The three survey methods principally used by archaeologists
The geophysical methods used in archaeology have been largely adapted from those used in geological contexts, such as with mineral exploration. There are many forms of geophysical survey – but there are three which are principally used in archaeology.
measures the magnetic field strength at particular locations at particular locations within a site. This means that buried features which are more magnetic than the surrounding natural geology, which might be for example sand and gravel, will be registered by magnetometry.
Example: pottery or bricks or hearths which have been subjected to considerable heat in the past, acquire a permanent magnetic field on cooling. Pits and ditches which have been in-filled with soil which is more or less magnetic than the natural undisturbed subsoil into which they have been cut will produce magnetic responses which are registered by the magnetometer.
measures variations in electrical resistance which are determined by the amount of moisture in an archaeological feature.
Example: electrical resistance will detect high resistivity archaeological features (features low in moisture) such as buried walls or surfaces and low resistivity archaeological features (features high in moisture content) such as buried ditches or pits which contrast with the surrounding natural subsoil.
Ground penetrating radar (GPR)
works by sending electromagnetic pulses into the ground and then measuring any reflected signals from sub-surface archaeological features. GPR can prove very effective at detecting archaeological remains beneath a wide range of surfaces where magnetometry and electrical resistance would not work such as concrete, stone or tarmac and it is therefore often employed in urban contexts where the technique is able to detect sub-surface archaeological remains.
Geophysical survey is a relatively inexpensive and rapid method for those involved in development to establish the presence of potential archaeological remains within a proposed site at an early enough stage for them to assess the risk involved and to form a strategy for managing the time and cost implications as the development moves forward.