# Isostatic regional/residual calculation for residual-regional separation

Hello everyone
please, I need your help in achieving isostatic regional computation from the topography grid. In my approach, I always obtain an isostatic regional grid which is similar to the topography grid. Surely, I must have made a mistake somewhere.
you can check the different parameters I used from the image attached below.

• edited July 2019
@QuentinMarcANABAFOTZ based on the picture you sent, I think that you are using the wrong Output mode in the Calculate Isostatic Regional Grid dialog; you are using "RootDepth" when you should be using "Isostatic". The "Isostatic" output will generate an isostatic regional grid, whereas the "RootDepth" will generate a grid that defines the depth (in metres below sea level) of the root used to calculate the isostatic regional, which is in effect the depth to the Moho.

The RootDepth output will resemble the topography you input, because according to Airy isostasy (in which a constant-density crust floats on a higher-density mantle) topography is determined by the thickness of the crust. As a result, topographic highs will correlate with thicker crust (and a deeper RootDepth), so "highs" in the topography grid will correspond with "highs" (=greater depths) in the RootDepth grid. If you compare the actual values of the Root Depth and Topography grids however you should see that they are quite different.

The isostatic regional should resemble the inverse of the topography, and should also somewhat resemble the regional wavelengths in your Bouguer corrected gravity data.
Darren Andrews
Technical Team Manager

• Thank you very much @DarrenAndrews2
• Sorry for my intrusion, I have a question related to yours.
I am trying to estimate the depth of the MOHO below my study area with this method, to use it as a parameter for a 3D geological model, because there is no data on the depth of the MOHO below my area.

First of all, do you consider this approach realistic?

The continental area is bellow sea water. So, is it ok to put 2,67 gr/cm3 as Bouguer density on land and Bouguer density on sea?? And for the depth of sea level compensation, some authors says that the crust is thin, about 26km. Should I put this (26km) as level compensation? or 30km as default?

Thank you very much for your help.