1. 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

2. 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

3. 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.

4. (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.

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

6. 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).

7. 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.

8. 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.

9. 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’

10. 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

11. 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

12. 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

13. 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.

14. 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.

15. 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

16. 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

17. 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

18. 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

19. 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

20. 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