Data Documentation
Data Recompiled and Processed by:
A database
development task under the Great Basin Center for Geothermal Energy’s -
Geopowering of the West Project -
Supported by
Department of Energy funding
under DE-FG07-02ID14311.
Content Links:
Task Description
Gravity
Aeromagnetics
Topography
Image and grid
Projections
Data Coverage Area
Data Formats
Task Description
Rationale
Well-crafted gravity and magnetic anomaly maps will aid
regional geothermal assessments by providing improved expression of buried
geologic structural patterns that may be used to link known geothermal
occurrences to a new discovery. Existing gravity and magnetic data displays and
grids compatible with the ArcView GIS system convey only a fraction of the
useful information present in these data sets.
Objective
This data compilation task seeks to make the Great Basin’s
existing regional gravity and aeromagnetic geophysical datasets more accessible
to geothermal industry explorers and researchers utilizing ArcView compatible
GIS compilations. Currently available GIS compatible geophysical data sets offer
only summary presentations of major geophysical features. Primary source
geophysical datasets, by definition, offer full spatial detail, but its
effective transformation into GIS compatible formats requires specialized
processing software and knowledge of geophysical processing methodology. For
best utility, the improved geophysical displays must have common projections,
areas of coverage, and data formats that allow their direct integrated into our
developing geothermal database compilations based on ArcView GIS compatible
formats. The spatial resolution offered by these new map products should allow
useful map associations and map images to be made at scale of 1:250,000. Under
this task we will complete all necessary specialized processing and provide the
significantly improved geophysical data products for results in web accessible
GIS formats.
Tasks:
1)
Identify and obtain best available public domain potential field data
sets.
2)
Reprocess, merge, edit and grid data sets in map projects; determine best
processing parameters.
3)
Produce preliminary image, grid and shape file formats and test in
ArcView and ArcExplorer GIS programs with other GB data base products.
4)
Create a task-level web site to make products available as downloadable
files for initial public and group use.
5)
Based on group feedback, refine image and map specifications; complete
production of all displays and make files accessible on task-level web site.
6)
Assist in integrating this geophysical data set collection into the Great
Basin Center database site.
Description of Geophysical Data and Products:
Gravity
Data
Products produced and made
available.
Complete bouguer anomaly
-
Observed gravity corrected for standard oblate spheroid earth model with
terrain corrections.
-
Contains effects of crustal thickness variations.
Isostatic residual
anomaly
-
Removes effects of a standard crustal thickness model from the
complete bouguer anomaly.
-
Reveals crustal scale density variations not predicted by standard
topographic loading model.
Shallow crustal residual
-
Further corrects Isostatic residual anomaly by removing broad
regional anomaly patterns with out regard to source.
-
Emphasizes density changes in upper few kilometers.
-
Best for showing basin patterns.
Deep crustal anomaly
-
The Isostatic residual anomaly with basin scale anomalies removed
(through upward continuation)
-
Reveals deep and broad density anomalies in the crust.
Gravity station locations
-
Presented as a shape files and a text database of gravity station
locations and reduced gravity values.
-
Station locations plots are essential overlays for meaningful use of the
gravity displays.
Gravity data sources:
-
National Geodetic Survey (NGS) 1999 gravity database from NGDC Gravity
Data 1999 CD-ROM..
-
USGS DDS-42 Gravity Map of Nevada Ponce Nevada 1997.
-
Recent western Nevada surveys previously compiled by Oppliger 2002.
Compilation methods:
-
Merging non-redundant coverage from the above gravity station databases
produced an improved database.
-
Gravity reduced values were recomputed; the results were cleaned of
inconsistency gravity values.
-
1 km resolution grids were generated using 1000 iterations of minimum
curvature gridding.
-
Quality high-resolution color shaded-relief displays and contour maps
were generated.
-
USGS Composite Magnetic Anomaly Grid of the Western U.S., Kucks 2001.
-
See
http://minerals.usgs.gov/sddp/doc/faq/westernAeromagnetics.faq.html and
-
ftp://musette.cr.usgs.gov/pub/GEOPHYSICAL_DATA/non-web_pf_comp/WUSA/ file
name wusa_1000ag_gxf.gz
-
The original IGRF corrected total intensity aeromagnetic grid (1km) is
reprojected for compatibility.
-
Quality high-resolution color shaded-relief displays and contour maps
were generated.
Elevation Model
(DEM)
-
Data Source: GTOPO30 - 30 second geographic grid USGS EROS Data Center.
-
Reprojected and regridded to 600 m grid cell size.
-
Provides a necessary reference surface for understanding the gravity and
magnetic images.
-
Quality high-resolution color shaded-relief displays and contour maps
were generated.
Specifications
for delivered data products:
Image and grid projections:
Data coverage area: –123W to –110W,
34.5N to 45N. degrees:
Data formats:
-Image formats: ArcView
compatible tif with wtif world registration file. (i.e ArcView
compatible)
-Grid formats: ArcView
compatible binary raster grid flt with hdr header file.
-Contour maps: ArcView shape
format shp.
Data
visualization methods:
-Images: color shade-relief surface
representations.
-Grids: raster data
values.
-Contour maps: contour line representations.
References:
OPPLIGER, Gary L., WIDMER,
Michael, FAULDS, James E., and HENRY, Christopher D.,
2002,
Extensional and Strike-Slip Faulting Interactions In The Northern Sierran –
Great Basin Transition Zone Inferred From New Integrated Gravity Data,
GSA Denver Annual Meeting,
Session No. 6 Geophysics/Tectonophysics/Seismology
(Oral) Sunday, October 27, 2002
Simpson, Robert W., Jachens, Robert C. and Blakely, Richard J., 1986,
A New Isostatic
Residual Gravity Map of the Conterminous United States With a Discussion on
the
Significance of Isostatic Residual Anomalies: J. Geophys. Res., 91, No.
B8, 8348-8372.