Are We Sinking? NGA Visits LSU Campus to Measure Subsidence
November 26, 2018
BATON ROUGE, LA – Throughout the month of October, 10 members of the National Geospatial-Intelligence Agency visited LSU’s campus and other statewide GPS sites to take gravity measurements for a research project spearheaded by LSU Center for Geoinformatics (C4G) Chief of Geodesy Cliff Mugnier.
Every five to 10 years, the NGA takes measurements of the earth’s gravity field to check for subsidence, a project Mugnier has been working on for nearly 30 years.
“LSU has the largest university-owned network of permanent GPS stations in the world, which reach from Louisiana throughout the Gulf Coast,” Mugnier said. “Over the years, I have asked the military to come in to take measurements at these stations. I first go to the New Orleans Corps of Engineers, whose colonel then writes a letter to the Pentagon asking if they can come to Louisiana and observe different places where we have our GPS antennas for our research on subsidence. The idea is if you come back to the same spot over a period of years and see an increase in the strength of the earth’s gravity, that means you’re getting closer to the center of the earth, which equates to subsiding.”
The NGA kicked off its recent Louisiana trip at LSU’s Absolute Gravity Station using instrumentation similar to the FG5-X Absolute Gravity meter that C4G purchased this year. C4G also houses two CG5 Relative Gravity meters. From LSU, the NGA team—consisting of two groups of five trainees from New Mexico, Whiteman Air Force Base in Missouri, and both the East and West Coasts—moved on to other GPS core sites across south Louisiana that included Oakdale, Alexandria, New Orleans, Slidell, Boothville, Lafayette, Lake Charles, Cocodrie, and Grand Isle. The NGA will take measurements in north Louisiana this winter to conclude their 2018 Louisiana campaign.
“The reason for these gravity measurements is to have a completely independent physical measurement of vertical movement as a function of gravity, rather than readings from the GPS satellites,” Mugnier said. “It all fits together for the subsidence research, but another reason is when people mortgage their homes, the banks require them to have flood insurance, which is based on elevation. The Corps [of Engineers] is also interested in subsidence because they have the responsibility of levees and flood control.”
The NGA hasn’t visited Louisiana since 2003, shortly after Mugnier arrived at LSU. Though subsidence is of vast importance to Louisianans, the NGA does not take measurements every year because subsidence is a slow process.
“If you do it too frequently, the observations get lost in what is called white noise,” Mugnier said. “Even though the instruments are super precise, you still have to have the time for the sinking to be enough for the instruments to detect.”
Mugnier’s research on Louisiana subsidence began when he taught at the University of New Orleans, where the first NGA measurements were taken in 1989. Since then, research has shown that UNO’s campus is sinking at a rate of 9.1 mm/year, which Mugnier said is significant when you consider that equates to 3 feet in 100 years.
“Three feet is a whole lot when you also consider sea level is rising about 2 mm each year,” he said.
Since the first reading taken at LSU around 2003 by the National Geodetic Survey, it has been discovered that LSU’s campus is subsiding around 5 mm/year near the fault line on Nicholson Drive. The campus is on the downside of the fault.
Since subsidence is an unstoppable force, Mugnier has made sure that C4G will continue his research when he is no longer at LSU. The man who started C4G, LSU Professor Roy K. Dokka, established the organization whose mission is in keeping with state law that says C4G is the source to keep track of elevations for the state of Louisiana.
“So, we have an organization that’s going to be here long after I’m gone,” Mugnier said. “And we have the perfect research project that will go on forever.”
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Contact: Erin Verpil
Center for GeoInformatics
Coordinator
225-578-8743
C4G Acquires Geodetic Instruments to Better Understand Subsidence Rates in Louisiana
The Center for GeoInformatics (C4G) in the Department of Civil and Environmental Engineering (CEE) recently received new geodetic instruments to model the Earth’s gravity field. A Scintrex CG-5 Relative Gravity Meter, Leica T60 Total Station, and Trimble R10 GPS Rover Kit were acquired as part of an enhancement grant sponsored by the Louisiana Board of Regents. Drs. George Voyiadjis (PI) and Joshua Kent (Co-PI) led the one-year project, which ended in June, 2017. The instruments are acquired to address the needs of three objectives: First, to develop a novel, high-resolution gravity model of sea level (i.e., geoid); second, to augment knowledge of existing subsidence rates and the driving mechanisms; and finally, promote advanced geodetic research at the University.
