to the interaction of the Yellowstone hot spot and the North
American plate around 16 Ma [Takahashi, 1998]. The Oregon-Idaho graben formed shortly after the CRB around 15.4 Ma [NHS Field Camp Report, 2011]. This formation is a north to northwest trending rift system spanning approximately 1100 km in length which includes the northern Nevada rift, the Baker graben, and the CRB dike swarms [NHS Field Camp Report, 2011]. Michael Cummings and others also indicate that the Oregon-Idaho graben’s northerly trend may be a structural expression of a craton margin [Cummings, 2000]. The WSRP is the youngest of the formations at around 9.5 Ma. Spencer Wood and Drew Clemens describe the WSRP as a simple basin and range structure with a length of about 300 km and a width of about 70 km bounded by normal faulting [NHS Field Camp Report, 2011]. The formation of this basin was initiated by magmatism caused by the migration of the Yellowstone hot spot
[Wood, 2002]. As the hot spot migrated eastward it heated and weakened the surrounding rock causing normal faulting of the basin. Rapid subsidence of the basin then took place, followed by Lake Idaho sedimentation from 9.5-1.7 Ma [Wood, 2002].
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1.3 - Geologic Conclusions
Considering the complex geology at NHS, Dr. Robert Raynolds created a simplified geologic map of the area. His approach was to separate volcanic rocks from sedimentary rocks and to define possible fault trends in the area. Figure 3 is a map of his interpretation of the local geology at Neal. His map shows that there is an abrupt transition from volcanic rocks east of the spring to sedimentary rock west of the spring [NHS Field Camp Report, 2011]. This could be due to a north to south trending fault with the sedimentary rocks lying on the graben block, and the volcanic rocks lying on the horst block [NHS
Field Camp Report, 2011]. With significant normal faulting in the area due to the Oregon-Idaho graben and the WSRP formations, and a past history of volcanic activity from the migration of the Yellowstone hot spot, it is likely that the geothermal fluid feeding the hot springs originates from a deep fault wich is heated at by hot igneous rocks [NHS Field Camp Report, 2011]. The surface expression of the fault is found just to the north of the production zone. It has a strike of N 15° W and an apparent dip of 60° to the west [NHS Field Camp Report, 2011]. Figure 4 illistrates a simplified geometery of the fault in profile from A to A’ cooresponding to the transect line in Figure 2.
According to Dr. Raynold’s interpretation of the geology the production zone lies directly over the fault in question. Evidence that hydrothermal fluid permeates through this fault at depth was strengthened while drilling well TG8. At approximately 3000 feet the drillers lost enough drilling fluid that they could not drill any deeper [NHS Field Camp Report, 2011]. It was concluded that the highly permeable zone they drilled into was in fact the fault plane. Furthermore, fault scarp found in the field marked the location of the surface expression of the fault and well logs from TG8 confirm that it was related to the hot springs [NHS Field Camp Report, 2011].
E
W
Figure 3: Dr. Robert Raynold’s interpretation of the geology at
Neal Hot Springs. Red= volcanic rocks. Yellow-=sedimentary rocks.
Figure 4: Simplified representation of the half graben structure at Neal.
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2.0 – Methods
2.1 - Summary
The geophysical methods utilized for my field experience include passive seismic, active seismic, magnetic, and CSAMT. The passive seismic data was collected every 45 days (approx.) from the passive network installed by the 2011 field camp. Active seismic data was collected by the 2011 field camp using vibroseis trucks. CSAMT and magnetic surveys were conducted by the 2012 acquisition campaign. The primary goal of that campaign was to gather CSAMT data for the National Student Geothermal
Competition.
2.2 - Passive Seismic
Eleven passive seismic stations were installed in the vicinity of the Neal production zone to ultimately determine if seismic events from fluid re-injection can be detected. Each station was equipped with an L-22 seismometer, a data acquisition system (DAS), a GPS clock, solar panel, and battery. The stations run continuously and record the vertical, northward, and eastward components of wave propagation at a rate of 250 samples per second. The DAS at each station stores 32 bits per sample with a storage capacity of about 4G, so service dates must be performed every month-and-a-half to avoid filling the flash drives completely.
