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 bishop_Geosoft_grids
<pre>

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
[https://docs.google.com/open?id=0B_notXWcvuh8dF92bWdXcDF0UUk]. This should automatically download the file to your computer.  Now open a terminal window and type the following dialog:

<pre>
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 [2]