1. Field observations which begin and end at the same point with a number of intervening observations. Obtaining data in loops (called looping) is useful in correcting for drift in gravity-meter observations or diurnal variation in magnetometer surveys, and in detecting faults or other cause of misclosure in seismic work. 2. An electrical circuit which provides feedback, as an AGC loop. 3. A part of a computer program in which the last instruction is to repeat the preceding series of operations (with or without modification) until some particular condition is reached. 4. A transmitting or receiving coil used in electromagnetic surveying.