Most of the equipment used for the passive network was borrowed from the Incorporated
Research Institutions for Seismology (IRIS). One of the conditions for the agreement between BSU and
IRIS is that we share all of the data collected with them. In order to share the data a series of processing steps must be made to convert the data into the format preferred by IRIS. This is done using the MacOX laptop provided by IRIS, which is a Mac Book with UNIX operating software. The following gives a description of how to perform these steps:


After a service date has been performed there should be 2 flash drives for each station that was serviced. These flash drives need to be written to Iris’s MacOX laptop using the provided card reader and Neo software. Neo will convert the data into zip files and store it on the MacOX in the directory /Volumes/Data/NealHotSprings/PSprocessed/Dump/. Note that once the zip files are complete we must make a directory within /Dump corresponding to the given service date. For example, typing the command mkdir June2012 in /Dump will make a directory called June2012.
Then, move the zip files into that directory using the command mv *.zip June2012. Be sure that the number of zip files in the new directory corresponds to the number of flash drives that were processed. And be sure to check that the file sizes are consistent with the service sheets. If there is a file that is only, say, 13kB (really small) then there may have been a problem when Neo created that zip file. Also, within /Dump make a directory called ServiceSheets. This is where the digitized service sheets should go.
Next, the zip files must be transferred to augustine@boisestate.edu. To do this, first log in to
Augustine and navigate to /pal/fieldcamp/2011/passive_seismic/data. Create a directory within data corresponding to the service date of interest. We use the convention that the directory is named yyyymmdd, so for a June 16, 2012 dataset we would name the directory 20120616. Next, create directories within 20120616 (for example) called raw and servicesheets. Once the
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 directories in augustine have been created go back to the directory
/Volumes/Data/NealHotSprings/PSprocessed/Dump/June2012 (for example) and send them to augustine using the command scp *.zip
$username@augustine.boisestate.edu:/pal/fieldcamp/2011/passive_seismic/data/$servicedat e/raw, where ‘$servicedate’ refers to whichever service date directory we are processing. In this case it would be 20120616. Also, use the scp command to transfer the digitized service sheets to augustine.
Now the data is safe. It is located on the MacOX laptop, on augustine which will create backups, and on the flash drives (at least until the next service data). Next, perform a flow of processing steps to 1) add data to our database for research purposes, and 2) transform the zip files to the format Iris wants them. The script for this step is at /pal/fieldcamp/2011/passive_seismic/codes.
 Finally, we need to FTP (file transfer protocol) the data to Iris’s Database Management Center. Open a terminal on augustine and type gui_DoFTP. Select Full Operation and navigate to the day_volumes directory in the particular service date directory. Select OK (use the default “FTP passive mode”). This will take quite a while!
After completing the steps above the data may be analyzed using the Antelope Relational
Database System, Datascope. A description of how to view data from each station is as follows:
 First, open a new terminal and log in to the augustine or sonic account. Access the root directory using the command cd / and navigate to
% cd pal/fieldcamp/2011/passive_seismic/data.
 Within this directory we can use Datascope commands to access the data. The
% dbe neal_HS & command will run the database window which is a general tool for navigating through the relational database. Since this window and the seismic database window will be running simultaneously run the dbe command first in the background (defined by the ‘&’).
 Once this is completed, one can now use the dbpick command to pull up the data:
% dbpick -ts 2011180:00:00:00 -nostarttalk -t 3600 -sc ‘*:EHZ’ neal_HS
Now there should be a rectangular dialog box and a square dbpick window that includes the data. With the above command, the time of the data begins on the 180th Julian Day of 2011. The 3600 represents the amount of seconds displayed in the dbpick window at one time, in this case one hour. The ‘*:EHZ’ describes the channel and the Z component. to pull up all three components, E,N, and Z, use the command ‘*’.
 Now it’s time to navigate through dbpick. The P-wave arrivals should be flagged, which correspond to earthquakes in the National Earthquake Information Center database that can be accessed by pressing the
origin button in the dbe dialog box. To find the events, first press f, which finds the first event in the database and from there press n to proceed to the next event, or p to go back to a previous event in dbpick window. You can also use the scroll on the bottom of that window to scroll through the seismic data.
