The lowering of various sensors within an instrumental package (a sonde) down a borehole. The output of these instruments is then measured to produce a series of well logs as the sonde is wound back to the surface. A wide variety of instruments can be combined: caliper, electrical, radioactive, temperature, etc. The term ‘well logging’ is also used to refer to the recording of the nature of drill cuttings encountered during the drilling of the well. See also borehole logging; mud log.