Geophysics globe: SP12RTS tomography "seismic matryoshka"

Global tomographic models of seismic velocity provide us with crucial information about the planet’s interior. Here, we represent S wave velocity variation such that slow velocities stand proud and fast velocities as depressions.

To capture the 3D nature of tomography models (in which velocity structure varies with depth), we draw analogy to Russian nesting dolls (Matryoshkas). Globes can be separated to reveal deeper layers.

The tomographic velocity model presented here is SP12RTS (GJI, Koelemeijer et al 2016) and shows structures up to spherical harmonic degree 12. We have selected layers at 50 km depth (shallow), 660 km (mantle transition zone) and 2850 km (core mantle boundary).

The models can be printed without support, though this gives a slightly messy interior at the top which may need cleaning before assembly so that everything fits snugly. The holes are sized for 5 mm diameter, 2 mm thick cylindrical neodymium magnets which can be glued in place for a snap-together fit. Alternatively, use the 5mm plug stl provided for a grip-fit.

How I Designed This

Grids, contours and continental outlines were prepared using the Generic Mapping Tools. These were composited to generate a bump map which was applied to a sphere in Blender using UV mapping.  Detailed methodology here: https://github.com/jeffwinterbourne/3D_globes/

The models were divided into hemispheres and hollowed out using boolean differencing between the outer and inner layers (modified to allow the parts to pass in and out without fouling) in meshmixer, though this could also be done in blender. The holes for the magnets were located and recessed at the same time.

Scale

The size of the innermost globe relative to the outermost globe is correct, such that the innermost globe reflects the size of the earth's core. The middle globe (660 km depth) is NOT scaled correctly - the topography created by visualising the tomographic velocities means that the outer and middle globes would interfere. It should be a little larger.

Contour intervals are in %age variation from the mean S wave velocity at that depth:

• 50 km (outer) - 0.5% dVs
• 660 km (middle) and 2850 km (inner) - 0.25% dVs

Category: Physics & Astronomy