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Combining different methods

How can you survey a landscape using ERT? And how can you combine different methods?
ANITA THOMPSON: So, the ERT survey behind us is on quite a small scale. How would you go about surveying a larger landscape?
KRISTIAN STRUTT: Well, the ERT equipment that’s set up here behind us, the probes have been set up at one metre intervals. Now, the way in which resistance survey works is that you’re generally dealing with the depth about half the distance between the probes in terms of the depth of the propagation of the current and your measurement. And, on a small scale like this, we can set the probes up at metre spacings and get quite a high resolution survey, going down in half metre increments of depth, and every metre along the profile. So, for an archaeological site like this, where we’re working on structures, warehouses, and things associated with the port, that’s the kind of resolution we want.
We want to see walls. We want to see variations in the stratigraphy, and so on and so forth. When it comes to surveying broad landscapes, you are more looking at the things to do with the geomorphology and the geoarchaeological landscape. Now, what we can do with this multiprobe array is, we can increase the spacing between the probes, up to five metres spacing. Now, patently when we set it up at five metre spacing, you’ll have a longer profile. You’ll cover a longer profile every time you do the survey. Your spacing at five metres means you’re getting down in 2 and 1/2 metre increments. So, you’re going down much deeper, much faster. But you’re also losing resolution.
You’re getting a reading every 2 and 1/2 metres rather than every half a metre. And this system is ideal, then, for looking at things like channels, canals, paleochannels, and much larger features. You wouldn’t pick up, using that resolution, things like walls, buildings. But you would pick up the broader variations and geomorphological changes in the landscape.
FRANZISKA MARCHESELLI: Can you put all the results of the different kinds of geophysics together to create a bigger image?
KRISTIAN STRUTT: You can. One of the things that we’re trying to do with the project of Portus is to ensure good integration of material from different methods, both geophysics and things like remote sensing. In terms of the geophysics, we’ve covered the landscape of Portus extensively with magnetometry, something to the tune of 250 hectares of survey around the port itself, and another 170 hectares south of the Fossa Traiana going down towards Ostia Antica. That’s a massive area to survey. We can do it with magnetometry. It’s another thing to try and do that with things like electrical resistivity tomography and ground penetrating radar, which runs at a slightly slower pace, especially when we’re dealing with single antenna or single sets of multiprobes.
So, in that instance, what we’ve chosen to do is target different areas across that landscape, using these other techniques. So, a lot of the work we’ve done close to the port here, using ERT and GPR, has allowed us to get really detailed coverage, where we did the magnetometry originally, and to find out what’s going on in terms of three dimensions in terms of the increasing depths of the site.
ANITA THOMPSON: That sounds like a very complicated process. Lizzie, could you give us a more basic understanding of why integration is important?
LIZZIE RICHLEY: Integration is important because each geophysical technique can reveal different aspects of the subsurface. Each technique measures a different quality. And, therefore, we can pick up different features and different amounts of detail from each technique. If we did one technique, we might miss obvious readings which we can pick up in another technique. For instance, Kris mentioned magnetometry. It can pick up highly volcanic rocks. If you’re doing the survey in a highly volcanic area, you’re not going to pick up any archaeology, at which point we need to do other techniques in that area. These will be on top of magnetometry if we have done it there, to then pick out the features which we wouldn’t see with this technique.
And so, when we can merge them all together, in both the graphical formats and also numerically, we can pick out stronger features which they all have either picked up or which only one has picked up. You can make a judgement as to whether they are more likely to be archaeology or more likely to be geological or natural occurrences within the data sets.
FRANZISKA MARCHESELLI: Finally, are there any other methods of studying a landscape, other than geophysics?
KRISTIAN STRUTT: Here, we use a lot of geophysics to study the area, from Portus all the way down to Ostia Antica. There are a number of other different things that we’re using in order to look at the area. We are, to a certain degree, restricted in terms of what areas we can access. They’re also pretty much like using varying techniques in geophysics, other methods that show different features in the landscape. So, in addition to geophysics, we’ve been looking at remote sensing imagery, satellite imagery, both panchromatic, colour image and multispectral, a band satellite imagery.
Now, what you can do with that is, you can set the different bands so you can see basically using different parts of the spectrum of light and see what features show up in terms of archaeology in the landscape. It allows you to highlight various features that may run across that landscape. In addition to that, we’ve been looking at things like the air photographic imagery that the RAF took in 1943 and 1944 as part of the pre-invasion and invasion of Italy during the Second World War. Our imagery is especially useful, because a lot of the area around Portus has been developed in the last 15 to 20 years.
Modern satellite imagery, there’s a lot of conurbation, industrial space, et cetera, that now cover a lot of the interesting archaeology. We can take these aerial photographs from 70 years back and see areas that 70 years ago were all just fields, and see features in those fields and be able to, again, interpret what archaeology is there that we cannot now survey because it’s part of our past. In addition, we’ve been using kind of platforms such as drones or balloons to take near infrared aerial photography as part of the seasons, with Belgian colleagues working on different parts of the port.
And again, that will help under different conditions during different parts of the year to highlight archaeological features in the port and in the immediate area. And then we can integrate all of this so we can see things in terms of spatially put together and produce interpretations and see exactly what’s going on in terms of archaeological features across the broader landscape.
ANITA THOMPSON: Lizzie, Kris, thank you very much. We look forward to seeing the results.
Kris has introduced nicely how the scale of geophysical surveys affects the resolution of the data and therefore what we can actually detect within the datasets.
An Electrical Resistivity Tomography (ERT) survey with probe intervals of 1m will detect much smaller features such as walls at shallower depths than a survey with probe intervals of 5m which will penetrate deeper, but only detect large subsurface features such as channels, canals and palaeo-channels. As such in a geophysical survey we tailor the survey to each individual site.
Students laying out an ERT transect at Portus
Students laying out an ERT transect at Portus – Hembo Pagi © University of Southampton
Surveying a large landscape such as the Portus landscape with ERT would take a very long time to undertake as it is a slow technique. Magnetometry on the other hand is quick and efficient. Over the port itself approximately 250 hectares of magnetic data has been collected with a further 170 hectares collected to the south of the Fosse Traiana towards Ostia Antica.
It would be time consuming to collect this amount of data with other techniques, so areas are targeted based upon the results of the magnetometer and the results integrated. Using ERT and GPR in small areas has enabled high resolution coverage over important aspects within the port itself including 3-dimensional information. Combining different methods is important so we can attain a more comprehensive knowledge of the site. Each technique measures different properties of the subsurface; for example magnetometry measures the magnetic field and electrical techniques measure the ability of the subsurface to conduct an electrical current. If we used one method only we could miss obvious features that would be detected by a different technique. In a high magnetic area, for example Pompeii, magnetometry would not work well (if at all!) so ERT or GPR would be appropriate methods.
The geophysical data can also be complemented and integrated with other methods such as remote sensing images. Data can be collected from satellites, airplanes and UAV (Unmanned Aerial Vehicles or drones) and includes panchromatic imagery, colour imagery, multi-spectral imagery and thermal imaging. These images can be processed to show different colour bands that highlight various features.
Aerial photography has been of great use at Portus, in particular imagery that was taken by the RAF during reconnaissance flights in 1943 and 1944. These images show archaeological features that were visible 70 years ago but are now covered by modern developments.
Electrical Resistance Tomography profile recorded at Portus
Electrical Resistance Tomography profile recorded at Portus – Kristian Strutt © University of Southampton
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