Ben Urmston's blog

Wessex Archaeology recently conducted a geophysical survey at St. John’s Church in Bemerton, a mile or so west of Salisbury, Wiltshire. Residents from the Bemerton Community Group are in the process of converting part of the church into a community centre, whilst maintaining part of the building as a worship space, and need to be connected to mains utilities. We were asked to survey the routes the utilities would take through the churchyard, from the road to the church building itself.

The construction of St. John’s Church began in 1859 and dedicated in 1861, so the team expected to find archaeology associated with the church, along with graves, up to 150 years old. Out of the techniques we commonly use, ground penetrating radar (GPR) was considered the most useful for this project as we can look at the results in three dimensions. The GPR sends pulses of energy into the ground and measures the time taken for them to bounce off buried features; it works in exactly the same way as radar used to find aircraft, only we point ours downwards!

The site was surveyed on a closely spaced grid, with each data profile showing the distance along the line and the time it took the GPR signal to travel through the ground, reflect off any buried features and return to the antenna. The data are processed so that we can peel layers away that took the same length of time to return to the GPR. We can work out the speed the GPR signal travels through the ground, so work out how deep the responses are.

The GPR is pushed around in a wheeled cart and the data appear on a screen in front of the surveyor as they push the GPR forwards. A group of children from Bemerton St John C of E Primary School were able to join us in the afternoon to help collect some of the data. It’s always a pleasure to meet the possible archaeologists and geophysicists of the future!

 

The results successfully showed the locations of a number of unmarked graves, as well as modern services and possible elements of the church foundations. The information provided by the survey will help with more detailed planning of the construction of the new community centre.

 

For more information about the work of the Bemerton Community Group, you can follow the progress of the project on their blog.

We recently completed a survey as part of the Practical Archaeology Course at Down Farm, Dorset. Following on from previous work, we surveyed an area south of the excavation to help place it in a wider context. This is the result of our day's work (the area was about 180m wide by 120m, and 1.3ha or just over 3 acres in size; click for a larger image):

The site is thought to be an Iron Age farmstead, and excavation has shown that a large ditch surrounds the settlement. Unfortunately, this doesn't appear in the geophysical survey! A modern trench for an electricity cable can be seen running from bottom-left to top-right, and some other anomalies are clear, showing as small dark blobs. The straight lines in the lower half of the results show where ploughing has disturbed the natural soil under the site, and the stronger line near the bottom may mark the limit of historic ploughing.

 

Until all of the information from the excavation has been entered into our computers, we can't be sure why the ditch doesn't show in the geophysical survey. It's possible that the ditch lies entirely in the area we couldn't reach because of the excavation, shown as the blue area at the top of the image. We'll keep you posted!

Gradiometer survey with barrowsThe magnetometer is the standard piece of kit that gets used on most of the sites we look at; we use one made by Bartington (one of the pictures on their website shows Ben in a yellow jacket!). It can cover large areas quickly, and can detect most kinds of archaeology from most periods. It works by sensing tiny differences in the Earth's magnetic field, which is much larger than the tiny signals produced by archaeological features, but the instruments we use are able to ignore this background field and are very good at picking up the relatively weak archaeology.

 

We can only detect features when there is a difference between the magnetic field occurring naturally and the fields caused by archaeology. Imagine a ditch being dug through the natural soil and rock; the ditch slowly fills up as soil from the site slides in and people throw rubbish into it; and finally the ditch becomes completely full. It is this change in the fill of the ditch from its surroundings that causes a difference in the magnetic field, which we see as an anomaly in the survey data.

 

This is a very different technique to metal detecting, mainly because we can look for archaeological features such as ditches and pits, but also because the only metal the magnetometer detects is iron. These ferrous objects act just like magnets and have 'north' and 'south' ends; they also tend to produce much stronger anomalies than most other things, and show as spikes from positive to negative over a short distance. Sometimes being able to detect iron can be useful, but mostly the ferrous anomalies we find are just bits of tractors and old steel cans! This also means that our clothes and shoes must be completely free of any zips and press-studs; shopping can be challenge...

 

We display the survey data so that negative readings are white, through shades of grey, to positive readings are black; we choose the values so that we get good contrast throughout the survey data. The magnetic background of the site shows as grey because the values lie around the middle of the display range.

 

The image shows part of a survey we conducted earlier this year. The two circular anomalies are barrows, or burial mounds; the larger one is about 40m wide, and the smaller one 33m wide. After we processed the GPS data we collected as part of the survey, it became clear that the barrows sit on top of a low ridge that was barely visible in the field.

 

Lots of positive anomalies appear inside the upper barrow and look like pits, but we can't tell from the geophysical survey alone how they are related. The slightly curved line near the bottom of the lower barrow is probably a ditch or old field boundary, but we can't say if it's from the same period. Faint stripes running at a slight angle from top to bottom are the result of ploughing; the field was under a young wheat crop at the time, but may have been ploughed for centuries. There are also some ferrous anomalies sprinkled throughout the data; these stand out because of their characteristic magnetic response.

Bartington gradiometer in actionWelcome to the blog of the Terrestrial Geophysics service here at Wessex Archaeology. As this is our first post, we should start by introducing ourselves and what we do.

 

The Terrestrial Geophysics service is led by Paul Baggaley, our Geophysics Manager; he's also in charge of the Marine Geophysics service. Ben Urmston is responsible for leading teams in the field, and is involved with the majority of the fieldwork.

 

Since going ‘live' back in January, when we began advertising commercially, we've carried out work all around the southern half of the country. Most of our work comes from new developments of houses, roads and so on. The developers have to make sure that any archaeology that might be damaged during construction is checked out and excavated if needs be.

 

Sometimes the areas that are being developed are huge and, for a long time, the only way to find previously unknown archaeology was to dig lots of trenches and hope that at least some might cover the archaeology. To make this job of finding sites easier and more reliable, geophysicists use instruments that can detect archaeology without having to dig it up. We apply different methods depending on whereabouts in the country we are, and what we might expect to find. Each has its own benefits and drawbacks, so we need to choose the equipment carefully to provide the archaeologists with the right information.