# Difference between revisions of "Bishop Model"

(basic wikifying, cat) |
(needs more work...) |
||

Line 1: | Line 1: | ||

==Overview of the Bishop 5 test dataset== | ==Overview of the Bishop 5 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). | + | 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). | + | 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: | + | All files are available as Geosoft binary grids (filename.grd) or GXF grids (filename.gxf) defined as follows: |

: ''x'',''y'' grid spacing 200 m<br> | : ''x'',''y'' grid spacing 200 m<br> | ||

: ''x'',''y'' nodes 1901 × 2011<br> | : ''x'',''y'' nodes 1901 × 2011<br> | ||

: ''x'' extent 2000 m to 382,000 m<br> | : ''x'' extent 2000 m to 382,000 m<br> | ||

− | : ''y'' extent 141000 m to 543000 m | + | : ''y'' extent 141000 m to 543000 m |

bishop5_basement depth to basement in meters – negative down<br> | bishop5_basement depth to basement in meters – negative down<br> | ||

Line 20: | Line 20: | ||

The approximated moho was calculated as follows:<br> | The approximated moho was calculated as follows:<br> | ||

1. A forward gravity calculation was generated using a sediment depth density function of:<br> | 1. A forward gravity calculation was generated using a sediment depth density function of:<br> | ||

− | + | : 0 to 1000 m 2.1 g/cc<br> | |

− | + | : 1000 to 3000 m 2.2 g/cc<br> | |

− | + | : 3000 to 5000 m 2.3 g/cc<br> | |

− | + | : 5000 to 7000 m 2.4 g/cc<br> | |

− | + | : 7000 to 9000 m 2.5 g/cc<br> | |

− | + | : 9000 m to basement 2.6 g/cc<br> | |

− | + | : basement to flat lower crustal layer at 20000m 2.70 g/cc <br> | |

− | + | : flat lower crustal layer to flat base crust at 32800 m 2.90 g/cc <br> | |

− | + | : mantle 3.30 g/cc <br> | |

− | 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 | + | 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<br> |

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

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. | 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. |

## Revision as of 13:55, 1 November 2012

## Contents

## Overview of the Bishop 5 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 data click here. Maybe there are multiple links here to grab the various pieces.

## Terms of use

The Bishop data set was created for an SEG workshop in 20??. In 2012 the data was installed in the SEG open data library.

## References

Are there some links to the workshop papers?

## Contact information

If you have any questions you can contact me at [1]