Why do we measure subsidence?
In the simplest terms, it keeps us from “flying blind”. Groundwater can be a significant freshwater source, but it is increasingly important that we use it wisely. The harmful effects of pumping too much groundwater must be minimized, and measuring subsidence plays a key role in this. The more data we have, and the more accurate that data is, the easier it is for us to maintain a necessary balance.
Measurements not only provide us with data on changes in land elevation, but they also provide us with the data necessary to calibrate models. Why is this important? It allows us to “see” into the future. Through these sophisticated groundwater and subsidence models, we can predict the results of future groundwater pumpage. This means we can plan ahead…developing groundwater regulations that will prevent foreseeable subsidence.
How do we measure subsidence?
The need for data and the distribution of that data is key. As early as 1906, surveys were conducted throughout the Houston area to establish permanent benchmarks (some of which are still used today). Over the years, subsidence measurement methods have evolved from manual site measurement of benchmarks to satellite-based technology, and for the Subsidence Districts, the goal has always been the same: to monitor the effects of groundwater withdrawal within our area(s), and to take appropriate actions based on those measurements.
All land measurement systems have been developed and controlled by the National Geodetic Survey (part of the National Oceanic and Atmospheric Administration – NOAA). From the creation of the HGSD and FBSD to present-day, the NGS has been an integral partner…serving as counselor, setting standards, studying and housing data, and much more.
Conventional Measurement Method
Also called “geodetic differential leveling”, this initial form of measurement originally consisted of the establishment of permanent benchmarks. Included in these benchmarks were precise elevations, latitudes and longitudes for each point.
As the land surface began to subside due to groundwater withdrawal, the need to relevel benchmarks became necessary. Over the years, new benchmarks were added (for a total of more than 2,500) and “relevelings” were conducted in 1978, and again in 1987. And although this measurement method provided excellent spatial subsidence data, the cost of the releveling procedure for a single epoch prohibited us from accessing up-to-date data at a rate necessary to sufficiently monitor the sometimes monthly, weekly, or even daily effects of subsidence. It was time to take advantage of new technologies that could provide us with the same accuracy, yet allow us to constantly monitor subsidence in a cost-effective way. In 1987, in conjunction with the conventional releveling, an experimental GPS releveling was initiated.
The first of thirteen (13) deep borehole extensometers (designed and installed by the United States Geological Survey – USGS – in the early 1960s) were used in preparation for the soon-to-be-built manned spacecraft center. Of the thirteen in operation today, six (6) of those are “subsidence” or “total depth” monitors (meaning their bottom is below the aquifers from which we draw water), and the other seven (7) are less than total depth, or “compaction” monitors.
What are they and how do they work? Borehole extensometers are deeply anchored benchmarks. To construct each, a hole is drilled to a depth at which the strata are stable. The hole is then lined with a steel casing with slip-joints to prevent crumpling as subsidence occurs. An inner pipe rests on a concrete plug at the bottom of the borehole and extends to the top. This inner pipe then transfers the stable elevation below to the surface. A measurement of the distance from the inner pipe to the surrounding land surface provides us with the amount of compaction that has occurred.
Although the accuracy of this measurement method is impressive, there is one drawback. The high cost to construct and install the equipment prohibits their use in sufficient numbers, resulting in a lack of adequate information for the entire Harris-Galveston and Fort Bend areas. And, as was stated in the overview, the sufficient amount of and wide distribution of data is extremely important. Over time, as technologies have evolved, we have moved toward more cost-efficient and equally accurate forms of measurement…but borehole extensometers are playing an important role in this new era. Three of our existing extensometers have been outfitted with GPS (Global Positioning System) antennas, and are now the only stable GPS points within the greater Houston area.
GPS – Using Technology from the World Above To Monitor the Land Below
From feeds to our television sets to national security, the use of satellites has become almost commonplace, and our world of subsidence measurement is no different. In fact, we began working with GPS technology as far back as 1987, and the class-A benchmarks established for that very GPS releveling have proven to be the most valuable benchmarks in the Houston area.
