An archaeological field survey is an examination of an area known or believed to be of archaeological importance. It may be intrusive or non-intrusive, depending on the needs of the survey team and the risk of destroying archaeological evidence if intrusive methods are used. It is a practical way to decide whether or not to carry out an excavation, and a way of recording the basic details of a possible site.
A special type of field survey is the rescue survey, used when a site of possible archaeological importance is under threat. This is usually connected to construction work, and is undertaken to decide whether an excavation is necessary or not before work can commence on the site.
Research and planning
A field survey is usually the result of a long period of research and planning. The process will normally start with the notion that a site is worth excavation or that it should at least be recorded in some way.
Indicators
There are several indicators that an area is a potential target for a survey. In no particular order, they are:
- Artifacts found: Locals have picked up artifacts, sometimes found in the local museum but more often in private homes or old buildings such as churches.
- Literary sources: Old literary sources, in some cases ancient Roman or Greek texts, have provided archaeologists with clues in many cases. Sometimes the texts may be quite recent; a book on local history that mentions an interesting area.
- Oral sources: In many locations, local stories contain some hint of a greater past, and there is often some truth to them. It is not uncommon for someone to remember that a grandfather who used to walk the hills around a town as a shepherd used to talk about columns from an old temple, without having actually seen these themselves.
- Local knowledge: In many cases, locals actually know where to find something that is of interest to archaeologist. They may not have reported it, either because it's simply a part of their world, or because they fear intrusions on their land.
- Previous surveys: In some places, a survey was carried out in the past, and is recorded in an obscure academic journal. It may have been disregarded at the time, but more recent technologies and finds from other sites might cast a different light on it.
- Previous excavations: Excavations carried out before the middle of the 20th century are notoriously poorly documented. They were also often carried out in a way that left much of the evidence the modern-day archaeologist is looking for behind, preferring to carry away only fine pottery, jewelry and statues.
Aerial photography
Main article: Aerial archaeology
Aerial photograhy is a very good tool when planning a survey. Remains of older buildings often show in fields as cropmarks, as they are often just below the topsoil and therefore affect the crops or grass. There should preferably be photographs of the same area at different times of the year, allowing the analyst to find the best time to see cropmarks.
Previous work on the site
If the indicator that started the process was not a record of previous work, the archaeologists will need to check if any work has been done. As many older surveys and excavations were published in papers that were not widely spread, this may be a difficult task. A common way to handle this is through a visit to the area, to check with local museums, historians and older people who might remember something.
Permissions
It is usually a simple matter to gain permission to perform a survey, especially a non-intrusive one. If the area is privately owned, the local laws may or may not require the landowners' co-operation. Permission for an intrusive form of survey may be more difficult to aquire, due to the fear of destroying evidence.
Intrusive vs. non-intrusive surveys
In a non-intrusive survey, nothing is touched, just recorded.
An intrusive survey can mean different things. In some cases, all artifacts of archaeological value are collected. This is often the case if it is a rescue survey, but less common in a regular survey.
Another form of intrusive research is bore holes. Small holes are drilled into the ground, most often with hand-powered bores. The contents are examined to determine the depths at which one might find cultural layers, and where one might expect to strike virgin soil. This can be valuable in determining the cost of an excavation - if there is a build-up of several meters of soil above the layers the archaeologist is interested in, the price will obviously be much higher than if artifacts are found only centimeters below ground.
Field walk
An important part of the survey is normally the field walk. The common way to perform it is to draw a grid, place the survey team on a line and then walk slowly through the area looking for artifacts or other indications. This works best on either plowed ground; as the soil is turned regularly artifacts will move to the top, or on ground that has not been cultivated since the period the archaeologists are interested in.
Modern technology such as GSP has made the process much easier, as coordinates can be taken well within the limits necessary for survey work.
In some areas, the field walk is quite different. When searching in dense jungle, buildings may be covered by vegetation, and are therefore virtually invisible even at short distances. The team will then need to look for unnatural changes in the vegeation and landscape to decide if a building is hidden below the vegetation.
Narrowing it down
At this stage, the problem is often that one knows the approximate area, but it needs to be narrowed down. During the field walk, the members of the team are likely to miss minor pieces of artifacts hidden in vegetation. However, if the are all trained to look for the same thing, it is likely that they will miss the same amount of artifacts, and the results of the survey can therefore still be used to draw a map of find frequencies within the grid system. This might in turn make it possible to identify the main site.
Geophysical survey
Main article: Geophysical survey
In recent years, there have been great advances tools for geophysical survey. The most important are resistivity meters and magnetometer. Both techniques are non-intrusive, and are often used as part of a field survey. While the magnetometer is better for certain locations, the resistivity meter is more often used because of the lower cost.
A resistivity meter passes an electrical current through the ground between two electrodes. A moist soil will offer comparatively low resistance, while drier and denser matter will give a higher resistance reading. In archaeology two, four, or more probes are fitted to a frame, with a waist-high handle for use. The display meter is usually fixed to the handle. Readings are usually taken along pre-surveyed lines at regular intervals, hopefully cutting across any features of interest. This method works best with an flat and well-drained soil with artifacts at a similar depth, natural variations can easily give misleading results. It is best suited to finding strong linear sources like walls or roads by measuring across a suspected placement.
Magnetometers detect variations in magnetic fields, so archaeologists can undertake a magnetic survey. The alignment of naturally occurring magnetic soil particles can be altered by a number of human activities. Simple soil-moving to form ditches or pits can be detected; solid constructions will often contain fewer magnetic material than surround soil; and high heat can realign magnetic particles - indicating the presence of furnaces, kilns or similar.
The difficulty with geophysical surveys is that they do not distinguish between the structures that are of interest to archaeologists and more recent ones.
Analysis
The most important part of the survey is the analysis, bringing all the parts together. At times, one part of the survey may not have yielded the evidence one wanted to find. For instance, very little may have been found during a field walk, but there are strong indications from geophysical survey and local stories that there is a building underneath a field. In such a case, the only way to decide if an excavation is worth the cost is to carefully analyze the evidence to determine which part to trust. On the one hand, the geophyics might just show an old and forgotten waterpipe, but it might also show the wall of just the building the archaeologists were looking for.
The analysis therefore includes careful examination of all the evidence collected. A method often used to determine its value is to compare it to sites of the same period. As the number of well-documented surveys grow, this becomes a slightly easier task, as it sometimes is easier to compare two survey results that a survey result and an excavated site.