Geophysical Survey: How Does It Work?

Over the last few weeks our students (see fig.1) have been undertaking geophysical surveys of the castle grounds under field technician Dr. Philip Rowe using Ground Penetrating Radar. GPR is a non-invasive technique which was selected to gain access to the areas of Berkeley Castle which cannot be excavated.

Fig. 1Students using GPR equipment.

High frequency radio waves are used by the GPR to detect subsurface features such as walls, buildings and ditches. The GPR system consists of an antenna with wheels, an interface display, a battery and a radar control unit (see fig. 2). The GPR detects buried features by emitting electromagnetic waves from the antenna. The transmitted waves bounce off buried objects, causing them to be reflected back to the GPR (see fig. 3), where the control unit transmits the data from the antenna to the user interface display where it can be viewed. It is commonly thought that GPR allows you to see through the ground like an X-ray. This is a common misconception – in reality GPR data can be difficult to interpret and only suggests the location of potential archaeological structures, where electromagnetic waves are being reflected off a buried feature. An example of the data received through GPR surveys can be seen in fig. 4 – potential structures are seen in the data as curves (these are indicated by arrows). The vertical axis represents the depth of the features – the deeper they are, the older they are. The horizontal axis shows where the features are in space.

Figure 2. An labelled digram of a GPR. 

Figure 3. The basic principles of GPR. 

Figure 4. Example of the data which results from GPR survey.

GPR can be heavily affected by soil type and weather, as moisture in the soil reduces the accuracy of the data – luckily we’ve had lovely weather the last few weeks. GPR has been especially useful in areas such as the inner keep of the castle and the carpark, while are covered in stone or tarmac and can therefore not be excavated. See fig. 5 for a guide to where GPR surveys have been carried out around the castle.

Figure 5. Map showing the areas of the castle which have been surveyed using GPR 

So far this year, geophysical survey using GPR has revealed several structures around the castle, which may include a well, a drain, and Victorian houses. However, we cannot know for certain what features are present until they are excavated, and this is unfortunately impossible in some areas of the castle. Advanced techniques such as GPR can hopefully be employed throughout the castle and grounds to help unravel more of Berkeley's rich history in the future. 

References:

Global GPR Services Inc. (Unknown). How a geophysical survey works. [Image]. Available at: http://www.global-gpr.com/gpr-technology/how-gpr-works.html. [Accessed on 06/06/16]

Google Earth. (2016). Satellite image of the Berkeley area with annotations. [Image]. Available at: https://www.google.co.uk/maps/@51.6895798,-2.4574207,421m/data=!3m1!1e3 [Accessed on 06/06/16]

Lambert Locations Pty Ltd. (2016).  Labelled diagram of a GPR. [Image]. Available at: http://www.lambertlocations.com.au/ground-penetrating-radar/groundpenetratingradar/. [Accessed on 06/06/16]

Tapatio. (2007). Example of geophysics data with annotations. [Image] Available at: https://en.wikipedia.org/wiki/Ground-penetrating_radar. [Accessed on 06/06/16]

-Hattie Ford