Estimating depth to Moho and Isostatic correction (T55)
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1. 005 Max_Iterations 6 MaxBesselSize 150000 Default Increases Bessel array size Parameters End Process End Library Help Top 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Estimating depth to Moho and Isostatic correction T55 6 Library Help Top 4 Back gt Frequently asked questions Library Help Top Q What does Dea Level Root depth mean This is the depth in metres of the Airy mass balance cylinder This is normally 32000m Sea_Level_Root_Depth 32 We have tried 30 km and found 32 km is better for Australia It is possible to try various column depths and densities quite quickly as the output is generated promptly and the reasonableness of the result is very sensitive to the parameters The aim is to get an output where there is no regional Q What are typical density values that I can use Here are some common density values Medium Density in kg m Sea water 1027 Crust density 2670 Mantle density 3300 2012 Intrepid Geophysics lt 4 Back gt2. INTREPID User Manual Estimating depth to Moho and Isostatic correction T55 1 Library Help Top lt 4 Back gt Estimating depth to Moho and Isostatic correction T55 Introduction The Intrepid Isostatic tool is a rewrite of the USGS fortran program airyroot Airyroot calculates an airy isostatic regional at sea level out to 166 7 km At present the Isostatic tool runs in a batch process only Usage isostatic exe batch jobfile The outputs from the program are 1 Depth to Moho Airyroot grid in kms 2 Isostatic gravity correction grid in mGals Then it is a simple matter of subtracting the correction grid from the Bouguer gravity grid to create an Isostatically corrected Bouguer gravity grid Earth curvature corrections are not done beyond 166 7 km see discussion below Note that you must work in projected coordinates Geodetic is not supported Sample task file example of Parker style isostatic correction job uses FFTW Process Begin Name isostatic Parameters Begin InputGridName DEM_Onshore_Offshore_Airy ers DepthMohoGridName depth_moho ers FilteredGridName isostatic_correction ers FftGridName iso_fft ers WindowedGridName iso_wind ers ExpandedGridName iso_exp ers AmplitudeGridName iso_amp ers CoefficientGridName iso_coeff ers Band 0 OutputPrecision IEEE4ByteReal DetrendDegree O must be DC RolloffType COSINE WindowType HANNING FillType ARTHUR R
3. ORY variable as discussed above If you have problems with memory you can use the FORCE_DISK option to force all memory usage to disk This will allow INTREPID to manage much larger input grids without over using physical memory However processing will be much slower using this method INTREPID creates large temporary files during program execution It removes most of these when the program finishes Two of the files remain after processing e Forward FFT F tGridName Expanded grid name ExpandedGridName We have left these in place for chacking purposes while the program is still in beta Further issues Please note the following warnings Avoid using high resolution grids for input The wavelengths in the isostatic are long so 1km cell grids are sufficient for computing the correction unless topography contains extreme gradients Do not use any detrending during the FFT processing as this will impact long wavelengths in the isostatic Use of the FFT based algorithm causes some errors on the boundaries of the isostatic gravity correction grid due to the wrapping procedure employed If you are concerend about this then we recommend that you make the input DEM or bathymetry grid approximately 30 larger than the area for which you are calculating the isostatic To remove any edge effects it is possible to use the Source_Grid option in the FFT options and set a subarea for the final result so that it is free of any edge effe
4. cts Library Help Top 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Library Help Top Estimating depth to Moho and Isostatic correction T55 5 q Back gt For an example of this see Job file example below example of Parker style isostatic correction job file using subset to limit problems with edge effects uses FFTW Process Begin Name isostatic Parameters Begin Subset Begin XLower 141500 0 XUpper 847500 0 YLower 5219000 0 YUpper 6338000 0 FFTborder 100000 0 SubsetGridName DEM_subset100k Subset End InputGridName DEM_Onshore_Offshore_Airy ers DepthMohoGridName depth_moho_subset100k ers FilteredGridName isostatic_correction_subset100k ers FftGridName fft_subset100k ers WindowedGridName wind_subseti00k ers ExpandedGridName exp_subset100k ers AmplitudeGridName amp_subset100k ers CoefficientGridName coeff_subset100k ers Band 0 OutputPrecision IEEE4ByteReal DetrendDegree 0 must be DC RolloffType Cosine WindowType None FillType SOURCE_GRID ReApplyTrendAfterReverseFft 1 ApplyMaskAfterReverseFft 1 UseSymmetry Yes DiskUsageRule AUTO FORCE_MEMORY FORCE_DISK now for the specific isostatic parms SupplyMohoGrid No Moho grid is calculated from DEM SubtractFlatEarthRoot Yes Subtract flat earth gt 166 7kms Sea_Level_Root_Depth 32 Crust_Density 2 670 Density _Constrast_Depth 0 6 Max_Tolerance
