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The location of a downhole downhole gauge may not match the formation top.

In order to translate it to the Datum one needs:

1. Estimate gauge readings at formation top 

   LaTeX Math Inline
   body --uriencoded--p_%7B\rm top%7D = p(z_%7B\rm top%7D)

   using the wellbore fluid density density 

   LaTeX Math Inline
   body \rho_f

   gradient: 

   LaTeX Math Inline
   body --uriencoded--\displaystyle p_%7B\rm top%7D(t) = p_%7Bz_%7B\rm gauge%7D%7D(t) + g \cdot \int_%7Bz_%7B\rm gauge%7D%7D%5e%7Bz_%7B\rm top%7D%7D \rho_f(z) dz

   
   
   
2. Recalculate the pressure at formation top to the Datum using regional hydrostatic pressure gradient 

   LaTeX Math Inline
   body GP

   : 

   LaTeX Math Inline
   body --uriencoded--\displaystyle p_%7B\rm dat%7D(t) = p_%7B\rm top%7D(t) + GP \cdot (z_%7B\rm dat%7D - z _%7B\rm top%7D)

where 

When wellbore fluid density is fairly condant between the gauyge location and formation top then ine can simplify the datum pressure calculation to:

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