EPT. The EPT is a device that measures the propagation time (TP1) and attenuation rate (A1) of a microwave frequency electromagnetic wave that is propagated through the formation near the borehole. These two measurements can be related to the (composite) dielectric constant of the formation close to the borehole. The EPT is a shallow investigation device that has a depth of investigation of 1 to 4 in., depending on the formation conductivity. As a result, the EPT responds primarily to the flushed or invaded zone of the formation. The utility of the EPT arises from two basic facts. First, the dielectric constant of earth formations is dominated by the amount of water contained in the rock pores. That results from the fact that the dielectric constant of water is an order of magnitude greater than that of the other constituents of reservoir rocks; namely, oil, gas, and the rock matrix. Second, at microwave frequencies, the dielectric constant of water saturated rocks is relatively independent of water salinity, except in ranges corresponding to very high salt concentrations. Those two facts imply that the dielectric constant inferred from the EPT measurements is effectively a salinity independent datum capable of distinguishing between water and oil in the zone of investigation. Also, these measurements can be used to derive values for formation porosity and water saturation that are essentially salinity independent. EPT is a mark of Schlumberger.