 An interesting feature in the origin window is that the earthquakes have their longitudes and latitudes listed, which are used to locate the epicenter of the earthquake. Go to Graphics and press Map and the earthquake location is presented in a new window.
The goal of obtaining data from the Passive network at NHS is to detect induced seismicity directly related to geothermal power production due to fluid movement in the subsurface. In theory, all events not associated with NHS can be flagged and disregarded.
To do this IRIS’s
Seismiquery tool is used from their website to generate data files of seismic events within three areas; global, regional, then local. Files of global events list all cataloged earthquakes of magnitude 5 or greater.
Files for regional events include all cataloged earthquakes of magnitude 3 or greater within an area
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stretching from southern Canada to central Mexico, and from the eastern edge of the Pacific Ocean to central U.S. Files for local events list all cataloged events of magnitude 1 or greater within an area containing eastern Washington and Oregon, western Montana, the western edge of Wyoming, the northern edges of Nevada and Utah, and all of Idaho. We use these files to generate markers in our data so we can filter out events not associated with the NHS area. What are left are seismic events that have taken place within the NHS vicinity. In practice, it is quite challenging to identify events originating only from within the Neal area. It is an ongoing process and so far we have identified two earthquakes that took place very close to Neal.
It is relatively easy to identify teleseismic events since P and S waves from distant earthquakes arrive at each station at the same time. An example is an aftershock from the Honshu earthquake near Japan which took place on June 24, 2011. Figure 5 shows this earthquake’s epicenter and Figure 6 shows the dbpick results. The first wiggles for each station are the P waves and occur at exactly the same time.
Notice that the P waves arrive shortly after the expected arrival time indicated by the ‘P’ flags in Figure 6. This is because the flags are generated based on the theoretical Earth Reference Model for rock velocities in the subsurface. In this case, rock velocities must be slower than
Figure 5: Map produced by the Antelope Relational Database System of the location of the Honshu aftershock those of the Earth Reference Model, which is why the P waves arrive later than expected. The larger amplitude wiggles are the S waves occurring about 15 seconds after the P wave arrivals.
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Figure 6: dbpick results of an aftershock of the Honshu earthquake which took place on June 24, 2011. The display shows the vertical component for each passive seismic station with a 1.0 Hz low-pass filter.
Not every earthquake is necessarily catalogued. With this in mind, another processing step is to identify events within our data that have not been catalogued. So far two events have been found that do not show up in any catalog. One took place on March 1, 2012 at 7:29 pm and the other occurred on
May 17, 2012 at 10:45 am. Dbpick results for the March 1 earthquake are shown in Figure 7. Although its exact location has not been pinpointed, it has been determined that its direction of propagation is from Northeast to Southwest, first arriving at passive seismic station 4. Its general direction of propagation is determined simply by correlating the first arrivals of the seismic waves at each station with the network’s spatial geometry. A midpoint between the stations experiencing the event first, in this case stations 4, 3, 10, and 5, respectively, are mapped and a line is drawn through that midpoint and perpendicular to the line connecting the stations. This line bisects the two stations exactly. The wave’s first arrival is then checked for each station. Since the first arrival occurs at station 4 it can be concluded that the direction of propagation is in the area containing station 4 and bounded by the line bisecting station 4 and the next station to receive the first arrival. In this case, station 3. The process is repeated for other stations until a general direction of propagation is established. Figure 8 illustrates this procedure. Its epicentral distance was found by multiplying the difference in P and S wave velocities by the difference in P and S wave arrival times at a station. For the March 1event described above an average P wave velocity is assumed to be 5000 m/s. The S wave velocity is assumed to be 2500 m/s. The difference in P and S wave arrival times is determined to be 0.6 seconds. Therefore, it can be said that this event took place 1500 meters away from the network.
Aside from the very nice dbpick results, Figure 9 shows a spectrogram illistrating that the event does not exhibit many low frequency waveforms. Low frequencies (less than 1 Hz) are generally associated with teleseismic events. In this case we see frquencies mainly from 5Hz to 20 Hz, indicative of a nearby earthquake. So far it cannot be confirmed that this event is related to the geothermal power
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plant but it provides a great example of what a nearby seismic event looks like with respect to the passive seismic network at NHS. It would be ideal to identify all events like this within our data from
May 2011 up to the time the power plant is fully operational. This will establish a seismic datum for the
Neal area in order to compare seismic results after power production begins.
