Presentation on theme: "Creator: Dr. Anthony Beck (School of Computing, Leeds University)"— Presentation transcript:
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.PPTResource Section: THEORYSuggested Prerequisites: NoneSuggested Level: Secondary, Tertiary, CPDKeywords: 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 CampanaSlide 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 techniqueReliant on specific seasonal and environmental conditionsIncreasingly extreme conditions are required for the detection of ‘new’ sitesLow understanding of the physical processes at play outside the visual wavelengthsSignificant bias in its applicationin the environmental areas where it is productive (for example clay environments tend not to be responsive)Surveys don’t tend to be systematicInterpretation tends to be more art than scienceThis 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 deficienciesMulti/hyperspectral sensors (including thermal)LiDAR (ALS) - High resolution topographic recordingGround 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 domainStarting from AP assumptions unlikely to be helpfulThis 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 vigourSoil mineralogyMoistureTemperatureUse of non-visual wavelengths has a number of benefits:Can extend the window of opportunity for archaeological identificationMay not require extreme environmental conditionsMay 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 signaturesImportantly 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:StructuresShadow marksSoil marksCrop marksThermal anomaliesInfluenced by effects of:WeatherSeasonSoil type and soil moistureCrop typeDoes anyone have a better slide. Rog any criticisms of this
12 First Principals – Archaeological Site Examples Micro-Topographic variationsSoil Marksvariation in mineralogyand moisture propertiesDifferential Crop Marksconstraint on root depthand moisture availabilitychanging crop stress/vigourNow you see meThe 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 identifyProxy Thaw MarksExploitation of differentthermal capacities of objectsexpressed in the visualcomponent as thaw marksNow 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 matrixA sensor that can ‘detect’ this contrastSensor utilised during favourable conditionsi.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 SignaturesRather: produce localised disruptions to a matrixThe nature of these disruptions vary and include:Changes to the soil structureChanges to moisture retention capacityChanges in geochemistryChanges in magnetic or acoustic propertiesChanges to topographyAt 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 typeCrop typeMoisture typeDiurnal temperature variationsExpressed contrast differences change over timeSeasonal variations impact on the above (crop, moisture, temperature in particular)Diurnal variations: sun angle (topographic features), temperature variationsExacerbated by anthropogenic actionsCroppingIrrigationHarrowing
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 visualLow 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 visualMedium 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 visualHigh Spectral Resolution.Banding from Airborne Visible/Infrared Imaging Spectrometer
20 SummaryNon-visual remote sensing has huge potential for the detection of archaeological featuresHowever, aerial photographic techniques are not a good starting pointRequires a thorough understanding of how archaeological contrast is produced so that the correct sensor can be applied at the correct time:(De) Formation processesLocal (contrasting) matrixAmbient conditionsSensor characteristics