Geophysical Surveys Services
Geophysical surveys are non-destructive and non-intrusive methods to detect buried features when their physical properties contrast measurably with their surroundings. Archaeological remains can be considered as a modification of the soil, and thus the evaluation of differences between the physical property of soils and archaeological remains contained therein basically leads to the identification of “geophysical anomalies”. Geophysical survey usually describes the process of imaging these subsurface anomalies both as a sequence of two-dimensional slices (e.g. ground penetrating radar and electrical resistivity tomography ERT) or as two-dimensional maps (ground penetrating radar, magnetometry and electrical resistance meter).
Services Details
A great variety of geophysical methods are currently available for specific archaeological demands and the choice of either a single method or combined geophysical applications depends on the particular characteristics and size of the site. In most cases, magnetometry and ground penetrating radar (GPR) yield the best results due the particular characteristics of the site. Certain conditions, such as an extensive investigation or a high conductivity ground, alternatively require the electrical resistivity tomography method (ERT). The geophysical surveys are also non-destructive practices that produce significant, new data while preserving the non-renewable archeological record. In any case, geophysical surveys are particularly needed prior to archaeological excavations as they represent a cost-effective and rapid means of assessing a site’s archaeology.
In this framework ArcheoRes operates across Europe, employing the most appropriate geophysical survey techniques to match local requirements to evaluate the unseen archaeology of a site beneath the soil. Thanks to the collaboration with the Center of Excellence of the University of Perugia (S.M.A.Art. – Scientific Methodologies Applied to Archaeology and History of Art), our services include intensive survey by ground penetrating radar, magnetometer, electrical resistance meter, electrical resistivity tomography covering sites of all sizes. We also deliver thermal imaging and metal detecting.
Ground Penetrating Radar
The ground penetrating radar, also known in the literature as GPR, is a versatile tool for geophysical survey. It involves transmitting high frequency electromagnetic pulses form a surface antenna into the ground (frequency range between 10 and 2000 MHz) and recording electromagnetic waves reflections by a receiver antenna. If electromagnetic waves strike an object they will be bounced back to the surface. The time elapsed between the pulse being sent and it being received back will provide evidence of depth. The reflections of EM waves can be caused by changes of electrical properties, water content and lithological properties of the ground. In GPR surveys for archaeological purposes, those reflections are usually produced by buried walls, hypogea, tunnels, metal objects and stratigraphic layers. The post-processing software allows to produce the so-called time slices maps, two-dimensional detailed representations of GPR anomalies at a given depth. The ground penetrating radar provides the best results when it is used on low attenuation soils, characterized by a low electrical conductivity, such as sand, rock, ice, etc. Otherwise, it is much less effective when operating in high-attenuation soils such as clay, saturated silts and soils containing water with high concentration of salts.
Magnetometry
Magnetometer survey offers the most rapid ground coverage of the various survey techniques and responds to a wide variety of anomalies caused by past human activity. It is cost-effective, it delivers reliable results and it ensures the coverage of large areas in a few hours of work (in good terrain conditions up to 10,000 square meters per day). It should thus be the first technique considered for detailed survey of an area and other, slower, techniques should usually follow afterwards, targeting smaller areas of interest identified by the wider magnetometer survey. Magnetometry is based on the measurement of localized variations in the earth’s magnetic field or its gradient. These variations – or magnetic anomalies – reflect the difference between the remaining magnetic susceptibility of archaeological targets and the magnetic susceptibility of the soil. It can also identify thermoremanently magnetised features such as kilns and furnaces as well as in-filled ditches and pits and areas of industrial activity (both recent and ancient). Unless composed of materials that contrast magnetically with the surrounding soil (eg bricks carrying a thermoremanent magnetisation), magnetometers do not usually detect wall footings directly and in this regard it is complemented by earth resistance survey.
Electrical Resistivity
Electrical resistance survey (also called earth resistance or resistivity survey) is a geo-electrical prospecting technique commonly used in archaeological research. Earth resistance was one of the first geophysical methods applied to archaeology, as early as 1946. It involves mapping buried features by feeding electric current into the ground and taking resistance measures at particular points. Such resistance is highly dependent on the distribution of moisture in soils affected by drainage, the presence of structures and soil porosity. In electrical resistance survey for archaeological purposes, an array of multiple electrodes is required to undertake fieldwork. Within two pairs of electrodes, current passes through one pair and resistance is measured by the other pair. There are different electrodes configurations and different distances between them, which provide significant variations in results (e.g. Wenner array, double dipole and twin-probes array, etc.). Compared to other geophysical methods, earth resistance is a relatively slow method since surveyors should push mobile probes into the ground every fixed distance, which takes some time. The most suitable conditions for earth resistance surveys are the presence of sedimentary soils, lack of rock outcrops on the surface, limited short vegetation and a well-drained season. In general, any buried archaeological remain have physical properties that differ from the surroundings, so that the electrical resistivity survey technique is very suitable for detecting walls, ditches, burials, roads, etc.
Electrical Resistivity Tomography ERT
The multi-electrode electric tomography (also called electrical resistivity tomography or ERT) is a geo-electric probing technique commonly applied in archaeological research to detect buried structures. The ERT technique provides the measurement of the specific electrical resistance of soil at high depth across a two-dimensional section. This technique differs from earth resistance method in that the depth of investigation is higher and it is largely controlled by using variable electrode spacing. Conventionally, ERT technique uses an array of probes and compares the resistance between each and every pair – according to different probes arrangements (Wenner, Schlumberger, Dipole-Dipole, Pole-Pole, etc.) – in order to produce a cross-section or ‘pseudo’-section through the ground. The graphical representation of this section is then produced during pro-processing by the means of Finite Element Method (FEM).
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