Resistance isn’t futile after all.

Today was another successful day. We had a short rain shower just before lunch but other than that, all went well.  We ran all three machines with good results.  Firstly the mag survey.  The first image shows the western side of our survey.  The block of data we collected today is in the lower left corner of the image.

The western side of the magnetometer survey.

The western side of the magnetometer survey.

The most obvious feature is the big, bright black and white object.  That is a galvanized steel water trough!  Across the SW corner is a dark diagonal line.  That is another stretch of the 1955 ditch — the first century boundary of the town — which Martin Atkin traced with a very early magnetometer in 1959 and 1960.  It was also the first feature we found in Verulamium Park in 2013.  Running diagonally across the survey from NE to SW is a linear feature made up of many light and dark lines which cuts across the western end of the hedge line and goes into the unsurveyed  grid square.  This is the line of one of the street which has buildings and other features along its length.  If we zoom in we can see more details.

The area surveyed with the magnetometer on day 20.

The area surveyed with the magnetometer on day 20.

The area to the SE of the water trough has a number of buildings shown clearly by white lines (representing a low magnetic response) especially either side of the NE–SW running road.  To the west, however, is very quiet with very few features showing.  The unsurveyed square, which is first on the list for tomorrow, should be very busy with the road and more buildings.  The eagle eyed amongst you will have spotted some vertical lines of uniform grey.  Ugh.  Another sensor freeze.  We will need to redo about a third of that square tomorrow.

The resistance survey continued on and completed another four 20x20m squares at 0.5m spacing,  Pretty good going.

The resistance survey after day 20.

The resistance survey after day 20.

In the resistance survey plot, black represents high resistance and white low.  High resistance features are things like roads and walls which do not hold moisture.  Low resistance features are things like ditches and pits which do hold moisture.  Generally, I have found, the high resistance features are easier to spot and interpret on the plots.  In the case of the image above, the two roads show clearly, especially the one in the NW corner.  The buildings along the roads also show well, including some internal walls.  Interestingly, not much shows in the middle of the insula (a “block” in a modern town).  Are they yard areas behind the buildings?

The GPR team managed an 80x40m block of data, plotted to the west in the following image.

GPR data. The two squares to the west were completed on day 20. This is the 15.5 to 18.5ns time slice.

GPR data. The two squares to the west were completed on day 20. This is the 15.5 to 18.5ns time slice.

The GPR has picked up the buildings along the two roads very nice.  It is also pleasing to see the plots from this year join up with the plots from last year so well.  Using a high accuracy differential GPS makes this quite easy to do!

In some areas we now have three data sets over the same area.

The earth resistance data overlain on the GPR data.

The earth resistance data overlain on the GPR data.

The resistance, magnetometry and GPR data all overlain.

The resistance, magnetometry and GPR data all overlain.

Obviously, combining all this data is going to be a challenge.  Looking at a few specific details is fascinating and informative, but how does one do this on a large scale?  There are techniques called “data fusion” that I am going to have to look into.

Again, many thanks to all the team who turned out today, and watch out for tomorrow’s exciting installment!

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