Satellite Gravity Data
This new 2008 satellite gravity product is the result of ‘stacking’ three independently derived solutions of free air gravity, (from satellite mission altimeter data), by three different authors; namely GETECH’s ‘Ultimate’ satellite gravity (2005), NDSC (National Danish Space Centre) NDSC08 and Sandwell and Smith v16.1 (2008). All three datasets over the continental margins of the world are now of similar general quality and resolution, but vary from area to area in their accuracy.
coverage of GETECH’s ‘Ultimate’ satellite gravity data
Since the data in each solution has been independently sampled, processed and interpolated, the resulting noise can be considered random between solutions, rather than systematic. Thus the ‘stacking’ process improves the signal-to-noise ratio of the final product by a factor of 1.7 over the separate solutions, and thus allows more reliable derivatives to be generated, which in turn improves the interpretation capability.
The figure below shows the correlation coefficient of the three separate global datasets; GETECH’s ‘Ultimate’ (2005), NDSC08, and Sandwell and Smith v16.1 (2008) with a ship-borne gravity survey in S E Asia. The results show all three solutions have similar correlations within the wavelength range 20 – 60km, (0.2 – 0.6 degrees). Comparisons in other areas of the world show similar results. The red dashed line shows the significantly higher correlation of the stacked combination (Trident) with the same ship-borne survey. Further tests showed the 50% coherency of the Trident solution with the ship-borne marine survey was at ~13km, with RMS error of 2-3 mGal.
The results of GETECH’s Trident study were presented at the 2008 AGU conference in San Francisco
Fairhead, J. D., Williams, S., Green, C. M., Fletcher, K. M. U. And Vincent, K. (2008). New satellite gravity solution ‘Trident’, AGU fall meeting, San Francisco.New satellite gravity solution ‘Trident’
Statistical comparison of GETECH’s ‘Ultimate’ satellite gravity, NDSC08, Sandwell and Smith v16.1 (2008), and Trident, with ship-borne gravity survey.
GETECH’s ‘Ultimate’ satellite gravity (2005)
covers all the continental margins of the World (see figure 6). It was generated using three innovative and crucial processing steps:
• The ‘first breaks’ of the original radar waveforms from the ERS-1 satellite were re-picked using synthetic waveform matching techniques.
• The altimeter profiles along every track were edited both automatically and manually for dc-shifted data, spikes and noise arising from near shore radar returns.
• The edited altimeter profiles were levelled using cross-over and micro-levelling techniques to calculate a geoid grid. Free air gravity was calculated from the geoid using FFT techniques.
The product was a gravity dataset in which:
• geologically meaningful and coherent gravity signatures to within 5 km of the coast were retained• ‘noise’ was minimised in the 30 – 100km wavelength range
• the resolution of the data was down to ~10km wavelength
In addition, an improved bathymetric dataset covering the continental margins were generated (Appendix 1) allowing more reliable Bouguer anomaly and Isostatic residual corrections to be calculated.
