Time-series analysis can be used to investigate log repeatability and to quantitatively determine the vertical resolution of a log. This technique involves comparing repeat runs of logs to differentiate between coherent signal and incoherent noise. No detailed knowledge of the sonde characteristics is required. The technique accounts for the degradation of the logging data due to noise (e.g. counting statistics) and sonde accelerations and decelerations, and, hence, results are representative of the vertical resolution achieved under realistic logging conditions.