How the data is collected for Satellite gravity inversion
Analysis of satellite gravity data offers an opportunity to rapidly evaluate the sedimentary structure of basins. The accurate and evenly distributed measurements of the gravity field determined from satellite orbits contain information about the bathymetry, age, sediment thickness and crustal structure of the world’s oceans. The data is collected from satellite based methods. Many organizations have carried out seismic surveys, opportunity vessels and satellite sensors to collect data for gravity inversion maps. These institutions have archived them and available to the general public.
For instance, the sea level data from satellite based altimeters is used and obtained from, for example, Archiving, Validation, and Interpretation of Satellite Oceanography Data (AVISO) operations canter. AVISO distributes sea surface heights (SSH) and sea level anomalies (SLA) measured by Jason and ERS–1/2 satellites (Al Saafani 2008) Jason and ERS–1/2 altimeter data helped in better resolving the mesoscale variability and the data provides more homogeneous and reduced mapping errors than the individual data set.
The National Oceanic and Atmospheric Administration (NOAA) as a federal US agency focused on the condition of the oceans have a lot of satellite data. The measurements made by the sensors mounted on board satellites are used, especially, Seas Surface Temperatures (SST) and the sea level heights are measured by the satellite based altimeters and the sea winds measured by the scatterometer.
Other profiling instrument such as the ’Conductivity–Temperature–Depth (CTD)’, ’Expendable Bathythermograph (XBT)’ and ’Mechanical Bathythermograph (MBT)’ and the ’Ocean Station Data (OSD)’, obtained using reversing bottles attached with reversing thermometers are used (Al Saafani 2008). The use of hyperspectral sensing as one of the remote sensing techniques can be used to generate maps of the seafloor and the bathymetry in littoral zones (up to 20 m. ).
A model technique is the Hyperspectral Mapping (HyMap) that is widely used in bathymetry and sea floor mapping. This process involves transformation of subsurface reflectance to the bottom albedo. According to Heege et al (2003), the unknown input value of depth is calculated iteratively in combination with the spectral unmixing of the respective bottom reflectance. The unmixing procedure produces the sea floor coverage of three main bottom components and the residual error between the model bottom reflectance and the calculated reflectance.
The final depth, bottom reflectance and bottom coverage is achieved at the minimum value of the residual error. The final step of the thematic processing classifies the bottom reflectance due to the spectral signature of different bottom types and species using a Fuzzy Logic method and assignment of individual probability functions for each defined sea floor component (Heege et al 2003). 6. 0 The gravity inversion maps 6. 1 Thermal Gravity anomaly Thermal gravity anomaly is generated by the gravitational admittance that modifies the topography.
The thermal gravity anomaly is attributed the early stages in the formation of divergent margins when the lithosphere experiences large changes in temperature which play a key role in the anomaly of the Gulf of Aden. Evidence suggest that in this regions, thermal anomaly in the upper mantle that has persisted after continental break-up. According to Chappell and Kusznir (2008), the oceanic lithosphere thermal model is always used to predict the lithosphere’s thermal gravity anomaly. In the deeper parts of the margin the heat flow is high and constant, but it decreases abruptly near the shelf-slope.
Thermal gravity anomaly parameters contain information on the state of isostacy for a surface topography feature. The sensitivity of the lithosphere thermal gravity anomaly and the predicted ocean depth from gravity inversion maps are always correlated. Variations of the sea floor bathymetry constitute a load distribution on the oceanic lithosphere. The presence of shallow-water sediments deposited after the opening of the Atlantic Ocean hints at lower subsidence than would have occurred in the absence of persistent thermal anomalies. Figure 5: Thermal gravity anomaly maps (mgal); a) with sed and b) with sed and volcanic corrections.
As shown in figure , the curve along the gulf is more shallower than towards the ocean, hence reduction in water discharge, the latent heat due to high temperatures and eddies contribute to the water depth anomaly in the shores of Yemen. 6. 1 Thinning Factor The lateral density changes caused by the elevated geotherm in thinned continental margins such as the Gulf of Aden and adjacent ocean basin lithosphere yield important thermal gravity anomaly. According to Leroy et al (2004), magnetic quiet zone corresponds to an area of thinned crust.
Lucazeau et al (2008) argues that the lithosphere in the deep margin should be locally hotter and more buoyant than any homogeneous margin. There are several methods to determine crustal depth, lithosphere thinning and the location of the ocean-continent transition at rifted continental margins using 3-D gravity inversion which includes a correction for the large negative lithosphere thermal gravity anomaly within continental margin lithosphere. Figure 6: Thinning Factor maps; a) with sed correction and b) with sed and volcanic corrections 6. 1 Sediment Thickness
The sediment thickness can be obtained from geophysical studies and associated gravity inversion maps. For instance, a three-dimensional Bouguer anomaly map is produced for dimensional inverse approach to gravity data interpretation. The Bouguer anomalies along the axial portion of the rift floor, as deduced from the results of the regional and residual separation, are mainly caused by deep-seated structures. Figure 7: Sediment Thickness map The use of satellite remotely sensed imagery and hyperspectral remote sensing gives precise information about the sediments and sea bed.
