First off I do not like the term “corrections.” By definition, a correction fixes something that is erroneous or wrong. Gravity data are NOT wrong or in error! Unless the gravity meter malfunctions or the operator makes a mistake; however, since academia and my peers have so strongly forced the idea that the observed gravity data are in error, I will sparingly use “correction,” so people will know which processing standard Earth model I am discussing. Next, individual gravity effects are additive, so each can be calculated individually.
If we look at a map of the observed land gravity data for North America, it is evident that all of the Earth and significant geology-based gravity anomalies are visible.
The first “correction” applied to the observed gravity data is removing the gravity effect of a theoretical Earth gravity model calculated at the gravity station’s location on the Earth’s geoid model.
Moreover, as geodesist update their geoids, the theoretical gravity models are updated. Additionally, in most cases, geophysicist don’t keep up with the latest model since it requires recalculating ALL of their gravity data. The continuous updating of the theoretical-gravity equation often results in mixed models in the gravity data. Data consistency is a bookkeeping problem for project consistency! World Geodetic System 1984;
Go = 978032.67714 * {[ 1+0.00193185138639 * sin^2(lat)]/[1-0.00669437999013 * sin^2(lat)]^(1/2)} [mGal]
The next “correction” is the free-air. The free-air, AKA Faye, a calculation, Z [meters] * 0.3086 [mGal/m] is a first order free-air is a first order estimate of the calculated change of the gravity for the gravity station elevation above and here is a problem, is it sea level or geoid? In the olden days, it was sea level. By the way, the application of the free-air does not change the location of the gravity station, unlike what is in the Wikipedia posting for the free-air anomaly.
Additionally, in regions of extreme elevation and complicated geology investigators often develop unique higher order free-air equations rather than use the first order free-air equation and estimate the gravity effect of the complex geology using one or more causal 3D GEOLOGY/DENSITY models. There it is again, the GEOLOGY getting in the way of a dogmatic and overly simplistic geology model.
Now in the case of NOAA’s Grav D project, www.ngs.noaa.gov/GRAV-D/, NOAA suggests using a higher order free-air equation based on a higher order approximation of the change in gravity with the change in elevation above the geoid.
In the end the North America free-air anomaly, FAA, looks like:
The FAA map is a compilation of the NRCAN and PACES land and TOPEX, topex.ucsd.edu/marine_grav/mar_grav.html, free-air anomaly, FAA. It is at this point when geophysicist need to earn the GEO part of the title geophysicist.
NO, I do not support using the complete Bouguer geology model or the isostasy calculation. These are entirely incorrect geology and mathematical models that do not reflect the universe’s density distribution that generated the observed gravity field. Additionally, they do not help illuminate the Earth’s geology.
I am not addressing the Eötvös, or tides models. The Eötvös mathematical model is for data collected using moving observation platforms, and tides is another very complicated gravity field generated by moving masses in the universe. While the mathematical tide model's give an insight to the general tidal gravity signal the in-situ local tide signal is the result of the moving masses the local and regional geology, and the crust’s geodynamic setting. The best case is to develop new continuously monitoring gravity/GNSS meters giving insight on the geologically constrained tide’s effect.
The PACES data repository is not operational, I have an old copy in shapefile format with the PACES principle data, contact me if you want a copy.
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