Figure 7: dbpick results for an event not associated with any catalogue. This low magnitude earthquake took place very close to the Neal area.
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Line bisecting PS3 & PS4 through their midpoint.
Midpoint of
PS3 & PS4
Figure 8: Map of the Neal passive seismic network illustrating the general direction of propagation for the event shown in figure 6. Stations 2, 3, and 10 are the first to detect the event.
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Figure 9: Spectrogram of PS4 from the figure 6 event near
Neal. The bright spots indicate large amplitude waveforms primarily at frequencies ranging from 5Hz to 20Hz.
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Activity from production and testing has also been detected by the network. Figure 10 shows events registering at all stations in the network during regular working hours at the site. This “noise” makes it quite challenging to interpret seismic signature related to fluid reinjection through the wells.
Sprectrograms of these events show that the waves propagate at a frequency of 2 Hz which is indicative of fluid flow. However, activity logs from U.S. Geothermal cannot confirm that reinjection took place during that time.
Figure 10: Production and testing “noise” during working hours. These waveforms may be due to fluid reinjection, but activity logs from U.S. Geothermal cannot confirm that claim.
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Figure 11: Spectrogram of the event shown in Figure 10.
Peak amplitude frequencies of 2 Hz may indicate fluid flow events. However, activity logs from U.S. Geothermal do not confirm that reinjection took place during this time frame.
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2.3 - Active Seismic
During the 2011 field camp a 2D Deep Seismic survey was conducted along Bully Creek Road beginning near the eastern edge of the reservoir and ending on the western flanks of the NHS area. The survey aimed to image large geologic features at a depth of up to 6 km. This survey was conducted with the use of geophones, vibroseis trucks, and a recording truck. A line of geophones were laid out along the road every 5 meters. The vibroseis trucks sent source waves along the road every 30 meters over a sweep of frequencies ranging from 8Hz to 80Hz. The high-end frequencies provide good resolution at shallower depths while the low-end frequencies penetrate deeper into the subsurface. During each shot
120 live geophones recorded and sent the data to the receiving truck. Ultimately the processed data provides a stack of seismic traces to produce an image of reflectors in the subsurface. The reflectors correspond to contrasts in rock density, which controls the “velocity of the rock”. Velocity of the rock refers to how fast seismic energy can travel through the rock. The more dense the rock, the faster a seismic wave can travel through it. Regarding the Neal area the changes in velocity alternate between volcanic rock and sedimentary rock, with volcanic rock being denser than sedimentary rock. In addition to imaging contrasts in rock velocities this method also images reflectors related to fractured zones and fault planes. This is important for the Neal area as it is thought that geothermal fluid migrates to the surface through fault planes as shown in Figure 4.
My analysis of the data focuses on a portion of the deep seismic line (dark green) outlined by the black box in Figure 12. The line was overlaid on a geological map to show normal faults intersecting the survey line and the lithological environment. Lithology in which the survey line crosses are described by the following: light green indicates sedimentary rocks of the Pliocene, beige indicates alluvium of the
Holocene, light purple is older sedimentary rocks of the middle Miocene, and fuchsia is Basaltic Andesite and Andesite of the Upper Miocene. The goal is to correlate the vibroseismic data with the lithology and with the normal faults trending from northwest to southeast through the center of the black box.
In order to obtain a decent image of the subsurface a flow of processing steps using the ProMAX seismic processing system is required. A subsurface image for the portion of the survey line described above was generated by four processes in the processing flow; Disk Data Input of the unmigrated stack,
Phase Shift Migration using a previously established 1-D velocity model, Time/Depth Conversion, and
Trace Display. Disk data input simply reads the seismic datasets. Migration involves the rearrangement of seismic information elements so that reflections and diffractions are plotted at their true locations, according to R.E. Sheriff. Phase Shift Migration is based upon the approach outlined by Gazdag (1978) in which data are migrated using a downward continuation approach. It is applied in two steps as follows.
First, the wavefield extrapolation step downward-continues the recorded data into a form corresponding to a repositioning of the recording plane through the use of the scalar wave equation.
