BATON ROUGE –Louisiana can be a very rewarding place to live. It also comes with its adventures. Many Louisianians live close to nature. All live near to nature. Mother Nature has her perks, warmth, verdant fields, beautiful moss laden swamps, and more, but there is also a price, mosquitoes, occasional flooding, and of course, hurricanes. Then there is the phenomenon of subsidence. The LSU Center for GeoInformatics (C4G) is devoted to studying and understanding the geodetic nature of our state. The center was founded by the late professor, Roy Dokka, in 2001. He aimed to observe the tectonic activity of Louisiana using permanent GPS stations (CORS) across the state. The center has grown its network of CORS that now number over 120.
The C4G was designated by the National Geodetic Survey to serve as the Louisiana Spatial Reference Center. The C4G network of over one hundred CORS has been named GULFnet across the state as well as with NOAA tide gauges along the northern coast of the Gulf of Mexico, from Corpus Christi, Texas to Jacksonville, Florida. The center uses the data from its network to provide a service called C4Gnet to subscribers across the state that include surveyors, mappers, environmental scientists, and farmers, among others.
In the aftermath of hurricanes Katrina and Rita, the GULFnet met its first severe test of public service. Many state national government and private agencies were in need of accurate data for mapping the effects, correcting maps and search and rescue. Since 2001 the benchmarks in Louisiana were declared unreliable due to subsidence by the NGS in its report to Congress. The C4G CORS were the only precise references for them to use. C4G was recognized for its contributions. Louisiana Revised Statute 50:173.1 identified the LSU C4G as the legal standard in the State of Louisiana to obtain elevations.
Crucial to determining actual elevations is knowing the earth’s geoid, what most people think is meant by “Mean Sea Level.” It is a useful image to think of the strength of gravity equal to that at sea level. The height one must be in order to find that strength varies depending upon the density of the earth’s crust in the vicinity. If denser, the height is higher than if less dense. This invisible surface is the geoid. Mapping that surface creates a geoid model. Mapping the geoid requires knowing where one is relative to the Earth’s center AND the strength of gravity at that point as well as how it changes in that place.
As subsidence continues to occur in Louisiana and surrounding regions, the geoid also changes. This necessitates the continuing observation of the values of gravity for continual modeling of the geoid. The frequency of repeat gravity campaigns will be determined by experience gained through the monitoring of the gravity observations and the rates of change.
The unit of measure is named in honor of Galileo Galilei; the Gal equals 1 centimeter per second squared (1 cm/s). Specialized equipment, gravimeters, are used to measure the acceleration of gravity at a place. There are two methods used. The simpler method observes relative gravity between points, like an ordinary bathroom scale only millions of times more precisely in units of microGals (1 μGal = 0.000 001 Gal) with a precision of about 5 μGal. The LSU Center for GeoInformatics (C4G) has two CG-5 relative gravimeters. They require a reference point that has a known absolute gravity value.
An absolute gravimeter is many times more difficult to construct and operate and determines the acceleration of gravity by actually dropping a mass many times to observe how it is accelerated at that place with a precision of 1/10 μGal (0.1 μGal = 0.000 000 1 Gal). A change of 1 μg ≈ 0.3 centimeters (0.01 in.) in elevation! The LSU Center for GeoInformatics (C4G) has purchased and received the first university-owned Absolute Gravity Meter in the Southeastern United States.
For decades, the LSU System has relied on Federal Agencies to come into Louisiana from time-to-time in order to observe the changes in the absolute gravity in the State. The U.S. National Geodetic Survey (NGS) first observed absolute gravity in New Orleans in 1989. The NGS has returned over a half-dozen times since then as well as at numerous other sites. Also, with the help of the New Orleans District, U.S. Army Corps of Engineers; the U.S. National Geospatial-Intelligence Agency (NGA) has also performed several absolute gravity campaigns in the State, the most recent one was observed in early 2018. As absolute gravity increases, the changing values indicate actual subsidence of the surface of the observation site and the distance to the center of the Earth.
The importance of this acquisition is that all elevation reference data in Louisiana is subject to subsidence. The federally published values of elevation benchmarks only last 3-5 years in South Louisiana, somewhat longer to the north. The nation’s surveyors depend on GNSS (includes GPS) observations to get elevation data, but GNSS does not provide elevations with respect to the geoid; GNSS only provides “geometric” heights. Precise knowledge of the Earth’s gravity field is necessary for the conversion from GNSS heights into elevations. Flood insurance is dependent on the results of flood elevation surveys performed by engineers and land surveyors. Planning for evacuation routes in anticipation of impending hurricane flooding do as well. The Absolute Gravity instrument now held by C4G is the basis for refining the accuracy of the Earth’s gravity field.
The FG5-X Absolute Gravity meter will be employed in conjunction with its CG-5s, to observe at all LSU CORS sites in the future on a revolving basis in order to track the crustal movement of subsidence, its changing values, and the changes in the rates of subsidence. Intermediate points of elevation, the passive benchmarks will be observed and re-observed by the CG-5 Relative Gravity meters owned by C4G along with the Absolute sites being used as reference standards.
June 21, 2018,
J. Anthony Cavell, PLS, CFedS,
LSU Center for GeoInformatics