Archaeologists employ a wide range of reconnaissance techniques to locate archaeological sites and investigate them without excavating. The four main categories that are commonly used to locate and explore sites before development:
- Desktop study – office-based research using existing records such as historical, legal, tax, economic, pictorial and archaeological records as well as ordinance survey maps. In addition to concrete written documents, oral accounts from farmers that may have knowledge on the particular land type may be of help.
- Surface study – field walking, surveying and planned aerial photography. Non-destructive visual surveys at ground level such as finding traces of unrecorded sites e.g. scatters of building rubble or slight undulations in the surface could indicate where buried walls or house platforms are. Samples are taken in a variety of ways including systematic (at regular intervals along a transect) and random (using randomly generated intervals plotted on GPS). Geochemical prospection enables highly advanced soil analysis that calculates the levels of phosphate (organic phosphate as waste from living organisms) present in the soil that may indicate settlements or animal enclosures.
- Geophysical or geochemical survey – Techniques that detect features through physical differences with the surrounding soil. The most common methods use detection of magnetic and electrical anomalies (these require a high level of skill to interpret). With the increasing popularity of archaeology there has been a shift amongst archaeologists to favour preservation rather than excavation.
A second option is resistivity surveying = the differences in the ability of different soils to conduct electricity. The electricity is conducted through the soil by mineral salts in the water. The higher the moisture content the better the conductivity – archaeological sites such as a buried ditch or grave will retain water more effectively than the surrounding soil. A buried wall or road will conduct poorly and therefore resist current more. This method works better in some kinds of soils over others – clay for example retains water so well that it may be difficult to detect differences in resistance as well as factors such as plants and rocks that create variations in depths of soil.
- Aerial survey – uses aerial photographs taken from balloons or light aircraft and sometimes even kites. These are used for mapping, finding new sites swiftly over vast areas of land and illustrating or exploring known sites.
The Archaeology Coursebook: An Introduction to Themes, Sites, Methods and Skills, Jim Grant, Sam Gorin and Neil Fleming