Bishop Model

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Bishop 5 Test Data Set

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 x 2011
x extent 2000 to 382,000m
y extent 141000 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 gm/cc
1000 to 3000 m 2.2 gm/cc
3000 to 5000 m 2.3 gm/cc
5000 to 7000 m 2.4 gm/cc
7000 to 9000 m 2.5 gm/cc
9000 m to basement 2.6 gm/cc
basement to flat lower crustal layer at 20000m 2.70 gm/cc
flat lower crustal layer to flat base crust at 32800 m 2.90 gm/cc
mantle 3.30 gm/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.

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