Geophysical Methods in Marine Archaeological Reconnaissance

Various methods of mapping or remote sensing are commonly used for geophysical and geological mapping, surveying and prospecting. These methods may often be a help in planning archaeological excavations, both on land and in areas covered by either salt or fresh water. The same methods may also be used with success for environmental or topographical mapping, as well as in the search for resources such as gravel, sand, clay or fresh water. These techniques can also be applied to a search for specific archaeological objects, when the purpose is to delimit the most promising areas for more detailed investigation.

Our purpose here is to give short descriptions of various useful indirect mapping methods as well as their possible applications and limitations. We will illustrate some of them with examples, although our own experience is based mostly on work with the methods of magnetics, gravity, seismics and geo-electrics. The methods are briefly listed and summarized in table 1, as are some of their characteristic properties and restrictions.

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Specific Methods of Reconnaissance: Air-Photography

This is probably the most powerful method of reconnaissance for archaeological sites on land or in shallow water, when environmental conditions are suitable. Some of the limiting factors on land (apart from the cost of flying) are the time of the day (because of the position of the sun and hence the shadows), the time of the year (because of vegetation), and the soil fertility and humidity (causing differential growth of the crops). Limiting factors over shallow sea areas are reflections in the water surface, reduced transparency and water depth, which should not exceed 2-3 m. For further discussion see Svejgaard (1990).

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Magnetics

Remanent magnetic measurements may be applied as a dating tool because of the magnetic memory in suitable materials. Furthermore, magnetic surveying by ground surface measurements, which has already been mentioned in a previous paragraph, is useful for mapping hidden objects or structures involving materials with magnetic properties that differ distinctly from the surrounding natural background. These include flex, stones, ceramics, bricks, slags, ancient fireplaces and metallic iron tools and weapons; even ditches filled with soil of a different humus content may be detectable.[…]

Geo-electrics (resistivity)

Geo-electric measurements may be used to map variations in the electrical resistivity of the subsurface materials. As the resistivity is very sensitive to the clay and water content in natural sediments, it is used onshore as a standard tool for the mapping of the superficial geological layers (geo-electrical sounding), as well as of the lateral variations in a certain depth-range (geo-electrical profiling).

In the archaeological context, geo-electrical profiling measurements may be useful in establishing background knowledge of the geological situation (e.g., the bottom of old lake or moor topography), and in some cases hidden old ditches, walls and pavements may be traced.[…]

Geo-radar

Geo-radar employs the seismic idea of recording reflected signals from subsurface objects or layers, but uses high-frequency electromagnetic waves (80-1000 MHz) instead of low-frequency sound waves (0.02-50 kHz). The signal reflected (or transmitted) is dependant on the contrasts in electrical permittivity (relative dielectricity), which is mainly dependant on the water content; the depth of penetration is strongly dependant on conductivity which typically limits the depth of penetration to 2 to 5 metres (which is quite sufficient in most archaeological contexts).

The method is well suited to detect near-surface structures on land, such as ditches and walls, as well as individual objects such as stone clusters, pavements, etc. The method is often still too expensive for routine archaeological use, as it is only available from commercial sources at the moment. (It is now mostly used for the solution of geotechnical problems).

Metal detector

The metal detector is a handy, cheap and very popular instrument. It is used to detect objects with a contrasting electrical conductivity, typically stones and metallic objects such as coins and tools (and modern metallic scrap). A circuit with a flat coil is tuned in a neutral (i.e., sterile, homogeneous) environment. When the coil is held near the surface of the ground, small objects with contrasting conductivity will change its self inductance. The tuning is then disturbed, and a tone is emitted by a small loudspeaker or a deflection is seen on a meter. The instrument may be quite sensitive (depending on its tuning and the homogeneity of the soil), but the depth of penetration is typically not more than 30 cm. When in-phase and quadrature-phase signals are combined, magnetic and non-magnetic conductive materials may be distinguished. This even makes marine measurements possible, although with reduced sensitivity.

The metal detector is most often used on land, but in specific circumstances it may also be used in a marine environment (e.g., Skaarup, in Møller et al. 1984). It is very useful during ongoing excavations, as an area may be rapidly scanned for objects, before the next 10-20 cm of the surface is removed.

Conclusions

Our conclusions about geophysical/archaeological reconnaissance works are summarized below.

1.No methods of reconnaissance are universally applicable.
2.A combined use of several methods may be advantageous in the early phases of selecting a site and planning the excavation.