[CIG-LONG] Fwd: Extension - Compressible Layer in Gale

[Louis Moresi]

Sorry, I should have forwarded this to the list !

---------- Forwarded message ----------
From: Louis Moresi <louis.moresi at monash.edu>
Date: 17 June 2010 13:38
Subject: Re: [CIG-LONG] Extension - Compressible Layer in Gale
To: "atyl0789 at uni.sydney.edu.au" <atyl0789 at uni.sydney.edu.au>


The question of strain in the "air" layer has been mentioned before on this
list and the conclusion is that, since the
layer is there for numerical convenience, the behaviour is not especially
important. If you don't plot this strain / strain rate
then the model looks pretty good.

As with any modelling, determining the applicability / suitability of the
tool with which you have been provided is
your task — like a pilot checking the plane before flying it, I suggest you
run a number of tests before you make
any bold scientific conclusions. Does the air layer matter ? A more relevant
question is how much / when does the air
influence the model. This is something you can determine empirically.

I'll leave the question of the hydrostatic correction to Walter or somebody
else to deal with.

L.

On 17 June 2010 12:59, atyl0789 at uni.sydney.edu.au <
atyl0789 at uni.sydney.edu.au> wrote:

> G'day all,
>
> My name is Aaron Tyler and I am currently undertaking my honours year in
> Geophysics at the University of Sydney, with a project focused on the
> modelling of tectonic inversion using the Gale software.
>
> As a first step I have been attempting to create a simple extensional model
> consisting of 8km of brittle crust overlying 2km of weak viscous material,
> extending at a constant rate of 1cm/year. I have been so far using the
> Gale_1_4_1 binary and have encountered a number of problems in getting this
> model to run. Using either iterative or direct solver, Gale generally
> crashes after around 50-60 timesteps coinciding with the development of high
> strain and the first glimpses of faulting. I initially increased resolution
> to 100m (x and y) in order to determine whether this was the issue, however
> the same problem was encountered.
>
> I read that John Naliboff encountered a similar sounding problem in the
> cig-long archive (March 2010), and following Guillaume and Walter's advice I
> attempted to implement a compressible air layer above the crust and added a
> basal velocity. The compressible 'air' layer was 2km thick, had a density of
> zero, a viscosity of 1e19, and compressibility factor of 10. The model seems
> to run smoothly, however I observe some extreme deformation of the air layer
> as extension proceeds, and I am unsure whether this is expected and/or
> whether it affects the deformation of the layers beneath it. I am also
> unsure whether this would begin to cause problems if I were to run the model
> longer. Finally, I am unsure how to use the hydrostatic function with the
> addition of this compressible layer. Is it possible to make it so that it
> affects only the lithosphere? I have attached an image of what my model
> output looks like after 152 timesteps (arbitrary, its just when i stopped
> the model).
>
> I have corresponded with John and he has told me that he was unable to get
> the compressible layer working with the hydrostatic term, and ultimately
> ended up using an a weak incompressible layer with a density of 1000,
> simulating a body of water above the crust. Is this a more effective
> approach for me to follow?
>
> I am unsure how to proceed at the moment, so any advice on how best to
> tackle my problems would be fantastic :) I have attached my initial input
> file without the compressible layer (1012), and an input file with the
> compressible layer (2001). They are relatively well commented so it should
> hopefully be clear what I have done. Thanks everyone!
>
> Cheers,
> Aaron
> _______________________________________________
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>
>
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