Posted tagged ‘how to’

Geophysics! With enough money, and insanity, you can do it at home!

July 8, 2008

Geophysics is like most TV archaeology programmes, the majority of the work is behind the scenes and unglamorous but important if any work is to be done. So how does one go about conducting a geophysical survey on an archaeological site?

What you’ll need
At least three people – one person to use the machine and collect the data and two “line monkeys” (and eventually one person needs to be able to use the software to view the results)
Many measuring tapes- preferably fiberglass and 60M long
Bamboo “canes”
At least 2 clotheslines with one meter intervals marked off with colored tape
Ranging rods (large metallic rods with alternating colors)
A geophysics recording device- resistivity or magnetic device depending on what you are looking for (Geoscan and Bartington are the two largest producers)
Interpretation software- geoscan seems to have the most widely used software
The right conditions- preferably flat, well drained and non magnetic soil with no brambles or flesh craving insect to be found

How do you get started?
A major concept to get your head around is the idea of a “baseline”. No, I’m not talking about the mating call of middle-class guys that you can hear before you even see their Jeep TJ or their Honda Civic. A baseline is a centerline that acts as the backbone for your survey. If you are doing a larger scale “landscape survey” then you would have a very long baseline which would be in the center of your area of interest, say a Rome fort. You are using this line to divide the landscape into smaller grids of 20 x 20M or 10 x 10M.

From this centerline you measure, or subdivided the line into, 20M intervals (or 10M ) and the triangulate the other two points. Yes that’s right, you have to use math in archaeology. Kids, remember to learn trigonometry and geometry. You have the length of two known points, how do you learn the unknown point(s)? Help me Pythagorean theorem! If you are making 20 x 20M grids then here is a hint, you make a triangular with measure tapes with one tape measuring 20M and one measuring 28.28M. Then you switch the tape measurements, make another triangle and now you have your first grid!

Now use the bamboo canes to stake in a tape along the base line. Imagine you have made a giant square on the earth and one side is now measurable. Stake in a tape opposite to the measurable side of the box and now you would have the “line monkeys” use the tapes as guides and stake the clotheslines over the tape. Now the data collection/fun can begin.

Simplifying matters (for those who aren’t experienced geophysicists), the surveyor would carry the geophysical device and collect data when the machine was over the colored tape of the clothesline. When one line is complete, they would move over a meter, turn around and start the process anew. Meanwhile the “line monkeys” would perform the “ritual dance” of unstaking the clotheslines and moving them into the next position. The line monkeys have to move around the surveyor so that the data collection is not stopped or compromised, least they have to redo the data collection for that line. Once the grid is complete, the tapes on either side are moved to the next grid, much like the movement of two inchworms, and the process begins again.

I hope that you understand some of the basics of a geophysical survey. Now here are some complications that occur. When you are working on a slope the tape measurements become inaccurate and the wind can catch the tapes like a sail and rip them out of your hands. When doing a magnetic survey the ends of the tapes have metal in them and have to be kept at least 2Ms away from the machine. As if thats not bad enough many farm animals enjoy chewing on the fiberglass tapes and knocking over the canes. Don’t believe me? Check out this picture.

Not the bambo cane!

Not the bambo cane!

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