For shallow profiling a Geonics, Inc. EM-31 Terrain Conductivity meter is used. One person can collect up to 10,000 data points per day with this instrument. The EM 31 uses an alternating electromagnetic field, which fills the space below and above ground, surrounding the transmitting coil. When the electromagnetic field couples with a conductor, for example a steel pipe under the ground, AC eddy currents are induced to flow in the pipe. This generates a secondary magnetic field, which is sensed by the co–planar receiver coil. Due to phase lag the computer on board can discriminate between the primary and secondary fields and outputs the measurements of the secondary field (thus, a conductive zone is sensed by the induced secondary magnetic field).