Geophysics



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Some experts - take their advice!


Ground Penetrating Radar Paper by B Strongman RCMP

Introduction>

Geophysics plays a large part in a multitude of disciplines. What is geophysics ? This is a technique which has recently begun to be used by a number of disciplines to give information about the underlying ground without having to engage in invasive digging of the area. The main uses of this surveying technique are within geology. Geophysics is the use of non-invasive techniques that can survey anomalies under ground without having to engage in any destructive digging of the area. Geophysics covers a number of techniques such as ground radar, resistivity, magnetometry and seismology. Also in cases of underwater surveying, sidescan sonar could be classified as a non-invasive surveying technique. The development of geophysics has been evolving since the 1950's after the Second World War, and new methods of automating geophysics, such as resistivity and magnetometry, are being developed now. Resistivity and magnetometry were primarily geophysical techniques used in the prospecting of geological features in the mining industry or in academic research. In the late 1950's the techniques were adjusted to meet the needs of archaeologists, and the potential of this as a pre-excavation survey was recognised. This technique was first used in urban archaeology in excavations in York (information supplied by the Royal Commission for Monuments). Later techniques that have been developed such as ground radar are now being introduced into archaeology. In this essay I hope to discuss these different techniques and their role in surveying in archaeology.

Resistivity and magnetometry are the two main types of geophysics used in surveying by archaeologists, and have made the identification of sites a considerably easier task. As the equipment becomes better adjusted to the needs of archaeologists, the ability to process information in a quicker way allows instant images of the data from the equipment - available in a couple of minutes. With archaeological work becoming more and more expensive, geophysics can save time and money which may otherwise have been incurred with digging experimental trenches which may not turn up very interesting results.

Resistivity. This surveying technique, as with many others, was adopted from another subject and applied to archaeology. This method can give very impressive results in a very short time, and is used as the preliminary resource of information on sites. Geological prospection is the main use of this surveying technique. This technique can be considered as a radar method of a type that can identify objects that can not be seen from the surface of the ground being surveyed. By passing electrical current through the sub-soil and moving the position at which the reading is taken, the resistivity of different places in the ground can be measured: this needs to done on a grid with readings taken at consistent intervals.

Geophysics can detect many underground anomalies such as wells, pits, post holes, gullies, ditches, walls and floors, basically any associated ground work that may be covered by top soil.

Resistivity works by measuring electro-physical properties of the soil by passing a current through from one probe to a stationary pickup probe. A measurement is made of the soil at a set of intervals. The flow of current from the total station resistivity meter moves in a circular movement, rather than a straight line. At a position where there exists an archaeological or geological feature the meter gives a high reading as it passes over, and as it moves away from the object the reading drops off. Below is a diagrammatic representation of the level of resistivity over certain geological features. The cause of these readings is the water content in the soil above the feature (similar to the way that crop marks are formed). The depth of soil above a stone wall would be quite short in comparison with the depth of soil on either side of the wall which would be quite deep.

Magnetometry works in a similar method but relies on a different system of electronics which detects the magnetic properties of the soil. The higher electromagnetic properties produce a higher reading, the lower magnetic properties produce a lower reading. Both of these techniques can only be used in appropriate conditions, since such things as the temperature and water content of the soil can affect the readings, as can previous treatment of the soil. This method enables surveying in areas where the conditions are such that resistivity would not work. The results can be of a very high resolution and so for sites that lie on the topsoil that may have been damaged , this method produces outstanding results that would have been missed on a resistivity survey.

Both of these techniques can give some outstanding results with very clear images of the underlying archaeological features. These techniques work on a grid plane. A grid is set out which corresponds to the Ordnance Survey map or site plan of the area. Readings are then taken at regular intervals, and any information of interest acquired from the survey can then be returned to and pinpointed for excavation. This method of pre-survey can save time in preliminary surveying and does not cost as much as , say, a trial excavation using trial trenches. Information gained by these techniques stored in the equipment can be downloaded into a site computer and a picture of the site can be instantly interpreted. The use of geophysics can identify sites of archaeological interest that might otherwise have been overlooked or misinterpreted. It can also rule out geological factors that might possibly be misinterpreted as archaeology from the surface, again allowing the budget of an excavation to be directed to the archaeology directly.

Other new techniques that are being applied to archaeology are ground-probing radar and seismology. Ground probe radar has more application than seismology, and is gaining considerable interest from archaeological geophysicists. This technique was first introduced 20 years ago by Mike Gorman, who was part of the British Antarctic Survey team at Cambridge. This technique was being used to measure the ice thickness. Around 5 years ago, ground-probing radar was first used in archaeology by Geoscan Ltd. at the charnel crypt in Worcester. The ground-probing radar gave clear images of the crypt that had been difficult to locate. Ground-probing radar not only identified the stone work in the crypt, but also showed the soil settled within the crypt. As can be seen in the hard copy print out, this technique uses aerials being moved over the site, usually on the side of a vehicle or roller. However the cost of this equipment is very high, at around 30,000, and the results are not exactly suited to the needs of archaeology. There are possibilities, however, to use this technique to produce a 3D representation. These techniques are all in development at the moment and the Royal Commission for Monuments, British Antarctic Survey, and geophysicists are taking a keen interest in its development. Stratoscan are at present the only company that have produced successful results.

Seismology

Seismology is of interest to archaeology as well, but is lagging behind as it requires explosives to be used as part of its methods to gain results. This is not really suited to archaeological sites, as it may endanger evidence on the site. Also it requires experts to handle the explosives. Both of these developing techniques give the stratigraphy of the soil below ground rather than an aerial view which is gained from resistivity and magnetometry.

Conclusion

Geophysics has revolutionised pre-excavation survey in archaeology. This saves a lot of time and money which can be used later in the actual excavation of the site. There is also the point that the data gives very clear pictures of the landscape and the archaeological features within it, which as previously mentioned allows direct concentration of later possible excavation to areas of interest. With all of these techniques there is a geophysical method suited to the area being surveyed. The emergence of 3D techniques for the plotting of anomalies, gives depth and position information at the finger tips of archaeologists. Geophysics is constantly developing and is at the forefront of new studies in archaeology.


Bibliography

Techniques of Archaeological Excavation pp. 61-68 P Barker
Dictionary of Archaeology
Archaeology Method C Renfrew

Acknowledgments

Mr A Johnson British Antarctic Survey
Mr P Baker Stratoscan
Mr A Payne English Heritage
Dr A David Royal Commission for Monuments
Mr R Walker Geoscan
Dr Kate Clark University Southampton
Mr Nick Bradford University Southampton
Student of the 1994 intake Department Archaeology
Students on this years Survey Course