Houston, Texas, is one of the earliest urban areas to employ
Global Positioning System (GPS) technology for land subsidence and fault
monitoring. As of 2020, the University of Houston and the Harris-Galveston
Subsidence District have integrated over 230 permanent GPS stations into
their routine GPS data processing for regional subsidence and fault
monitoring. This article summarizes the GPS geodetic infrastructure in the
Greater Houston region. The infrastructure is comprised of two components: a
dense GPS network (HoustonNet) and a stable regional reference frame
(Houston20). Houston20 is realized by 25 long-history (>8 years)
continuous GPS stations located outside the subsiding area and is aligned in
origin and scale with the International GNSS Reference Frame 2014 (IGS14).
The stability of the regional reference frame is below 1 mm yr-1 in all
three directions. GPS-derived ground deformation rates (2010–2019) within
the Greater Houston region are also presented in this article.
HoustonNet permanent GPS stations in the Greater Houston region as
of early 2020. The red lines represent active faults mapped by USGS (Shah
and Lanning-Rush, 2005); the orange filled polygons represent salt domes
mapped by USGS (Huffman et al., 2004).
Introduction
For over 100 years, the city of Houston, along with the greater metropolitan
region, have been impacted by land subsidence and fault movements. The
Greater Houston region covers an area of approximately 22 500 km2 (150 km by 150 km) and centers on Harris County (Fig. 1), the third-most populous
county in the U.S. The Chicot, Evangeline, and Jasper aquifers are the major
aquifers underlying the Greater Houston region. Land subsidence in this
region is primarily caused by the compaction of clay lenses in the Chicot
and Evangeline aquifers throughout the region and the Jasper aquifer in the
northern part of the region due to groundwater withdrawal (e.g., Kearns et
al., 2015; Turco et al., 2015; Kasmarek and Ramage, 2017; Qu et al.,
2019). Ground deformation associated with subsidence and faulting has caused
widespread damage to residential, commercial, industrial buildings, and
public infrastructure since the 1950s. Long-term subsidence also increases
the flooding risk, which has a particular concern in many areas within
Houston along with low-lying areas near the bayous and Gulf coast. To
prevent land subsidence that contributes to flooding and infrastructure
damages, the Texas Legislature created the Harris-Galveston Subsidence
District (HGSD) in 1975 and the Fort Bend Subsidence District (FBSD) in 1989
to regulate groundwater withdrawal in areas within their respective
jurisdictions. Subsequent to establishing these two subsidence districts,
the Texas State Legislature established two groundwater conservation
districts, Lone Star Groundwater Conservation District (LSGCD) (2001) and
Brazoria County Groundwater Conservation District (BCGCD) (2005). These were
created to conserve, protect, and regulate groundwater resources in
Montgomery County and Brazoria County.
Accurate and long-term monitoring of ground deformation is critical to
establishing effective groundwater regulations for mitigating damages due to
subsidence and faulting. Prior to the 1990s, subsidence within the Houston
area was measured using levelling surveys and extensometers. The Global
Positioning System (GPS) has gradually replaced conventional levelling
surveying and has become the primary tool for subsidence monitoring in the
Greater Houston region since the 1990s.
The accuracy of GPS measurements (site velocities) does not solely rely on
GPS equipment (antennas and receivers), but largely depends on the available
regional geodetic infrastructure, which can be defined by two fundamental
components: a dense continuous long-history GPS network and a stable
regional reference frame. The former is often referred to as the hardware
component and the latter is referred to as the firmware component of a
geodetic infrastructure. This article aims to expound the geodetic
infrastructure within the Greater Houston region as of 2020.
HoustonNet: a permanent GPS network consisting of over 230 stations
In the early 1990s, HGSD established a surveying network of approximately 20
permanent GPS stations for the purpose of subsidence monitoring. These GPS
sites are also called Port-A-Measure (PAM) stations (Zilkoski et al., 2003).
PAM stations were installed on free-field and designed as a campaign-style
long-term GPS monitoring solution. On average, the GPS data was continuously
collected for one week per month at each PAM site prior to 2005 and one week
every two months since 2005. The PAM network has continuously expanded,
reaching100 total stations in 2019. In addition to the PAM GPS network,
there are currently over 130 continuously operating GPS stations within the
Greater Houston region. These continuous GPS stations are installed on
free-fields or one- to two-story buildings and are operated by joint efforts
of the University of Houston (UH), Texas Department of Transportation
(TxDOT), National Geodetic Survey (NGS) at the National Oceanic and
Atmospheric Administration (NOAA), SmartNet, the City of Houston, and other
agencies.
