The measurement versus depth or time, or both, of one or more physical quantities in or around a well. The term comes from the word "log" used in the sense of a record or a note. Wireline logs are taken downhole, transmitted through a wireline to surface and recorded there. Measurements-while-drilling (MWD) and logging while drilling (LWD) logs are also taken downhole. They are either transmitted to surface by mud pulses, or else recorded downhole and retrieved later when the instrument is brought to surface. Mud logs that describe samples of drilled cuttings are taken and recorded on surface.