Ground Probing Radar

Why use GPR? A ground probing radar (GPR) survey is often the only geophysical technique that can be used where cluttered urban conditions prevent other techniques from being used. Its capacity to work through a wide variety of surface materials from soft landscaping through to hard surfaces such as tarmac, concrete both inside and outside buildings makes it an ideal technique for many archaeological applications. Typical locations can range from gardens, courtyards, car parks and derelict land through to floors of churches, basements and graveyards. Structures can also be investigated normally using higher frequency antennas.

A GPR survey being carried out over the floor of the Orangery at Hampton Court Palace using a 300MHz antenna

How it works

The radar system works by transmitting a pulse of radiowaves at a set central frequency into the material being examined. Frequently the transmitter and the receiver are combined for convenience into one antenna. The receiver is set up to scan for returns from the transmitter, normally 40 scans per metre. These scans produce a scrolling image on a video display as the antenna is moved across the search area.

The success of GPR relies on variations in the velocity of tranmission of radio waves in different sub-surface materials. These variations are determined by the dielectric constant of each material. The greater the velocity change at each sub-surface interface, the greater the amplitude of the response.

Reconnaissance or Detail

If the area of interest is a fairly large site with little information available it may be more cost effective to undertake a Reconnaissance survey with survey lines every 5m or 10m. From this, areas of potential interest can be targeted for intensive orthogonal follow-up surveys.

If however only a small area is to be inspected say 25m x 25m it would be possible to survey this intensively using a 0.5m parallel grid. This could be carried out within one day's fieldwork which is normally the minimum charge for a survey team. The relative close centres for the transects allows interpretation of the data to be made with confidence so providing a real understanding of the sub-surface. A further advantage of using 0.5m centres is that time slice plots can be generated.

Sites that are known or are suspected to contain small, extremely complex features such as church floors and spires, complex archaeological sites or environmental applications such as badger set mapping may require an even more detailed survey. Typically for sites like these we would deploy the radar over an orthogonal 0.5m or 0.25 grid.

Air voids Vaults Tombs Tunnels Chambers Caves
	An example radar section over a brick lined tunnel
Structures Foundations Floor examinations Rubble fill Garden Paths
	The remains of a buried brick wall at a depth of 0.8m
Ditches Enclosure ditches Moats Trenches Robber Trenches
	A medievel ditch
Discrete Features Large metal objects Hearths Fire Pits Garden features
	A buried garden wall at a moated manor house
Stratigraphy Prominent horizons Water table Stratigraphic layers Garden horizons
	A section displaying 3 stratigraphic horizons