The Laboratory conducts small and large scale geophysical surveys of archaeological sites for mapping the archaeological relics and increase the efficiency of the excavation activities. Both, geophysical prospection and satellite remote sensing contribute in the mapping of the subsurface archaelogical monuments, the management and conservation of the archaeological sites and the better exploitation of the environmental and cultural resources.

Geophysical prospection techniques and satellite remote sensing/G.I.S. are used for the preservation, monitoring and management of the environmental resources (natural and cultural). The Lab is also involved in research of the environmental consequences of technical construction works, environmental pollution and geological mapping/hydrogeology. Geographic Information Systems, classification techniques and thematic mapping is applied together with image processing techniques for the assessment and management of the natural and cultural resources (landuse, protection areas, urban planning, etc.).
The Laboratory is also conducting research on the biological remais from archaeological excavations, aiming to the reconstruction of the ancient environment.

The Laboratory of Geophysical - Satellite Remote Sensing & Archaeo-environment has performed a number of geophysical prospection surveys in Greece and abroad (Cyprus, Egypt, Hungary, a.o.), offering its services to the Greek Archaeological Service, Universities and Higher Institutions. The researchers of the Laboratory are also dealing with the training of researchers and students through hand-on experience in the field and lecture series.

The Laboratory uses the most modern and precise equipment for fieldwork and sophisticated computer facilities and software for processing and mapping of the geophysical and remote sensing data. Geophysical investigations include the use of caesium, proton and fluxgate magnetometers/gradiometers, soil resistivity and conductivity techniques, microgravity, seismic, electric tomography, ground penetrating radar (with a variety of antennas) (G.P.R.), Global Positioning Systems (G.P.S.) and magnetic susceptibility measurements. The Lab is also equipped with the necessary instruments for macro and micro analysis of archaeo-environmental remains. Digitization boards, scanners, color plotters and printers guarantee the high quality presentation of the results and maps.

Services

• Near-surface mapping of the archaeological sites with aim to guide excavations or surface surveys, locate and map archaeological remnants and estimate the wider area of archaeological interest.
• Estimation of depth to the archaeological relics and verification of the occupation layers. Magnetic properties of the archaeological soils and materials.
• Satellite Imagery and Aerial photography. Image processing and Geographic Information Systems (G.I.S.). Emphasis on the archaeological site assessment and management. Predictive modelling & Settlement patterns.
• Planning and management of actions for the development of archaeological sites. Assessment of the effects of construction works in archaeological areas.
• Digitization and scanning of topographic maps, geological maps, architectural drawings, pictures, images, etc.
• Digital Elevation models (DEM). Thematic mapping (2-D & 3-D).
• Environmental consequences on archaeological sites. Conservation areas. Cultural and natural national parks. Assessment of archaeological parks.

In large development projects, geophysical investigations constitute a necessary tool for the assessment and management of archaeological sites. Aerial photography and satellite digital imagery are able to locate and confine areas of probable archaeological interest.

Surface surveys in conjunction to other remote sensing techniques (geophysical, aerial, satellite) can be combined through a Geographic Information System for a better geographic registration of the archaeological finds and a productive management of archaeological information.

Project planning

A geophysical prospection project consists of two phases: the field work and the lab (data analysis and processing) phase. The fieldwork usually takes a period of a week for covering an area of about 5-8,000 m2, with a 1m sampling interval. Geophysical measurements are conducted in specific regions of interest, following the consultation of the supervising archaeologist.

The technical support of the Laboratory includes the mapping of large areas with a dense sampling interval, which is usually specified depending on the needs of the survey and the land cover. In this way, it is possible to locate architectural remnants, wall structures, trenches, kilns, roads, firehearths, pits and other features of archaeological interest. A much more systematic approach could even estimate the depth to the archaeological features and identify the different occupation layers of a site. Processing of the geophysical data is followed by the production of maps which represent the geophysical targets of probable archaeological nature.

The analysis and processing of the geophysical data includes the mapping of surface features, the application of mathematical filters, the drawing of the geophysical grids, image processing techniques, modelling and mapping of the geophysical anomalies, etc. The final report is in most cases ready within a period of 3-6 months after the end of the fieldwork period, depending on the availability of related information and urgency of the project. In a few cases, a first-hand processing and mapping of the data is possible on the site, providing (almost) ´´real-time´´ information on the results of the geophysical survey.

