PROCEEDINGS OF THE REMOTE SENSING AND PHOTOGRAMMETRY SOCIETY CONFERENCE 2012“Changing how we view the world”. University of Greenwich, London, 12-14 September 2012.

esa_pubs@cornell.edu

Keywords: Archaeology, Hyperspectral, airborne, DART prject, field, NERC ARSF

1. Introduction

Aerial photography (AP) has substantially increased our understanding of the nature and distribution of Archaeological Residues (ARs). However, it is over-reliant on the visual component of the electromagnetic spectrum. The collection technique and the technology utilised militate against using any other sensor (i.e., peripatetic surveys are directed by visual observation from a plane and collected using an optical system, a camera out of a window). This is further exacerbated by the policy frameworks that define the application of aerial detection: it is predominantly used on specific, free draining soils where crop marks more regularly occur. Such biases mean that the large proportion of the UK under clay, pasture and other ‘difficult’ conditions are rarely flown (Mills, 2007). This was dramatically detailed in the 1996 flights over the Cambridgeshire clays where a serendipitous vertical survey undertaken by the County Council provided unprecedented information on the buried landscape. Landscape, as opposed to feature, surveys using non-visual sensors will help address many of the biases associated with traditional archaeological aerial photography. Hyperspectral remote sensing (RS) techniques have a huge potential for use in archaeological prospection but are currently underutilised (National Heritage Science Strategy (NHSS) reports (Williams 2009)).

The current resistance in using hyperspectral RS results from the fact that the science underpinning how archaeological contrasts form and their sensor detection characteristics is not well understood and so there are difficulties in their optimal deployment. Archaeological sediments create localised physical, chemical and biological variations in the soil matrix (Schiffer, 2002) which can be detected directly or by proxy (normally through the detection of crop-marks: localised crop stress/vigour variations, but also as temperature, topographic and soil structure anomalies). The subtleties associated with archaeological formation means that detectable contrast alters as environmental conditions change. Hence, there is a seasonal and diurnal dimension to detection. The challenge is to detect both the ephemeral expression of contrast and the environmental process that can accentuate or diminish this response.

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The driest spring in 100 years: hyperspectral imaging and spectro-radiometry of vegetation over known archaeological features

Boyd, D.S., Beck, A., Stott, D., Llewellyn, G. and Carrol, Q.

School of Geography, University of Nottingham, Nottingham NG7 2RD, UKContact details: 01159 514182; doreen.boyd@nottingham.ac.uk

AbstractThe aim of this study was to understand how hyperspectral remote sensing can be optimally utilised for the detection of archaeological residues. In June 2011 the UK NERC ARSF acquired Eagle/Hawk hyperspectral data over an area of arable farming (Diddington, Cambridgeshire, UK) in June 2011. Concurrent field spectra were collected and in addition were collected at dates earlier and later in the crop growing season. Both the field and airborne hyperspectral data were pre-processed to derive absolute reflectance, first derivative reflectance, the red edge, vegetation indices and continuum removed metrics to detect sites of localised contrasting responses. Preliminary results illustrate the value of the hyperspectral approach which does allow for much improved archaeological prospection. This is the first study of this kind to be undertaken and once results are transferred has the potential to reveal archaeological residues as yet unknown. Since this study occurred in non-normal conditions (after the driest spring in 100 years) it provides a benchmark for future multi-temporal studies in the area (which took place during for 2012), as well as similar areas and conditions worldwide.

PROCEEDINGS OF THE REMOTE SENSING AND PHOTOGRAMMETRY SOCIETY CONFERENCE 2012“Changing how we view the world”. University of Greenwich, London, 12-14 September 2012.

This paper presents preliminary results from an investigation using hyperspectral data acquired by the UK’s Natural Environment Research Council’s (NERC) Airborne Research and Survey Facility’s (ARSF) Eagle/Hawk sensors, and in-field spectraradiometry, to understand archaeological detection dynamics, particularly with respect to the extrapolation of local detection to the landscape. The research complements that of previous work supported by NERC ARSF (e.g., Bennett et al., 2011) and extends it by focussing on active arable landscapes. Land management techniques on these landscapes are recognised as a significant threat to ARs (e.g., deep ploughing).

This investigation forms part of a wider research project supported by the UK’s Arts and Humanities Research Council/Engineering and Physical Sciences Research Council funded Detection of Archaeological Residues using remote sensing Techniques (DART) project (http://dartproject.info/) which has the overall aim of developing analytical methods for identifying and quantifying gradual changes and dynamics associated with near-surface archaeological residues under different environmental and land-management conditions. An understanding of how RS technologies detect contrast caused by different underlying factors under different environmental conditions, can lead to the deployment of sensors, and survey techniques, which are likely to detection a greater range of ARs (Beck 2011; Verhoeven and Doneus, 2011), in particular those that express their contrast outside the visible or in very small wavelength bands. This understanding will allow different sensors to be deployed at the times when the ARs have the greatest likelihood of being detected.

