Geophysics – large enclosure – September 2014

A team of AVAS members met early(ish) on Sunday morning to survey a large cropmark near Fordingbridge.  This is in the same field as the previously surveyed oval barrow.  Here is an aerial photo of the cropmark, with the 20m square we chose to survey highlighted in yellow.  The crop mark continues below the area shown on the photograph, and appears to be closed off by a terminal ditch, forming a large enclosure.

Enclosure cropmark near Fordingbridge

One of the problems with this cropmark was trying to locate its actual position on the ground.  GPS on our phones came to the rescue, allowing us to roughly position the square to within a few metres.  The yellow square above is actually a GPS track, hence the sides not being completely straight due to the slight variation in the GPS signal.  We chose this particular location as it was where the main side ditch and a cross ditch intersected.

After quickly setting out a baseline, and then the 20m square, we had soon completed the first resistivity survey.  The results are shown in the image below (click on the image to see it at full size).

Resistance survey for first square

Resistance survey for first square

The results were a slight disappointment as the overall pattern appears to be a natural gradation of readings from high resistance in the bottom right to low in the top left, leading to the rainbow pattern.  With a little imagination there are faint suggestions of a couple of linear patterns that roughly correspond with the cropmarks, but these are far from clear.  We then completed a partial survey of the square immediately to the right before having to head off for Sunday lunch.  The combined results are shown below (agains, click the image to see full size).

Resistance survey for second square

Resistance survey for second square

As can be seen, the pattern continued into this second (partial) square.  The images below show the cropmark to the left and the resistivity results overlaid onto the same area to the right.

Enclosure cropmark near Fordingbridge - enlarged Enclosure cropmark near Fordingbridge - enlarged - surveyGiven the results were not as good as were expected, we will have to reconsider whether we continue to survey this feature using geophysics.  It might be worth waiting until the soil is wetter before attempting another survey.


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