Methodology

Electrical Methods

One of the most commonly used geophysical techniques in the detection of "shallow structures" is the electrical method, which is also known as "Direct Current method". The purpose of the method is the determination of the subsurface resistivity contribution, by conducting measurements at the surface of the earth.

To achieve this, electric current is inserted into the ground via two electrodes and the potential difference, which is caused by the inserted current, is measured in two other electrodes. The measured potential difference gives an image for the difficulty of the current flow through the subsurface. This is an indication of the electrical resistance of the subsurface. In figure 1 a typical array of four electrodes A, B (current electrodes) and M, N (potential electrodes) is presented.

The resistivity method is widely used in Hydrogeology to detect aquifers, in Technical Geology to find the stable rock and the cavities and in assessing the hydraulic properties of the subsurface e.t.c., for environmental purposes in detecting the ground-water pollution, in the search of geothermic areas and mines and in Archaeometry.

The resistivity depends on hydrological-hydrogeological conditions, the chemical composition of the water, the dissolved ions in it, the porosity of the formation, the possible fractures, the temperature and pressure and the topography.

As it was mentioned above resistivity depends on many factors and it doesn´t comprise a distinguishing property of specific formations, since the resistivity variability may have a large range in the same formation. Thus the interpretation of resistivity measurements must be treated with caution and must always depend on the available geologic information of the area (geologic maps, drills).

The procedure of measurements´ acquisition depends on the use of four electrodes, two in order to insert current and two to measure the potential difference. The current probes are inserted into the ground in a depth of a few centimeters and distances that vary from a few to several hundred of meters.

Due to the fact that earth is inhomogeneous and anisotropic the measured resistivity depends on the subsurface geoelectrical distribution and the geometric arrangement of the probes. In order to include these factors in the actual measurements the term apparent resistivity is used.

Resistivity Survey Methods

Soundings: The purpose of the vertical electrical resistivity sounding (VES) is to investigate the variation of the resistivity with depth. The whole procedure is based on the assumption that the subsurface has a horizontal stratigraphy. In other words it consists of discrete, horizontal, homogeneous and isotropic layers. The array that is commonly used in VES is the Schlumberger array. VES are mainly used in Hydrogeology.

The measurements are taken with gradually increasing distances of the current electrodes (the potential electrodes remain constant). As the distance between current probes is increased, there is also an increased in the depth at which the current penetrates below the surface of the ground increasing the depth of investigation. In this way, an estimate of the vertical resistivity distribution below the centre of the array is determined.

Profiles: Profiles are used to detect lateral resistivity changes. In contrast to the VES, the distances between the probes remain stable and measurements are taken by moving the whole array with constant step interval. By this way the lateral resistivity changes at a steady depth are mapped. The most widely used arrays for profiling survey are Wenner, Pole - Pole, Dipole - Dipole and Twin - Probe array.

Especially in Archaeometry the Twin - Probe (Figure 3) array is very popular because the data can be collected within a small period of time and are easily interpreted. The method also has a relative good spatial distribution. Two remote probes faraway from the survey area are used (one for the current and one for the potential), which there are placed at a distance equal 30 times of the distance between the remote probes. (e.g. 15 meters away if the distance between the mobile probes is 0.5 meters). The mobile electrodes (one for the current and one for the potential) are moved simultaneously with a constant step. The spatial resolution ability of the method is 1.0a, while the depth of investigation can be 1.0 - 2.0a, where a is the mobile probes´ separation. The accuracy of the measurements is of the order of 1 - 0.1 O.


Tomography (2-D imaging): 2-D electrical surveys are employed for gathering information for the horizontal and the vertical variation of the resistivity. These type of measurements are also used in the quantitative interpretation of buried structures (determination of depth, size, shape of the body). 2-D electrical tomography is also applicable in detecting buried antiquities.

Nowadays 2-D resistivity surveys have been developed and can be used in large scale surveys. A series of electrodes are placed at the surface of the ground and via a multiclone cable and a multiplexing system, the resistivity measurements are obtained automatically along profiles, with a gradually increasing distance between the probes.