UH and HGSD have integrated over 230 permanent GPS stations within the
Greater Houston region into their routine data processing and analysis for
subsidence and fault monitoring (Fig. 1). This collection of GPS stations
forms a network called HoustonNet.
A stable regional reference frame: Houston20
GPS positions are initially provided as a set of coordinates with respect to
a global reference frame. In general, a global geodetic reference frame is
realized with an approach of minimizing the overall movements of a group of
selected reference stations distributed worldwide. As a result, the
GPS-derived movements at a site with respect to a global reference frame are
often dominated by factors such as the long-term drift and rotation of the
tectonic plate on which the site is located, glacial isostatic adjustment,
and other minor secular motions (Wang et al., 2020). Localized and temporal
ground deformation, such as subsidence and fault creeping, could be obscured
or biased by those common motions. A stable regional reference frame is
designed to exclude those common ground motions and highlight localized
ground deformation.
Site velocities of 25 reference stations.
Ref.Location Site Velocity Site Velocity GPS(Degree) (IGS14) (mm yr-1) (Houston20)∗ (mm yr-1) Long.Lat.EWNSUDEWNSUDLESV-93.26931.142-12.8-1.6-1.2-0.2-0.10.9OAKH-92.65730.816-12.6-1.3-3.0-0.1-0.1-0.7THHR-92.08130.529-12.3-1.1-0.80.10.01.6TXAU-97.75630.312-12.3-2.5-1.30.10.4-0.2TXBE-97.73528.424-12.0-3.4-1.4-0.1-0.4-0.4TXBT-97.47931.033-12.1-3.0-2.00.5-0.1-0.6TXCK-95.43631.323-13.4-2.4-1.3-0.7-0.20.4TXCU-97.27629.134-12.2-3.4-1.8-0.1-0.5-0.7TXFL-98.14229.161-12.0-2.90.20.10.21.2TXGN-95.13631.061-12.5-2.5-2.10.1-0.4-0.7TXH1-96.60230.893-12.7-2.4-1.6-0.10.2-0.1TXHP-93.86531.334-12.3-0.9-2.90.40.7-1.5TXLF-94.71831.356-12.8-2.5-1.4-0.1-0.60.5TXLG-96.84829.917-12.3-2.4-2.20.00.2-1.1TXMX-96.52431.595-13.7-2.0-0.70.30.50.9TXNA-96.53932.042-12.6-2.5-0.80.20.00.9TXNC-94.66931.668-13.1-1.5-1.7-0.30.40.3TXPI-95.59531.724-13.2-2.2-2.2-0.40.0-0.3TXPV-96.61928.638-11.9-2.4-1.00.10.20.2TXRU-95.12631.785-12.6-2.1-2.00.20.00.0TXS1-94.12831.526-13.1-1.5-2.2-0.40.2-0.1TXSE-97.99829.591-12.0-3.0-0.30.20.10.8TXSM-97.90329.878-12.2-2.50.10.10.51.2TXTA-97.44530.564-12.6-2.9-1.1-0.1-0.10.2TXWA-97.11131.578-12.4-3.0-1.90.4-0.2-0.4Root Mean Square 12.62.41.70.30.30.8
∗ The site velocities are estimated by a linear regression fit to the displacement time series within the period from 2010 to 2019. All reference stations have a minimim 8 year dataset. The uncertainties (95 % Confidence Interval) of the velocities are at 0.3 to 0.5 mm yr-1 for the horizontal components and 0.5 to 0.9 mm yr-1 for the vertical component.
