Preliminary gravity inversion model of Frenchman Flat basin, Nevada Test Site, Nevada

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Frequently anticipated questions:

What does this data set describe?

Preliminary gravity inversion model of Frenchman Flat basin, Nevada Test Site, Nevada
The depth of the basin beneath Frenchman Flat is estimated using a gravity inversion method.
This dataset is distributed as an XYZ ASCII format data file generated from the 2002 gravity inversion model of the bedrock beneath Frenchman Flat basin. Data forms a rectangular grid of data values in the defined projection. The data values are the depth below the land surface,in meters, of the modeled location of the Pre-Tertiary basement surface. The report which accompanies the data explains the modeling process and results.
  1. How might this data set be cited?
    Phelps, Geoffrey, 20020930, Preliminary gravity inversion model of Frenchman Flat basin, Nevada Test Site, Nevada: U.S. Geological Survey Open-File Report 02-363, U.S. Geological Survey, Menlo Park, CA.

    Online Links:

  2. What geographic area does the data set cover?
    West_Bounding_Coordinate: -116.08897159
    East_Bounding_Coordinate: -115.81127475
    North_Bounding_Coordinate: 36.93573514
    South_Bounding_Coordinate: 36.71733292
  3. What does it look like? (PDF)
    Printable version of the colorized model, 1.6 megabytes
  4. Does the data set describe conditions during a particular time period?
    Calendar_Date: 2002
    ground condition
  5. What is the general form of this data set?
    Geospatial_Data_Presentation_Form: raster digital data in ASCII XYZ format
  6. How does the data set represent geographic features?
    1. How are geographic features stored in the data set?
      This is a Raster data set. It contains the following raster data types:
      • Dimensions 49 x 49 x 1, type Grid Cell
    2. What coordinate system is used to represent geographic features?
      Grid_Coordinate_System_Name: Universal Transverse Mercator
      UTM_Zone_Number: 11
      Scale_Factor_at_Central_Meridian: 0.9996
      Longitude_of_Central_Meridian: -117.0
      Latitude_of_Projection_Origin: 0.0
      False_Easting: 500000.0
      False_Northing: 0.0
      Planar coordinates are encoded using row and column
      Abscissae (x-coordinates) are specified to the nearest 500.0
      Ordinates (y-coordinates) are specified to the nearest 500.0
      Planar coordinates are specified in meters
      The horizontal datum used is North American Datum of 1927.
      The ellipsoid used is Clarke 1866.
      The semi-major axis of the ellipsoid used is 6378206.400000.
      The flattening of the ellipsoid used is 1/294.978698.
  7. How does the data set describe geographic features?
    XYZ ASCII file of modeled depth to Pre-Tertiary basement formatted as single lines of text of northing,easting,depth (in meters below ground surface)
    For example:

Who produced the data set?

  1. Who are the originators of the data set? (may include formal authors, digital compilers, and editors)
    • Geoffrey Phelps
  2. Who also contributed to the data set?
  3. To whom should users address questions about the data?
    Geoffrey Phelps
    U.S. Geological Survey (USGS) Western Earth Surface Processes Team (WESP)
    345 Middlefield Road, MS 989
    Menlo Park, CA

    650-329-4922 (voice)
    650-329-5133 (FAX)

Why was the data set created?

These data are intended for science researchers, students, policy makers, and the general public. The data can be used with geographic information systems (GIS) or other software to display a geophysical model of the elevation of the basin beneath Frenchman Flat, Nevada Test Site, Nevada.

How was the data set created?

  1. From what previous works were the data drawn?
  2. How were the data generated, processed, and modified?
    Date: 30-Sep-2002 (process 1 of 2)
    The depth to the Pre-Tertiary basement was determined by conducting a gravity inversion modeling process described in the U.S. Geological Survey Open-File report 02-363. The gridded data are converted to an XYZ ASCII format file (northing, easting, depth below surface in meters).
    Date: 26-Aug-2003 (process 2 of 2)
    Creation of original metadata record Person who carried out this activity:
    U.S. Geological Survey (USGS) Western Earth Surface Processes Team (WESP)
    Attn: Geoffrey Phelps
    USGS, MailStop 989, 345 Middlefield Road
    Menlo Park, CA

    650-329-4922 (voice)
  3. What similar or related data should the user be aware of?

How reliable are the data; what problems remain in the data set?