Here, as in many river deltas around the world, land surfaces are sinking due to subsidence. On average, southern Louisiana experiences ~10 millimeters per year of subsidence. Understanding the mechanisms that drive subsidence is essential for mitigating risk and promoting sustainability. The CG-5 relative gravity meter supports these goals by measuring the relative differences in the Earth’s gravity across southern Louisiana. 
Surveys using the total station and R10 rover kit are currently underway to geodetically correlate the CG-5 data with absolute gravity readings collected in the early 2000s by the National Geospatial-Intelligence Agency and the National Geodetic Survey. The updated gravimetric surveys conducted by C4G researchers and staff will deliver much needed insight into the variety of geophysical processes driving the spatially and temporally heterogeneous rates of subsidence measured across the state.
In addition to the subsidence research, this enhancement grant will directly and indirectly benefit Louisiana’s geodetic stakeholder and consumer communities. For nearly a decade, the C4G has provided tools, services, and other geodetic resources dedicated to precise positioning throughout the state and across the region. 
Central to these resources is the C4GNet real-time reference network. The network includes more than 50 continuously operating GPS reference stations (CORS) installed across Louisiana.
Over the next five years, the C4G plans to geodetically correlate the gravity measurements with antenna heights at each station. Extended surveys will include CORS in neighboring states. When completed, the data will contribute to the creation of a novel, high-resolution geoid model that will allow the geodetic community to accurately and precisely measure elevations above sea level.
The instruments acquired by this grant represent an investment into the geodetic research capacity at the C4G and CEE. In addition to the above goals and objectives, these resources have already been selected for use by investigators in two external funding proposals, both of which will rely on the precision of these instruments to deliver meaningful geodetic solutions. These instruments not only promote research activities, they have galvanized national and international collaborations with partners across the US Gulf Coast and western Europe. More information about these instruments and geodetic models is available at the C4G website www.c4g.lsu.edu or gravity.c4g.lsu.edu.
C4G is Hiring!
Do you have what it takes to be our Coordinator?
Join the C4G Team!
The LSU Center for GeoInformatics is so busy doing important work for Louisiana and the Gulf Coast that we need an office manager to help coordinate our office and publish what we accomplish to the public, our clients and partners, i.e., Toot Our Horn!
We are looking for an organized, detail-oriented individual who is interested in what we do at C4G and has the skills to coordinate our efforts and share the results using social media, web based content management systems and through LSU system newsletters.
Don't Let the Techno-Babble Scare You Off!
C4G has all the resources necessary to help You Succeed!
We just need You to bring the following:
- Office administration skills.
- Basic accounting skills.
- Creative writing and publishing skills
- Team oriented enthusiastic nature.
- A desire to be part of something you can be proud of!
We will provide the rest of the tools and
resources you will need to SUCCEED!
| Apply to Join Us |
2010 South Louisiana GNSS Network Results Webinar
Note that the LSU Center for GeoInformatics is actively pursuing the funding necessary to do the control work needed to create better GEOID models in Louisiana. Feeding well distributed vertical control data into future GEOID models will improve the vertical accuracy of these models and is perhaps our only hope of ever creating a ±1cm GEOID in the state of Louisiana. |
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The NGS presentation on Friday March 15, 2013, pointed out that GEOID12a produces 95% confidence at ± 4 to 8 cm in Louisiana. The NGS conclusion states that the problem is too large for NGS to handle alone and we all need to work together to get better height results. They also recommend partnering with locals to leverage existing resources and that a plan needed to be created to move forward with improving heights. |
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The presentation was given by NGS geodesists / scientists, Michael Dennis and Dan Roman.
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Big picture ideas to consider from the presentation
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Recently, NOAA’s National Geodetic Survey released updated orthometric heights for Southern Louisiana relative to the September 2010 GNSS Height Modernization project. These heights represent the most up to date heights available for the region. On March 15, 2013, NGS hosted a free, on-line webinar to present the results of this project. |
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Visit the NGS page for this event To download the .mp4 file, click here. Link to the presentation: |
C4G Gravity Study Accelerates!
