Limitations on the resolution and suitability of global gravity and magnetic models for geological interpretation: A user health warning!
Abstract
There have been major advances in the generation of global gravity and crustal magnetic models that are now down loadable from the Internet. All models have variable spatial resolution which is not always readily apparent when viewing these models. The global models include: 1) Free air gravity: EGM08 (5'), Danish DTU(1') and Sandwell and Smith V16.1 (1'). 2) TMI crustal magnetic: WDMAM (3') and EMAG3 (3'), where grid cell size is in arc minutes, where 1'≈ 2 km Both models can be spectrally divided into Long wavelength components: that are very well constrained and are derived from satellite observations. For Free air gravity: wavelengths > 150 km from Grace and for TMI crustal magnetic:wavlengths > 400 km from CHAMP MF6 Shorter wavelength components: The spatial resolution is very dependent on the data coverage of the terrestrial (ground, ship and airborne) surveys and are discussed below Gravity Model: The gravity model is expressed as the Free air gravity anomaly. For marine and large inland water areas the data used are derived from satellite altimeter measurements that generate a spatial gravity resolution of 15 to 20 km (full wavelength) from 3 to 4 km spaced orbital track data. For onshore areas the data coverage is highly variable from no data (e.g. interior Angola) to more than adequate coverage (e.g. Europe). To infill data gaps, free air correction values derived from the SRTM topography data were used. This results in a free air anomaly grid that appears to have full spectral content down to the resolution of the model. Locating where the data gaps exist is difficult, since the land gravity survey coverage for EGM08, used by all models, has not been released. A further resolution problem is that many large surveys used in EGM08 have been decimated to preserve their commercial value (e.g. GETECH input grids were decimated to at least 15' grid). TMI Crustal Magnetic Model: For marine areas these models suffer even larger problems in that the sparsity of magnetic data making gridding problematic. Instead of using minimum curvature as in WDMAM, the EMAG3 model uses the plate tectonic isochron model to directionally control grid interpolation in order to honour the strong 2D remanence (reversal magnetic field patterns) generated from Mesozoic to Recent sea floor spreading. For onshore areas many of the same problems encountered with the gravity field are present. Airborne magnetic coverage is near complete for Europe/Asia/Australia/North America but is poor in many parts of Africa, South America and Antarctica. In addition the models have upward continued the field to 4 or 5 km and many of the commercial surveys have been decimation. The talk will present many examples of these resolution limitations. For reliable geological interpretation one first needs to be sure of what the survey coverage is present and has it been decimated and/or, upward continued. Finally just because a global model is provided as a 1' (~2km) grid it does not imply that the wavelength resolution is everywhere down to 4km (twice grid cell spacing), e.g. satellite derived gravity data for the oceans has a resolution of only 15 to 20km.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFMGP33B..03F
- Keywords:
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- 1200 GEODESY AND GRAVITY;
- 1500 GEOMAGNETISM AND PALEOMAGNETISM