It is quite difficult to interpret these kinds of data instantly. The data must be processed with the algorithms of non-linear 2-D inversion. It must be noted that sections of this type are carried out of specific regions of interest, because of the difficulties that have to be encountered during the conduction of the field survey.

Gravity and Magnetic Methods

Gravity and magnetic methods consist of one of the first used techniques in the detection of subsurface structures. The relatively easy procedure in data acquisition combined with the low cost in relation to other geophysical methods (seismic reflection) make them very popular methods.

As their name reports, using these methods, it is tried two potential fields to be measured. For this reason these methods are mainly known as "potential field methods". There are cases where these methods can be used simultaneously, as the transition from one field to another can be accomplished via the Poisson relation. Poisson equation connects the gravity and the magnetic potential that a body causes, provided the density and the magnetization distribution is allocated uniformly in his whole volume. Furthermore many of the applied processing techniques to the collected data are common to the both methods.

Magnetic Techniques

The goal of the magnetic methods is to detect the subsurface´s magnetic changes because of the presence of structures which are found under the surface of the earth. During the application of a magnetic survey the local magnetic field of the earth is measured at a distance above its surface. The height of the sensor ranges from 0.5 meter, (in the case of buried antiquities detection), to some hundred of meters above the surface´s topography of an area, for the anomalies´ detection which are related with the geology of this area.

In order to measure the magnetic field, either magnetometers which measure the total intensity of the magnetic field (proton magnetometers) or gradient magnetometers (proton or flux) which measure the vertical gradient of the magnetic field are employed. The accuracy of these instruments is of the order of 0.01 - 1 nT. In the detection of archaeological remains the measurements are taken with a steady sampling step in rectangular small dimensional grids (10x10 ? 20x20), placing the sensor to a small and steady distance from the surface of the earth.

Underground bodies which have different magnetic properties from the rest of the subsurface, change to a smaller or bigger degree the local magnetic field. The magnetic field´s deformation is observed as an "anomaly" to the measurements. These anomalies are caused by different reasons which vary according to the intensity of the magnetic field and the geometric shape of these. Ditches, places of burning, kilns, architecture structures or concentration of organic material can cause these anomalies. The magnetic anomalies depend on the direction of the earth´s magnetic field and on the direction of the magnetization vector. For this reason the magnetic anomalies are mainly dipolar.

The magnetic anomalies are directly related to the soil´s magnetic susceptibility. Regions with an increased magnetic susceptibility (related to the environment) appear as positive magnetic anomalies while regions with decreased magnetic susceptibility appear as negative anomalies. Both of these kinds of anomalies are equally interesting in the procedure of the magnetic data interpretation.

In general the existence of buried antiquities in the subsurface is usually accompanied by an increase in the magnetic susceptibility causing a weak magnetic field which alters the local magnetic field of the earth. Total field magnetometers measure the resultant of the weak local magnetic field and the stronger earth´s magnetic field. Gradient magnetometers (flux magnetometers) measure the vertical or the horizontal component of the magnetic field. These kinds of magnetometers comprise the most efficient instruments that measure the local magnetic field and its variations due to shallow depth remains.

Generally the variation of the local magnetic field because of the existence of subsurface archaeological remains is relatively small, because of the weak remanent magnetization. This variation is increased as the magnetic susceptibility of archaeological targets is increased (burning phenomena, density in iron components e.t.c.). The instruments that are required for the detection of archaeological ruins must have high accuracy, great sensitivity and being trustworthy. These instruments measure the magnetic field in an accuracy of order of 0.1 - 1 nT (0.1 - 1 x 10-9 T). Even bigger accuracy of the order of pT (0.001 - 0.01 nT) is possible using the Cesium magnetometers, but there is the danger to insert high level of outside noise.

It must be noted that the earth´s magnetic field isn´t stationary but it changes with time. The one variation that is the most interesting is the daily one. These transient variations affect the magnetic measurements and can not be predicted.

Under normal circumstances the intensity of the magnetic field varies from 50 - 100 nT. In case that a magnetic storm occurs the magnetic field is more active and the diurnal variations are of the order of 100 - 500 nT. For this reason it is a necessity to observe the change of the magnetic field while the magnetic survey is conducted, using a second magnetometer. The use of gradient magnetometers has the advantage to eliminate the drastic changes of the magnetic field and to rebate the geologic influence.

