Zond 12e (GPR)
Utility Scan 400 (GSSI)
Utility Scan 270 (GSSI)
Radar MALA Easy Locator (Gazomat)
2150 GR (Ditch Witch)
GPR sends into the ground an electromagnetic pulse. The frequency range applicable to image structures the size of utilities is typically from about 50MHZ to 500 MHZ. Some ratio of this pulse signal is transmitted through "boundaries" and some is reflected from the boundary back to the receiving antenna. The boundaries are a function of the dielectric properties of adjacent materials. Overall propagation into the ground is a factor of frequency, power, and soil resistivity. Detection occurs when the boundary of the utility is different than the surrounding soil. A vastly different boundary would produce the best reflection (i.e., metallic utility buried in dry sand). The smaller the utility, the higher the frequency is needed to image it. This is a direct effect of the wavelength to pipe size ratio. However, the higher the frequency, the quicker the signal will dissipate as a function of depth, and the lower the depth of penetration. A general rule of thumb for current technology in practice is a depth to diameter ratio of 12 to 1 under ideal conditions (i.e. a 1" utility at a 1' foot depth). The more conductive is the surface (steel reinforcement, de-icing salts, saline water, and soil with high organic or iron contents) the less effective are GPR systems. Ideal conditions for GPR include dry sand while least optimal conditions are represented by marine clays and similar highly conductive soils. Other challenges associated with GPR systems include referencing the location of the machine and complex and time consuming data interpretation. Advantages presented by GPR include: a) the ability to locate non-metallic utilities; b) even if the utility itself cannot be imaged, often the trench can be detected, suggesting the presence of a utility; c) relatively accurate depth determination. Current advanced in GPR technology include GPS integration, laser-based referencing, and multiple antenna arrays and stepped frequency capabilities.