In  Hong  Kong,  underground  utility  detectors  are  normally  of  two  types: Electromagnetic detector and Ground Penetrating Radar (GPR).

Electromagnetic  detectors  can  detect  the  signals  emitted  by  metallic  utilities  (Passive mode). While in active mode, the detector has to pick up the signals through a transmitter  connected directly to the non-metallic utility. Both the alignment and depth can be found in active mode while only alignment can be found in passive mode. The electromagnetic detectors available in market can detect utilities up to depth of 3m.

Ground Penetrating Radar sends radio waves into the ground and receives signals from  reflections  from  utilities.  It  has  the  advantage  of  locating  both  the  depth  and alignment  of  utilities.  More  importantly,  it  can  detect  both  metallic  and  non-metallic utilities.  However,  it  suffers  from  the   disadvantage  that  it  is  quite  expensive  and interpretation of data is not simple. Reference is made to LD, DSD (2000).