Reference stations
In the surveying and geodesy community, a regional reference frame is often
developed through a simultaneous Helmert transformation from a
well-established and broadly used global reference frame. A group of common
points, known as reference stations, are used to link these two reference
frames. A stable Houston reference frame was initially established in 2013
and tied to the International GNSS Service (IGS) Reference Frame 2008 (Wang
et al., 2013). The regional reference frame firstly provided a platform to
integrate long-history GPS datasets collected by number of agencies in
various areas with different equipment into a uniform reference frame. The
initial reference frame was updated in 2015 by adding approximately two more
years of observations from 10 reference stations (Wang et al., 2015). A
recent update was completed in 2016 with over 7 years of continuous
observations from 15 reference stations (Kearns et al., 2019), which was
designated as Houston16. IGS updated its official reference frame from IGS08
to IGS14 in 2017 (Rebischung et al., 2016). Consequently, the reference
frame of all IGS satellite orbit products was updated to IGS14. JPL's GipsyX
software package (Version 1.2) is employed to calculate single-receive
phase-ambiguity-fixed Precise Point Positioning (PPP) solutions for our
routine processing (Bertiger et al., 2010). The PPP solutions are aligned to IGS14. Thus, a local reference frame tied to IGS14 is needed.
Locations and horizontal velocity vectors (referred to IGS14) of 25
reference stations utilized to realize Houston20.
This study updates Houston16 by removing stations that are no longer in
operation or have poor data quality, while adding additional reference
stations to improve the overall geographic distribution and data redundancy
(Fig. 2). Since GPS data as available in early 2020 is used for realizing
the reference frame, the updated reference frame is designated as Houston20.
The detailed methods for realizing a stable regional reference frame and the
criteria for selecting reference stations are addressed in recent
publications for establishing regional reference frames in North China (Wang
et al., 2018), Houston (Kearns et al., 2019), and the Caribbean (Wang et
al., 2019).
Realization of Houston20
The original PPP solutions are defined in an Earth-Centered-Earth-Fixed
(ECEF) Cartesian coordinate system that represents a position as a pair of
X, Y, and Z coordinates. The ECEF-XYZ coordinates with respect to Houston20
can be obtained by the following transformation, Eq. (1):
X(t)H20=X(t)IGS14+Tx′⋅(t-t0)+Rz′⋅(t-t0)⋅Y(t)IGS14-Ry′⋅(t-t0)⋅Z(t)IGS14Y(t)H20=Y(t)IGS14+Ty′⋅(t-t0)-Rz′⋅(t-t0)⋅X(t)IGS14+Rx′⋅(t-t0)⋅Z(t)IGS14Z(t)H20=Z(t)IGS14+Tz′⋅(t-t0)+Ry′⋅(t-t0)⋅X(t)IGS14-Rx′⋅(t-t0)⋅Y(t)IGS14
where X(t)IGS14, Y(t)IGS14, and
Z(t)IGS14 are the ECEF-XYZ coordinates (at epoch t) of a
site with respect to the global reference frame IGS14; X(t)H20, Y(t)H20, and Z(t)H20 are the ECEF-XYZ coordinates of the site with respect to
Houston20 at epoch t. The daily positions with respect to IGS14 (X(t)IGS14, Y(t)IGS14, Z(t)IGS14) can be obtained by the PPP processisng. The units of these
XYZ coordinates are meters. t0 is the epoch that aligns the
coordinates with respect to these two reference frames. t0 is
fixed at 2016.0 (year) for Houston20. A site retains identical XYZ
coordinates at epoch 2016.0 with respect to IGS14 and Houston20.
Tx′, Ty′, Tz′Rx′, Ry′, and Rz′ are
constant parameters indicating the rates (one-time derivates) of three
translational shifts and three rotations between two reference frames along
the x, y, z coordinate axes. These seven parameters: t0,
Tx′, Ty′, Tz′,
Rx′, Ry′, and Rz′, are
listed in Table 2.
Seven Parameters for Realizing Houston20.
Parameters∗UnitsIGS14 to Houston20t0year2016.0Tx′m yr-11.4040400×10-2Ty′m yr-19.6139040×10-4Tz′m yr-17.2404862×10-3Rx′radian yr-1-9.8590126×10-10Ry′radian yr-1-1.7311089×10-9Rz′radian yr-11.3205311×10-9
∗ Seven parameters are used to transform ECEF-XYZ coordinates from IGS14 to Houston20 according to Eq. (1). Counterclockwise rotations of the x, y, and z axes are positive.
