Pecube Learning Exercises

Pecube Learning Exercises

March 13, 2007

Todd Ehlers

Additions by D. Whipp

Revised by J. Barnes –August 2007

1) Make a topographic input file. Use ArcGIS and the make_topo.sh script to extract topography from a DEM for your study area.Pay attention to variables make_topo.sh outputs to the screen – you’ll need them. Instructions for extracting a DEM for your study area are as follows...

1.Open ArcGRID in a new command window in windows

  • Click on the Start button, then on Run...
  • type 'cmd'
  • change to the directory with your ArcGrid folder

·cd “C:\path\to\folder”

  • type 'arc', followed by 'grid' once ArcInfo starts

2.Create a 3 column ASCII file from your ArcGrid file

·<file>.xyz = sample (<gridfile>)

3.Copy the output 3 column ArcGIS file to the Pecube working directory

4.Run make_topo.sh and give the ArcGIS file as input and any filename as output

5.Open Pecube.in and change the topography file used (option 2) to be the output file created from make_topo.sh

6.Change the number of longitude and latitude points to those printed when make_topo.sh is run

7.Change longitude and latitude spacing to the values make_topo.sh gives

Note:

·VERY IMPORTANT:

·You must set the coordinate system origin in the Pecube.in file to be offset from zero by half the resolution of the DEM you input to have the topography in the correct locations

·e.g.: If your DEM has a 250 m resolution, your coordinate system origin would be 125, 125 m

·The product of the longitude and latitude points, set in Pecube.in, must not exceed the number of topography values in the input topography file.The finite element model (FEM) mesh cannot be at a finer resolution than the digital elevation model (DEM)

·If the product is smaller, Pecube will still run.However, the topography read in by Pecube will be garbage.This means that the extent of the topography file from Arcgis must be equal to the size of the FEM.

·Note: You can use a more coarse FEM mesh but the mesh must be a multiple of the DEM resolution (e.g. skipping factor in Pecube.in file)

·Make sure to set input unit type (degrees or kilometers) to match the topography file generated

2) Start a sample Pecube simulation.You will need to edit the Pecube.in file to set up your simulation.See comments in this file for what to do – use the default values in the file now for things like material properties, erosion rate, etc (start simple!).After your Pecube.in file is ready you first compile pecube by typing “./build_pecube.sh” (no quotes), then run Pecube by typing “./pecube” at the command line.

3) Practice plotting in Tecplot.After the Pecube simulation is finished, look in the output directory (RUN00). Start tecplot and learn how to make contour plots of the output temperatures (Temps_tecXXX.dat) and ages (Ages_tecXXX.dat). Make the following plots in Tecplot (Note: you might want to learn how to make a macro to these, you’ll repeat them in following steps).

A)3D contoured block plot of model temperatures.

B) learn how to take two 2D slices through your block and show isotherms.

C) Contour plot the elevations of your topography (instead of temperature) for the block in A).

D) Use the extract data option to extract 2-3 temperature depth profiles from your 3D domain.Pick areas for each profile that might be interesting (e.g. valley bottom and ridge crest).Plot these geotherms in and x-y plot in a separate frame.Calculate the surface heat flow from the top 1 km of each profile.

E) Make a contour plot of the AHe and AFT ages output from your model.

F) Extract a slice or profile through your ages.Plot in two frames ages vs. distance along the profile and a topography vs. distance along the profile.

G) Try extracting and plotting an age-elevation relationship across an interesting topographic feature.

4) Exploring parameter space in Pecube.You will now set up several different Pecube simulations that explore how different parameters in the Pecube.in file influence temperatures and predicted ages.For each of the combinations of parameters below make several of the plots listed in the previous item.In particular, make sure you create plots A, B, D, F, G.Use the tecplot macro option to make your life simpler.

Parameters to explore:

4A) Run simulations that hold topography constant and use vertical erosion rates of 0.1, 0.5, and 1.0 mm/yr (3 simulations).Use a diffusivity of 31 km^2/myr, heat production of 0, at atmospheric lapse rate of 6 C/km. Turn isostasy off.

4B) Run simulations that vary the thermal diffusivity with a fixed erosion rate of 0.5 mm/yr.Keep all other parameters the same as A).Use thermal diffusivities that 0.75, and 1.25 the value used in A). Calculate what thermal conductivity might result from these different diffusivities. Plot results.

4C) Run simulations that vary the heat production with a fixed erosion rate of 0.5 mm/yr.Keep all other parameters (including diffusivity) the same as in 8.Use 2 different heat productions of 0.5 uW/m^3 (a crustal average), and 4.0 uW/m^3 (a hot granite).Note you’ll need to convert units to Pecube format (C/My). To convert divide your volumetric heat production (uW/m^3) by rock density times heat capacity. Use a density of 2750 kg/m^3, heat capacity of 800 J/kg K. Check your units!

4D) Exploring changes in relief and ages.Use the parameters in step A) and an erosion rate of 0.5 mm/yr.Now, modify the Pecube.in file and set up simulations that increase relief by factors of 0.5 and 2.0 over a 10 Myr time period.Also do a static shift in the elevation so that the mean elevation of the model stays the same during the relief change.Plot your results.Note – you can also plot erosion rates across the domain by plotting the erate_tecXXX.dat files (they will be spatially variable while relief is changing).

End of exercises.