Data Recompiled and Processed by:

Dr. Gary L. Oppliger
Arthur Brant Laboratory for Exploration Geophysics
University of Nevada, Reno
Mackay School of Mines /172
Reno, Nevada 89557-0138
E-mail: oppliger@mines.unr.edu

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 Data

Aeromagnetic Data

Topography (Digital Elevation Models)

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.

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.

Aeromagnetic Data

-         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.   

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.