2. Methodology and Data

The study area for this research is a field farmed at Diddington in Cambridgeshire which is part of the Thornhill Estate. This Estate has a known and identifiable archaeological repertoire, suitable arable and soil conditions and some land under permanent pasture. The clay rich field west of the A1 is associated with a linear feature close to a ring ditch of presumed Iron Age date. Soil profiles developed within these deposits have been homogenised by ploughing and are therefore immature. A typical profile comprises a 0.15m-thick humic A horizon overlying a 0.20-0.35m-thick argillaceous B horizon, while the C horizon (i.e. the till) consists grey-brown clays with variable amounts of sub-angular chalk pebbles-cobbles and occasional sub-rounded flint pebbles.

The ‘Pasture Field’ east of the A1 and is associated with a ditch feature with Iron Age pottery that was sealed by a thick, possibly alluvial, deposit. This field is located on deposits mapped as Quaternary sands and gravels of the lowest two (undifferentiated) terraces of the River Ouse. The soil profile away from the archaeological features comprised a 0.15m-thick humic A horizon overlying a 0.20m-thick silt/clay B horizon. The C horizon consisted of c 0.20m of homogeneous yellow-brown silt/clays, i.e. a floodplain deposit, while the terrace gravels outcropped at >0.55m below ground surface. In other words the present soil in the sampled field has developed in overbank alluvium rather than terrace gravels. Spectro-radiometry transects are located in the adjacent arable fields indicated by the red line in Figure 1. This site extent intersects with the scheduled Boughton Deserted Medieval Village.

Two sets of hyperspectral data were acquired: these were (i) airborne- and (ii) field-sourced. These benefit from concurrent soil moisture profile, temperature profile, weather, geo-technical and geo-chemical data provided by DART.

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Figure 1: An aerial photograph showing the archaeological repertoire of the clay field at Diddington. The red line denotes the feature

and associated transect studied.

PROCEEDINGS OF THE REMOTE SENSING AND PHOTOGRAMMETRY SOCIETY CONFERENCE 2012“Changing how we view the world”. University of Greenwich, London, 12-14 September 2012.

The airborne data were collected as part of a pilot study supported by NERC ARSF award (GB11/01) in which Eagle/Hawk hyperspectral data were collected in 14th June 2011. The size of

the archaeological residues (1m or less) requires that fine spatial resolution data are acquired, thus the Eagle/Hawk sensors were deployed at an altitude of 600m. The field data were collected along a GNSS located transect 15m long, centred over the known archaeological feature. Under ideal illumination conditions transect readings were taken at 1m intervals, with closer readings over the known archaeology. Throughout the year hyperspectral data were taken by deploying an ASD FieldSpec Pro System at these dates: 08/06/11; 14/06/11; 28/06/11 and 15/07/11. Figure 2 illustrates the impact of the AR on crop growth.

The image and field spectroscopy data were preprocessed (i.e., image mosaicing and atmospheric correction) using both bespoke (e.g., APL) and proprietary software (e.g., ENVI)) to derive spectral reflectance (and their derivatives, such as red edge, vegetation indices (ND680; ND705; PRI; SIPI; PRSI) and continuum removed metrics (such as BDNA).

3. Analysis and Results

Preliminary investigation concentrated on qualitative comparisons of metrics providing contrast between archaeology and non-archaeology. A temporal dimension to the investigation was provided with the field data and comparisons made with the airborne hyperspectral measurements made on the 14 th June. The raw field spectra over the four dates are illustrated in Figure 3 where a temporal effect is evident on the spectral contrast between the archaeology and non-archaeology.

Figure 3: Average reflectance spectra for each of the four date across the growing season.

In the early part of the growing season (i.e., June 8th and 14th) the archaeology is mainly detectable as variations in LAI / canopy structure rather than changes in photosynthetic pigmentation. This is demonstrated by contrast between the archaeology and non-archaeology, as

demonstrated in Figure 4, in the ND-based indices as opposed to small variations in leaf pigments demonstrated by continuum removed spectra and the PRI vegetation index. This alters during ripening and senescence of the canopy (June 29th, July 15th), where variations in maturation distinguish the difference between the archaeology and non-archaeology, and demonstrated by changes in leaf pigmentation.

