A first method consists in comparing porosity curves, to superpose them from where the name of Overlay or Quick look. These methods are working only when the logs are recorded with compatibles scales .To be able to compare two logs of porosity, like the density and the neutron for example, it is necessary that the scales are suitably selected. If the neutron is limestone calibrated for example, with 3 units of porosity (in %) per division, and the density with 0,5 g/cc per division, the scales are known as compatible and if we bring 0 % of porosity of the neutron on the value of the limestone matrix density(2,71 g/cc) both logs must be superposed on the noise close in, so far as we have a limestone formation with water.

Synthetic example of quick-look
with diferent lithologies (with 10% porosity).