The knowledge about the gravity field plays a crucial role in understanding the system Earth. The heterogeneous gravity field shapes the mean sea level surface and it is used in many different research topics such as in determining ocean surface currents, unifying height systems globally, and mapping mass distributions that mirror the processes in Earth’s interior, namely plate tectonics, mantle convection, seafloor spreading and volcanic eruptions.

The Earth’s gravity field can be mapped globally based on satellite observations and surface gravity measurements. However, the spatial resolution is limited due to the band-limited spectral content of the input data. The GRAV4GEO project aims to expand the resolution of global gravity field models beyond their limits using a forward gravity modelling technique. In our method, the Earth gravitational potential is computed by numerical integration of Newton’s law of gravity based on very high-resolution digital elevation data and laterally varying mass density information.  Such an approach allows for complementing the medium to very-short wavelength information of the global gravity field. The numerical forward modelling is performed along a sequence of thin shells (as shown in Figure 1). In this context, the GRAV4GEO project uses an ellipsoidal approximation and the outcome of the project are high-resolution gravity field models in terms of ellipsoidal or spherical harmonic coefficients for the Earth’s upper crust, defined as the masses between the Mariana trench and top of Everest in this project. 

The project is a continuation of our previous activities in the topographic gravity field modelling (Abrykosov et al. 2019; Ince et al. 2020) where we used constant density values for rock, ocean, lake, and ice. GRAV4GEO now aims to introduce laterally varying high-resolution density models together with a state-of-the-art global digital elevation model. The density model (UNB_TopoDens_2v02) planned to be used in the project is developed by scientists from University of New Brunswick (Sheng et al. 2019), whereas the used merged global digital elevation model has been created by the project scientists at GFZ (GDEMM2024, Abrykosov et al. 2024). The latter is based on a consolidation of recent elevation models that consists of a global suite model for surface, bedrock, and ice.

As proven by our preliminary work (Figure 2), the outcomes of the GRAV4GEO project are expected to be:

1. reduction of the omission error (Figure 2c) and enhancement of the spectral and spatial resolution of global gravity field models (Figure 2b)
2. delivery of topography/density-based gravity information particularly for physically inaccessible areas
3. improved reduction of the gravity measurements for the topographic effect (Figure 2a) to investigate the residual signal of deeper Earth layers. This should help in the 3D crustal and lithospheric modelling especially in geologically complex areas.

Moreover, improvement in the accuracy of gravity modelling is expected due to the use of laterally varying density instead of the commonly used averaged density values.

The contribution of forward modelling will be tested in different areas such as Antarctica (see Figure 3) since the high-resolution global gravity field model merging scheme will be different for different regions depending on the availability and the quality of the terrestrial gravity measurements.

A high-resolution topographic gravity field model will be delivered at the end of the project which will also increase the spatial resolution of the recent global gravity field models up to ~2 km. Such a high-resolution global model will also lead to an improved global gravity field expanded up to degree/order 10800. As a result, a more accurate reference surface for global vertical datum and basis for better geophysical modelling especially in the regions of density discontinuities are expected.

The project is funded over three years (2022-2025) by the German Research Foundation (Deutsche Forschungsgemeinschaft – DFG, grand No. 505165206).

References:

Abrykosov, O., Ince, E.S, Förste, C., Flechtner, F. (2019): Rock-Ocean-Lake-Ice topographic gravity field model (ROLI model) expanded up to degree 3660. GFZ Data Services. https://doi.org/10.5880/ICGEM.2019.011

Abrykosov, O, Ince, E.S., Förste, C. (2024): GDEMM2024: 30 Arcsec Global Digital Elevation Merged Model 2024, a suite for Earth relief. GFZ Data Services. https://doi.org/10.5880/GFZ.1.2.2024.002.

Ince, E.S., Abrykosov, O., Förste, C. et al. Forward Gravity Modelling to Augment High-Resolution Combined Gravity Field Models. Surv Geophys 41, 767–804 (2020). https://doi.org/10.1007/s10712-020-09590-9

Ince, E. S., Förste, C., Abrykosov, O., Flechtner, F. (2022): Topographic Gravity Field Modelling for Improving High-Resolution Global Gravity Field Models. - In: Freymueller, J. T., Sánchez, L. (Eds.), Geodesy for a Sustainable Earth, (International Association of Geodesy Symposia ; 154), Cham : Springer, 203-212.
https://doi.org/10.1007/1345_2022_154

Sheng, M. B., Shaw, C., Vaníček, P., Kingdon, R. W., Santos, M., & Foroughi, I. (2019). Formulation and validation of a global laterally varying topographical density model. Tectonophysics, 762, 45-60.