GIGeoscientific Instrumentation, Methods and Data SystemsGIGeosci. Instrum. Method. Data Syst.2193-0864Copernicus PublicationsGöttingen, Germany10.5194/gi-6-447-2017Geoelectric monitoring at the Boulder magnetic observatoryBlumCletus C.https://orcid.org/0000-0003-1001-4636WhiteTimothy C.SauterEdward A.StewartDuff C.BedrosianPaul A.LoveJeffrey J.jlove@usgs.govUS Geological Survey, Geomagnetism Program,
Box 25046 MS 966 DFC, Denver, Colorado 80225, USAUS Geological Survey, Crustal Geophysics and Geochemistry Science
Center, P.O. Box 25046 MS 964 DFC, Denver, Colorado 80225, USAJeffrey J. Love (jlove@usgs.gov)2November20176244745214March20177June201731August201714September2017This work is licensed under the Creative Commons Attribution 3.0 Unported License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/3.0/This article is available from https://gi.copernicus.org/articles/6/447/2017/gi-6-447-2017.htmlThe full text article is available as a PDF file from https://gi.copernicus.org/articles/6/447/2017/gi-6-447-2017.pdf
Despite its importance to a range of applied and fundamental studies, and
obvious parallels to a robust network of magnetic-field observatories,
long-term geoelectric field monitoring is rarely performed. The installation
of a new geoelectric monitoring system at the Boulder magnetic observatory of
the US Geological Survey is summarized. Data from the system are expected,
among other things, to be used for testing and validating algorithms for
mapping North American geoelectric fields. An example time series of recorded
electric and magnetic fields during a modest magnetic storm is presented.
Based on our experience, we additionally present operational aspects of a
successful geoelectric field monitoring system.
Map of geoelectric monitoring deployment at the Boulder
observatory. North is up.
Introduction
Geoelectric fields are induced in the Earth's electrically conducting
interior by time-dependent geomagnetic field variation sustained by dynamic
processes operating in the ionosphere and magnetosphere. This induction
occurs all the time, during both magnetically calm and stormy conditions.
During intense storms, induced geoelectric fields can drive quasi-direct
currents in bulk electric-power grids of sufficient strength to interfere
with their operation, sometimes even causing blackouts and damaging
transformers (e.g., Boteler et al., 1998; Piccinelli and Krausmann, 2014).
Notably, the magnetic storm of March 1989 (e.g., Allen et al., 1989) caused
the collapse of the Hydro-Québec power-grid system in Canada, leaving 6
million people without electricity (Bolduc, 2002; Béland and Small,
2005). More recently, the Halloween storm of October 2003 caused operational
failures in parts of the Swedish power-grid system (Pulkkinen et al., 2005).
Some scenario analyses anticipate that the future occurrence of a rare but
extremely intense magnetic superstorm could cause widespread and long-lasting
loss of electric power (e.g., Kappenman, 2012) and entail substantial
economic cost (e.g., Baker et al., 2008).
In support of a project for modeling and evaluating geoelectric hazards
(e.g.,
Thomson, 2007; Love et al., 2014), in June 2016 the Geomagnetism Program
of the US Geological Survey (USGS) commenced long-term geoelectric field
monitoring at its Boulder, Colorado, magnetic observatory (BOU). The Boulder
geoelectric monitoring project partially fulfills a directive in the United
States National Space Weather Action Plan (NSTC, 2015; Goal 5.5.4) (one of
many given to different agencies) for the Department of Interior to “assess
and pilot a geoelectric monitoring capability”. It is further consistent
with strategic goals of the USGS Hazard Mission for enhancing observations,
pursuing fundamental understanding, and improving hazard assessments (Holmes
et al., 2013, Goal 1). Geoelectric field monitoring is a natural extension of
the geomagnetic monitoring that is already the responsibility of the USGS
Geomagnetism Program (Love and Finn, 2011), and it is similar to long-term
geoelectric monitoring projects supported in other countries, including Great
Britain (Kelly et al., 2013) and Japan (Fujii et al., 2015), and to
shorter-term campaign-style measurements common to magnetotelluric surveys
(e.g., Ferguson, 2012). From 1932 to 1942, analog geoelectric measurements
were supported at the Tucson magnetic observatory (Rooney, 1949); from 1988
to 1995, geoelectric monitoring was performed in Parkfield, California, as a
part of an earthquake research project (Park, 1997). Otherwise, there has
been very little multi-year geoelectric monitoring carried out in the United
States.
