Physical Geodesy is the science of studying 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 been necessarily 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.
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-lntelligence Agency (military).
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. Some of the C4G CORS sites will be visited based on personnel availability and scheduling, but their generosity cannot be extended to over 100 of our CORS sites! Our own FG-5X Absolute Gravity meter is now a necessity to support our two CG-5 Relative Gravity meters in our continuing, never-ending 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 nowadays and for the future to provide observations for that purpose. The fly in the ointment, 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 which the satellites orbit about. A mathematical model of the Earth's gravity field is necessary to translate from "height" to "elevation," and that model is called the "geoid." 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 new public utility offered by C4G, but 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.