CSIRO Marine and Atmospheric Research
Past Seminars

Wednesday 17 September, 10.30 am (Tas time)

CSIRO Auditorium, Hobart

Dr Cedric Griffiths
CSIRO Petroleum Resources, Perth

The Australian Seabed Model

The CPR contribution proposes a combination of numerical models adapted from oil-field use. Key to this is the 'Sedsim' model. Sedsim is a three-dimensional stratigraphic forward modelling program developed originally at Stanford University in the 1980's and extensively modified and extended in Australia since 1994. Sedsim is a collection of linked modules capable of replicating most of the physical processes which influence sediment deposition. These include sediment transport in fluvial, lacustrine and marine environments, sediment deposition and erosion, sea level and tectonic effects, carbonate growth, compaction, isostatic loading, slope failure, debris and gravity flows, geostrophic currents, wind and wave effects as well as storm modelling.

Fluid flow modelling in Sedsim uses an approximation to the Navier-Stokes equations in 3-D. The full Navier-Stokes equations describing fluid flow in 3-D are currently impossible to solve due to limitations in computer speed. Sedsim instead simplifies the flow by utilising isolated fluid elements to represent continuous flow (Tetzlaff and Harbaugh, 1989). This approach allows for a massive increase in speed of computation and simplification of the fluid flow equations.

Fluid elements travel over an orthogonal grid describing the topographical surface, reacting to the local topography and conditions such as the flow density and the density of the host medium (e.g. air, sea water or fresh water). Fluid elements are treated as discrete points with a fixed volume, an approach known as "marker-in-cell". Several simplifications are made to the Navier-Stokes equations, comprehensively described in (Tetzlaff and Harbaugh, 1989). The net result of these simplifications is that the Navier-Stokes equations are modified into non-linear Ordinary Differential Equations (ODEs). These equations are now solved using a modified Cash-Karp Runge Kutta scheme (Press et al., 1992) that ensures stable and accurate 4th order solutions in time.

Sediment is assumed to move at the same rate as the fluid element. At each time step it is either deposited onto the surface, transported across the surface or further sediment is eroded into the fluid element. Additionally, sediment can also be reworked by slope failure.

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