Contrast, archaeological site detection and the non-visual component of the electromagnetic spectrum Creator: Dr. Anthony Beck (School of Computing, Leeds University) Author(s): Dr. Anthony Beck (School of Computing, Leeds University) Stakeholders: N/A

Contrast, archaeological site detection and the non-visual component of the electromagnetic spectrum Resource Reference: AARG_THEORY_CONTRAST_01_01.PPT Resource Section: THEORY Suggested Prerequisites: None Suggested Level: Secondary, Tertiary, CPD Keywords: contrast, archaeology, remote sensing, aerial photography, satellite imagery, spectrum, formation, proxy, detection

Contrast, archaeological site detection and the non-visual component of the electromagnetic spectrum In recent decades advances in sensor technology have led to a range of ground, airborne and spaceborne imaging instruments that can be applied to archaeological and heritage management problems. However, the development of the archaeological detection techniques associated with these technologies have evolved independently with variable understanding of the physical, chemical, biological and environmental processes that determine whether archaeological residue contrasts will be identified in one or any sensor. This presentation will explore some theoretical issues surrounding archaeological contrast identification.

(Re-)use statement Insert here (Lyn: please advise) The slides do not have to be used in this order. Where there is not enough descriptive information in the slide itself further details can be found in the notes section.

Slide courtesy of Stefano Campana Re-use agreed with Stefano Campana Slide courtesy of Stefano Campana

EM spectrum and Aerial Photography (Log scale) The point of this slide is to demonstrate that archaeological features will express contrast across the whole electromagnetic spectrum. Traditionally Aerial Photography has relied exclusively on the visible and near-infra-red wavelengths. This is a very small component of the EM spectrum (note this is a logarithmic scale).

Aerial Photography and archaeology Most successful archaeological detection technique Reliant on specific seasonal and environmental conditions Increasingly extreme conditions are required for the detection of ‘new’ sites Low understanding of the physical processes at play outside the visual wavelengths Significant bias in its application in the environmental areas where it is productive (for example clay environments tend not to be responsive) Surveys don’t tend to be systematic Interpretation tends to be more art than science This slide does not directly address contrast identification but does provide appropriate background during a presentation.

Remote sensing and archaeology New and different sensors/technologies can address some of these deficiencies Multi/hyperspectral sensors (including thermal) LiDAR (ALS) - High resolution topographic recording Ground geophysics (magnetometry, resistivity) GIS/IP software – improved processing (getting the best out of what we have) Will require going back to first principles to model how archaeological anomalies occur in each domain Starting from AP assumptions unlikely to be helpful This slide does not directly address contrast identification but may stimulate discussion and lead into contrast identification/detection.

Why Non-Visual Remote Sensing? Many archaeological contrasts are easier to identify in non-visual wavelengths: Crop stress and vigour Soil mineralogy Moisture Temperature Use of non-visual wavelengths has a number of benefits: Can extend the window of opportunity for archaeological identification May not require extreme environmental conditions May be applicable in ‘non-responsive environments’

First Principals - Archaeological Site Detection Discovery requires the detection of one or more site constituents which are sufficient to suggest that a site might be present. The important points for archaeological site detection are that: Archaeological sites are physical and chemical phenomena. There are different kinds of site constituents. The abundance and spatial distribution of different constituents vary both between sites and within individual sites. These attributes may be masked or accentuated by a variety of other phenomena. Importantly from a remote sensing perspective archaeological site do not exhibit consistent spectral signatures Importantly from a remote sensing perspective archaeological site do not exhibit consistent spectral signatures: Spectral signatures are used by many remote sensing specialists to identify an object. A spectral response has unique properties which, depending upon the spectral and radiometric resolution, allows one to determine what it is (type of soil, rock or plant). Also see slide titled Example: Multi/Hyper-spectral remote sensing