  1. How well have the observations been checked?
    The accuracy of the calculation of depth to pre-Cenozoic rock is dependent on many factors, including: the accuracy of the gravity measurements, the relative spacing of the measurements on the ground, the accuracy of the constraints and assumptions built into the reduction process (e.g. assuming an average crustal density of 2670 kg/m3 for complete Bouguer and isostatic corrections), the assumption of uniformly dense, horizontal layers, the density constraints used in the inversion, and the interpolation algorithm used between the data points.
    The primary source of uncertainty is the density-depth function used to calculate the thickness of the basin fill. To investigate the variation in the final model a sensitivity analysis (Hill, 1998) was performed. The model was perturbed systematically, and 1% scaled sensitivities were calculated for each model parameter (layer). The sensitivities represent the approximate change in depth resulting from a 1% change in the parameter (density) value. We can use them to gain insight into how the parameters quantitatively contribute to the model. However, the sensitivities do not scale; in other words, multiplying one result by 10 does not yield the exact amount the depth would change given a 10 percent change in density. This is because the model is non-linear, and so the values are valid only within a short range of the solution. Figures 8-11 show the 1% scaled sensitivities for the final model in Frenchman Flat. The largest range in sensitivity, indicating the largest overall influence on the model, occurs in the second layer, while the fourth layer has the greatest influence over the deepest part of the basin. Laterally the shallower layers of the model have a greater affect than the deeper layers. The deeper layers influence the deep part of the basin only; this follows from the simple fact that a layer cannot affect the model if the predicted depth is not deep enough to include the layer.
    The general range of the sensitivity values is from a few meters up to 30 meters. This implies the model is fairly sensitive to density changes, since fluctuations of a few percent imply tens of meters fluctuation in the depth estimates.
  2. How accurate are the geographic locations?
  3. How accurate are the heights or depths?
  4. Where are the gaps in the data? What is missing?
  5. How consistent are the relationships among the observations, including topology?
    Values within the raster dataset are valid depth values. Values represent the center of cells 500 meters by 500 meters. From the original rectangular raster dataset, only values with valid depths are present in the ASCII data file (all null values have been omitted).

How can someone get a copy of the data set?

Are there legal restrictions on access or use of the data?
Access_Constraints: none
Please recognize the U.S. Geological Survey (USGS) as the source of this information. USGS-authored or produced data and information are in the public domain.
  1. Who distributes the data set? (Distributor 1 of 1)
    United States Geological Survey (USGS) Information Services
    Box 25286
    Denver, CO

    (888)ASK-USGS (voice)
  2. What's the catalog number I need to order this data set? USGS Open-File Report 02-363
  3. What legal disclaimers am I supposed to read?
    Although these data have been used by the U.S. Geological Survey, U.S. Department of the Interior, these data and information are provided with the understanding that they are not guaranteed to be usable, timely, accurate, or complete. Users are cautioned to consider carefully the provisional nature of these data and information before using them for decisions that concern personal or public safety or the conduct of business that involves substantial monetary or operational consequences. Conclusions drawn from, or actions undertaken on the basis of, such data and information are the sole responsibility of the user. Neither the U.S. Government nor any agency thereof, nor any of their employees, contractors, or subcontractors, make any warranty, express or implied, nor assume any legal liability or responsibility for the accuracy, completeness, or usefulness of any data, software, information, apparatus, product, or process disclosed, nor represent that its use would not infringe on privately owned rights. Trade, firm, or product names and other references to non-USGS products and services are provided for information only and do not constitute endorsement or warranty, express or implied, by the USGS, USDOI, or U.S. Government, as to their suitability, content, usefulness, functioning, completeness, or accuracy.
  4. How can I download or order the data?
    • Availability in digital form:
      Data format: x-coordinate, y-coordinate, depth in meters below ground surface in format Columnar text comma-separated values, one point location per line Size: 0.0654
      Network links:
    • Cost to order the data: none

Who wrote the metadata?

Last modified: 13-Jun-2016
Metadata author:
Peter N Schweitzer
USGS Midwest Area
Collection manager, USGS Geoscience Data Clearinghouse,
Mail Stop 954
12201 Sunrise Valley Dr
Reston, VA

703-648-6533 (voice)
703-648-6252 (FAX)
Metadata standard:
Content Standard for Digital Geospatial Metadata (FGDC-STD-001-1998)

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