Diurnal experiments at Cirencester
This post will be revised as corrections are sent through by colleagues to correct my ignorance on certain scientific issues and what happened in practice.
We proposed a thermal imaging experiment at the RAC in Cirencester. This was originally instigated after viewing the diurnal temperature variations from the embedded probes and wondering what the impact of the vegetation canopy would be on any sensor. This was given life with follow up conversations with John and Rosie Wells (West Lothian Archaeology).
Between the 18th and 22nd June 2012 two diurnal measurement experiments were conducted at the Harnhill study site in Cirencester. Diurnal refers to patterns or activities that occur over a daily cycle. In this instance we are interested in solar insolation, how this is emitted in the thermal wavelengths and how this varies throughout the day. It was hypothesised that different surface and subsurface thermal variations throughout the diurnal cycle will lead to different readings with different sensors. We proposed to study this by conducting diurnal measurements with thermal imaging and video cameras and diurnal measurements with the ERT. The former experiment will examine how emitted radiation at the thermal wavelengths changes with the diurnal cycles and if this can be used to detect archaeological features. The latter experiment will look at how subsurface temperature variations in conjunction with the electrolytic medium change electrical resistance conditions which in turn impact on the sensor readings. This experiment will be reported by Rob Fry.
The conditions for the experiment were far from ideal. Torrential rain was forecast. Fortunately the rain only started in earnest from midnight on the Wednesday.
The aim of the experiment was fourfold:
- to collect temperature variations throughout a diurnal cycle to understand the impact on temperature dynamics on archaeological detection.
- to determine the impact of the crop canopy on the detection of archaeological features.
- to evaluate the use of Fourier Transform Infra-red on archaeological detection.
- to compare the imaging measurements with the results from in-situ subsurface and ambient temperature sensors.
Three sensors were used to measure the thermal infra-red: a low cost auto-ranging thermal video camera (30Hz NTSC Flir PathFindIR, Model 334-0001-00) , a quantitative thermal imaging camera (model Tetso 875?) and a FTIR (described in a subsequent post). The PathFindIR thermal camera was provided by the West Lothian Archaeological Trust. This imaging camera has a capture resolution of 320 x 240 pixels, a field of view of 36 x 27 degrees and a spectral range of 8-14 micrometre. Whilst this camera is not designed for quantitative use it has a quoted radiometric resolution of 0.1°C which means that it can adequately detect the relative temperature variations observed in the probes in Quarry Field. The thermal sensitivity of the PathFindIR is given as 100mK at + 25°C. Further details on the PathFindIR can be found here. This camera is also light (360g excl. battery): this means it can be easily deployed on either a UAV or kite. The video output from this camera was captured on a dedicated hard drive DVR (with a small PVR used as a monitor) meaning that video was captured for the full diurnal cycle.
The Tetso 875 (model 1 or 2) thermal imaging camera was provided by XXXX at Kings College, London. This imaging camera has a capture resolution of X by Y pixels, a field of view of Z and a spectral range of A. This camera is designed for quantitative scientific analysis, is calibrated and has a radiometric resolution of B. This camera is manually triggered with images taken at half hour intervals. Images are stored in a proprietary format on an internal SD card. The ***** FTIR was provided by NERC Field Spectroscopy Facility (FSF). This device collects a single pixel thermal hyperspectral stack covering wavelengths (A, B, C). If has a Z field of view. Data is stored…..
In order to examine contrast between the sub-surface archaeological feature and the surrounding soils it is important to achieve an Instantaneous Field of View (IFOV) that encompasses both the archaeological feature and the surrounding matrix. Based upon the field of view of the imaging cameras and guidance on safe operating height a height of approximately 8m is required to collect a 5m wide image on the ground. In addition it is necessary to compare changes over time so it is important to co-register images taken at different time frames. With these two factors in mind it was decided to employ a scaffolding tower to provide both elevation and a stable platform for repeat measurements and to place ground control markers for rectification. Candles in jars provided this purpose. Three lit candles were placed over known points for a period of 10 minutes and removed prior to taking readings. This warmed the soil providing both rectification points and a target to focus the camera. After thinking about this, and recognising that the thermal camera measurements are based on the emmisivity of the objects under study. Emissivity is a dimensionless quantity: in general, the duller and blacker a material is, the closer its emissivity is to 1. The more reflective a material is, the lower its emissivity. The emmisivity values for bare metal is significantly different from soil and vegetation. Hence, we just inverted the tealights and recorded the different value of the aluminium base.
