Archaeological Prospecting

By: Elizabeth K. Ralph

Originally Published in 1969

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As all readers of Expedition know, the basic technique of archaeology is excavation. But, as labor costs become higher all over the world and as modern civilization encroaches upon ancient sites, there is a need to facilitate the finding of structures at known sites and to locate unsuspected or lost cities and sites before they are lost to us forever. For this purpose, Dr. Rainey asked the help of the physical scientists in developing and experimenting with a variety of instruments for the detection of buried archaeological features. Since physicists and geophysicists are in short supply in our MASCA group, I have been fortunate in having had the opportunity to carry out most of the field trials and surveys.

The first requirement of an instrument for archaeological prospecting is that it be portable. The obvious initial step was to adapt or to use the basic principles of operation of a number of devices that had already been developed for geophysical prospecting. Those which have been found most suitable for archaeology detect changes in either ground conductivity (conversely, resistivity) or variations in magnetic intensity. A variety of instruments have now been tried and some are being used by a number of groups. The one that we have found most satisfactory for resistivity surveying is the Gossen Geohm. For magnetic detection, we started with the Elsec proton magnetometer designed by the Oxford laboratories. For reasons explained previously (Expedition, Vol. 7, No. 2), we are now using the more sensitive cesium magnetometers. The first type was developed specifically for the University Museum by Varian Associates after initial trials with the Varian rubidium magnetometer. More recently Varian has designed an even lighter and more portable cesium magnetometer for other purposes, but this unit is also suitable for our archaeological prospecting although not quite so sensitive.

During the past eight years, we have used these gadgets at 34 different sites in eight different countries. This includes ten field seasons abroad, of average duration of three months each. We have learned that magnetic prospecting is much the fastest of all methods tried and that where sufficient magnetic contrast exists, some types walls and roof tiles can be detected at depths of five meters or more. However, at historical sites in the USA and Canada where walls are less massive and not buried deeply and where there is often magnetic clutter from modern civilization, the Geohm is the optimum instrument.

A summary of 34 different sites is given in the accompanying Table. Surveys at many of the local sites were conducted over weekends only, and not all were selected as being ideal for prospecting. Some were chosen as convenient spots for testing particular instruments, and others as training grounds for students. I have chosen only a few to describe in greater detail than that given in the Table.

A full summer season was spent on the Ile-aux-Noix, Quebec, in 1964, both to expose students in archaeology to all of the new ‘tools’ which were then available, and to provide an extensive test of the different types of equipment; and also, of course, to find the most likely regions for excavation. The entire island, about a mile long, was covered with the proton magnetometer and experiments were conducted with the rubidium one which was then in the developmental stage. The magnetometers indicated where regions of disturbance were to be found, but it was the Geohm that pinpointed the foundations precisely. An example of the precise delineation of the hospital foundations is shown. The outline of the resistivity contours corresponded exactly with the outer walls, the high density contours near each end represented hearths, and the bulge at the northern end reflected an unsuspected doorway.

At extensive sites with structures buried in alluvial plains, however, magnetometers are the most suitable. Eight seasons of work on the plain of Sybaris, Italy may not have delimited the archaic city precisely, but the site has been an excellent testing ground for the development of better magnetometers. Also, the extensive drilling programs and a few test excavations have assisted in the interpretation of the anomalies found.

As one may note in the Table, many trials, short in duration, have been made at sites that were not suitable for magnetic prospecting for one reason or another. This was particularly true in Greece until Prof. Marinatos, Director of Antiquities, asked our help in surveying ancient Elis (about 70 km. north of Olympia). This proved to be a perfect site for magnetic detection. Many of the walls of the 5th and 4th century B.C. were massive and made of reused roof tiles. (Roof tiles and bricks are much more magnetic than unfired clay and normal stone.) They were found at depths of approximately one meter, but extended downward to depths of four to five meters. In some places there were Roman structures overlying the earlier ones, but on the basis of fairly extensive excavations at this site, it seems likely that the Roman walls followed the pattern of the ones below.

Elis, leading city of the second largest province (Elis) of the Peloponnese, lies along the banks of the River Peneus, between a hilly eastern region and a fertile, western, coastal plain. Originally settled in dispersed villages, the city was founded in 471 B.C. by bringing together many of these smaller populations into a large and open town. It played a minor role in ancient politics, generally amenable to Spartan policies, and preferring quiet country life. It was, however, noted for its horses and high-quality flax and achieved its greatest fame as administrator of the important sanctuary of Olympia and president of the Olympic games. These quadrennial competitions, one of the greatest of Panhellenic festivals, were traditionally reputed to have begun in 776 B.C. Elean control was definitely established by about 572 B.C. and was maintained into the Roman era.

The site of the ancient city has long been known, observable in many (Roman) brick and stone fragments scattered over an area three to five kilometers in circumference. Excavation was begun by the Austrians in 1910-14, resumed in 1931-32 and again, in a joint Greek and Austrian campaign, in 1960.

As well as offering readily detectable magnetic anomalies, most of the surface now consists of farmland uncluttered by modern civilization, except for a small village at the southern edge of the area of interest. The survey here was not only successful, but urgently needed because of the construction of extensive irrigation ditches by the Greek Government. The first tests with our cesium magnetometer were made in September 1967, supposedly in the proposed path of the large irrigation ditch. When I returned in May 1968, I found out that not only had the route of the ditch been changed, but that it was already under construction. The Greek archaeologists had been excavating extensively during the winter and early spring in the new path of the ditch. Since many of the structures were still exposed, it was possible to correlate visible lines of walls with anomalies in adjacent magnetometer grids.

Complete grids were made over one-half square kilometer, roughly a quarter of ancient Elis, and its approximate limits were determined by running long radial lines out from the regions of known anomalies. It extends over approximately 2 sq. km. Within the grids, the walls and rows of houses showed up so clearly that it appears to be possible to recreate part of the ancient city plan. A possible row of houses is illustrated in Grid. No. 19. In the upper part of the grid, one sees closely spaced contours running east to west, which continue in adjacent grids. It is presumed that these represent rows of walls made of bricks or a combination of stones and roof tiles.

At this writing, the final plan of the city is not complete. However, this will probably be the first example of the reconstruction of the plan of a Greek city by means of magnetic contouring. Here, it has been demonstrated also that it is now possible to map a good part of a large ancient city and to find its limits, all within a period of eight weeks. Certainly, much more was detected in these few weeks than had been uncovered or suspected during sixty years of intermittent excavations.

Cite This Article

Ralph, Elizabeth K.. "Archaeological Prospecting." Expedition Magazine 11, no. 2 (January, 1969): -. Accessed May 18, 2024. https://www.penn.museum/sites/expedition/archaeological-prospecting/


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