Create Isostatic Regional Grid

Use the Isostatic > Calculate Regional Grid menu option (AIRYROOT GX) to calculate the isostatic regional response, isostatic root depth, or topographic load from a topo grid.

Create Isostatic Regional Grid dialog options

Input root depth/topo grid (meters)

Input the topographic grid or the root depth grid (.grd file) - depending on the setting in the Input mode field (see below). The elevations are in meters.

Input mode

Choose the type of input:

  • "Topo"

  • "Root Depth" - this is the geometry of the root used to calculate the isostatic regional (depths in meters below sea level).

Fill input topo grid

Select between "Yes" and "No" to turn the filling on or off.

Output grid

Specify the output grid. The generated grid will be, depending on the Output mode setting set below, one of these: isostatic regional (in mGal), root depth (in meters below sea level), or load (in gm/cm).

Output mode

Choose the output type:

  • "Isostatic"

  • "Root Depth" - the geometry of the root used to calculate the isostatic regional.

  • "Load" - simply the elevation multiplied by the appropriate Bouguer density.

Depth of sea level compensation (km)

Specify the depth (in km) to the Moho to use for elevations at or below sea level.

Bouguer density on land

Specify the density of the terrain above sea level. (in g/cm3)

Bouguer density at sea

Specify the density used to fill oceans (in g/cm3). Sea water density (1.027 g/cm3) is subtracted from this number for calculations.

Moho density contrast, land

Specify the density contrast across continental Moho. (in g/cm3)

Moho density contrast, sea

Specify the density contrast across oceanic Moho. (in g/cm3)

Application Notes

This GX uses a modified version of the USGS AiryRoot algorithm to calculate the isostatic regional gravity from a topographic grid. It expects the topographic grid to be in meters. It first calculates a depth to the Moho (the "root") from the topographic grid, terrain density, Moho density contrast, and depth of sea level compensation. It then calculates the 3-D gravity response of that root, at sea level, out to 166.7 km. The output must be combined with a solution beyond 166.7 to make a complete Airy regional.

The GX looks at the X, Y length units specified in the projection area of the header of the topo grid to scale the grid dimensions. If the length units are not set correctly, the results from Airyroot will definitely be wrong. You can check and change the length units settings via Grid Info | Projection.

  • The input grid needs to be "filled" (i.e., no "dummies").

    • Airyroot checks the grid header and aborts if the number of dummies is > 0. The input grid must be smaller than 2 million cells (i.e., nx*ny < 2000000).

    References

    • [1] R. W. Simpson, R.C. Jachens, R. J. Blakely, 1983, "AIRYROOT; A Fortran Program for Calculating the Gravitational Attraction of an Airy Isostatic Root Out to 166.7 KM", U.S. Geological Survey, Open-File Report 83-883, 66 p.
    • [2] R. W. Simpson, R. C. Jachens, R. J. Blakely, R. W. Saltus, 1986, "A New Isostatic Residual Gravity Map of the Conterminous United States with a Discussion on the Significance of Isostatic Residual Anomalies", Journal of Geophysical Research, vol. 91, pp. 8348-8372.

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