geophysical prospection, aerial photography

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AND CHEMICAL ANALYSIS IN ARCHAEOLOGY. 1. 1. 1. 1. Luis Barba ... spot test phosphate distribution. PRINCIPAL COMPONENTS ANALYSIS AND MDT.
GEOPHYSICAL PROSPECTION, AERIAL PHOTOGRAPHY, TOPOGRAPHY AND CHEMICAL ANALYSIS IN ARCHAEOLOGY 1Luis Barba, [email protected] Agustin Ortiz, [email protected]. 1Jorge Blancas, [email protected]. 1 Instituto de Investigaciones Antropológicas, Laboratorio de Prospección Arqueológica, UNAM. Circuito Exterior S/N, Ciudad Universitaria, Del. Coyoacan, C.P. 04510, México, D.F.

UNIVERSIDAD NACIONAL AUTONOMA DE MEXICO INSTITUTO DE INVESTIGACIONES ANTROPOLOGICAS

LABORATORIO DE PROSPECCION ARQUEOLÓGICA ARCHAEOLOGICAL PROSPECTION LABORATORY

GROUND PENETRATING RADAR (GPR, GEORADAR) INTRODUCCTION The purpose of the geophysical prospection is investigate the site response to different geophysical technics, to define the position of interesting structures that can be investigated by archaeologists. Surface geophysical methods are used for delineation of various anomalies including; buried walls, ditches, burial mounds, graves, ancient water courses, field boundaries, and sometimes for forensic applications. The main goal for using geophysics is to identify areas for detailed investigation and as follow up to conventional archaeological dig. Geophysical survey allows to create maps of subsurface archaeological features. Geophysical instruments can detect buried objects when their measurable physical properties contrast with their surroundings. In some cases individual artifacts, especially metal, may be detected as well. Readings taken in a systematic way can become a data set that be represented as distribution maps.

Ground penetrating radar (GPR) uses high frequency electromagnetic waves to acquire subsurface information. Energy is radiated downward into the ground from a transmitter and is reflected back to a receiving antenna. The reflected signals are recorded and produce a continuous cross-sectional image (radargram) or profile of shallow subsurface conditions. Reflections of the radar wave occur where there is a change in the dielectric constant (RDP) or electrical conductivity between two materials. Changes in conductivity and in dielectric properties are associated with natural conditions such as bedrock, cementation, moisture, clay content, voids, and fractures. Large changes in dielectric properties (RDP) often exist between geologic materials and manmade structures such as buried features or archaeological structures. The highest amplitude radar reflections usually occur at an interface of two relatively thick layers that have greatly varying properties. If the target archaeological features are composed of almost exactly the same material as the matrix, or have the same physical and chemical properties, there will no variation RDP between them, and therefore little or no reflection will occur at their interface. GPR can be a powerful tool in favorable conditions (sandy soils). The main disadvantage of GPR is that it is highly limited by the high electrical conductivity of fine-grained soils (clays and silts) causes conductive losses of signal strength; discontinuos bedrock and heterogeneous sediments scatter the GPR signal. Another disadvantage is that data processing is relatively slow.

Survey results can be used to guide excavation and to give archaeologists insight of non-excavated parts of the site. Unlike other archaeological methods, geophysical survey is neither invasive non destructive. For this reason, it is often used where preservation is the goal rather than excavation, and to avoid disturbance of culturally sensitive sites.

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GPR data can also be used to construct 3D-models and depth slices of the anomalous reflections and hence visualize the extent of potential structures.

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AERIAL PHOTOGRAPHY THE METHODOLOGY TO STUDY ARCHAEOLOGICAL SITES north

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An aerial perspective produces immediate visual information that is not discernable from the ground. Even the most well trained eye of a specialist archaeologist can miss outstanding features if the archaeology site is only surveyed at ground level. Archaeological prospecting can be greatly enhanced by aerial photography before excavation. A strong visual contrast of ground-based features is presented by aerial photography of an excavation site. In the experience of our laboratory balloon photography has provided detailed information through cheap low-altitude color photographs in archaeological sites. The visual information, in the form of changes in color or relief shadows from the surface, interpreted along with available cartography and environmental data, provides the basis for further research.

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CHEMICAL ANALYSIS The application of chemical analysis to the study of archaeological floors is recent. The history of phosphate analysis in archaeology shows that, as a result of its evolution this analysis is no longer a mere agricultural technique adapted to archaeology, but has become an analytical technique especially designed for field archaeology, as it demonstrates a close relationship to human activities. In r ecent times more spot test have been incorporated to include organic indicators of human activities. Since phosphorus is not the only element present in settlement areas, many of the elements determined by geochemistry such as calcium, magnesium, copper, potassium, and zinc may be useful to archaeology. Substantial increases in the concentration of these elements in occupied areas have been reported. More recently other researchers has proposed the use of some other chemical elements related with human activities. Some authors have implemented a set of chemical elements analyzed by XRF and ICP to locate middens and activity areas.

TOPOGRAPHY Topographical changes often contains important information on the conditions and nature of an archaeological site and the probable existence of structures buried beneath the soil. The changes in topography can also have a greatinfluence in the size and extent of buried archaeological structures. Therefore it is important to produce a detailedand complete topographic survey as part of the field survey of any given site. This usually involves the recording of elevations and depressions across a grid of high resolution, but also the recording of points on known breaks of slope, to emphasis archaeological features in the landscape. Topographical survey with diferential GPS equidistance between levels 0.1 m

PRINCIPAL COMPONENTS ANALYSIS AND MDT

spot test phosphate distribution

MAGNETIC METHOD Magnetic prospecting is used to study extensive areas and probably is the most used method in archaeology so far and produce very good results with high accuracy. Total field measurements (made with one magnetometer) and gradient measurements (made with two magnetometers) are commonly used. Magnetic gradient measurements are made by a gradiometer, which includes two magnetic sensors separated by a constant offset. Magnetic gradient measurements are often used to enhance the anomalies produced by shallow buried objects. Modern portable gradiometers measure the vertical gradient of the Earth's magnetic field in nT/m. Different materials below the ground can cause local disturbances in the Earth’s magnetic field that are detectable with this technique. Gradiometers are more sensitive to archaeological features like kilns, iron, bricks, burned soil, and many types of rock. Where these highly magnetic materials do not occur, it is often possible to detect very subtle anomalies caused by disturbed soils with a minimum amount of magnetic minerals. The main limitation of magnetic survey is that subtle features of interest may be obscured by modern materials or a highly magnetic geology.

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Digital terrain model produced by diferential GPS data combined with vertical gradient data

ELECTRICAL RESITIVITY METHOD The electric direct current (DC) method for resistivity measurements gives the distribution of the electrical or potential gradient of a direct current. Electrodes are used to introduce a current into the ground and to map the potential distribution. There are different types of electrode configurations like dipole-dipole, pole-dipole, and the Twin Electrode configuration that has been developed specially for archaeology. Electrical resistance of the ground is a lmost entirely dependent on the water content. Archaeological features can be mapped when they are of higher or lower resistivity than their surroundings. Hearths, habitation sites, graves, foundation walls, burned structural poles and rocks used for construction are just a few of the archaeological targets detectable with the electrical resistivity method.

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Shaded relief map of vertical gradient of the magnetic field Electric anomalies of ancient settlements

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