For some time now the course of prehistoric Archaeology engages in different survey methods which include aerial archaeology and geomagnetic prospection of archaeological sites. Since summer 2010 those survey methods where intensified and expanded as part of the project “Aerial Archaeology in Westphalia”. The focus lies on the combination of the different archaeological survey methods such as:
LIDAR (Light Detection And Ranging), also known as airborne laser scanning, is based on airborne terrain scans to create digital terrain models that provide a detailed image of the terrain surface, even in densely forested regions. The system is based on the emission of laser beams at regular intervals and can therefore cover large areas in a very short time. The laser is installed on the aircraft floor and the laser beams are reflected by the ground, vegetation and buildings. The individual points of the measurement can be created in three-dimensional space by calculating the angle and time of flight of the laser beam.
Geophysical prospection uses the physical properties of the earth or the subsurface such as the earth's magnetic field, the electrical conductivity of the soil, the earth's gravitational field, thermal properties, propagation speeds of vibration waves or acoustic signals, electromagnetic phenomena or natural radioactive radiation. As early as the 19th century, it was recognized that ceramics and fired clay were weakly magnetic. In 1955, Le Borgne discovered the stronger magnetizability of the surface soil. In 1964, J. C. Alldred developed and used the first fluxgate gradiometer for archaeology. As with aerial archaeology, the measurement methods were subsequently optimized, refined and simplified.
The most important geophysical prospection methods in archaeology
include: Geomagnetics, geoelectrics and ground penetrating radar, with geomagnetic prospection and ground penetrating radar measurements being used at the Institute of Archaeological Sciences.
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Geomagnetic prospection is particularly suitable for finding settlement structures such as trenches, pits, postholes and house foundations and is a non-destructive method that does not require any intervention in the ground. Magnetic prospection examines the various influences that different materials beneath the surface have on the earth's magnetic field in contrast to their undisturbed surroundings. We use a highly stable fluxgate gradiometer, Grad601 (Bartington Instruments), with a sensor that can also detect anomalies at depth (Fig. 01). Two measuring sensors are arranged one above the other in the measuring rod, each of which records the magnetic field and displays the difference between the measurements of the two sensors as a grayscale image in nanoteslar (nT). This fluxgate gradiometer is used to take measurements at 0.25, 0.5 and 1.0 meter intervals.
The Institute's measuring device normally measures at a distance of 1.0 meter. The device can only be used to examine measuring fields (grids) measuring a maximum of 40 × 40 meters. The second measuring device (Fig. 02) is a 5-channel sensor (Sensys Version 1.x). The advantage over the single-sensor measuring system lies in the increased grid size, as a maximum area of 50 × 50 meters can be measured here, and above all in the significantly increased measuring speed. The processed grayscale images are linked to various thematic maps in ArcGIS (ESRI), which forms the basis for the creation of various maps and the comprehensive evaluation of the sites.
The ground penetrating radar device used is manufactured by the American company GSSI (www.geophysikal.com) and distributed in Central Europe by Allied Associates (www.allied-associates.co.uk/files/contactus.html). The measuring system, consisting of an antenna and a portable computer, makes it possible to excite electromagnetic waves that propagate in the subsurface and are reflected there at electromagnetic discontinuities (e.g. geological layer boundaries) and others. Waves that propagate back to the antenna due to their path can then be digitally recorded. The depth of a reflector can be determined by knowing the propagation time and wave velocity. The resolution and penetration depth of electromagnetic radar waves work against each other. High frequencies correspond to a small wavelength, which in turn can resolve smaller heterogeneities in the subsurface. At the same time, high-frequency waves are more strongly attenuated than low-frequency waves, so that the penetration depth is lower in comparison. For this reason, both a 200 MHz and a 400 MHz antenna were used for the ground penetrating radar measurements, which have a vertical spatial resolution of around one and half a decimetre respectively in average underground conditions and guarantee a penetration depth of one to several meters. During the ground penetrating radar measurement, the antenna is pulled along the profile line over the ground. The distance is measured via an impeller. Different ground conditions at different locations are taken into account by the respective calibration of the impeller. The ReflexW program (www.sandmeier-geo.de) is used to evaluate the amplitude-time traces. ReflexW makes it possible to display profiles in two dimensions and to evaluate sections of several parallel profiles in three dimensions.
