GROUND PENETRATING RADAR - GPR,
COMPUTER SYSTEMS FOR GPR,
COMPUTER DATA ANALYSES FROM GPR
METAL DETECTORS,
LONG RANGE LOCATORS,
DOWSING OF MAPS BY PROFESSIONAL MAP DOWSERS,
COMPUTER SYSTEMS,
GEOLOGICAL EQUIPMENT
COMPUTER GENERATED MAPS AND DIGITAL IMAGES FROM GPS

   Ground penetrating radar (GPR, sometimes called ground probing radar, georadar, subsurface radar, earth sounding radar or "radar terrestre penetrant") is a noninvasive electromagnetic geophysical technique for subsurface exploration, characterization and monitoring (history).  It is widely used in locating lost utilities, environmental site characterization and monitoring, agriculture, archaeological and forensic investigation, unexploded ordnance and land mine detection, groundwater, pavement and infrastructure characterization, mining, ice sounding, permafrost, void, cave and tunnel detection, sinkholes, subsidence, karst, and a host of other applications.  It may be deployed from the surface by hand or vehicle, in boreholes, between boreholes, from aircraft and from satellites.  It has the highest resolution of any geophysical method for imaging the subsurface, with centimeter scale resolution sometimes possible.
    Resolution is controlled by wavelength of the propagating electromagnetic wave in the ground.   Resolution increases with increasing frequency (shorter wavelength).  Depth of investigation varies from less than one meter in mineralogical clay soils like montmorillonite to more than 5,400 meters in polar ice.  Depth of investigation increases with decreasing frequency but with decreasing resolution.  Typical depths of investigation in fresh-water saturated, clay-free sands are about 30 meters.  Depths of investigation (and resolution) are controlled by electrical properties through conduction losses, dielectric relaxation in water, electrochemical reactions at the mineralogical clay-water interface, scattering losses, and (rarely) magnetic relaxation losses in iron bearing minerals.  Scattering losses are the result of spatial scales of heterogeneity approaching the size of the wavelength in the ground (like the difference between an ice cube and a snowball in scattering visible light).  Detectability of objects in the ground depends upon their size, shape, and orientation relative to the antenna,  contrast with the host medium, as well as radiofrequency noise and interferences.  This is representative but greatly oversimplified: see tutorial.