A density, sonic, or resistivity log run over the entire wellbore. Records weak electrical currents that flow in the rock next to the wellbore and shows the thickness and boundaries of the rock layers.  These logs help determine the amount of salt water present and the permeability.

Logs are a record of certain electrical characteristics (such as resistivity and conductivity) of formations traversed by the borehole. It is made to identify the formations, determine the nature and amount of fluids they contain, and estimate their depth. If the pores are partially filled with gas or oil, which are resistant to the passage of electrical current, the bulk formation resistance is higher than for water filled pores. For the sake of a convenient comparison from measurement to measurement, the electrical logging tools measure the resistance of a cubic meter of formation. This measurement is called resistivity.

Induction log
An electric well log in which the conductivity of the formation rather than the resistivity is measured. Because oil-bearing formations are less conductive of electricity than water-bearing formations, an induction survey, when compared with resistivity readings, can aid in determination of oil and water zones.