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Directory Identifier: GRAVCD-id_ter
Directory Title: Idaho Batholith Study Area Density Grid
Principal Investigator: Numerous

Contributing Organization: U.S. Geological Survey
EROS Data Center User Services
Sioux Falls, SD 57198

Summary: A 2 kilometer terrace-density grid for the Idaho batholith study area. Number of columns is 331 and number of rows is 285. The order of the data is from the lower left to the right and then up one row.

TERRACE-DENSITY GRID

A terrace-density grid is the result of a terracing operation (Cordell and McCafferty, 1989; Phillips, 1990) that was performed on the gridded isostatic residual gravity data. The terrace method transforms the smoothly varying gravity field into a stepped field of uniform domains separated by discontinuous boundaries. The result is a physical-property (density) map that mimics a geologic map by theoretically outlining geologic structures and terraces associated with gravity anomalies. The terracing procedure emphasizes measurable density contrasts caused by lithologic contacts, abrupt facies changes, or structural breaks and mathematically minimizes minor changes internal to each uniform domain in order to emphasize regional patterns in the gravity field.

The density values in the density-terrace grid are not calibrated because they are not corrected for depth to sources or thickness of sources causing density contrasts. Calibration can be done by statistical correlation of the terrace map with density data from measured rock samples or by using density models calculated from topographic data. Earlier studies (McCafferty and others, 1989) represented the density model used in calibration by a slab with constant thickness and flat top. Such density models cannot be applied to this study area, where depth and thickness of gravity sources are widely varying.

CAVEAT: Caution is recommended when using these data. This data increment was provided to NGDC with minimal documentation. NGDC will continue to seek improved documentation as part of a routine and on-going data rescue effort.

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