One of the other concerns was that after removal of the crop the soil would take some time to return to it’s nominal diurnal dynamic temperature range (i.e. there would be a lag after strimming while the soil achieved it’s normal operating temperature range). We therefore decided to strim 50% of the IFOV to provide the maximum amount of time for the soil to get back up to temperature. After strimming the soil was raked to remove the majority of vegetation. A full day of recording would take place over the 50/50 split. The equipment would then be transferred to the other field to conduct the same measurements whilst the other 50% of vegetation was removed. The following day the monitoring equipment would be returned to the original location.
As the weather conditions were against us, rather than taking measurements on different days with and without the crop it was decided to strim half the IFOV prior to measurement to directly measure the contrast between soil and crop. The rationale was based on the lag effect that would occur after the crop was harvested which means that the subsurface soils would take some time to achieve the ambient dynamic temperature range of bare soils throughout the diurnal cycle. It was, therefore, proposed that any site would have two batches of diurnal measurements: one with the crop half removed from the IFOV and the other with the crop fully removed from the IFOV.
Fieldwork commenced on the 18th June 2012. The survey areas in Cherry Copse and Quarry Field were set out. The appropriate segments were strimmed with the surface raked to clear vegetation to leave stubble and bare soil exposed. The scaffolding towers were erected and generators installed. Thermal video and imaging measurements commenced at Cherry Copse at 16:15 on Monday the 18th June and concluded at 16:20 on the 19th June. The weather was clear for the duration of the experiment. The remaining part of the area in Cherry Copse was strimmed and the surface raked. The equipment was re-established at Quarry Field with thermal video and imaging measurements commencing at 17:45 on the 19th June. The ditch feature was easily identified in the crop on the first thermal image. Detecting features in heavy, clay, soils represents the largest gain to the archaeological community. As the weather forecast predicted rain on the Thursday it was decided to continue measurements at Quarry Field. The first diurnal cycle measurements concluded at 17:45 on the 20th June. At this time the remaining part of the area in Cherry Copse was strimmed and the surface raked. Measurements continued in Quarry Field until 17:45 on the 21st June.
The weather conditions were too poor to continue in Cherry Copse. However, the initial readings from Quarry Field did not provide the same degree of differential measurements as predicted from the in-situ temperature probes. This could be due to a number of factors that includes: the change in conditions between the probes and the measurement sites, the attenuation of the soil and crop on thermal emmissions, differences in wavelength sensitivity of the different measuring devices (probes, thermal imager and thermal video), trampling of the exposed surface impacting on temperature ?????, not enough time was given to (work this up later). To address this further thermal imaging readings were taken at exposed holes adjacent to the subsurface probes. These holes, excavated to provide topsoil and sub-soil for the plant growth experiments, were situated over the archaeology and in the natural soil. Measurements were taken with the thermal imaging camera between the hours of solar noon and 3 pm which represent the period of maximum temperature differential between the archaeological sediments and the surrounding soil matrix from the subsurface temperature probes.
We will be working on the data preparation and analysis and will post further information here. John and Rosie have posted some images on the West Lothian Archaeology website: http://www.armadale.org.uk/dart.htm
This experiment was a real team effort thanks are due to a whole range of people including:
Tony Norris (and everyone else on the RAC Harnhill estate)
Rosie and John Wells (also thanks for getting a picture of the UAV before I crashed it!)
RAC and Leeds University finance offices (for being understanding)
Three sensors were used to measure the thermal infra-red: a low cost auto-ranging thermal video camera (30Hz NTSC Flir PathFindIR, Model 334-0001-00) , a quantitative thermal imaging camera (model Tetso 875?) and a FTIR (described in a subsequent post). The PathFindIR thermal camera was provided by the West Lothian Archaeological Trust. This imaging camera has a capture resolution of 320 x 240 pixels, a field of view of 36 x 27 degrees and a spectral range of 8-14?m. Whilst this camera is not designed for quantitative use it has a quoted radiometric resolution of 0.1°C which means that means that it can adequately detect the relative temperature variations observed in the probes in Quarry Field. This camera is also light (360g excl. battery): this means it can be easily deployed on either a UAV or kite. The video output from this camera was captured on a dedicated hard drive DVR (with a small PVR used as a monitor) meaning that video was captured for the full diurnal cycle.