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You Ain’t Seen Nothing Yet …

You Ain’t Seen Nothing Yet
Geophysical Surveying Methods as a Tool for Cultural Heritage Protection

Presentation from A. Posluschny at the WORLD UNIVERSITIES CONGRESS in Çanakkale (Turkey), October 2010.


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  • Dear colleagues, During the next 15 minutes or so I am going to talk about ways of finding archaeological features in order to be able to protect them for cultural heritage purposes. Those of you that are archaeologists or are working in Cultural Heritage Management might already be aware of the methods I am talking about - you can lean back and enjoy the pictures. Those of you that are not archaeologists and who just came in because of the nice rock song title of my talk, will hopefully find the things that I am talking about interesting and might learn about archaeology, that is not so visible. Those of you that do work or will work in construction or land planning or in related fields might find some of he information useful for their further work and when collaborating with archaeologists or cultural heritage managers.
  • I will start with a short introduction to the general problem of invisible features in archaeology. One method, or a couple of methods to overcome this problem is the use of geophysical survey, which I will explain quite briefly. Some examples from geophysical surveys will then show you, how helpful geophysics can be and in the end I would like to show you some more, also technically advanced methods for archaeological surveys.
  • Before I start with the main part of my paper let me please very short explain what my research background is. I am working for the Roman-Germanic Commission of the German Archaeological Institute. While the DAI is operating worldwide - which you can see on this already outdated map - the Roman-Germanic Commission is concentrating on the archaeology of Europe north of the Alps and on the Balkan. We have a strong focus on landscape archaeology as well as on the use of modern surveying and research methods. wech?
  • I am responsible in the Roman-Germanic Commission for the management and coordination of a EU funded networking project, called ArchaeoLandscapes Europe. It deals with all aspects of modern archaeological surveying such as aerial archaeology, remote sensing, LiDAR scanning and last but not least with geophysical surveys - which will be my main topic of today’s talk. As a landscape archaeologist with a strong focus on spatial techniques and on computer based methods, I am speaker of the German branch of the international society “COMPUTER APPLICATIONS AND QUANTITATIVE METHODS IN ARCHAEOLOGY” (CAA) - this might explain my point of view to a certain extent. wech?
  • When we talk about archaeology or Cultural Heritage in general we usually think of well known objects like the Egyptian pyramids, a Roman temple, the library of Ephesos or large effigy mounds in the United States . They are so well known mainly because they are large and of course also because they are visible! But that of course is just a very small portion of what ancient cultures have left as traces of their former activities. Many structures and features in the soil are often not known at all – which makes it difficult to protect them as defined by the European Convention of the Protection of Archaeological Heritage.
  • Features like pits, ditches and wells might be covered by soil and are invisible to the human eye. Even when excavated most of them do not look very spectacular.
  • Yet they can contain such precious finds like this gilded horse head from a life size bronze statue of a Roman Emperor which was found in a well near Frankfurt, 11 meters below the surface - an object which is quite singular in Europe.
  • What is left from many features, buildings, ditches and so on after a couple of hundreds or thousand years remains invisible at least when seen through the human eye. But still there are traces in the ground, traces that differ from their surrounding soil in their colour, texture and other physical and chemical properties.
  • Some of these features can be discovered when parts of it - like stones from a wall or sherds from within a pit - are uncovered by ploughing. That is the basis for field walking surveys which usually is a good instrument to detect unknown archaeological sites. But it is not suitable in all cases as the following example shows: In 1982 this large ditch structure was discovered from an aerial archaeologist in northern Bavaria, Germany (KLICK) . No archaeological finds were known from that site so far though some field walking has been carried out before.The ditch is part of a Roman military camp from which you can see a small part on the excavation plan on the right hand side. It shows a large amount and a great variety of features, not only from the camp but also from neolithic burials and from an Iron Age settlement. All structures - besides the ditch - couldn’t be seen on the aerial photograph, but they were clearly visible in a magnetometric survey, which has been carried out a short time after the detection of the ditch from the air.
  • If those investigations wouldn’t have been conducted, the whole area would have been overbuild after a few years and because no relevant archaeological traces were known before, the archaeology underneath the soil would have been destroyed if none of the building workers would have reported archaeological features during the construction of houses and streets. What we know now about the internal structure of the site, the area it covered and the features that were in it, is not only the result of an archaeological excavation but mainly coming from the geophysical survey. It was carried out in all rectangular areas that are marked on the left hand side plan while excavations (KLICK) were only undertaken in the areas marked here with the blue circles.
  • Geophysical surveying methods comprise a variety of different techniques which I will introduce in a minute. They all have in common, that they are non-destructive, they are machine-based, in most cases they are less expensive than excavations and they can cover much larger areas than field walking or trial trenches or even large scale excavations. Their disadvantage is the expert knowledge one has to have in many cases to be able to handle the data derived from various measurements.
