Sagar Rawal Neither the sonic, nor the density read directly the porosity of the formation.

The sonic or, more exactly, the P-modulus (product of density and squared P-velocity) reacts to the changes in bulk and shear moduli of the formation; of course, some changes are the consequences of variations in porosity and fluid contents; other factors account like consolidation status, pore connectivity. The relationship of sonic and porosity is not linear. A simple approach is to use the BGK poro-elastic model to model sonic (and shear sonic) in clean rocks.

The density measures the electron density of the formation. Electron density and bulk density are related:

- rhoe = rhob.(2 Z/A) ; rhoe: electron density, rhob bulk density, Z atomic number, A atomic weight; for a molecular substance, replace Z by the sum of all atomic numbers and replace A by the molecular weight.

The density tool is calibrated in freshwater-filled limestone to obtain an apparent bulk density rhoa; the calibration process is to ensure that calibrated water density is equal to the true water density (1.0 g/c3):

- rhoa = 1.0704 rhoe - 0.1883

Apparent density and true bulk density are very similar for most minerals/materials found in reservoirs (calcite, quartz, dolomite). They differ for some like halite, sylvite, gypsum, coals.