Why C4G Has Taken On The Study of Gravity In Southern Louisiana
In March 2013, NGS made a presentation on it's 2010 GNSS Height Modernization project in Southern Louisiana where they pointed out that GEOID12a produces 95% confidence at ± 4 to 8 cm in Louisiana. This is a problem since the goal of the NGS GRAV-D project is to establish a 2 cm GEOID for much of the country. The NGS conclusion states that the problem in Louisiana is too large for NGS to handle alone and that we all need to work together to get better height results. They also recommend partnering with locals (like the C4G LSRC) to leverage existing resources and that a plan needed to be created to move forward if we expect to improve heights in the 2022 vertical datum which will be based on a gravimetric geoid.
Big picture ideas NGS presented to consider regarding the GEOID problem in Southern Louisiana
- Short term (through ~2015)
- Extend 2009 MS leveling into and across LA
- Establish GNSS infrastructure to monitor subsidence
- New ~2015 GNSS Height Mod survey (gives 10-year delta time)
- Medium term (~2016-2022)
- Terrestrial gravity surveys
- Incorporate GRAV-D aerial gravity into geoid model for region
- Long term (~2022)
- New vertical datum based on gravimetric geoid
Because C4G already had the GNSS infrastructure in place and in its role as the Louisiana Spatial Reference Center (LSRC), C4G was already an active Height Mod partner, in 2013, the LSU Center for GeoInformatics started actively pursuing the funding necessary to do the terrestrial gravity surveys and control work needed to create better GEOID models in Louisiana. Only by feeding well-distributed vertical control data into future GEOID models will we be able to improve the accuracy of future GEOID models. And that is perhaps our only hope of ever creating a ±2cm GEOID in the state of Louisiana.
Through funding from Height Modernization and Board of Regent grants, we acquired two relative gravity meters. C4G then self-funded an absolute gravity meter with the intention of capturing the data needed to feed future GEOID models and help NGS reach their 2 cm GRAV-D goals in Louisiana.
The Gravity of our Positions!
Physical Geodesy is the science of the variations of the Earth's gravity field. The Geodesy of Subsidence has been an active topic of research in the LSU System since the middle 1980s. The intensity of the Earth's gravity at any spot on the surface is a function of the density of the Earth's crust at that spot and the distance to the center of mass. In a homogeneous sedimentary basin (Lower Mississippi Valley and the Delta), the density is practically constant except for occasional salt domes; therefore any variation in the gravity field may be attributed directly to changes in elevation, which usually relates to subsidence in Louisiana. Some "noise" can result from significant changes in groundwater.
Relative gravity surveys have determined the magnitudes of change from one elevation benchmark to another in the Gulf Coast Region, including Louisiana but the initial starting point has necessarily had to be assumed until the advent of Absolute Gravity observations made in the early 1990s. Re-observation of Absolute Gravity in New Orleans has been performed six times since then, and each new observation has shown a constant subsidence rate of 9.1 millimeters per year (3.5 inches per decade).
In the early 2000s, the LSU Center for Geolnformatics began installing GPS receivers at public buildings throughout the State of Louisiana and has continuously recorded the heights of the receivers as they have changed through the years. Most of these sites have shown varying rates of subsidence from place to place, based solely on GPS observations. A series of Absolute Gravity observation campaigns commenced in 2002 at some of the C4G GPS CORS sites by the National Geodetic Survey, and in 2006 was observed by the National Geospatial-Intelligence Agency.
No further observations of Absolute Gravity were performed in the State of Louisiana until 2018 when the National Geospatial-lntelligence Agency returned through the official requests of C4G and the Commander, New Orleans District Corps of Engineers. NGA agreed to visit some of the C4G CORS sites based on personnel availability and scheduling, but their generosity cannot be extended to over 100 of our CORS sites! Our own FG5-X Absolute Gravity meter is now a necessity to support our two CG-5 Relative Gravity meters in our continuing campaign to observe, record, and map the ever-changing gravity field of the State of Louisiana.