Gravity Techniques

Gravity methods aim to determine the subsoil´s gravity changes conducting measurements of the gravity field on the surface of the earth. Namely gravitometry aims towards the detection of structures with different density compared to their environment (either positive or negative). These methods depend on the Newton´s gravity law. This law defines the attractive force which is exercised between two bodies with a specific mass and they are separated of a specific distance.

The modern instruments that are used to measure the gravity field are called gravitometres. Their main function principle depends on the existence of a spring. At the edge of this spring a mass is hanged. The springs which are known as "zero length springs" are commonly used nowadays to the modern gravitometers.

Gravitometers are very sensitive instruments and they are affected by temperature and pressure changes. For this reason gravitometers are placed in boxes, where temperature and pressure are maintained steady using various techniques. Furthermore the changes of the springs´ elastic properties have to be considered although many times it is difficult to predict them.

In order to measure the gravity field, it is necessary to locate a base station in the area of interest. Afterwards a net of points that comprise a grid or they are at equal spaces along a profile must be determined. All the necessary corrections (reduction to the same latitude, "free-air" reduction, Bouguer reduction, topographic reduction) must be applied to the collected data, before they will be processed and interpreted.

The advantage of these methods in relation to the magnetic methods is the fact that the anomalies are unipolar. This means that they maximize or minimize directly above the subsurface body that causes the irregular gravity field. This is ought to the fact that gravity anomalies depend only on the body´s gravity distribution. In contrast magnetic anomalies depend on the earth´s magnetic field direction and on the direction of the magnetization vector. This fact causes the magnetic anomalies to appear as dipolar.

Ground Penetrating Radar

 

Radar is the acronym for RAdio Detecting And Ranging. It is a system that uses the high frequency electromagnetic radiation.

The Ground Penetrating Radar method has the same operation principle with the seismic reflection method. This geophysical technique is applicable to strata mapping in the cases of soils and rocks and depends on the different electrical properties that various materials have. The development of the method started mainly in 1956, but accelerated considerably after 1970 as a result of the tremendous progress that took place in electronics and computer technology after 1960.

G.P.R. can be used in a series of applications like the mapping of the bedrock depth, the determination of the stratum thickness and the aquifer depth, the location of physical and artificial cavities in the subsurface, cracks in the bedrock and the tracing of the changes in the rocks´ composition. The method is specially used in Archaeometry for the detection of buried antiquities.

A high frequency electromagnetic radiation is transmitted in the ground and the reflected waves are recorded. The propagating electromagnetic energy in the ground depends on the subsoil´s electrical properties, the conductivity and the dielectric constant. Basically the method depends on the record of the waves reflected on surfaces that divide regions with different electrical properties.

G.P.R. is similar to the seismic reflection method. A high frequency, small duration electromagnetic pulse is transmitted into the ground. This pulse (signal) is diffused in the subsurface materials and its direction depends on its properties. A part of the pulse energy is reflected on the surface that separates materials with different properties and is recorded at a receiver on the surface. The remaining pulse energy is diffused at deeper levels.

The time between the transmitting and the receiving pulse depends on the velocity along the trace the pulse followed. This time can be measured and if the electromagnetic wave propagation velocity is known then the depth of the reflector can be determined. In most of the geologic materials the conductivity and the dielectric constant (relative permittivity) are the main parameters that affect the pulse. Furthermore the absorption of the signal depends mainly on the antenna frequency, the conductivity and the dielectric constant.

The maximum penetration depth of the G.P.R. depends on the absorption of the electromagnetic waves. The absorption increases with frequency and thus a smaller frequency is used for detecting the deeper targets. On the other hand the resolution of the method is decreased as the frequency is increased. For instance G.P.R. system working at the range of 25-50 MHz can investigate depths over 50 meters in soils with low conductivity (smaller than 1ns/m) like sand and gravels.

The Radar recordings (radiograms) are placed one beside the other so as to construct a section that simulates the real subsurface electrical section, producing information of the changes of the electrical properties with depth.