Re: Gravity effect of a layer

Thanks for the pointers Joaquim. I was just looking at the documentation that was installed with 5.1.0 and -Q was not listed (spotted the entry on the command line for grdgravmag3d!)
 
Good to know the values are about right, at least for a sloping layer model. Doing a much coarser model run would be more efficient.
 
Cheers
Lester
 
Date: Thu, 13 Mar 2014 11:50:43 +0000
From: [email protected]
Subject: Re: [GMT-HELP] Gravity effect of a layer
To: [email protected]


  
    
  
  
    

    
      I tried a small region and the program ran; clearly
        it was working on the big grid but very very slowly!

      
    
    

    Yes, it is slow but one often do not need to compute with a very
    high resolution. Bhattacharyya prisms would be a bit faster but
    probably not that much

    

    
      The output is very much affected by boundary
        effects. There is no way to set a larger dimension to work in
        other than cut a larger grid area to process and set a region of
        interest (-R).  With grdfft etc we can set a -N option to force
        a larger dimension, but not here.

      
    
    

    Yes, there is. See option -Q (documented) and the, sorry my fault,
    the -R option that is only mentioned on the on line help

    

    
       Given that this is a small region, the gravity
        signal computed seems rather large? The thickness range is
        3600-10400m.

      
    
    

    They looks perfectly reasonable values. A max of ~700 mGal compare
    very well with the Bouger plate that gives (with you density)

    

     2*pi*2000*6.67e-11*10000*1e5 = 838 mGal

    

    
       

        Attached a quick image of the original data and output.

         

        Cheers

        Lester

         

        
          Date: Wed, 12 Mar 2014 07:45:33 -1000

          From: [email protected]

          Subject: Re: [GMT-HELP] Gravity effect of a layer

          To: [email protected]

          

          Is there a -Vl option to see progress? If not then please
            add.

            

            Sent from my iPad
          

            On Mar 12, 2014, at 6:29 AM, "J. Luis" <[email protected]>
            wrote:

            

          
          
             Lester,

              

              grdgravmag3d is a slow program (it computes the effect of
              all the triangles (two per cell) at all the nodes of the
              destination grid), and your grid isn't exactly small (in
              terms of grdgravmag3d reference frame)

              Are you sure it hanged? The processor was working at 100%?

              Can you cut a smaller piece of the grid and try again? If
              it still hangs open an issue in the gmtrac page and append
              the file so that we can debug it.

              

              Joaquim

              

              
                
                Hello,

                   

                  I have a grid of sediment thickness data and would
                  like to correct the gravity data for this.

                   

                  I have looked at grdgravmag3d and this should be fine,
                  but when I tried this route it gets stuck:

                   

                  grdgravmag3d -C2000 -GSeds_thick_clipped_grav.grd -V
                  -fg Sediment_thickness_clipped.grd -I1m -Z

                  grdgravmag3d: Grid dimensions are nx = 2581, ny = 1441

                  grdgravmag3d: Allocates memory and read data file

                  Line = 0         of = 1441  <----- hangs here!

                   

                  What I am looking to do is a sediment corrected
                  Bouguer.

                   

                  Any pointers would be useful.

                   

                  Thanks

                  Lester

                   

                   

                
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