Modulated Active Magnetic Surveys For Sub-Surface Utility Mapping

A quasi-static magnetic method has been developed to indicate the exact location of sub-surface<br>utilities. Standard radio frequency EM transmitter methods are complicated by distortions<br>associated with induction in secondary targets including underground services and surface<br>metallic artefacts. Similarly, standard high definition magnetic methods are subject to high<br>ambient noise levels and ambiguities associated with superposition of responses from multiple<br>sources. The modulated active magnetic system (MODAM) has now been developed to avoid<br>these complications. Static magnetic fields are readily removed from the total field response by<br>comparing on-time and off-time magnitudes, allowing spatial vectors (defining the transmitter<br>location) to be resolved with great precision. Results obtained from MODAM field trials have<br>been confnmed with electronic total station surveys, giving an agreement better than 0.05 m at<br>12 m depth.