: Principles
Ground Penetrating Radar (GPR) is used in archaeological surveys to produce detailed images of the sub-surface. The degree of success obtained with GPR depends on a number of factors, including the type of equipment used, the nature of the survey target, the soil type and the experience of the operator.
The Discovery Programme does not have the equipment to undertake this type of geophysical survey but it has been applied by geophysical specialists on selected projects including the Tara archaeological survey.
The Discovery Programme does not have the equipment to undertake this type of geophysical survey but it has been applied by geophysical specialists on selected projects including the Tara archaeological survey.
Basic principles
The GPR method works by transmitting a pulse of electromagnetic energy into the earth from a transducer located on the ground surface. Part of this energy pulse is reflected back to the surface from different subsurface features, such as large stones, metal objects and soil layers. These reflections arise from the contrast between the electrical properties, such as electrical conductivity and the dielectric constant, of the different soil layers and storied objects. The variation of these electrical properties depends on factors such as the composition of the soils, their moisture content and their depositional history.
If the time it takes for the electromagnetic pulse to travel to the subsurface feature of interest (the target) and back to the surface again is measured, and if the velocity at which the pulse moves through the ground is known, it is then possible to calculate the depth of burial of the target as follows:
d = t x v,
where d = depth of- burial (metres) , t = travel time of the pulse to and from the target (seconds), and v = velocity of the pulse through the ground (metres/second).
If the target is located at too great a depth beneath the surface only a very weak reflected signal is received at the surface because the transmitted pulse loses energy as the distance travelled increases. This is known as attenuation, and the rate of attenuation depends on both the properties of the material through which the pulse is travelling and the nature of the pulse itself. Clay-rich and/or wet soils have a high rate of attenuation, while clay-poor and/or dry soils have a lower rate of attenuation. Clay-rich and/or wet soils are often good conductors of electrical current and are referred to as 'conductive', while clay-poor and/or dry soils often resist the passage of current and are referred to as 'resistive'.
An appendix in the 1997 Discovery Programme publication, Tara: An archaeological survey describes in detail the principles and application of this technique. This was written by Peter O’Connor (now Apex Geoservices) and is now available as a pdf download, with two jpeg enlargements of the profile results.