The ggm_bg.ipynb Jupyter Notebook in this repository presents a procedure to obtain a global map of the topography-reduced gravity disturbance (or complete Bouguer disturbance, or No Ellipsoidal Topography of Constant density (NETC) disturbance), starting from the spherical harmonics coefficients of a global gravity field model and a synthetic model of the gravity effect of topography, ice, and water. It relies on SHTOOLS (Wieczorek and Meschede, 2018, doi:10.1029/2018GC007529), using pyshtools functions and classes (link to documentation).
It was set up to try SHTOOLS out of curiosity, to get familiar with Jupyter notebooks and their deployment on Binder, and, to successfully compute a Bouguer disturbance using global gravity models, which should be a trivial task. The operation which we perform here follows the definition in Eq. 33 of ICGEM report STR09/02.
This notebook was set up with the following gravity models, downloaded in the gfc format from ICGEM:
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Input global gravity model: XGM2019e, Zingerle, P., Pail, R., Gruber, T., Oikonomidou, X. (2019): The experimental gravity field model XGM2019e. GFZ Data Services. doi:10.5880/ICGEM.2019.007.
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Topographic effect model: dV_ELL_Earth2014, Rexer, M., Hirt, C., Claessens, S., Tenzer, R. (2016): Layer-based modelling of the Earth's gravitational potential up to 10km-scale in spherical harmonics in spherical and ellipsoidal approximation. Surveys in Geophysics. doi:10.1007/s10712-016-9382-2.
When deployed in a BinderHub instance, these two models are downloaded and unzipped after the container is built, using the PostBuild script provided in this repository.
If run locally, PostBuild must be run beforehand and the downloaded files moved accordingly.
In alternative, the argument of read_icgem_gfc may be edited with the correct path (an url to the gfc or zipped gfc may be also provided).
The code should work with any other combination of models. Note that using models expressed to a different maximum spherical harmonics degree may lead to unconsistencies (if the maximum degree of the two models differ, the model with the lower one gets zero-padded to the higher one).
Note that the topographic effect model used in the example does not include the normal potential.
Keep in mind that this example deals with computing a topography-reduced gravity disturbance, as described in Eq. 33 of STR09/02, using SHTOOLS and gfc files downloaded from ICGEM. Its aim is keeping formal consistency in the computation - a trivial task, albeit prone to errors. Apart from this, for this purposes, any other issue was deemed of secondary importance (to a reasonable extent). Thus I must warn against using the 'Bouguer disturbance' here obtained in any application.
A derived version of this procedure and plots, adapted for the purpose, was used in Fig. 7 of the "Gravity" entry by Braitenberg C. in Encyclopedia of geology (2nd ed., Vol. 1, pp. 706-718, Elias, S. A., & Alderton, D. H. M., editors).
The contents of this repository is licensed under the Mozilla Public License 2.0 - see the LICENSE file for details. The coefficients of the cited models are downloaded at build time, no part of them being stored in this repository.