Second, the imaging step outputs a portion of the migrated data corresponding to the zero travel time of the repositioned dataset. The data are recursively migrated by using the output of one wavefield extrapolation for the next. Time/Depth Conversion converts the input section from two-way travel time to depth. The step uses the previously established 1-D velocity model to convert from two-way time to depth. Based on the velocity model, one millisecond corresponds to one meter. The last processing step,
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Trace Display, is a general purpose tool for displaying and interacting with prestack and poststack seismic data. Figure 13 is the final image for my portion of the 2D deep seismic survey line.
Figure 12: Geology of the Neal area with the 2D Deep Seismic line (dark green).
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Figure 13: Processed image for the portion of the 2D deep seismic line shown in Figure 11.
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2.4 - Magnetic
Conducting magnetic surveys has proven to be very reliable in determining changes in the magnetic susceptibility of rock units at the surface and in the subsurface. Magnetic susceptibility refers to how easily a rock can be can be magnetized based on the content of iron and/or other magnetic elements contained in the rock. The magnetic method detects and records the magnitude of the response of magnetic susceptibility of differing formations which gives insight into where those formations are located spatially across an area. In the case of Neal Hot Springs magnetic surveys detect boundaries between sedimentary and volcanic rock formations and also provide clues to where normal fault planes may be located. Magnetic data coupled with gravity data provides an even more convincing representation of geological structure in the subsurface, as will be described.
All geophysical techniques have inherent anomalies associated with them which must be accounted for before interpretation, including the magnetic method. In this case, the major source of error is the diurnal variation (or drift) of Earth’s magnetic field. This occurs as electric currents are induced in the Earth from electric currents in the ionosphere. The changing currents in the ionosphere are considered to be the source of induction and are driven by solar activity. In order to correct for diurnal drift a magnetic base station is required for magnetic surveys. The base station records changes in the magnitude of the magnetic field in one location near the area in which a magnetic survey is to be conducted. In other words, the base station records the diurnal drift at the location near the area of interest. Other factors contributing to errors in magnetic data show up as spikes, noise, or dropouts on a plot of magnetic field strength vs. time. These are caused by abrupt changes in the local magnetic field to due to cultural sources such as cars, power lines, metallic fences, etc. Errors such as these are identified as spikes and/or dropouts in the data and filtered before interpretation.
During my field experience the magnetic survey was conducted across an area to the north of
Bully Creek Reservoir. The survey line was carried out on foot spanning 5.5 miles from the main road near the power plant to a point coinciding with the eastern edge of Bully Creek reservoir. The base station was set up just to the south of our line near passive seismic station PS4. The goal was to fill in data gaps in a magnetic data-set acquired by Clinton Colwell, a Geophysics graduate student at BSU. The data were gridded with a grid cell size of 10 meters, and a blanking distance of 100 meters using Oasis
Montage processing software. The blanking distance extends the grid for 100 meters on either side of the data points for visual interpretation. Our line crossed over many volcanic and sedimentary formations as well as the previously mapped basin-forming CMF. From this survey alone it is shown that differing rock units correspond well with abruptly changing colors of the magnetic line (Figure 14). When tied to Colwell’s magnetic data patterns are noticed in each line’s color contrasts. In particular are contrasts on different mag lines that correspond to the mapped faults. The black box in Figure 15 outlines a portion of Fault A in which this is very obvious; the same magnetic contrasts are visible in three magnetic lines that intersect Fault A. Purely geophysical evidence for the existence of this fault is even more convincing when paired with gravity data (Figure 20).
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Cottonwood Mtn. Fault
Figure 14: Geology at Neal with the magnetic line acquired during the 2012 Field Camp. Color contrasts along the mag line correlate well with changing lithology.
20

Fault A
Figure 15: Field Camp 2012 magnetic line together with previously acquired magnetic lines. Contrasts in the mag lines correlate well with some of the mapped faults, especially with Fault A.
