Application of drillhole vector magnetic measurements to resolve the position of existing underground structures

Total field magnetic surveys are commonly utilized for mapping geological structure and the detection of underground ferrous metal objects such as ordnance, pipes and services. The method is directionally insensitive, especially in locally strong magnetic fields, introducing an unquantified component of error to detailed dimensional analysis of the source. For a number of years the potential advantages of employing three component vector downhole magnetic measurements to calculate the full vector orientation of the magnetic field, and hence define the position of a buried magnetic ore body in three dimensions, has been recognized by the mining exploration industry. This paper presents the application of this vector magnetic downhole technique to the 3D mapping of ferrous metal in underground structures, such as reinforcing bar, pipes, foundations and also soil nails. The technique is discussed and a brief case history relating to the proposed alignment of a small bore tunnel is given.