In this category of methods, are various means of following the path of a known utility pipe or cable in between known location points where there are connections to the surface. Some of these methods involve impressing a signal of some kind onto the utility in question at the surface connection point and then using an instrument traversing the ground surface to pick up the strongest signal being emitted from the buried utility. This is normally directly over the buried utility. This approach can be used, for example, for directly tracing electrical cables from their 50Hz or 60Hz electromagnetic field or by impressing an electrical current onto a conductive pipe and again measuring the resulting electromagnetic field. For non-conducting pipe materials carrying a fluid, e.g. plastic water pipes, pressure waves can be introduced into the pipe and the acoustic signal emitted from the pipe monitored at the ground surface to trace the pipe alignment. For non-metal pipe systems, it is also possible to insert a sonde device into the pipe. As the sonde moves through the pipe, its electromagnetic signal can be traced at the ground surface and its horizontal alignment and depth indicated. Instead of using a sonde and a surface tracing technique, it is also possible to trace the alignment of an underground pipe using accelerometer-based or gyroscopic-based motion tracking as the carrier device passes through the pipe. For larger pipes or tunnels, such tracking can be combined with laser-point-cloud imaging to build up an accurate position and cross-sectional record of an underground structure’s geometry. A separate type of approach to trace the location of defects in a system or to establish the nature of interconnections in a system is to use smoke or dye testing in a pipe network. In smoke testing, a portion of an empty pipe network is pressurized with air and smoke. As the smoke moves through the system, points of leakage will allow the smoke to potentially become visible at the ground surface. In dye testing, a traceable or visible dye is introduced into a pipe system and its presence monitored for at other points in the system. This will help establish the nature of the connectivity within the buried pipe system.