The seabed, basement and crust boundaries are defined by a series of triangular facets with their sizes varying as the amount of constraining data changes. In this case, the sediment and basement boundaries and the base of the crust are defined by larger facets than those defining the bathymetry. Figure 8: Volcanic addition map (m) 6. 1 Mantle Residual Anomaly There are prominent slow anomalies within the Gulf of Aden and the entire Indian Ocean unlike the fast the central Atlantic and the older parts of the Pacific.
Since most of the large slow anomalies define geoid highs, Leroy et al (2004) indicates that there is poor overall correlation between velocities in the upper mantle and the geoid because subduction zones in general are associated with geoid highs and regions of fast velocity below. Figure 9: Mantle Residual Anomaly (mgal); a) with sed correction and b) with sed and volcanic corrections These maps identify target area of study using satellite gravity inversion. The maps are compiled from seismic data, surface and wave data and contoured to produce the exact heights of the crust at various points.
Embedded thermal correction and prediction of Gulf of Aden crust thickness is used to map crustal thickness. The map shows thickening of the crust from Red Sea and Gulf of Aden eastwards and northwards. This crust is inherent in the middle of the Arabian plate. 7. 0 Summary The water depth anomaly has been analyzed to describe the vertical and horizontal structure of Red Sea. The outflow water in the western Gulf of Aden, at the location where it is first injected into the open ocean is one contributor to shallowness despite the high atmospheric temperatures and eddies among the contributing factors.
An important region for oil exploration and shipping, the application of 1km resolution imagery aids in the studies and development of this region. This project is focused on how to create gravity inversion maps and the general application of satellite data in bathymetry, SST estimation, thinning and age determination. It is evident that the Gulf of Aden existing in the slow Indian Ocean margin is experiencing water depth anomalies especially during summers.
Remote Sensing and Earth observation does not only cover the global and regional survey of geophysical parameters by satellite- or airborne radars, it also includes local observations by ground based radar techniques. 8. 0 References Al Saafani (2008). Physical Oceanography of the Gulf of Aden. PhD Thesis. National Institute of oceanography. Goa. Bower, A. S. , Johns, W. E. , Fratantoni, D. M. and Peters, H. (2005). Equilibration and Circulation of Red Sea Outflow Water in the Western Gulf of Aden. Journal of Physical Oceanography. Vol 35: pp 1963-1985 Bower, A. S. , D. M. Fratantoni, W. E. Johns, and H.
Peters (2002), Gulf of Aden eddies and their impact on Red Sea Water, Geophys. Res. Lett. , 29(21), 2025, doi:10. 1029/2002GL015342 Chappell, A. R. ; Kusznir, N. J. (2008). Three-dimensional gravity inversion for Moho depth at rifted continental margins incorporating a lithosphere thermal gravity anomaly correction. Geophysical Journal International, Volume 174, Issue 1, pp. 1-13. Cochran, J. R. (1982) Simple models of diffuse extension and the pre-seafloor spreading development of the continental margin of the Northeastern Gulf of Aden. Oceanologica Acta sp. , 155–165 (1981). d’Acremont, E. , Leroy, S.
, Beslier, M. O. , Bellahsen, N. , Fournier, M. , Robin, C. , Maia, M. and Gente, P (2004). Structure and evolution of the eastern Gulf of Aden conjugate margins from seismic reflection data. Geophysical Journal International, Volume 160, Issue 3, pp. 869-890. Girdler, R. W. , Brown, C. , Noy, D. J. M and Styles, P. (2009). A geophysical Survey of the Westernmost Gulf of Aden. Royal Society. Vol 298, Pg 1-43. Hastenrath, S. and Lamb, P. J. (1979). Climatic Atlas of the Indian Ocean, Part 2. The Ocean Heat Budget, University of Wisconsin Press, Madison, U. S. Heege, T. , Hase, C. , Bogner, A. , Pinnel, N. , 2003.
Airborne Multi-spectral Sensing in Shallow and Deep Waters. Backscatter 1/2003: 17-19 Heileman, S. 2008. I. West and Central Africa. In: The UNEP Large Marine Ecosystems Report: A Perspective on Changing Conditions in LMEs of the World’s Regional Seas.. Sherman, K. and G. Hempel. UNEP. Nairobi, Kenya. pp. 101–142. King, S. D. & Ritsema, J. (2000). African hot spot volcanism: Small-scale convection in the upper mantle beneath cratons. Science 290, 1137–1140 Kusznir, N. J. & Karner, G. D. Continental lithospheric thinning and breakup in response to upwelling divergent mantle flow: Application to theWoodlark, Newfoundland and IberiaSample Essay of EduBirdie.com