ANITA THOMPSON: What are the different types of geophysical survey?
KRISTIAN STRUTT: We have a number of different types, depending on whether you’re wanting to do kind of extensive survey and cover large areas very quickly. Or whether you’re trying to target different areas. Or whether you’re trying to do tomography and survey in three dimensions. So, generally, in archaeology the kind of workhorse of archaeological geophysics has been magnetometry and resistance survey. And then you get into things like Ground Penetrating or Probing Radar, GPR for short. Or Electrical Resistivity Tomography, or ERT for short. But there are the techniques that are used as well. You can use things like magnetic susceptibility survey. And you can use electromagnetic survey as well. So there are a variety of techniques.
FRANZISKA MARCHESELLI: Can you explain a bit more about what exactly ERT is?
KRISTIAN STRUTT: ERT is imaging basically through sectioning or imaging sections through a particular thing. And in terms of archaeological survey, Electrical Resistivity Tomography uses apparent resistivity measurements to do that. And Lizzie’s been working on some of that here at Portus, so she will be able to tell you more.
FRANZISKA MARCHESELLI: So Lizzie, you have been doing ERT here at Portus. Can you explain a bit more about exactly what you are doing?
LIZZIE RICHLEY: So we are currently taking a series of sections across Portus using Electrical Resistance Tomography. And it works by taking a series of different levels through the ground. Initial level is quite close, and we always use four probes to take each data point. But the further apart these probes are, the deeper we can actually go. So the more probes we use, the deeper we can potentially measure the depth of. And with this technique, we can then pick out different areas of resistivity, which is highly dependent upon the actual materials below the ground and the actual water content of the ground itself.
And so we can hopefully pick up different bits of walls, bits of masonry flooring, and feasibly even the canals which were present here, and feasibly even down to the natural geology beneath.
ANITA THOMPSON: So Kris, you mentioned magnetometry. Could you explain a bit more about that?
KRISTIAN STRUTT: Yeah, of course. Basically, in archaeology, you use an instrument called a flux gradiometer. And it has a number of sensors attached to it. You walk across a survey area, and the sensors measure the changes in the Earth’s magnetic field across the area, at different points in the area. The Earth itself has a magnetic field. We all know about that. It’s generated by current moving in the centre of the Earth. Different materials under the ground locally can be measured using local variations in that magnetic field. So anything that has been fired above its Curie point , things that are built out of volcanic rock, kilns, furnaces, et cetera. They can be located using magnetometry.
Also any changes in the soil where you have an increase in magnetite or maghemite in the soil, things like ditch fills. They can also be recognised because they can be picked up as a local variation in the electromagnetic field. So you survey using magnetometry of an area. It means you can carry out a survey very, very quickly, covering a hectare and a half or two hectares of a site in a day. And it can be used for a kind of very broad landscape approaches to look at the types of archaeology that are under the ground.
FRANZISKA MARCHESELLI: Earlier you also mentioned Ground Penetrating Radar. Can you go into a bit more detail about that please?
LIZZIE RICHLEY: So Ground Penetrating Radar can be used across a wide variety of sites as well. Much like magnetometry and resistance. You can use it in most places. It is highly dependent sometimes depending upon what sort of ground penetration you use. We use one on a cart here, which means we can take it over quite diverse topology. And this machine works by having a small antenna and a receiver. And our one is incorporated into one box. And we are sending out an electromagnetic wave into the ground, or a radar wave. And we are then measuring the speed at which this wave returns, is reflected off different interfaces and different materials in the ground and is then returned to the receiver.
The speed of this is highly dependent upon what is beneath the ground, the soil type. Once again, the water content, as with ERT. This can conduct the EM wave away from us. And we then do not get a very strong signal. At which point, sometimes it can be very hard to do some GPR in some survey conditions. But with this, we can get high resolution images of the ground, and also the depth of features. So we can see the relative depth between one feature and another and are able to produce planned type images when we join together a series of traverses of GPR.
And we can create a plan image tracing walls of features and buildings and feasibly getting a whole image of the site that we are currently surveying. We can get different sorts of radar as well. We have different bandwidths of EM wave. And so our one we use is 400 megahertz one. We can get data to approximately 5 metres with quite high resolution. We can use a 200 megahertz one, which goes down a lot deeper. We can get up to about 11 metres with it if we’re lucky. And you can use 1,000 megahertz antennas, which only go down about 50 cm, and gets very high detailed resolution of behind walls and features just below the surface.
So depending on what you’re looking for, we tailor our survey to that.
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