Bishop Model

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Overview of the Bishop5x test dataset

Topographic data for a portion of the volcanic tablelands area north of Bishop, CA has been upscaled by a factor of 30 in x,y, and z dimensions and then shifted in the depth direction such that the structures are all now subsurface (approx 100 to 9300 meters subsea) to use as a test data set for gravity and magnetic calculations (see image below).

Forward calculations of the gravity and magnetic fields were done using GM-SYS-3D from Northwest Geophysical using default extrapolation parameters for a series of models using the depth model with some arbitrary additional susceptibility boundaries added to represent basement lithologic changes/intrusives (image below). The magnetic field was calculated for each model using inclinations of 0, 30, 45, 60 and 90 degrees and a total field of 50,000 nT. Additionally the fields were calculated using varying depths to the base of the magnetic layer (15000 m, 20000 m, 30000 m and a “moho” varying from approximately 33000 to 22000 m generated to approximate a passive margin setting).

All files are available as Geosoft binary grids (filename.grd) or GXF grids (filename.gxf) defined as follows:

x,y grid spacing 200 m
x,y nodes 1901 × 2011
x extent 2000 m to 382,000 m
y extent 141000 m to 543000 m

bishop5_basement depth to basement in meters – negative down
bishop5_mag_base_*_i** calculated magnetic field
where * indicates depth to basement of model
and ** indicates inclination
bishop5_susceptibility basement susceptibility
bishop5_gravity calculated gravity (see details below)
bishop5_moho depth to moho in meters - negative down (see details below)

The approximated moho was calculated as follows:
1. A forward gravity calculation was generated using a sediment depth density function of:

0 to 1000 m 2.1 g/cc
1000 to 3000 m 2.2 g/cc
3000 to 5000 m 2.3 g/cc
5000 to 7000 m 2.4 g/cc
7000 to 9000 m 2.5 g/cc
9000 m to basement 2.6 g/cc
basement to flat lower crustal layer at 20000m 2.70 g/cc
flat lower crustal layer to flat base crust at 32800 m 2.90 g/cc
mantle 3.30 g/cc

2. An inversion was done on the top of the mantle surface to generate an approximated moho (base crust) that would bring the observed gravity field to 0

3. The inverted base crust was then filtered at 300 km to remove local features and generate a reasonable looking base crust (moho) surface.

The calculated gravity file was generated by using the depth/density parameters above with the approximated moho as base crust. A constant of 4000 mGal was subtracted from the calculated gravity to make the numbers more reasonable.

How to obtain a copy of the data

To download a copy of the Geosoft binary grids data on a linux computer click here [1]. This should automatically download the file to your computer. Now open a terminal window and type the following dialog:

cd
find . -name bishop_Geosoft_grids.tar.gz

On my computers it finds the file as Downloads/bishop_Geosoft_grids.tar.gz or /tmp/bishop_Geosoft_grids.tar.gz. Continue the dialog with:

mkdir open_data
mkdir open_data/bishop
cd open_data/bishop
mv ../../Downloads/bishop_Geosoft_grids.tar.gz .
tar -xvf bishop_Geosoft_grids.tar.gz

This will untar the directory. When it completes, type:

cd Geosoft_grids
ls

This will list the Geosoft grid files.

GXF grids have not been uploaded for distribution, but should be availble by the end of 2012.

Terms of use

The Bishop data set was created for an SEG workshop in 2006. In 2012 the data was installed in the SEG open data library.

References

To download a copy of some papers about the Bishop model on a linux computer click here [2]. This should automatically download the file to your computer. Now open a terminal window and type the following dialog:

cd
find . -name bishop_Bishop_Workshop.tar.gz

On my computers it finds the file as Downloads/bishop_Bishop_Workshop.tar.gz or /tmp/bishop_Bishop_Workshop.tar.gz. Continue the dialog with:

mkdir open_data
mkdir open_data/bishop
cd open_data/bishop
mv ../../Downloads/bishop_Bishop_Workshop.tar.gz .
tar -xvf bishop_Bishop_Workshop.tar.gz

This will untar the directory. When it completes, type:

cd Bishop_Workshop
ls
acroread PhillipsWorkshop.pdf

You will see other pdf and doc files you can read.

Contact information

If you have any questions you can contact me at [email protected]