  • I am not going into the technical details now, first of all because I am not a technician and second because of the limited time I have. Magnetometric survey is used to rapidly generate data of large scale areas, showing a wide variety of anomalies that have been caused by different kinds of human activities. It uses one or more sensors to measure the gradient of the magnetic field i.e. the difference between the natural magnetic field of the Earth and the structures that have been caused by human impact. Because every kind of material has its own magnetic property (even those kinds of material that we think of as “non-magnetic”), they all result in a different disturbance of the Earth’s magnetic field, showing the strongest impacts from metals like iron and steel or from burned soils and bricks. Also some ferrophagous bacteria produce a certain amount of magnetite when living in certain soils that are enriched by humous remains, caused by past human activities. Other than earth resistance surveys, magnetometers do not usually directly detect walls or other stone structures (if not burned).
  • It is possible to measure an area of up to 2 hectares per day with a handheld array with 5 sensors; the average coverage is – depending on the terrain – 1 hectare per day. Modern systems with 16 sensors on a car-driven device can measure up to 30 hectares per day, the average still being 5 to 10 hectares. Systems like that have been used to investigate this lareg-scale area of the multi-period site of Vr abl e in Slovakia. over 100 hectare?
  • The picture becomes complete when we add the results of traditional field-walking which delivers dated material. In green you can see the remains of an early neolithic settlement (ditches, houses), in red the early bronze age ditch, pit and house structures and in blue the remains from roman military camps. Large scale investigations like this can only be carried out with a highly sophisticated system, they produce results in a much shorter time than excavations, still being very precise in the location of features and therefore being a fantastic basis for further research and also for the management of large sites in order to protect them.
  • Earth resistance survey (geoelectric survey) is measuring the local electrical resistance by inserting two or more sensors into the ground, which produce electrical circuits. The resulting data can be used to map (archaeological) features of higher or lower resistivity. Features like ditches often contain more moisture than the surrounding soil and therefore have less resistivity while wall structures, foundings and so on usually have a higher resistivity. The main disadvantage of a resistance survey is the limitation caused by the need for the sensors to make direct electrical contact by the insertion of electrodes. As a result resistance survey is mainly used for smaller areas, even if recent developments (like mounting the electrodes on a frame for a faster, automated data measurement) increased the possible speed of a survey.
  • Ground-penetrating radar (GPR) is often used because of its abilities to measure not only planar features but also to estimate the depth of features. A radar signal or electromagnetic impulse is send into the ground, which causes different kinds of reflections (travel time of signals), depending on the depth and the structure of the soil and of buried features The resulting data represents a profile information, that can also be interpolated into a planview map by taking into account the results of several, densely measured profiles The main disadvantage of GPR measurements is its dependency on ideal soil conditions. Fine-grained soil sediments like clays or silt cause losses of signal strength whilst rocky or very heterogeneous sediments cause a scattered GPR signal. Another problem is the low speed of measurements, especially when used for larger areas
  • I have already shown you an example for an aerial photography which is another advanced surveying technique used for the detection and the monitoring of both visible and hidden archaeological features as well as for the management of landscape changes and their impact on archaeological sites. XXX ZUSTANDEKOMMEN DER MAKRS ERKLÄREN? XXX erwähnen, dass es auch noch snow marks und water marks gibt
  • The result and the success of an aerial survey is highly depending not only on the skills and the experience of the aerial archaeologist but also on weather conditions, general soil conditions and others. With a little bit of luck these conditions have been suitable when high resolution pictures from satellites have been taken, so tools like GoogleEarth also offer the opportunity to detect archaeological sites from your armchair - a good technique for those that otherwise would suffer from air sickness. This example shows a quite impressing earthwork in Romania with a multiple ditch system.
  • Satellite images have been a source for archaeological and landscape feature detection since they are more or less easily available for research purposes. The development of the satellite cameras is rapidly increasing. While 10 years ago they produced images with a 1 m ground resolution the newer satellites like GeoEye have a resolution even below this 1m threshold. The large scale availability makes them a perfect instrument to not only detect new sites but especially to monitor changes of know sites (like the one shown here on the right from Sudan) and also to assess landscape features and to classify landscape for archaeological and other purposes.
  • One very new technique is the use of Airborne Laser Scans, also known as LiDAR scans. This is not just a visibility technique, it is also a method to measure landscape. While flying over a landscape a plane or a helicopter can take pictures of the landscape but it can also make radar based measurements of the terrain. The big advantage when using LiDAR scans is the possibility to distinguish between different kinds of signals or signal echoes and thereby simply calculating a terrain even when it is covered with trees and bushes with a very high resolution.
