There are two main categories of activities on archaeological sites: invasive and non-invasive. The first contains activities including earth work (e.g. excavation), the second contains continuously developing fields called site diagnostics. This latter category mainly uses geophysical methods and analyses archaeological sites non-invasively. By this method we can gain information on subsurface features relatively quickly without excavation. This information often increases the efficiency of the excavation to be carried out.
Archaeological problems related to developments
Concerning rescue or preventive excavations, developers (state or private) are liable for the expenses, known as the polluter-pays principle. Nevertheless, the amount of time and costs needed are difficult to calculate without prior knowledge about the site, therefore this situation poses a considerable risk for a development to be carried out.
In some cases, before purchasing or expropriation of a property, there is archaeological information available about the area that needs to be excavated. Nevertheless, archaeological topographic survey of Hungary covers only a limited area. Therefore, from the aspect of the development, receiving information about the archaeological coverage of an area may be too late, since the development is already in the process of authorisation. In these cases, the lack of information increases the amount of time needed for the excavation and also the costs of the development. This results in that the business plan of private investments may topple over, but state developments are also jeopardised because of EU financed developments have strict clearing obligations.
How to decrease these risks?
Archaeological coverage of an area can sufficiently be surveyed by geophysical methods. The cost of this survey is considerably smaller than that of the excavation. After a geophysical survey, costumers are provided with an impact study containing all the results of the survey. Accordingly, developers can calculate the cost and time of the excavation, or if it is possible the archaeological site can be avoided by the development.
Archaeological geophysical survey
Among the several methods of geophysical surveys in archaeology (near surface geophysics) we can use any of the three most wildly used and reliable methods. All these methods use high capacity measuring equipment with RTK GPS. Our colleagues have several years of experience in geophysical surveys. A fundamental difference is that as opposed to companies dealing with engineering geophysics, we are specialised in the identification of archaeological features. The difference is mainly that we have a dedicated equipment for archaeology and a specialized know-how, which are fundamental for the survey to be successful.
The most commonly and reliably used method in archaeological geophysics is magnetometry, which mainly identifies subsurface dug in features (ditches, pits, houses). Its advantage is that measuring and data assessment is fast, its disadvantage is the sensibility of the equipment, therefore it is most suitable for using it in green area excavations. We mostly use the German SENSYS gradiometers consisting of two single-axis Fluxgates that are aligned vertically to each other in a distance of 650 mm. Measurements are carried out by both manually or pulled by a quad bike. In the latter case even 20 ha a day can be surveyed. The assessment of the measurement is fast and flexible, therefore, if needed, surveying strategy can be adjusted on the site.
Ground Penetrating Radar
GPR surveying methods have gone through the most development recently. The number of channels and used frequencies increasing, but recently step-frequency radar systems are also used. These latter methods collect data in changing frequencies during the same measuring process. Since it is an active equipment (emits electromagnetic waves and measures back scattering) it is less sensitive to environmental conditions. For archaeological purposes GPRs with 200–1000 MHz wavelengths are used. For soils in Hungary 400–450 MHz are used but this also depends on the depth of archaeological features. It is mainly suitable to survey subsurface walls, built structures, walking surfaces, burnt surfaces and holes.
We use the Swedish ImpulseRadar (CO4080 and RAPTOR) for this survey. The advantage of the system is that it is reliable and fast (even though slower than magnetometric survey). However, the effectiveness of the survey is influenced by the humidity of the environment. We mainly use GPR to identify built heritage in urban area where there are cables and public works are in the way. During GPR survey, subsurface features are detected by the variations in the electromagnetic waves received. The advantage of this method is that in the case of walls and features with different infills, we receive good quality and accurate measurements. Its disadvantage is that in humid and too clayey environment it is less effective, and the processing of measurement is slower.
This is the most active and also the slowest method, it has numeral possible applications. During this survey, electrical current is passed through the ground at regular points. The patterns of resistance in the ground are recorded. Electrical resistance is affected by the presence of archaeological features. In archaeological survey it is very useful, since it identifies subsurface stone walls and floors since these have different surface resistivity. Its advantage is that it can be used very well in forested areas or on disturbed surfaces to identify walls or holes in the ground. The depth of the features can also be measured and it provides suitable data even for a depth of 10 m. For this survey we use the UK’s GeoScan and the Czech ARES.
Archaeological geophysical methods considerably help targeted and efficient archaeological interventions and the planning of costs. Therefore, the use of these methods is suggested in all cases where information is needed on subsurface features before the intervention.
Impact study and Preliminary Archaeological Documentation (PAD) based on field walking
For the preparation of developments and action plans we advise the preparation of an impact study and PAD along with site diagnostics. The impact study and PAD includes results from field walking, data archives and from the official site registry. During the preparation of these documentations archaeologists survey the area with GPS and the collected data are mapped. We query site data from the official site registry, but apart from including these data on a map, the archives of local museums are also necessary to consider in order to provide the most accurate spatial distribution of a site. These data form the basis to calculate the time and cost needed for the excavation.
We have been carrying out aerial survey for a long time. Aerial photographs may be suitable for assessing the area to be excavated and to identify certain features, such as fortifications, ditches, walls and in some cases graves. Besides the survey, the general overview picture of the site provides a good visual aid to assess possible correlations between features.