Geometry description

Earth structure

Danton models the Earth’s interior structure according to the Preliminary Reference Earth Model (PREM). That is, the Earth is composed of concentric layers, ranging from the very core to the surface, with varying bulk densities depending on the depth.

Note

The legacy PREM Earth is covered by a 3 km deep ocean, without any land. This can be disabled however (see the ocean attribute of the Geometry class).

Composition

The PREM is derived from seismic data and does not specify the composition of the Earth. Thus, Danton assumes a uniform composition aligned with standard rock, with the exception of the optional ocean layer.

Earth shape

Danton model’s the sea level according to a geoid. Three models are supported (see the geoid attribute of the Geometry class).

The default is to consider a spherical geoid of radius 6371 km, according to the PREM. As a refinement, the geoid can instead be modelled by an ellispoid, specifically the WGS84 ellipsoid. An additional level of accuracy is provided by the EGM96 model (also included in Danton), whose undulations are defined with reference to the WGS84 ellipsoid.

Scaling

The Earth interior structure is scaled radialy such that the ocean layer (i.e. \(x=1\), according to the PREM) matches the geoid. Note however that, except for a spherical geoid, this does not result in inner layers interfaces having a constant altitudes (or depth) w.r.t the reference ellipsoid.

Topography

The Earth’s surface can be modelled according to a Global Digital Elevation Model (GDEM), such as SRTMGL1. Alternatively, a constant topography elevation can be specified for the entire Earth.

Important

Danton always assumes that topography elevation values are given w.r.t. the geoid.

Oceanic regions

In the event that a topographic model is being specified, the PREM ocean layer is removed. However, negative topography elevations (e.g. from bathymetry data) indicate oceanic regions, ranging from the negative topography elevation values up to \(z=0\), which corresponds to the geoid.

Atmosphere

Danton considers that the Earth’s atmosphere adheres to the U.S. Standard Atmosphere (USSA) density profile (utilising Corsika’s parameterization), assuming a uniform composition. As for the Earth’s interior, the atmosphere is partitioned into layers, which are scaled to the geoid.

Outer space

The Monte Carlo geometry extends up to an altitude of 100 km (unscaled), which is considered by Danton to be the boundary with outer space.