1. A difference in values between two measurements, especially well-log-based measurements. 2. The difference in resistivity readings from two logging tools which have different depths of investigation (see Figures I-4 and M-10). Low-resistivity mud cake (which becomes thicker where formations are more permeable) causes the apparent resistivity of a shallow measurement to be lower than that of a deeper measurement, a situation called positive separation. See microlog and movable-oil plot. 3. Apparent displacement on a fault. See Figure F-2. 4. Displacement between source and receiver.