Figure 4: Difference in calculated VI between archaeology and non-archaeology (14/06/11).

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Figure 2: Shows the impact of ARs on the crop. Image is taken looking NNW from the centre of the area highlighted in Figure 1.

PROCEEDINGS OF THE REMOTE SENSING AND PHOTOGRAMMETRY SOCIETY CONFERENCE 2012“Changing how we view the world”. University of Greenwich, London, 12-14 September 2012.

As was found with the field data, examination of the airborne data for the same transect revealed wavelengths in the NIR spectral region to provide the greatest contrast between the archaeology and non-archaeology. The contrast was seen to be greater in particular wavebands/lengths (for example at 1158nm which is attributable to lignin in the leaf).

4. Discussion and Conclusions

The preliminary results presented here demonstrate that use of hyperspectral RS allows for the detection of biochemical and structural differences in the crops being sensed occur as a result of underlying archaeological residues. The variations in vegetation response across the electromagnetic spectrum means that the crop acts as a proxy bio-marker for sub-surface archaeological activity. It was also evident that this difference was temporally dynamic. Continued research is required to determine if this is exclusive to this point in time (i.e., after the driest spring in 100 years) and to capture the dynamic processes that led to this state. It is understood that subsurface archaeological residues have a localised impact on the property of soils which can be expressed as localised crop stress and vigour differentials or temperature changes. Models need to be developed that clearly identifies these and other contrast determinants, the conditions under which they occur and which spectral wavelengths and hence sensors, can be used to detect them. In the visible near-infrared we are looking at vegetation response (red-edge etc.) and mineralogy (secondary iron species) and in the short-wave infrared we are looking at leaf-water content and mineralogy (clay species, carbonate content and sediment particle size may also be a determinant).

The combination of multi-temporal spectro-radiometry and airborne imaging will also allow us to examine scaling issues (particularly, pixel mixing) and environmental dynamics. The different spatial resolutions of the aerial sensors will be key for examining issues of scale. The data from in-situ probes and weather stations currently operating as part of the DART project will highlight key environmental events. The results from the spectro-radiometry transects will be extrapolated to the local area through the hyperspectral images. The aerial archaeological flights will provide some temporal control. As the footprint is small and the soils and land-management techniques are known then localised variations should be mainly due to variations in crop. Given that the data presented in this paper were collected in non-normal conditions, although of great value, questions still remain surrounding the applicability of hyperspectral sensors to detect sub-surface archaeological features. Such questions include: (i) what are the best ways to employ the different sensors in a multi-sensor approach for the greatest heritage return (for example, can the increased spectral sensitivity in the visual wavelengths provide by hyperspectral sensors improve archaeological detection); (ii) what are the best environmental or operational conditions for deployment (e.g. season, time of day, weather, crop cover, angle of view etc.); (iii) how do we improve the detection of ARs in those areas which have proved difficult (e.g. on ‘heavy’ (clay rich) soils and permanent pasture); (iv) Can hyperspectral techniques extend the window of opportunity for detection outside the expression of contrast in the visual wavelengths. Answers to these should arise as part of the on-going DART project.

Acknowledgements – We are extremely grateful to The Thornhill Estate and NERC ARSF.

5. References

Beck, A.R., 2011, Archaeological applications of multi/hyper-spectral data - challenges and potential. In Remote Sensing for Archaeological Heritage Management, D.C. Cowley (Ed.) (EAC Occasional Paper: Europae Archaeologia Consilium) pp. 87-98.

Bennett, R., Welham, K., Hill, R.A., and Ford, A.L.J., 2011, Making the most of airborne remote sensing techniques for archaeological survey and interpretation. In Remote Sensing for Archaeological Heritage Management, D.C. Cowley (Ed.) (EAC Occasional Paper: Europae Archaeologia Consilium)pp. 99-106

Mills, J., 2007, Bias and the world of the vertical aerial photograph. In From the Air: Understanding aerial archaeology, K. Brophy and D.C. (Eds.) (Stroud: Tempus) pp. 291-299.

Schiffer, M.B., 2002, Formation Processes of the Archaeological Record (Utah: University of Utah Press).

Verhoeven, G., and Doneus, M., 2011, Balancing on the Borderline – a Low‐cost Approach to Visualize the Red‐edge Shift for the Benefit of Aerial Archaeology. Archaeological Prospection. doi:10.1002/arp.420.

Williams, J., 2009a, The role of science in the management of the UK’s heritage (No.1). NHSS Report. Retrieved from http://nhss.english-heritage.org.uk/upload/pdf/nhss_report_1_web.pdf?1317030601.

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