To study ground deformation at the Earth's surface, firstly, the ECEF-XYZ
coordinates are transformed to Houston20 from original IGS14; secondly, the
geocentric XYZ coordinates are converted to a geodetic orthogonal
curvilinear coordinate system (longitude, latitude, and ellipsoid height)
referencing the GRS80 ellipsoid; thirdly, the longitude and latitude
coordinates are projected to a two-dimensional (2D) local horizontal plane
for tracking surface ground deformation in the north-south (NS) and
east-west (EW) directions at each site. The change of ellipsoid heights over
time is used to depict the vertical displacement (subsidence or uplift). In
practice, the vertical displacements derived from ellipsoid heights retain
the same measurements as those derived from orthometric heights (Wang and
Soler, 2014).
Stability of Houston20
The stability of a reference frame determines its ability to extrapolate
station coordinates accurately into the past and the future beyond the frame
range. Since blocks of the Earth's crust are not strictly rigid, it is a
challenge to establish stable regional reference frames. To the most
stringent users, the stability of a reference frame defines the essence of a
successful reference frame. In practice, the stability or precision of a
regional reference frame is often evaluated by the average velocity
(root-mean-square) of all reference stations with respect to the reference
frame (e.g., Blewitt et al., 2013).
The useful lifetime of a regional reference frame depends on its stability.
According to our previous investigations, the root-mean-square (RMS)
accuracy of the PPP daily solutions is about 3 to 4 mm in the horizontal
directions and 6 to 8 mm in the vertical direction within the Greater
Houston region (e.g., Yu and Wang, 2017; Wang et al., 2017). For Houston20,
the frame range is approximately 10 years from 2010 to 2019. According to
the statistics listed in Table 1, the stability of Houston20 is at a level
of 0.3 mm yr-1 in the horizontal directions and approximately 0.8 mm yr-1 in
the vertical direction. The reference frame may result in an accumulated
positional-error (uncertainty) of 3 mm in the horizontal directions and 8 mm
in the vertical direction over a 10 year period, which are comparable with
the RMS-accuracy (repeatability) of the PPP daily solutions. That is to say,
the regional reference can be confidently used for approximately 10 years
beyond the frame window (2010–2019) without causing positional errors larger
than the accuracy of daily PPP solutions. Nevertheless, special attention
should be drawn when applying Houston20 for delineating ground deformation
at a sub-millimeter per year level. The stability of the regional reference
frame may be further improved in future updates when a longer time span of
observations and more reference stations become available.
Subsidence (black) and uplift (red) time series at typical GPS
sites located in different groundwater regulation zones within the Greater
Houston region.
Subsidence derived from GPS observations (2010–2019)
Long-term GPS observations accumulated by HoustonNet provide fundamental
datasets for delineating spatial and temporal variations of ground
deformation over time and space. Figure 3 illustrates the subsidence history
at several sites located in the subsidence and groundwater regulation zones
within the Greater Houston region. The locations of these sites are marked
in Fig. 4. HGSD is divided into three regulatory areas: Area 1, Area 2, and
Area 3. FBSD is divided into two regulatory areas: Area A and Area B. The
Richmond/Rosenberg (R/R) area is a sub-area within the regulatory Area A.
PAM station PA01 is located in Jersey Village, a northwest suburb within the
HGSD regulatory Area 3, which was not scheduled for a 30 % groundwater
withdrawal reduction until 2011. PA01 recorded substantial subsidence
(∼4 to 5 cm yr-1) before the 2000s. The subsidence rate
reduced to about 2 to 3 cm yr-1 during the 2000s, and further reduced to 1
to 2 cm yr-1 during the 2010s (Fig. 3a). The reduction of the overall
subsidence rates within the Greater Houston Region is attributed to the
reduction of groundwater withdrawals due to HGSD and FBSD regulations (e.g.,
Yu et al., 2014; Shah et al., 2018).
Map depicting subsidence-rate contours (mm yr-1) and horizontal
velocity vectors (with respect to Houston20) derived from GPS observations
within the period from 2010 to 2019. The colored areas represent the
groundwater regulation zones.
