We have seen that, in good conditions, we can draw on the S.P. log the Shale Base Line that corresponds to a volume of 100%clay.It is also possible to draw the Sand Base Line which represents 0%clay. This particular facility provided by the S.P. log enables us to estimate the volume of clay at a given point.

The volume of clay at point X is calculated as follows:

with: Vsh = Volume of clay in [%] S.S.P.= Maximum value of S.P. deflection in the given interval which corresponds to the sand line S.P.[mV] S.P. read at point X = valeur of S.P. deflection at given depth.