  • This example shows a digital elevation model, based on the LiDAR scan of an Early Iron Age hillfort near Frankfurt, Germany. You can clearly see shallow remains of a former rampart and ditch system (marked here with the arrows) that are not visible from the ground.
  • With the help of these data we were also able to detect grave mounds in the forest that where unknown to the archaeological heritage management authorities before.
  • One can only protect what he knows - so large scale surveying to discover hidden subsoil archaeological features is not only a technique for archaeological research but also for Cultural Heritage Management. The large variety of geophysical and other modern surveying methods have the great advantage of being non-destructive, they can they can amend each other and they deliver a very precise and nearly complete picture of what is hidden to the human eye. Data from geophysical surveys are an ideal basis for decision making in urban land-use planning, it can be used to assess the threads from erosion, looting and plundering or from ploughing and it can be used for monitoring archaeological sites. During building and construction planning the areas of archaeological interest can be taken into account, building sites can be replaced or at least the amount of pre-building research activities to excavate archaeological features can much better be assessed. So not only archaeology or Cultural Heritage Management benefits from a large scale geophysical survey but also investors and stakeholders from construction companies.
  • The ideal way of dealing with surveying is of course a combination of several techniques: Geomagnetic surveying to detect ditch and pit structures, geoelectrical survey to find wall structures and ground penetrating radar to know more about the depth of the hidden features - and maybe also some field walking or trial trenching to know more about the dating and the condition of the features. Speaking about the role and the responsibilities of universities in future, I think that it should be their aim to provide expert knowledge and to teach students modern surveying methods - not necessarily enabling them to conduct these investigations themselves but to be able at least to work with data derived from such techniques and to be able to critically interpret these data and to be able to judge the interpretations of others
  • I hope I could show you some interesting aspects of modern archaeological survey techniques and their use for Cultural Heritage Management and am looking forward for your questions.
  • Transcript

    • 1. You Ain’t Seen Nothing Yet Geophysical Surveying Methods as a Tool for Cultural Heritage Protection World Universities Congress Çanakkale, 23 October 2010 Dr. Axel G. Posluschny Roman-Germanic Commission of the German Archaeological Institute Palmengartenstr. 10-12 D-60325 Frankfurt [email_address]
    • 2. Content
      • Introduction
      • Problems of invisible features in Cultural Heritage Management
      • Traditional field walking
      • What is geophysical surveying?
      • Making the invisible visible
      • Other methods of modern surveying
      • Resume
    • 3.
      • Worldwide research projects of the German Archaeological Institute (DAI)
    • 4.  
    • 5. Features in Cultural Heritage Management http://www.ohiotraveler. com/images/serpent %20mound.jpg http://www.online-reisefuehrer.com/bilder-reisen/tuerkei/ephesos.jpg http://www.zum.de/Faecher/G/BW/Landeskunde/w3/provence/vienne/augustus1.jpg http://www.aegypten-spezialist.de/uploads/pics/gizeh-cheops-sphinx.jpg
    • 6. Invisible Features in Cultural Heritage Management
    • 7. Invisible Features in Cultural Heritage Management
    • 8. Invisible features in Cultural Heritage Management Braasch/Christlein1982
    • 9. Traditional fieldwalking Germania 71, 1993 Ber. RGK 72, 1993
    • 10. Geophysics and excavation Ber. RGK 72, 1993
    • 11. What is geophysical surveying?
      • Geophysical surveying methods comprise a variety of different techniques:
        • Magnetometer survey
        • Earth resistance survey (geoelectric survey)
        • Ground-penetrating radar (GPR)
      • They are:
        • non-destructive
        • machine-based
        • in most cases less expensive than excavations
        • can cover much larger areas than field walking, trial trenching or even large scale excavations
      • Their disadvantage is the expert knowledge one has to have in many cases to be able to handle the data derived from various measurements
    • 12. What is magnetometric surveying?
      • Is used to rapidly generate data of large scale areas, showing a wide variety of anomalies that have been cause by different kinds of human activities
      • It uses one or more sensors to measure the gradient of the magnetic field i.e. the difference between the natural magnetic field of the Earth and the structures that have been cause by human impact
      • Every kind of material has its own magnetic property, they all result in a different disturbance of the Earth’s magnetic field
      • Other than earth resistance surveys, magnetometers do not usually detect walls or other stone structures (if not burned) directly
      D. Peters English Heritage 2008
    • 13. Examples of magnetometric surveys Batora et al. in press
    • 14. Examples of magnetometric surveys Batora et al. in press
    • 15. What is earth resistance surveying?
      • Earth resistance survey (geoelectric survey) is measuring the local electrical resistance by inserting two or more sensors into the ground, which produce electrical circuits
      • Features like ditches often contain more moisture than the surrounding soil and therefore have less resistivity while wall structures, foundings and so on usually have a higher resistivity
      • The main disadvantage of a resistance survey is the limitation caused by the need for the sensors to make direct electrical contact by the insertion of electrodes. As a result resistance survey is mainly used for smaller areas
      English Heritage 2008 English Heritage 2008
    • 16. What is ground-penetrating radar (GPR) ?
