Pertaining to techniques in which the same quantity is measured at different times in the life of a reservoir. Normally the only change in a time-lapse measurement or survey will be due to changes in water or gas saturation. Thus, a comparison of two logs run at different times, such as a year apart, should simply reflect the change in fluid saturations in the pore space. The most common time-lapse logs are made with pulsed neutron capture, pulsed neutron spectroscopy and borehole gravity measurements.