Chair: Michael Kuhn (Australia)

A vast number of data describing the Earth’s shape and structure (elevation, density distribution models for crust and mantle) are currently available. Several of these data are given globally with a continuously increasing resolution. One of the greatest density anomaly, for example, is given nowadays by the topographic and ocean water masses as modeled by global digital elevation models (DEM) with resolutions down to 1 km x 1 km. Apart from these (mostly geometrical) data, there also exist global geological and geophysical information about the Earth’s interior, describing mainly the structure of the Earth’s crust and mantle. The increasing number of these data allows the use of forward gravity field methods (direct application of Newton’s integral) in order to perform gravity field recovery and interpretation. In geodesy first attempts in this field have been made in the framework of the former IAG Special Study Group 3.177 (http://www.cage. curtin.edu.au/~will/iagssg3177.html) with very promising results for more detailed studies. Such forward modeling results are of great significance to gravity field modeling and interpretation. Furthermore, the comparison of the forward models with existing gravity field models reveals useful information on the dynamics of the Earth’s interior as well as the validity of the forward gravity modeling techniques.

The SG can be seen as a continuation of the IAG SSG 3.177 with a special focus on forward gravity field modeling as well as the modeling of gravity inside the (topographic) masses. Therefore it follows one of the recommendations made in the final report of IAG SSG 3.177.

The main scope of the SG will be the employment of recently released global digital databases with elevation data as well as information on the structure of crust and mantle for gravity field recovery and interpretation. The high resolution of the currently available global data permits the evaluation of high-degree and -order gravity models, while enabling the recovery of the high-frequency content of existing topographic/isostatic models. Furthermore, the forward gravity models can be used to study the behavior of gravity within the (topographic) masses, which is a crucial point in gravity field determination.

The SG will mainly focus on the following items:

- Construction of forward gravity field models using geophysical data.
    - Construction of forward gravity field models from existing global information on the Earth’s shape and structure (e.g. global DEM (topography, bathymetry) such as DTM2000.1, global crustal models like CRUST 2.0, global mantle models on seismic velocity and density).
    - Different approaches can be used and tested, such as the numerical integration of Newton’s volume integral or the expression of Newton’s gravitational potential in spherical harmonics.
    - Following the ideas of SSG 3.177 these forward gravity models also can be used as synthetic Earth gravity field models.

- Interpretation of forward gravity field modeling results.
    - Comparison of the forward gravity models with existing models from satellite and/or terrestrial gravity measurements (e.g. global gravity field models from CHAMP and GRACE).
    - Construction of synthetic Earth Gravity Models after attempting to identify bandwidths with known or apparent geophysical implications.
    - Geophysical interpretation and numerical assessment of the forward gravity field models with case studies over tectonically active regions (e.g. Isostasy and flexure of the lithosphere, dynamics of the mantle).

- Application of forward modelling results in gravity field determination
    - Modeling of gravity inside the topographic masses, which is required in gravimetric gravity field determination (e.g. geoid determination using Stokes’s theory).
    - Study the forward gravity field modeling results under the scope to improve the computation of different gravity reductions/effects such as:
        - computation of terrain reductions,
        - downward continuation of gravity anomalies,
        - orthometric corrections,
        - mean gravity and mean gravity gradient inside the (topographic) masses.
    - Study the possibility to express the Earth’s gravity field by forward modeling only.

Program of Acitivities

Membership

Michael Kuhn (Chair) (Australia)
Dimitris Tsoulis (Vice Chair) (Germany)
Hussein Abd-Elmotaal (Egypt)
Irek Baran (Australia)
Miroslav Bielik (Slovak Republic)
Heiner Denker (Germany)
William Featherstone (Australia)
Jakob Flury (Germany)
Thomas Gruber (Germany)
Simon Holmes (USA)
Michael Kern (Austria)
Jon Kirby (Australia)
Pavel Novak (Czech Republic)
Spiros Pagiatakis (Canada)
Roland Pail (Austria)
Gabor Papp (Hungary)
Nikolaos Pavlis (USA)
Gabriel Strykowski (Denmark)
Gyula Toth (Hungary)
Tony Watts (UK)