Mapping the Underworld – State-of-the-art review

A major UK initiative, entitled Mapping the Underworld (MTU), is seeking to address the serious social, environmental and economic consequences arising from an inability to locate accurately and completely the buried utility service infrastructure without resorting to excavations. One of the four MTU projects aims to develop and prove the efficacy of a multi-sensor device for accurate remote buried utility service detection, location and, where possible, utility identification. This paper aims to introduce the MTU programme followed by a state-of-the-art review of the three essential technologies that are to be combined in the device – ground penetrating radar (GPR), low-frequency quasi-static electromagnetic fields and acoustics – and a summary of the influence of different soil types and states on the transmission of the various signals, and therefore how the techniques might be optimised from a knowledge of the ground instead of using very broad simplifying assumptions. The latest developments in impulse GPR, frequency modulated continuous waveform (FMCW) GPR and stepped frequency continuous waveform (SFCW) GPR are described and previous attempts to combine GPR with other sensing technologies are introduced. The work on quasi-static fields explores the ‘fields-of-opportunity’ related to the 50 Hz currents flowing in existing underground power circuits and the electric field variations when low-frequency current in actively induced into the ground. Acoustic techniques have been primarily used for leak detection and the review focuses on the potential for their application to buried utility service location. The paper concludes with a discussion of the facilities required, and currently available, for comprehensive assessment and independent verification of the performance of both existing devices/technologies and of the multi-sensor device under development.