The pedological investigations are primarily used to be able to address the anomalies measured by geomagnetics in detail, but also to find out the structure of the individual layers in the subsoil. In the project, the pedological investigations are carried out by Dipl.-Geogr. K. Röttger. A gouge auger (Pürckhauer) is rammed into the ground by hand using a soft-face hammer. The drill bit has a diameter of approx. 2 cm and can be extended to a depth of 2.50 m. Once it has been driven into the ground, the boring bar must be pulled out manually. Based on the resulting drilling profile, the structure of the subsoil can be described and cultural layers can be recognized or anomalies can be interpreted more clearly from the greyscale images of the geomagnetics. Selected samples (charcoal, bones, etc.) taken from the drill stem are forwarded to Zurich (Laboratory of Beam Physics Radiocarbon Dating) for AMS-14C dating: Under the direction of Dr. I. Hajdas, ten samples from the Corvey site were examined.
Since the 1990s the Institute of Archaeological Studies has provided a laboratory for aerial archaeology, which is equipped with extensive tools of modern technology. The laboratory research and educational capabilities are available in many areas.
Primarily the laboratory for aerial archaeology deals with the archaeological interpretation of aerial images which have not been taken for archaeological purposes. Generally, it concerns itself with the following aspects: the research of existing external images, the purchase of ideal archaeological images, the archiving of acquired images and the archaeological interpretation of such.
In aerial archaeology a various number of sources of information are exploited. They include the archives of military reconnaissance, geological and geographical remote sensing, land surveying, etc. Through the use of aerial image catalogues and indicators within aerial images – depending on the objectives – certain images may be selected and viewed. Among others, the criteria for image selection include image types (vertical or angled images, image overlapping, etc.), image scale and recording time (time of year and time of day). The selected aerial images are obtained either by loan or purchase. Later, these images are archived and prepared for interpretation. Depending on the aim and individual requirements the actual interpretation can be divided into different working phases, such as the preliminary interpretation, interpretation, and the verification by fieldwork. The archaeological contents of aerial images are recognized, interpreted and depending on the aim, documented in a variety of ways (preferably notationally and cartographically).
At the Institute of Archaeological Studies the Aerophotogrammetry occurs digitally. Both mapping photographs (purpose is for survey of vertical aerial images) and aerial images of the airborne prospection (usually angled images) can be rectified or alternatively georeferenced. The main reason for the geometric image rectification is to correct the lens distortion which occurs due to the central projection of the aerial images. After the graphic rectification, aerial images with certain map properties can be created. Therefore, these aerial images can be further compared to aerial maps with less effort. Furthermore, aerial images can be projected into a geographic coordinate system or into a coordinate system of the national survey (georeferenced). This can serve as the basis for an archaeological information system.
The versatility of aerial archaeology allows the recording and exploration of archaeological sites that would usually only be discovered by accident or could only be surveyed with great effort.
Aerial archaeology is a method of discovery, observation and documentation for archaeological sites. While archaeological aerial image interpretation is mainly used for field surveys of above ground preserved sites, it also can be used on completely levelled terrain to find preserved underground sites. Aerial survey is the direct flyover of important archaeological areas. Usually a two- or four-seated sporting aircraft like a Cessna 150 or Cessna 172 is used. If a new site is discovered it will be charted and subsequently geolocated via a digital camera GPS. The combination of a small aircraft with an easy to operate camera system allows for the observation and documentation of the sites through different perspectives and flight levels at any time of the year. Through that process it is possible to document and investigate archaeological sites which are usually discovered by accident.
Sites with different archaeological features can be recorded cartographically in the form of grid-based digital aerial maps and vector-based line drawn maps. This option is important for regions and countries in which topographical maps are either missing or unavailable. At the Institute numerous aerial maps with different scales and an atlas based on the archaeological information system was created.
Archaeological information usually consists of spatial data. The best option to systematically record, evaluate and utilise the data is by using Geographic Information Systems (GIS). GIS consists of a system of computer software, hardware, data and personnel by which spatial data can be processed, analysed and presented. Currently the institute uses Esri ArcGIS (campus licence). For the purposes of several archaeological research projects, GIS proved to be a useful tool in cultural heritage administration and archaeological studies. The institute has extensive theoretical and practical experiences with GIS.
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Last update: Jun 20, 2024