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2.5 - CSAMT
Controlled-Source Audio-Frequency Magneto-Telluric method measures subsurface resistivity as a function of depth. Unlike other electromagnetic methods CSAMT is favored when geophysicists are interest in deep resistivity measurements up to 2 kilometers beneath the surface. The method’s ability to penetrate to such depths is due to an electromagnetic wave’s frequency-dependent skin depth. This principle describes how energy is transferred to a dispersive medium from an electromagnetic wave.
The skin depth is the distance the wave travels in the dispersive medium until a certain percentage of the wave has dissipated (NHS Field Report, 2011). Since the skin depth of an electromagnetic wave is inversely proportional to its frequency the resistivity in the subsurface can be measured from the surface at high frequencies to significant depth at low frequencies.
Set up of CSAMT acquisition includes a source station, a receiving station, and in our case a repeater for radio communication. The source dipole is used to transmit a controlled signal at a range of frequencies into the ground. The receivers, which are placed at a distance from the source, measure the induced electric and magnetic fields. The ratio of the horizontal electric (E x
) and magnetic field (H y
) values can then be used to calculate the resistivity structure of the subsurface
[Zonge, 1992]. Our set put the source station about 15 kilometers away from the the receiving location because of terrain and access permissions. This was unfortunate because the further away the source is from the receiver, the shallower the depth of measurement. An ideal distance between source and receiver is 10 kilometers, providing measurements up to 2 kilometer deep. We collected reliable data only up to 500 meters.
CSAMT was the most challenging of the methods carried out during my field experience.
Although the 2012 CSAMT campaign was largely successful, there were many setbacks due to logistical errors and mechanical problems. The setup itself required a large area over rugged terrain, constant communication between the source and receiving teams, and dependence on multiple synchronized machines. The first problem encountered was due to lack of radio communication. Once our source station was installed we discovered that it was in a shadow zone of the repeater that was set up by a local communications company. Having no contact with the receiving team we had to relocate the entire source station about a mile uphill, on foot. Another problem that occurred well into acquisition was battery failure in the GDP. Another day was wasted as part of the team had to drive to Ontario, OR for new batteries. Yet again we ran into problems when the belt in the generator failed. Luckily a volunteer team member recognized the problem and installed a spare belt. Despite the drawbacks, we collected data on two of the three originally planned transects. However, some data was lost when the receiving unit battery failed which explains the coverage gap on the northern transect shown in Figure
17. Because the gap distance is significant we divide the line into two separate transects for processing purposes.
Data processing was done on campus with software provided by Zonge International. Text files of the data were compiled to produce an inversion model of subsurface resistivity along Transect 1.
Transect 1 is thought to intersect the production fault. The production fault dips to the west and trends north to south approximately 500 meters east of the power plant. The inversion model exhibits a sharp
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decrease in resistivity between stations 775 and 825. This is where the survey intersects the production fault. It is noted that well log information correlates exactly with the low resistivity area on the inversion model between stations 825 and 1125. Well logs from NHS8 indicate a sharp temperature spike and a change in lithology from Drip Spring Sediments to Hunter Creek Basalt.
E
Station:
W
Production Fault
Conductive geothermal fluid.
Figure 16: Inversion model for Transect 1. Note the large change in resistivity from station 775 to station 825. This is where Transect 1 is thought to have intersected the production fault plane. The circle indicates the area of low resistivity which is interpreted as conductive geothermal fluid. The purple line is the interpreted production fault.
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Transect 3
Transect 2
Production fault
Transect 1
Figure 17: Locations of the three CSAMT transects. The yellow line indicates the general location of the production fault.
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3.0 – Conclusions
3.1 – Integrated Magnetic and Active Seismic
Based on the 2D deep seismic and magnetic surveys it is concluded that the basin forming CMF may not have been mapped in the correct location. The geological map provided by Evans through Mark
Ferns to U.S. Geothermal (Figure 14) shows the CMF trending from northwest to southeast and located east of Bully Creek Reservoir. However, evidence from the described geophysical methods and from a digital elevation model of the area indicate that the CWF intersects the reservoir and trends along the same direction as described by Evans. Figure 18 shows the magnetic survey lines overlaid on Evan’s geologic map. The seismic stack was scaled and aligned with the corresponding vibroseis stations 1100 and 1200. We notice strong magnetic contrasts indicated by the boxes have a similar trend as the previously mapped CMF. We also notice a significant gap between two groups of strong reflectors on the seismic stack at a depth of 650 meters. The gap suggests that large scale faulting has occurred in this region. The interpreted CMF is shown by the yellow line. When projected to the surface CMF lies within meters of the magnetic contrasts near station 1100. More evidence for the location of CMF is provided by a Digital Elevation Model of the NHS area (Figure 19).
