Wednesday, February 11, 2009

How to be an Archaeologist: Geophysics

Next to trowelling into a previously undiscovered make of amphora, or planting your spade through the centre of some prehistoric skull, geophysics is one area of the archaeological universe that generates a massive amount of enthusiasm. That’s because it’s a bit like digging, only without all the hard work. It usually only requires one desperate willing student who’s spent too much time down the pub to pace up and down for several hours lugging the equipment about, leaving everyone else to stand around discussing said student critically and looking very important.
On the off chance that you’ve been in a coma for the last seventy years, or have never watched an episode of Time Team, or live at Grange Park or something, we’d better explain what geophysics actually is.
Geophysics is the method by which signals are sent into the ground, meet with various elements and then report back to a computer programme, which analyses the data and prints up a blurred but exciting pattern that inevitably leads to hours of heated debate and the occasional black eye.
The upshot of a geophysical survey is that stuff underground, like walls, and floors, and hypocausts, and drains, and coffins, and big lumps of rock, and cars buried for insurance purposes and so on, can all be detected without ever having to hoist your spade off your shoulder. Of course, the proof of the pudding is in the eating, but geophysics does help the overworked archaeologist narrow down those areas requiring further excavation to a few, more reasonably sized, locations.
There are several types of geophysical equipment available, such as magnetometers and other unpronounceable stuff, but the one pictured below with Danielle (on the top of Bourne Hill) is a Ground Resistivity…er…Machine...thing. (You can see I know this subject inside out, can’t you?)


The one thing that all geophysics equipment has in common is that it’s massively expensive. It might not look it. In fact it looks a bit like something that’s been cobbled together in a back garden shed with some old bedsteads and the remains of a ZX Spectrum. But trust me, it’d cost more to replace than one of Steve Austin’s limbs. That’s why only universities and councils tend to own them – another good reason to keep on the right side of those in possession of such valuable commodities.
So, how do you carry out a geophysical survey then?
Well, the first thing you do is grid up the area under investigation. Below is a photograph that shows Chris and John doing just that.

So that the computer programme can put all of the data gathered into some semblance of order and then build an image out of it, the person/student controlling the Ground Resistivity Machine thing needs to walk it in a set pattern. These patterns are usually squares measuring 10 metres by 10 metres, 20 metres by 20 metres, 30 metres by 30 metres… (Editor: We get the idea.)
Each reading the Ground Resistivity Machine thing takes covers one square metre, so the operator walks one metre along one side on the square, presses the Ground Resistivity Machine thing’s prongs into the earth, takes a reading, and then moves on by another metre before doing the same again.
And again.
And again.
Until they reach the end of the row, at which point they turn round and come back, pushing the prongs of the Ground Resistivity Machine thing into the soil and taking a reading every metre…again.
Over and over again.
Until they reach the place where they first started.
Whereupon, they turn around and make a start on the second row, which is a metre along…sideways…if that makes sense.
This goes on for several hours, whilst the rest of us go down the pub. It’s a lonely life being a geophysicist.
If you’re particularly bad at planning, occasionally the ‘Pegging Out’ crew overlaps with the ‘Geophysics survey team’. At this juncture cables, dumpies, measuring tapes, Ground Penetration Machine thing prongs and all sorts of other stuff get entangled with each other:


But, of course, it’s worth all the hassle in the long run. There’s nothing like being confronted at the end of the day with a geophysical image displaying the ruins of a Roman bathhouse or a mediaeval tithe barn in such startling detail that you can almost see the builder’s initials on the brickwork.
Geophysical images like this:


8 comments:

Andrew said...

Ok, I laughed when the geophysics person ensnared with pegger outerers. Education with humour.

Brian Hughes said...

Andrew,

This is what happens when the surveyors take too long over their meat and tatty pies.

John said...

This kind of thing is also good for finding dinosaur skeletons. Of course, here in the states you have a better chance of finding a dinosaur than a lost civilisation, so maybe that's why we look for dino's over here with ground penetrating radar and the like.

Really interesting stuff. I look forward to your report from Bourne.

JOHN :0)

PS I laughed as well.

Brian Hughes said...

"I look forward to your report from Bourne."

Same here John...hopefully by the end of this weekend we should have completed our preliminary 'scans', and, by the next meeting, had everything printed up and analysed.

chris2553 said...

It turned out that the markings on the stick that John and I found was a Fleetwood form of Oggum script. When decyphered, it was found to say: "If the bloke with the dodgy flask and wearing the trench coat sits down on that green foldaway chair again, hit him with this stick"

Brian Hughes said...

Chris

"It turned out that the markings on the stick that John and I found was a Fleetwood form of Oggum script."

That'd be the same sort of writing that Codheads use to decorate the inside of tram shelters.

"...it was found to say: "If the bloke with the dodgy flask and wearing the trench coat sits down on that green foldaway chair again, hit him with this stick""

I didn't sit down in my chair once last week, if you remember. That was because I forgot to take it with me, I admit, but still...

RVB said...

Great post. We learned about something similar in Biology, but rather about Fossils and how Biologists use technology to measure and locate them.

But why would anyone bury a car for insurance purposes? Don't people know how dangerous insurance company goons are?

Brian Hughes said...

Reuben,

Clapped out old banger, won't get through its next MOT, convenient hole in the ground, report of stolen vehicle, claim to the insurance company for several grand under theft, brand new car, Bob's your Uncle, problem solved. Unless of course Harry Seacombe and Michael Bentine are on the case...