5. eApplyTrendAfterReverseFft 0 ApplyMaskAfterReverseFft 1 UseSymmetry Yes DiskUsageRule AUTO FORCE_MEMORY FORCE_DISK now for the specific isostatic parms SupplyMohoGrid No Moho grid calc from DEM SubtractFlatEarthRoot Yes Subtract flatearth gt 166 7 Sea_Level_Root_Depth 32 Crust_Density 2 670 Density _Constrast_Depth 0 6 Max_Tolerance 005 Max_Iterations 6 MaxBesselSize 150000 Default Increases Bessel array size Parameters End Process End Library Help Top 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Estimating depth to Moho and Isostatic correction T55 2 Library Help Top Input files 4 Back gt Input is a standard INTREPID or ERMapper format grid of topography At present INTREPID does not support the surface load grid option allowed in Airyroot Contact Intrepid Support if you would like this option to be added to the Isostatic tool see discussion below regarding this option The topographic grid must be in a projected coordinate system INTREPID does not support geodetic projections and coordinates The topography grid must contain elevation values in metres at grid points Ocean depths must have 50 000m subtracted from them as a flag to distinguish negative on land elevations for example Death Valley If a surface load grid then values at grid points are equal to the mass excess or deficiency if negative resulting from the topographic features in unit
6. grid containing the airy root depth in km INTREPID uses this grid to derive the second output This is the gravitational attraction in milligals at sea level of an Airy root system out to a distance of 166 7 km from each observation point The output grid must be combined with a solution beyond 166 7 km to make a complete Airy regional The break occurs at 166 7 km because Terrain corrections for Bouguer gravity generally stop at this distance The earth s curvature which is not accounted for in this program starts to become important at about this distance 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Estimating depth to Moho and Isostatic correction T55 4 Library Help Top lt 4 Back gt Memory management Depending on the memory or swap space available on your computer you may need to adjust some INTREPID parameters such as INTREPID_MEMORY which defaults to 512 Mb Reducing the size of this variable will result in use of less physical memory and force the program to page to disk For instrucitions on chanign this parameter see Ways of setting INTREPID system parameters and environment variables in INTREPID system parameters and install cfg R07 Alternatively you can use the DiskUsageRule parameter in the job file The options are DiskUsageRule AUTO or FORCE_DISK or FORCE_MEMORY The default is AUTO and in this case the program manages memory usage in conjunction with the INTREPID_MEM
7. k gt INTREPID User Manual Estimating depth to Moho and Isostatic correction T55 3 Library Help Top 4 Back gt FillType ARTHUR ReApplyTrendAfterReverseFft No ApplyMaskAfterReverseFft Yes UseSymmetry Yes DiskUsageRule AUTO FORCE_MEMORY FORCE_DISK now for the specific isostatic parms SupplyMohoGrid Yes Supply a moho grid SubtractFlatEarthRoot Yes Subtract flat earth gt 166 7kms Sea_Level_Root_Depth 32 Crust_Density 2 670 Density_Constrast_Depth 0 6 Max_Tolerance 005 Max_Iterations 6 MaxBesselSize 150000 Default Increases Bessel array size Parameters End Process End Earth curvature corrections Output files Library Help Top Gravitational attraction of Airy root on a flat earth is calculated using Parker s fast Fourier transform algorithm Attraction of a flat earth root beyond 166 7 km is calculated using an approx to an integral containing a Bessel function This latter is subtracted subroutine subtract_far to leave attraction of root out to 166 7 km Beyond that approximate distance the curvature of the earth becomes important The calculation and subtraction of the flat earth root correction is provided as an option in the batch file SubtractFlatEarthRoot 1 or yes turns the calculation on Believers in the flat earth hypothesis can turn this calculation off INTREPID writes outputs as standard INTREPID or ERMapper format grids The first output is the Moho depth
8. s of g cm Note that ocean depths give a negative load because the water ought to be granite in this scheme A load grid is necessary as input if lakes glaciers and variable densities for topographic features are to be properly handled Using a depth to Moho grid from another source Library Help Top You can use a depth to Moho grid from an alternative source In the job file set the parameter SupplyMohoGrid Yes or 1 The parameter DepthMohoGridName is now the input grid for the isostatic correction and INTREPID does not calculate the Moho grid from the input DEM If you are using a DEM to calculate the Moho the grid must be depth to Moho in km as calculated The grid can be a different size or have a different cell dimension to your DEM which is not used in this case Process Begin Name isostatic Parameters Begin InputGridName DEM_Onshore_Offshore Airy 500m ers DepthMohoGridName depth_moho_500m_user ers Subset Begin XLower Null XUpper Null YLower Null YUpper Null FFTborder 5000 0 Subset End FilteredGridName isostatic_correction_MyMoho ers FftGridName isostatic_fft ers WindowedGridName isostatic_wind ers ExpandedGridName isostatic_exp ers AmplitudeGridName isostatic_amp ers CoefficientGridName isostatic_coeff ers Band 0 OutputPrecision IEEE4ByteReal DetrendDegree 0 must be DC RolloffType COSINE WindowType NONE 2012 Intrepid Geophysics 4 Bac
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