The Boulder site
The Boulder magnetic observatory facility (Love et al., 2015) is located on a
flat-top butte, north of the city of Boulder, Colorado, and east of the
United States Rocky Mountains. The land is rocky and sandy, sparsely covered
with grass and cacti. The climate is semiarid; summers can be hot
(> 30 ∘C is common) with occasional thunderstorms;
winters can be cold (often < -5 ∘C) with occasional
snowfall. The Boulder observatory is 1 of 14 supported by the USGS
Geomagnetism Program, and it is part of the International Real-time Magnetic
Observatory Network (www.intermagnet.org; Love and Chulliat, 2013). The
observatory is also used by Geomagnetism Program engineers and technical
staff to develop and test new sensors, acquisition systems, and operational
procedures. The geoelectric monitoring system described herein is located
southwest of the observatory's office building and primary geomagnetic
monitoring systems; see Fig. 1.
The Borin Stelth® electrodes used for
geoelectric monitoring at the Boulder observatory.
Electrodes and their installation
Geoelectric data are obtained by measuring the voltage between pairs of
non-polarizable electrodes over time. For geoelectric monitoring at Boulder,
Borin Stelth® two silver–silver chloride
(Ag–AgCl) electrodes were selected for their thermal stability, low noise
characteristics, long expected service life (> 30 years), and
relatively large surface area (200 cm2); see Fig. 2. Electrode noise
levels have been estimated to be significantly less than 1 mV based upon
long-term measurements of electrode potential in a temperature- and
salinity-controlled tank. In June 2016, six electrodes were installed: two
located near the data acquisition system and one each located 100 and 200 m
to the west and south from there. The electrodes were buried to reduce
grounding changes caused by time variation in soil moisture content and
temperature that can impart unwanted types of voltage variation. As shown
schematically in Fig. 3, at each electrode location, a 1 m deep hole was
dug; this was then partially filled with a thick layer of bentonite clay, a
substance that is very absorbent and commonly used as a barrier against
groundwater. A 20 cm diameter, 1.25 m long, open-ended, polyvinyl chloride
(PVC) pipe was placed vertically in the hole and in contact with the
bentonite; an electrode was placed in the bottom of the tube with connecting
wires leading out the top end. The tube was then partially filled with
additional bentonite until the electrode was covered; the rest of the tube
was backfilled with sand; the space around the outside PVC tube was filled
with native rock and sand. The electrodes are connected to the acquisition
system using shielded coaxial cables further protected by PVC conduit. Two
electrodes are located near the acquisition system; one is used for the
100 m dipoles and the other is used for 200 m dipoles. Additional empty
PVC pipes were installed in parallel for possible future electrode
emplacement that might be needed for testing and to provide redundancy.
Contact resistances between each pair of electrodes range from 200 to
300 Ω.
Schematic of electrode installation.
Geoelectric field data acquisition system at the Boulder
observatory. (a) View to the northeast of the data acquisition system.
Electric field lines, protected by PVC conduit, extend west and south. (b) Interior view of data acquisition system in environmentally sealed
enclosures. Electric and optionally magnetic inputs are brought into the
right enclosure with shielding tied to the observatory grounding system. The
left enclosure containing the ObsRIO is in the upper left, the switching
power-source controller is on the right, and a cellular model in the center.
Data acquisition and management
Electrode voltage measurements are acquired using the Observation
Reconfigurable Input and Output System (ObsRIO) that USGS engineers developed
in-house using the CompactRIO (cRIO) hardware platform manufactured by
National Instruments Corporation; the system is solar powered (see Fig. 4).
The two standard data types acquired by ObsRIO are discrete 10 Hz values and
discrete 1 s values. Ten hertz data values are formed from a digital
filtering of 100 Hz analog-filtered samples, while 1 Hz (1 s) values are formed
from a digital filtering of 10 Hz values. This data construction process
reduces aliasing from geoelectric variation with periods of less than 0.1 s
(frequencies greater 10 Hz). Data from the Boulder ObsRIO systems are
transmitted to the USGS database system, EdgeCWB (Patton et al., 2015), in
Golden, Colorado, via internet protocols in near-real time. Geomagnetism
Program personnel make regular checks of the Boulder geoelectric data to
guard against artificial interference and to ensure continuity of operations.
Three days of geomagnetic and geoelectric field variation recorded
at the Boulder observatory: (a) north (blue) and east (gray) geomagnetic
components; (b) north geoelectric component, 100 (black) and 200 (red)
dipole; (c) east geoelectric component, 100 (black) and 200 (red) dipole.