The significance of this research is that traditional surveying methods used to determine elevations of benchmarks have become inordinately expensive in the 21st century, and GPS is the only technology available currently to provide observations for that purpose. The issue, however, is that GPS does NOT provide "elevations" related to the concept of "mean sea level." GPS only provides a geometric "height" with respect to the center of mass of the Earth about which the satellites orbit. A mathematical model of the Earth's gravity field is necessary to translate from "height" to "elevation," and that is called the "geoid model." Where does the geoid come from? It comes from knowledge of the Earth's gravity field.
The more exquisite the knowledge we have about the gravity field, the more accurate we then can model the geoid, providing more reliable elevations for flood insurance, flood control, planning for evacuation route changes based on continuing subsidence, and variations in subsidence rates throughout the State of Louisiana. The LSU GPS Continuously Operating Reference System (CORS) network is a public utility offered by C4G, and that network is dependent on maintaining and improving the knowledge of the geoid for elevation control in support of Louisiana Revised Statute 50:173.1 which names C4G as the State Reference Standard for elevations.
C4G Acquires Geodetic Instruments to Better Understand Subsidence Rates in Louisiana
The Center for GeoInformatics (C4G) in the Department of Civil and Environmental Engineering (CEE) recently received new geodetic instruments to model the Earth’s gravity field. A Scintrex CG-5 Relative Gravity Meter, Leica T60 Total Station, and Trimble R10 GPS Rover Kit were acquired as part of an enhancement grant sponsored by the Louisiana Board of Regents. Drs. George Voyiadjis (PI) and Joshua Kent (Co-PI) led the one-year project, which ended in June, 2017. The instruments are acquired to address the needs of three objectives: First, to develop a novel, high-resolution gravity model of sea level (i.e., geoid); second, to augment knowledge of existing subsidence rates and the driving mechanisms; and finally, promote advanced geodetic research at the University.
Here, as in many river deltas around the world, land surfaces are sinking due to subsidence. On average, southern Louisiana experiences ~10 millimeters per year of subsidence. Understanding the mechanisms that drive subsidence is essential for mitigating risk and promoting sustainability. The CG-5 relative gravity meter supports these goals by measuring the relative differences in the Earth’s gravity across southern Louisiana. Surveys using the total station and R10 rover kit are currently underway to geodetically correlate the CG-5 data with absolute gravity readings collected in the early 2000s by the National Geospatial-Intelligence Agency and the National Geodetic Survey. The updated gravimetric surveys conducted by C4G researchers and staff will deliver much needed insight into the variety of geophysical processes driving the spatially and temporally heterogeneous rates of subsidence measured across the state.
In addition to the subsidence research, this enhancement grant will directly and indirectly benefit Louisiana’s geodetic stakeholder and consumer communities. For nearly a decade, the C4G has provided tools, services, and other geodetic resources dedicated to precise positioning throughout the state and across the region. Central to these resources is the C4GNet real-time reference network. The network includes more than 50 continuously operating GPS reference stations (CORS) installed across Louisiana. Over the next five years, the C4G plans to geodetically correlate the gravity measurements with antenna heights at each station. Extended surveys will include CORS in neighboring states. When completed, the data will contribute to the creation of a novel, high-resolution geoid model that will allow the geodetic community to accurately and precisely measure elevations above sea level.
The instruments acquired by this grant represent an investment into the geodetic research capacity at the C4G and CEE. In addition to the above goals and objectives, these resources have already been selected for use by investigators in two external funding proposals, both of which will rely on the precision of these instruments to deliver meaningful geodetic solutions. These instruments not only promote research activities, they have galvanized national and international collaborations with partners across the US Gulf Coast and western Europe.
First Louisiana Gravity Campaign of 2018 Report Completed
The United States National Geospatial-Intelligence Agency (NGA) has delivered its final report of their first Absolute and Relative Gravity Survey of February-March 2018. The survey has been conducted in support of requests from the New Orleans District, U.S. Army Corps of Engineers (USACE) and the Center for GeoInformatics of the Department of Civil and Environmental Engineering, Louisiana State University.