What a difference a year makes…
I write this intended as a quick piece on the changing earth resistance anomaly which is being detected in Cherry Copse, Cirencester. This, our most obvious ditch to detect (silty loamy ditch fill cut into weathered free draining limestone bedrock) has shown some interesting variations over the past year. To mark our full year of survey over this feature, I have included a graph to show how the ditch appeared in my Earth Resistance dataset in June 2011, and how it appeared last month, June 2012.
These measurements are taken during my monthly multiplexed-twin probe Earth Resistance readings (using a Geoscan RM15 data logger, with multiplexer attached to a PA20 frame – allowing for twin probe measurements at a= 0.25m, 0.5m, 0.75, and 1m respectively. - The increasing ‘a’ spacing can be equated with increasing depth of investigation beneath the ground – but I digress..). The results shown demonstrate the collected readings at a=0.5m which is generally the standard spacing for most twin-probe archaeological surveys.
The investigation of what we continuously term ‘seasonality’ has apparently failed this year. The weather has behaved in a way we could not foresee – in essence – we haven’t had ‘seasons’. One major risk with only having C.15 months of survey time over our ditch features supposed that the years of 2011-2012 would be ‘normal’ 0r at least ‘seasonal’. Instead we ended up with one of the hottest summers in 2011, the warmest and driest winter on record, drought (and associated hosepipe bans) in January and February – followed by the wettest April and June (and I suspect July) on record. It wouldn’t surprise me if Summer 2011- Autumn 2012 will end up being one of most unpredictable and extreme years on record. One thing this does offer, however is an intriguing look into one of the wildest years there has been, and how this has changed the way we can (or cannot) detect the archaeology. – after all – that’s what DART is all about, right? These extreme conditions may well be a catalyst in determining further the causes and effects of the environment on some aspects of archaeological detection.
This effect of the environment can be seen in the graph below. The data is comprised of taking the average of twenty Earth Resistance traverses taken over the ditch feature at Cherry Copse – 12 months apart (readings have been taken every month of the year, but are not presented here).
From the graph it is quite apparent that the two months do not show the same or even similar anomalies over the ditch feature – even though they are both taken at the height of summer, at the end of June. The general background resistivity of the site has decreased by around 60 Ohms over a year – indicating that the soil composition is vastly different to the same month a year ago. Compared to June 2011, the June 2012 readings show that the soil is almost at a state of saturation. Our anomaly, although still visible in the dataset, has shrunk dramatically, and is only visible as a contrast of around 5 Ohms compared to the background response. As a short post, I do not intend to go into the detail of weather histories and the like – or any further statistical analysis (for that you’ll have to read the DART publication at the end of the project – or my PhD Thesis) however, I feel it does illustrate our core problem which we will hopefully be able to find at least some answers for.
Comments are of course welcome.