Example data
In Fig. 5 we show 3 days of Boulder geomagnetic and geoelectric data
recording a geomagnetic storm that occurred in October 2016. In global terms,
the storm attained a maximum Dst =-104 nT and a maximum Kp = 6. Local
to Boulder, however, the east geomagnetic component saw a very abrupt and
high amplitude signal (∼ 150 nT), which induced a large geoelectric
signal in the north geoelectric component. The lowest-frequency (diurnal)
signals observed in the geomagnetic time series are not reproduced in the
geoelectric data due to a 30 000 s high-pass analog filter within the
acquisition system. All the measured geoelectric field variation is well
correlated with geomagnetic variation and is consistent with induction in the
solid Earth. Note, furthermore, the consistency between the geoelectric time
series for the 100 and 200 m dipoles.
Using the data
A high priority for monitoring and assessing geoelectric hazards is the
development of capabilities for making maps of the geoelectric field,
especially in real time (e.g., NSTC, 2015; Action 5.5.6). One approach to
regional- and continental-scale geoelectric field mapping is convolving maps
of Earth impedance with maps of geomagnetic activity (e.g., Thomson, 2007;
Love et al., 2014). Toward this end, long-term surface geoelectric field
data, spanning both quiet and storm times, are critical to validating
predicted field data and to benchmarking different modeling approaches (e.g.,
Kelbert et al., 2017; Bonner and Schultz, 2017). Additionally, surface
impedance functions can be calculated from synchronous electric and magnetic
time series, as is commonly done with magnetotelluric survey data. Long-term
(months to years) geoelectric time series data, as described here, constrain
estimates of surface impedance to longer periods than traditional
magnetotelluric studies, facilitating investigations into deep-Earth
conductivity structure. Finally, continuous recording of geoelectric and
geomagnetic time series data, particularly at sampling frequencies sufficient
to capture the magnetotelluric “dead band” (10–0.1 Hz), can serve as
remote referencing for regional magnetotelluric surveys (e.g., Gamble et al.,
1979; Egbert 1997). The availability of such data in near-real time can
reduce the logistics and costs associated with such surveys, and it can lead
to improved data quality.
Operational aspects of long-term electric field monitoring
Long-term electric-field monitoring introduces technical challenges that are
distinct from traditional magnetotelluric campaign or array deployments. As
with other monitoring studies, power supply, telemetry, and system
reliability are important design considerations for a successful
electric-field monitoring system. Furthermore, long-term electrode
deployment adds additional critical design elements, including thermal
stability, moisture stability, and lightning suppression. We describe below
aspects of the Boulder installation that we consider important to achieving
continuous, stable, low-noise geoelectric field data.
Long-term electric-field measurements can be improved with the use of stable,
low-noise, non-polarizable electrodes. A variety of electrode chemistries
exist, with Ag–AgCl and Pb–PbCl2 being two of the more commonly used
types. Both of these electrode types are known for their low noise levels,
small thermal coefficients, and long-term stability (e.g., Clerc et al., 1998;
Petiau, 2000). The Earth environment in which the electrodes are placed is
additionally important. In particular, greater thermal and moisture stability
reduces non-inductive signals (e.g., diurnal signals due to surface
temperature variations). Toward this end, the USGS electrodes are buried 1 m
deep. Electrode noise further scales with the contact resistance between the
electrode and the ground. To minimize both, soil within an area of
0.5 m2 was removed and replaced with an electrically conducting
bentonite slurry. A 1.25 m long (1.0 m below ground) PVC tube was emplaced
into the bentonite to facilitate the installation and, if necessary,
replacement of the electrodes. The electrodes were placed into the bentonite
slurry, covered with an additional layer of bentonite, and the remainder of
the PVC tube was filled with sand. Caps were subsequently installed to seal
the tube and prevent loss of moisture. Additional electrodes were similarly
installed for redundancy.
The electrodes are connected via coaxial cable to the data acquisition
system. Coaxial cable is selected to reduce the introduction of capacitive
noise via the long cable length deployed for this application. The cable
shields are grounded near the data acquisition system but isolated at the
electrode ends to avoid creating ground loops through the shielding. The
coaxial cable was further installed in PVC conduit to protect the cables
from damage due to wildlife. Strain relief was added to the PVC conduit at
30 m intervals to prevent damage to the conduit and cable that can be
caused by seasonal thermal expansion and contraction.