First Principals – Archaeological Sites Archaeological sites show up as: Structures Shadow marks Soil marks Crop marks Thermal anomalies Influenced by effects of: Weather Season Soil type and soil moisture Crop type Does anyone have a better slide. Rog any criticisms of this

First Principals – Archaeological Site Examples Micro-Topographic variations Soil Marks variation in mineralogy and moisture properties Differential Crop Marks constraint on root depth and moisture availability changing crop stress/vigour Now you see me The proxy thaw mark example is not archaeological however it demonstrates the effect. The contrast between the temperature profiles of soil and water change throughout the day (due to emmisivity characteristics of water and soil). At two points in the day the measurable thermal response from soil and water is the same and hence they can not be distinguished using temperature measurements alone. At all other times there is a contrast between the two materials. The trick is to find the time when this contrast is easier to identify Proxy Thaw Marks Exploitation of different thermal capacities of objects expressed in the visual component as thaw marks Now you dont

First Principals 3 - Contrast Types Direct -where a measurement, which exhibits a detectable contrast with its surroundings, is taken directly from an archaeological residue. In most scenarios direct contrast measurements are preferable as these measurements will have less attenuation. Proxy - where a measurement, which exhibits a detectable contrast with its surroundings, is taken indirectly from an archaeological residue (for example from a crop mark). Proxy contrast measurements are extremely useful when the residue under study does not produce a directly discernable contrast or it exists in a regime where direct observation is impossible.

Contrast and Archaeological Detection The nature of archaeological residues and their relationship with the immediate matrix determines how easily residues can be detected. Detection requires the following: A physical, chemical or biological contrast between an archaeological residue at its immediate matrix A sensor that can ‘detect’ this contrast Sensor utilised during favourable conditions i.e. you’re unlikely to detect thaw marks in summer using photography! Although you could detect the underlying thermal anomalies using a different sensor at this time. Here the underlying process remains the same (a thermal variation) and the detecting sensor is in part determined by the environmental conditions. It is this contrast between an archaeological feature and its matrix that one is wanting to observe.

Detection and (De-)Formation Processes Unfortunately archaeological sites do not produce distinct Spectral Signatures Rather: produce localised disruptions to a matrix The nature of these disruptions vary and include: Changes to the soil structure Changes to moisture retention capacity Changes in geochemistry Changes in magnetic or acoustic properties Changes to topography At least one of these disruptions will produce a contrast which is detectable

Environmental and ambient conditions Local conditions structure how any contrast difference is exhibited: Soil type Crop type Moisture type Diurnal temperature variations Expressed contrast differences change over time Seasonal variations impact on the above (crop, moisture, temperature in particular) Diurnal variations: sun angle (topographic features), temperature variations Exacerbated by anthropogenic actions Cropping Irrigation Harrowing

Example: Multi/Hyper-spectral remote sensing Dimension and number of recordable wavelengths. There is NO archaeological spectral signature. Allows one to select the portion of the spectrum where there is the most contrast. Hence, an improvement in archaeological detection. Poorly understood outside the visual Low spectral resolution

Example: Multi/Hyper-spectral remote sensing Dimension and number of recordable wavelengths. There is NO archaeological spectral signature. Allows one to select the portion of the spectrum where there is the most contrast. Hence, an improvement in archaeological detection. Poorly understood outside the visual Medium Spectral Resolution. Banding from Landsat Thematic Mapper satellite sensor

Example: Multi/Hyper-spectral remote sensing Dimension and number of recordable wavelengths. There is NO archaeological spectral signature. Allows one to select the portion of the spectrum where there is the most contrast. Hence, an improvement in archaeological detection. Poorly understood outside the visual High Spectral Resolution. Banding from Airborne Visible/Infrared Imaging Spectrometer

Summary Non-visual remote sensing has huge potential for the detection of archaeological features However, aerial photographic techniques are not a good starting point Requires a thorough understanding of how archaeological contrast is produced so that the correct sensor can be applied at the correct time: (De) Formation processes Local (contrasting) matrix Ambient conditions Sensor characteristics