Manhole detection and location for urban pavement

An underground practical sensing system was developed to detect covered-over manholes along the sewer lines under the urban pavement. The sensor head of the system is composed of a sonde and some transmitting and receiving circuits. Coil-type antennas has been designed and implemented base on the induction principle and numerical simulation. The transmitting circuits generate excitation current for the transmitting antennas, and the signal received by the receiving antennas is processed with the receiving circuits. A windows-based application software can calculate phase shift as well as amplitude value of the receiving signal. And there is also a distance-measurement device to provide information about the location of the covered-over or missing manholes. Satisfactory results have been achieved from the field tests of the system in actual manholes. For loop antennas with a spacing of 0.5 m, the system has a phase resolution of about 1/spl deg/ and an amplitude resolution of about 0.5%.