      • Ground-penetrating radar (GPR) is often used because of its abilities to measure not only planar features but also to estimate the depth of features. A radar signal or electromagnetic impulse is send into the ground, which causes different kinds of reflections (travel time of signals), depending on the depth and the structure of the soil and of buried features
      • The resulting data represents a profile information, that can also be interpolated into a planview map by taking into account the results of several, densely measured profiles
      • The main disadvantage of GPR is its dependency on ideal soil conditions. Another problem is the low speed of measurements, especially when used for larger areas
      English Heritage 2008 Kvamme et al. 2006
    • 17. Other methods of surveying soil marks crop marks Braasch 2010 Landesamt 1997
    • 18. Other methods of surveying
    • 19. Other methods of surveying
      • Satellite images
      • increasing resolution
      • large scale availability
      Cowley et al. 2010
    • 20. Other methods of surveying
      • LiDAR => Light Detection And Ranging = Airborne Laserscan (ALS)
      • A high-resolution digital surface model (DSM) can be derived and also a „vegetation-free“ digital elevation model (DEM)
      • Many national cartography agencies produce these scans which can also be used for archaeological purposes
      English Heritage 2010
    • 21. Other methods of surveying
    • 22. Other methods of surveying
    • 23. Resume
      • One can only protect what he knows - so large scale surveying to discover hidden subsoil archaeological features is not only a technique for archaeological research but also for Cultural Heritage Management
      • The large variety of geophysical and other modern surveying methods have the great advantage of being non-destructive, they can amend each other and they deliver a very precise and nearly complete picture of what is hidden to the human eye
      • Data from geophysical surveys are an ideal basis for decision making in urban land-use planning, it can be used to assess the threads from erosion, looting and plundering or from ploughing and it can be used for monitoring archaeological sites
      • During building and construction planning the areas of archaeological interest can be taken into account, building sites can be replaced or at least the amount of pre-building research activities to excavate archaeological features can much better be assessed. So not only archaeology or Cultural Heritage Management benefits from a large scale geophysical survey but also investors and stakeholders from construction companies
    • 24. Resume
      • The ideal way of dealing with surveying is of course a combination of several techniques: Geomagnetic surveying to detect ditch and pit structures, geoelectrical survey to find wall structures and ground penetrating radar to know more about the depth of the hidden features - and maybe also some field walking or trial trenching to know more about the dating and the condition of the features
      • Speaking about the role and the responsibilities of universities in future, I think that it should be their aim to provide expert knowledge and to teach students modern surveying methods - not necessarily enabling them to conduct these investigations themselves but to be able at least to work with data derived from such techniques and to be able to critically interpret these data and to be able to judge the interpretations of others
    • 25. References
      • Bátora , J., Eitel, B., Hecht, S., Koch, A., Rassmann, K., Schukraft, G. and Winkelmann, K., in press. Fidvár bei Vráble (Kr. Vráble, Südwestslowakei). Untersuchungen auf einem äneolithisch-frühbronzezeitlichen Siedlungshügel. Germania (Zabern Verlag, Mainz).
      • Bofinger , J., 2007. Flugzeug, Laser, Sonde, Spaten – Fernerkundung und archäologische Feldforschung am Beispiel der frühkeltischen Fürstensitze / Aircraft, Laser, Sensor, Spade – Remote Sensing and Archaeological Fieldwork Using the Example of Early Celtic Princely Seats (Esslingen)
      • Cowley , D. C., Standring, R. A., Abicht, M. J. (eds.), 2010. Landscapes through the Lens. Aerial Photographs and Historic Environment. Occasional Publication of the Aerial Archaeology Research Group No. 2 (Oxford, Oakville)
      • English Heritage 2008. http://www.english-heritage.org.uk/publications/geophysical-survey-in-archaeological-field-evaluation/geophysics-guidelines.pdf
      • English Heritage 2010. http://www.english-heritage.org.uk/publications/light-fantastic/light-fantastic.pdf
      • Kvamme , K., Ernenwein, E., Hargrave, M., Sever, Th., Harmon, D., Limp, F., Howell, B., Koons, M. and Tullis, J., 2006. New Approaches to the Use and Integration of Multi-Sensor Remote Sensing for Historic Resource Identification and Evaluation. SERDP Project SI-1263 (Fayetteville). http://www.serdp.org/Research/ upload/SI-1263-FR.pdf
      • Landesamt für Denkmalpflege Hessen (ed.), 1997. Zeitspuren. Luftbildarchäologie in Hessen (Wiesbaden)
    • 26.
      • Thank you very much
      • for your attention
      • [email_address]