The subsidence time series illustrated in Fig. 3a indicate that subsidence
rate varies considerably over time and space. The main reasons causing the
spatial and temporal variations include: differences among sites in the
ratios of sand, silt, and clay; differences of previously established
pre-consolidation heads over space; and differences in rates and amount of
groundwater withdrawal over space and time. In general, subsidence
associated with groundwater pumping does not follow a strict linear trend
over a long-term period (e.g., >10 years) (Fig. 3a). However, a
linear trend is still an efficient tool for assessing subsidence over a time
span between five to ten years (Wang et al., 2017). Figure 4 illustrates the
estimated subsidence-rate contours and horizontal velocity vectors derived
from GPS observations during the last decade (2010–2020). The velocities are
referred to Houston20. The contour map clearly depicts a spatial pattern of
the ongoing subsidence. Subsidence in downtown Houston and Galveston County
(HGSD Areas 1, 2) has ceased; slight land rebound (∼1 to 3 mm yr-1) has been recorded at several sites located in HGSD Area 1, Area 2,
and FBSD Area B (Fig. 3b). However, rapid subsidence (>1 cm yr-1) is occurring in The Woodlands (LSGCD), Jersey Village and Spring
(HGSD Area 3), and Katy (FBSD Area A) areas. Moderate subsidence (between 4 mm yr-1 and 1 cm yr-1) is taking place in a large part of Montgomery County, northwest Harris County, and Fort Bend County (Fig. 4).
The horizontal velocity vectors depicted in Fig. 4 suggest that only a few
sites experienced localized horizontal movements larger than 1 mm yr-1 over
the past decade. Several GPS stations adjacent to fault lines show
approximately 3 to 5 mm yr-1 horizontal motions, which may be associated
with localized faulting and subsidence activities. The applications of the
regional geodetic infrastructure in urban faulting studies are illustrated
in a recent article (Liu et al., 2019).
Summary and Conclusions
This study summarized the geodetic infrastructure in the Greater Houston
region. The primary product from this study is the determination of the
seven parameters (Table 2) needed for converting the ECEF-XYZ positional
coordinates from IGS14 to Houston20. The regional geodetic infrastructure
provides a consistent platform for studying local ground deformations over
space and time and serves the broad research and surveying communities. For
example, geologists and hydrologists may apply HoustonNet data and the
regional reference frame to study ground deformation due to faulting, salt
dome uplift, drought, aquifer deformation, seasonal hydrologic and
atmospheric pressure loading; meteorologists may use HoustonNet data to
study regional water vapor profiles that are critically important for
improving numerical weather prediction models for forecasting hurricanes;
civil engineers may apply the rigorous reference frame to conduct long-term
structural health monitoring for critical structures, such as high-rise
buildings, highway bridges, dams, and levees. The regional geodetic
infrastructure also makes it possible to integrate observations from
different remote sensing techniques (e.g., GPS, InSAR, LiDAR,
Photogrammetry) to a unified geodetic reference and enables
multidisciplinary and cross-disciplinary research.
The frame stability of Houston20 is approximately 0.3 mm yr-1 in the
horizontal directions and 0.8 mm yr-1 in the vertical direction. Houston20
can be confidently used for 10 years beyond the frame window from 2010 to
2019. Special attention should be drawn when applying Houston20 for
delineating sub-millimeter per year ground deformation. The regional
reference frame will be incrementally improved and synchronized with future
updates of IGS reference frames.
Data availability
Raw GPS datasets applied for this study are available to the public through the data archive facilities at Harris-Galveston Subsidence District (https://hgsubsidence.org, last access: 8 March 2020), UNAVCO (https://www.unavco.org/data, last access: 8 March 2020), National Geodetic Survey of U.S. (http://geodesy.noaa.gov/CORS, last access: 8 March 2020), and Texas Department of Transportation (http://ftp.dot.state.tx.us/pub/txdot-info/isd/gps/RINEX, last access: 8 March 2020).
Author contributions
GA and GW prepared the original draft. All authors edited, reviewed, and improved the manuscript.
Competing interests
The authors declare that they have no conflict of interest.
Special issue statement
This article is part of the special issue “TISOLS: the Tenth International Symposium On Land Subsidence – living with subsidence”. It is a result of the Tenth International Symposium on Land Subsidence, Delft, the Netherlands, 17–21 May 2021.
Acknowledgements
The authors acknowledge UNAVCO for archiving GPS data from the HoustonNet stations operated by the University of Houston.
This study was supported by the University of Houston and
the Harris Galveston Subsidence District. Yan Bao's research at the
University of Houston was supported by the Beijing University of Technology
and the National Science Foundation of China (No. 51829801).
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