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Archaeological geophysics, known to television viewers as ‘geofizz’, gives us a group of techniques which can be used to detect buried archaeological remains. Some of the more common ones include magnetometry, magnetic susceptibility mapping, resistivity and ground-penetrating radar (GPR). These are known as ‘remote-sensing’ or ‘prospection’ techniques. They help archaeologists locate buried structures and deposits and are very useful for helping to target excavation areas. From experimental beginnings in the 1950’s and 1960’s, geophysical techniques have grown into standard archaeological field practice in Britain and elsewhere, both on research and rescue projects.

Still one of the fastest-developing aspects of field survey, it relies heavily on computer technology to translate electrical and electronic measurements taken in the field into maps and plots which can be interpreted archaeologically. The areas which can be surveyed effectively using geophysics have grown from a few square metres in the 1950s to many thousands of square metres today, making it an important tool for investigating landscapes as well as individual features and sites.
Often the results can be displayed immediately after the survey work has been carried out, using field-based computers.  The results are sometimes very impressive, producing computer plots of recognisable structures which seem to reveal a complete picture of the archaeology before any excavation has taken place.  People sometimes question the need for any further investigation once they have seen the survey results.  However, geophysics is not ‘magic’ and has its limitations just like any other technique.  All that geophysics can do is to measure certain types of physical property in the soil, such as magnetism or electrical resistance. These properties can be also affected by non-archaeological factors. 
There are many kinds of archaeological evidence which are not well-suited to geophysical investigation because they do not produce the types of physical property which are easily detectable by geophysical methods.  Equally importantly, geophysical results need careful and unhurried interpretation. Some knowledge of the background science, and experience of looking at geophysical plots, is essential so the results are not ‘over-interpreted’ and misunderstood.
Further Reading:
A. Clark “Seeing Beneath the Soil” (Batsford, London, 1990)
V. Gaffney and J. Gater "Revealing the Buried Past, Geophysics for Archaeologists"  (Tempus 2003)