1100
1200
NHS
Strong Reflectors
CMF
Figure 18: Geologic map integrated with magnetic and vibroseis data. The black boxes indicate magnetic contrasts from CMF. The seismic stack has been scaled to align with vibroseis stations 1100 and 1200. The gap in strong reflectors suggests that large scale faulting has occurred there. The yellow line is the interpreted CMF.
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Figure 19: Digital Elevation Model of the NHS area showing the basin-forming CMF.
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3.2 – Integrated Gravity and Magnetic
Magnetic and gravity data confirm the previously mapped Fault A. Figure 20 is an integrated magnetic -gravity map showing how well the magnetic and gravity data line up with the fault.
NHS
Figure 20: Gravity and magnetic data correlate very well with the previously mapped Fault A. The three mag lines outlined by the box all lay on the same gravity gradient.
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3.3 - CSAMT
The inversion model produced from CSAMT data along Transect 1 exhibits a sharp decrease in resistivity between the stations 775 and 825 to a depth of 500 meters (Figure 16). It is concluded that the production fault was imaged and that the low resistivity region (red, Figure 16) is conductive geothermal fluid. This conclusion is confirmed by well log NHS8.
3.4 - Passive Seismic
With respect to the passive seismic network at Neal Hot Springs it is concluded that little or no induced seismicity has been detected. With over one years’ worth of data we have identified two earthquakes that have taken place very close to Neal that are not associated with any catalogue, but it cannot be said that these events are associated with the geothermal system at Neal. Furthermore, we have not detected natural seismicity within the immediate project vicinity. We have detected a great deal of signal from the development of NHS. Once drilling and testing have ceased, and the power plant is in continuous production, it will be interesting to compare new data to the noisy data acquired thus far.
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References
Clutter, T.J., 2010. Neal Hot Springs Promises Oregon Commercial Geothermal Power. [online] Available at: <http://www.renewableenergyworld.com/rea/news/article/2010/10/neal-hot-springs-promisesoregon-commercial-geothermal-power> [Accessed 7 June 2011].
Colorado School of Mines Field Camp Report: <http://geophysics.mines.edu/GEO-Field-Camp>
Cummings, Michael L.,James G. Evans, Mark L. Ferns, and Kate R. Lees. “Stratigraphic and structural evolution of the middle Miocene synvolcanic Oregon-Idaho Graben.” Geological Society of America
Bulletin (May 2000), 112(5):668-682
Gazdag, J., 1978, Wave equation migration with the phase-shift method: Geophysics, 43, no. 7, 1342-
1351.
NHS Field Camp Report, 2011:
<http://palwiki.boisestate.edu/index.php/NHSreport2011#Geological_and_Geophysical_Investigations>
Takahahshi, Eiichi, Katsuji Nakajima, Thomas L. Wright. “Origin of the Columbia River basalts: melting model of a heterogeneous plume head.” Earth and Planetary Science Letters. Volume 162, Issues 1-4,
October 1998, Pages 63-80.
Wood, Spencer H. and Drew M. Clemens. “Geologic and Tectonic History of the Western Snake River
Plain, Idaho and Oregon.” Idaho Geological Survey Bulletin 30, 69-103. 2002.
Zonge, K.,1992. Introduction to CSAMT. After Northwest Mining Association, 1992. Practical Geophysics
II. 2 nd
Ed. USA: Northwest Mining Association.
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Photos
Hank and Dan installing the source dipole for CSAMT.
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Dan and Jason moving the generator to the second source-station location. Heavy!
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Truck here.
Clint, Hank, and Jason moving equipment up the hill. Whew!
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Brian and Jason at the CSAMT receiver.
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Hank and Clint packing up the source station for the long walk back to the truck.
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Dan’s first mag line.
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“Almost Cowboy”, in his element.
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Same beautiful sunset every night at Bully Creek Reservoir.
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