GPR Survey Explained: Why It Comes Before Every Dig

Ground Penetrating Radar transmits a short electromagnetic pulse into the ground and records what comes back. A reflection occurs wherever the pulse meets a change in dielectric property — the boundary between soil and a pipe, or between soil and air in a void. That is why radar sees plastic pipe an electromagnetic locator cannot, and equally why it can miss a small target in ground offering little contrast. The method detects difference, not objects.

What sets the depth limit

Conductive ground absorbs the pulse rather than reflecting it, which is why saline coastal clay may allow 1.5 metres where dry sand allows four. Antenna frequency then trades resolution against reach: a 1.6 GHz system resolves rebar in a slab but will not reach a trunk main under two metres of fill, while a 250 MHz system reaches depth but will not resolve the rebar. Converting a reflection time into a depth requires the ground's velocity, which varies with moisture and material, so calibration is taken against the actual ground rather than a table value.

Where the method genuinely fails

Radar does not read through competent rock or through reinforced concrete beyond its cover. In heavy clay it may not reach the depth a design needs. Where several services run parallel and close, individual lines may not resolve. A small target below the line spacing is stepped over entirely, which is why the grid matters as much as the equipment. None of this makes radar unreliable; it makes an unqualified drawing unreliable. A survey that states its limitations is more useful than one that implies uniform certainty across ground where none existed.

Beyond the basics

Is GPR the same as a metal detector? No. A metal detector responds to conductive material; radar responds to a change in dielectric property, which is why it sees plastic and voids as well.

Why do two surveyors give different depths? Usually because they calibrated differently. Depth is derived from velocity, and a table value rather than a site calibration produces a plausible but wrong figure.

Can GPR see through rock? Not through competent rock. It maps the rock surface accurately, which is often the more useful output on hard ground.

Does rain affect a survey? Saturation does. Water raises attenuation, so the same line surveyed wet and dry will not return the same achievable depth.

What is the single most important survey decision? Line spacing. A grid set for a 300 mm main steps over a 50 mm service, and no amount of equipment quality compensates for that.

Terms used on this page

Dielectric property — the electrical characteristic that determines how radar travels through a material. Velocity — the speed of the radar pulse in that ground, needed to convert reflection time into depth. Attenuation — the loss of signal strength with depth, higher in conductive and saturated ground. Resolution — the smallest separation at which two targets can be distinguished. Line spacing — the distance between survey passes, which sets the smallest target that can be found.