1. n. [Formation Evaluation]
The effect on a propagation resistivity or induction log of charge buildup at the boundary between two formation layers with different dielectric properties. In a vertical well with horizontal layers, the current loops generated by the tool in the formation are parallel to the layers and do not cross bed boundaries. However, with an apparent dip between borehole and formation, the loops cross the bed boundaries and generate a charge buildup at the boundaries. The charge buildup acts like a secondary transmitter that increases the measured resistivity. The result is a spike to high resistivity as the tool crosses the bed boundary. In deviated or horizontal wells, polarization horns on measurements-while-drilling propagation logs often are used to detect a bed boundary. The spike increases with apparent dip and resistivity contrast between beds. The magnitude of polarization spikes varies with tool type and spacing, being larger for the propagation tools.