Lightning suppression, attenuation, and protection are of the utmost
importance in collecting continuous long-term geoelectric field data. On two
separate occurrences, buildings at the Boulder observatory have been struck
by lightning. Effective measures must be taken to protect recording systems
from damage under such conditions. The USGS has installed a grounding system
at the data acquisition site, consisting of a large steel ground rod driven
2.34 m deep. The coaxial shields are all connected to this ground, which
provides an electrical path for lightning-induced signals and other noise
sources incident on the shields. The electrodes are further connected to a
pre-amplifier and lightning isolation circuit board. The board was originally
designed for the NIMS portable MT system, developed by Narod Geophysics Ltd.
There are two components of lightning protection integrated on this board.
First are a series of 75 VDC spark gap devices, connected
individually to each incoming electrode connection. Additionally, varistors
are used in a suppression mode to shunt excessive currents incident on the
incoming channels. This board has been used to collect hundreds of thousands
of hours of data for the EarthScope US Array program with very few cases of
failure from lightning.
Amplification and filtering can be an important component of electric-field
monitoring depending upon the application as well as the sensitivity and
dynamic range of the data acquisition system. Quiet-time electric-field
amplitudes are on the order of 0.1 mV km1- or less; hence measured
voltages across electrode pairs separated by ∼ 100 m may be on the
order of 0.01 mV. Instrument gain is commonly used to amplify the raw
signals; the gain at the Boulder monitoring station is a factor of 10.
Filtering may also be beneficial to obtaining quality electric-field data.
There are two analog filters incorporated within the Boulder data acquisition
system. A notch filter attenuates 60 Hz signal, common to North America's
power distribution network, by a factor of at least 20 dB. An additional
analog high-pass filter, with a time constant of roughly 10.5 h, can
optionally be turned on. This filter may be used to attenuate long-period
signals, including diurnal variation arising from thermal drift in the
electrodes and long-term drift in grounding.
A low-noise, high input-impedance data acquisition system with moderately
high sample rate is needed for geoelectric field monitoring. To meet this
need we developed the ObsRIO based on the cRIO hardware platform manufactured by
National Instruments. A key design aspect of the ObsRIO platform is its
modularity and ability to change configurations in response to rapidly
changing scientific needs. Minimal development time is required to create
new images of ObsRIO for different scientific applications. The USGS has,
for example, designed a portable magnetotelluric variant of the system which
acquires both electric and magnetic field data and is battery powered.
The ObsRIO employs four-channel simultaneous sampling on a 24 bit, ±10 VDC
(direct current voltage) analog-to-digital converter (ADC, NI 9239). The ADC
is configured to sample at a frequency of 10 kHz. A box-car filter is used
to decimate the data from 10 to 1 kHz and ultimately to separate 100, 10,
and 1 Hz data output streams for logging and transmission. A GPS clock was
used to discipline the FPGA (field-programmable gate array) clock, a process
where the GPS signal is used to constantly calibrate the FPGA clock.
Time-stamped samples are passed into a first-in, first-out (FIFO) memory buffer
for further processing, logging, and transmission on the real-time controller
for the cRIO chassis. Pairing the ObsRIO system with a cellular device allows
for real-time data collection and transmission.
The data acquisition system was finally designed with automated switching
power-source control. ObsRIO automatically charges one battery, while
powering the system from a separate battery electrically isolated from the
charging source. A series of programmable relays and low-resolution ADCs are
used to set the power supply state for the system and switch charging and
load batteries as needed. This is an important aspect of the ObsRIO system,
as noisy power sources (such as solar) are kept from contaminating the
desired geoelectric fields. This feature is additionally critical to
campaign style deployments, which rely on batteries and solar power for
power.
Boulder geoelectric field data can be viewed on the USGS
geomagnetism plots page (http://geomag.usgs.gov/plots/, USGS
Geomagnetism Program, 2016) and downloaded from
(http://dev-geomag.cr.usgs.gov/ws/edge/).
The authors declare that they have no conflict of
interest.
Any use of trade, firm, or product names is for descriptive
purposes only and does not imply endorsement by the US government.
This article is part of the special issue “The Earth's magnetic
field: measurements, data, and applications from ground observations
(ANGEO/GI inter-journal SI)”. It is a result of the XVIIth IAGA Workshop on
Geomagnetic Observatory Instruments, Data Acquisition and Processing,
Dourbes, Belgium, 4–10 September 2016.
Acknowledgements
We thank Jill McCarthy and Janet L. Slate for reading a draft manuscript. We
thank Thomas Theissen from Borin Manufacturing Inc. for providing the
electrodes used at the Boulder magnetic observatory. Edited by: Kusumita Arora Reviewed by: Anatoly
Soloviev and Nandini Nagarajan
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