The beast

Anyone new to this blog or geophysics in archaeology is recommended to read the material on the “Geophysical survey in archaeology” page.

I have a fondness for electrical survey techniques. It may be something to do with only having access to an Earth Resistance meter for the first twenty years of my surveying career. UCL’s new RM85, which we got last summer, has been a real treat, speeding-up survey times and giving excellent results. One technique I have seen in print that I wanted to try at Gorhambury, uses “the beast”…

Fig. 1: “the beast”, aka, a Geoscan RM85 with a 1.5m beam.

As you can see from Fig. 1, the beast is the RM85 can be fitted with a 1.5m beam and six probes (not Fergus the dog, CAGG’s mascot).  Why?, you may ask.  The depth that Earth Resistance survey is measuring is proportional to the width that the probes on the frame are spread.  Our usual 0.5m spread gives us a depth of around 50–70cm.  By using the multiplexer — basically a fancy programmable switching box — built into the meter, we can take six readings at six different spacings and thus get six depths: 25cm, 50cm, 75cm, 100cm, 125cm and 150cm.  Just for good measure I got the machine to also take a reading using a 50cm spacing Wenner array, i.e., the two outer probes are passing the current and the two inner ones are measuring the voltage potential.

The downside?  Take a slow survey method, and make it really slow…  We managed two 20x20m grids in a day, and it is unlikely one would manage three without starting very early and ending very late.  Fig. 2 gives one a sense of how “quick” the method is.

Fig. 2: the string movers under pressure [not].

Our aim is to survey the “House on the Hill” using this technique so we have some nice comparative data to compare with the two GPR surveys.  It will take three days to complete six grid squares.  Here are the interim results.

Fig. 3: the results of the first day of multiplexed survey.

I have done the minimum of data processing to each of the images in Fig. 3: despiking to remove high points caused by rocks and some clipping to show the image more clearly.  As can be seen, the northern wall of the corridor seems to have quite substantial foundations.  I wonder if the house was terraced into the hill a little way, and this wall was a retaining wall down slope?  This isn’t going to be something we do often, but it is an interesting test.

The GPR team continued north.

Fig. 4: the loneliness of a long distance GPR-pusher.

They managed an excellent 80x40m block, good going on the slope and with the long grass.  Here are a selection of slices:

Fig. 5: a montage of time slices of today’s block.

As can be seen, we have a large building in the bottom centre of the block. This matches-up with yesterday’s building to give quite a curious looking structure.

Fig. 6: GPR survey, day 7.

It seems to have a very long corridor running down the SW side, the the rest of the structure to the NE is hard to see. There is then more rooms running SW-NE with some substantial “blobs” in the middle.  Clearly this strip of buildings running NE-SW across the site is very busy.  The next image (Fig. 7) shows the underlying mag.

Fig. 7: the mag data in the same area as the GPR data from days 6 and 7 shown in Fig. 6.

It seems to be a good year for our old friends the fairy rings…

Fig. 8: fairy ring.

Many years ago my brother wrote in my autograph book: “how do you get four elephants in a mini?  Two in the front and two in the back…”  I think Mike knows the feeling…

Fig. 9: equipment transportation.

Many thanks to Adrian, Dave, Mike, Jim, Pauline and Ellen for an excellent day’s survey.  We’ll be back tomorrow, and then two day’s off for a well-earned rest.


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