So what do we gain from using GPS, and how does it differ from previous measurement methods? One of the most important advantages to GPS is the ability to have constant data. Using dual-frequency, full-wavelength GPS instruments (with geodetic antennas), data is collected at 30-second intervals and averaged over 24 hours. That means that specific stations being monitored can be assessed on a daily basis. And just as important, the measurements are more reliable and handled at a fraction of the cost. Improved GPS techniques and processing have reduced the cost of releveling from millions of dollars to less than $100,000, and the data provided is accurate to + or – one centimeter. Now that’s progress!
Where are GPS measurements taken ? GPS measurements are taken using a system of CORS and PAM’s. Sounds complicated, but it’s really quite simple. Because of the broad extent of subsidence in the Houston-Galveston area, there were no stable benchmarks. Therefore, stable borehole extensometers were equipped with GPS antennas to provide a reference frame to measure subsidence at other stations throughout the area. These permanent stations are known as local GPS Continuously Operating Reference Stations, or CORS. In the mid 1990s, the District and NGS began developing the use of GPS Port-A-Measure, or PAM’s., to provide subsidence measurements.
Seven, portable trailers were built to house and secure GPS receivers and associated equipment (batteries, recording equipment and solar panels). The trailers are moved weekly to different PAM stations where they record Phase data every 30 seconds, allowing for a week’s worth of observations on each PAM, every month. The District also operates four (4) permanent CORS, which provide Phase data continuously, providing a basis from which change comparisons may be made and analyzed.
In addition to the points operated by the District, there are a number of additional CORS and Cooperative CORS which can also be used for monitoring purposes. They include:
- Eight (8) CORS operated by TXDOT
- Seven (7) CORS operated by the City of Houston
- A CORS in Angleton operated by the U.S. Coast Guard
- A WAAS (Wide Area Augmentation System) CORS in Houston operated by the FAA
- Six (6) other Cooperative CORS throughout the area
All additional CORS are relatively new and will require several months before they can be reliably used for monitoring.
Historical comparisons between the existing CORS and PAM’s. have indicated that some sites are subsiding at rates of seven (7) centimeters per year. This correlates well with rates observed at the Extensometers.
The District plans to double the number of PAM’s. from twenty-eight (28) to fifty-six (56), and this will be accomplished without an increase in personnel, equipment and overhead costs. Improvements in GPS equipment have recently eliminated the need for the seven trailers, and they will be phased out in the near future. The expansion of the monitoring network will not only permit a more comprehensive view of what is occurring in Houston and the surrounding areas, but will also serve as a future model for other localities facing similar problems.
With the equipment and technologies available, we’re confident we can continue to accomplish our goal of curtailing, and eventually eliminating subsidence.
Measuring Subsidence in the Future
The Evolution of GPS
In the GPS arena, a new Civil Frequency (L5) is in the planning stages for future satellites. Although not scheduled to be deployed for five years, this advancement will greatly increase the accuracy of GPS and decrease the time required for high-accuracy applications. High-accuracy positioning on moving platforms (auto, plane, boat) will be possible in real time, without the need to post process data through a computer to obtain solutions.
Also, the former Soviet Union and European Space Agencies will launch their own Global Satellite Navigation Systems which can be integrated with our current GPS system. As the number of space vehicles (satellites) increase, so to will the accuracies that can be obtained.
LIDAR (Light Detection and Ranging) and INSAR (Interferometric Synthetic Aperture Radar): these and other interferometric imaging techniques will play a major role in future subsidence detection and tracking as sensors and science improve.
Pulsars and Quasars and Stars
In the not-so-distant future, as technology is developed to efficiently and affordably manufacture powerful semiconductors, GPS will likely be replaced by a system which will use stars, Pulsars, Quasars and more as a signal source. This futuristic navigation system will be more precise than GPS and will be available on a galactic scale!