Tag Archives: multiplexed survey

The beast lives on

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

Yesterday saw Kris, Pauline and Jim wrestling with the beast once more. It really is something one only does once in a while simply because one would die of boredom to spend too much time on it. The tick-tick-tick-tick-tick-tick-cherble cannot go fast enough. We are going to do one more day so that we have covered the whole of the house on the hill. Here are the results so far:

Fig. 1: the multiplexed Earth Resistance survey, day 2.

As we saw yesterday, the downslope wall seems to have much deeper foundations than the upslope ones.  I’ll do a more detailed write-up when I have the last day’s worth of data and I can compare the images to the GPR slices.

The GPR team of Mike, Nigel, Adrian and Dave completed yet another 80x40m block. This time they headed south.  The block crosses the ‘1955 ditch’, the late first century boundary of the town. Figure 2 shows a composite of time slices.

Fig. 2: Composite of GPR slices (top row, then second row etc.).

As can be seen, there is a rectangular building in the top-middle of the plot. This seems to be a simple building with four downstairs rooms, the thin one of which is probably a staircase to an upper floor.  It lies parallel to the ‘1955 ditch’ which shows as a lighter band running across some of the slices from about 0m 30 to 40m 0.

The new software allows me to undertake an “overlay” analysis which produces a composite image from different time-slices.  This should help see the whole of a structure when different parts of it are at different depths.

Fig. 3: Overlay analysis of the GPR results from day 8.

The overlay shown in Fig. 3 certainly shows part of the 1955 ditch more clearly.

Fig. 4: the mag data in this area. The yellow box represents the area surveyed on day 8 shown in Fig. 3.

One curious thing about the 1955 ditch is how varied its response in the magnetic data is.  Here, the ditch shows as a strong black and white feature running across Fig. 4 from the NW corner.  There is, however, quite a sudden change just at the edge of the GPR grid shown in yellow. Clearly, the ditch must have had quite a complex history and that the story of “dug c.AD 80, out of use c.AD 125″ is probably too simple.  In the southern part of the town, we found a later building constructed over the line of the ditch, but here the ditch seems to have remained clear of buildings, at least for some of its length.  There are some quite large circular black blobs in the magnetic data (“circular magnetic anomalies” in the jargon) which are probably large pits.  A few of those show in the GPR data when one compares the two data sets carefully, but you wouldn’t notice them otherwise.  Although some things show in both data sets, such as the building we found in the latest GPR results, some only show in one or other.  It is definitely worth doing both.

On Wednesday we will be back at Gorhambury, one team using “the beast” and one using the GPR.  We’ll be back to “normal” Earth Resistance survey on Thursday.

Many thanks to everyone who helped.

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.