Electrical resistance map of ancient Aphrodisias
Geophysical survey most often refers to subsurface imaging techniques used in archaeology. Broadly defined, the term could refer to any systematic collection of geophysical data, but other disciplines have developed unique terminology (see: exploration geophysics; Nondestructive testing). Remote sensing and marine surveys are also used in archaeology, but are generally considered separate disciplines. Other terms, such as "geophysical prospection" and "archaeological geophysics" are generally synonymous with geophysical survey.
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Overview
Geophysical survey is used to create maps of subsurface archaeological features. Features are the non-portable part of the archaeological record, whether standing structures or traces of human activities left in the soil. Geophysical instruments can detect buried features when their electrical or magnetic properties contrast measurably with their surroundings. In some cases individual artifacts, especially metal, may be detected as well. Readings taken in a systematic pattern become a dataset that can be rendered as image maps. Survey results can be used to guide excavation and to give archaeologists insight into the patterning of non-excavated parts of the site. Unlike other archaeological methods, geophysical survey is not invasive or destructive. For this reason, it is often used where preservation (rather than excavation) is the goal.
Although Geophysical survey has been used in the past with intermittent success, good results are very likely when it is applied appropriately. It is most useful when it is used in a well-integrated research design where interpretations can be tested and refined. Interpretation requires a knowledge both of the archaeological record, and of the way it is expressed geophysically. Appropriate instrumentation, survey design, and data processing are essential for success, and must be adapted to the unique geology and archaeological record of each site. In the field, control of data quality and spatial accuracy are critical.
Methods
EM conductivity survey
Geophysical methods used in archaeology are largely adapted from those used in mineral exploration, engineering, and geology. Archaeological mapping presents unique challenges, however, which have spurred a separate development of methods and equipment. In general, geological applications are concerned with detecting relatively large structures, often as deeply as possible. In contrast, most archaeological sites are relatively near the surface, often within the top meter of earth. Instruments are often configured to limit the depth of response to better resolve the near-surface phenomena that are likely to be of interest. Another challenge is to detect subtle and often very small features ? which may be as ephemeral as organic staining from decayed wooden posts - and distinguish them from rocks, roots, and other natural ?clutter.? To accomplish this requires not only sensitivity, but also high density of data points, usually at least one and sometimes dozens of readings per square meter.
Most commonly applied to archaeology are magnetometers, electrical resistance meters, ground-penetrating radar (GPR) and electromagnetic (EM) conductivity. These methods provide excellent resolution of many types of archaeological features, and are capable of high sample density surveys of very large areas and of operating under a wide range of conditions. While common metal detectors are geophysical sensors, they are not capable of generating high-resolution imagery. Other established and emerging technologies are also finding use in archaeological applications.
Electrical resistance meters can be thought of as similar to the Ohmmeters used to test electrical circuits. In most systems, metal probes are inserted into the ground to obtain a reading of the local electrical resistance. A variety of probe configurations are used, most having four probes, often mounted on a rigid frame. Capacatively coupled systems that do not require direct physical contact with the soil have also been developed. Archaeological features can be mapped when they are of higher or lower resistivity than their surroundings. A stone foundation might impede the flow of electricity, while the organic deposits within an outhouse pit might conduct electricity more easily than surrounding soils. Although generally used for planview mapping, resistance methods also have a limited ability to discriminate depth and create vertical profiles. When many measurements are made, on the surface or between bore holes, a cross sectional image can be reconstruced. This technique is called Electrical Resistivity Tomography.
Electromagnetic (EM) conductivity instruments have a response that is comparable to that of resistance meters (conductivity is the inverse of resistance). Although EM conductivity instruments are generally less sensitive than resistance meters to the same phenomena, they do have a number of unique properties. One advantage is that they do not require direct contact with the ground, and can be used in conditions unfavorable to resistance meters. Another advantage is relatively greater speed than resistance instruments. Unlike resistance instruments, conductivity meters respond strongly to metal. This can be a disadvantage when the metal is extraneous to the archaeological record, but can be useful when the metal is of archaeological interest. Some EM conductivity instruments are also capable of measuring magnetic susceptibility, a property that is becoming increasingly important in archaeological studies.
Magnetic gradiometer map of Prehistoric fire-hearths
Magnetometers used in geophysical survey may use a single sensor to measure the total magnetic field strength, or may use two (sometimes more) sensors to measure the gradient of the magnetic field (the difference between the two sensors). In most archaeological applications the latter (gradiometer) configuration is preferred because it provides better resolution of small, near-surface phenomena. Magnetometers may also use a variety of different sensor types. Proton precession magnetometers have largely been superseded by faster or more sensitive fluxgate, cesium, or overhauser instruments.
Every kind of material has unique magnetic properties, even those that we do not think of as being ?magnetic.? Different materials below the ground can cause local disturbances in the Earth?s magnetic field that are detectable with sensitive magnetometers. Magnetometers react very strongly to iron of course, and brick, burned soil, and many types of rock are also magnetic, and archaeological features composed of these materials are very detectable. Where these highly magnetic materials do not occur, it is often possible to detect very subtle anomalies caused by disturbed soils or decayed organic materials. The chief limitation of magnetometer survey is subtle features of interest may be obscured by highly magnetic geologic or modern materials.
GPR survey
Ground-penetrating radar (GPR) is the perhaps the best known of these methods (although it is not the most widely applied in archaeology). The concept of radar is familiar to most people. In this instance, the radar signal ? an electromagnetic pulse ? is directed into the ground. Subsurface objects and stratigraphy (layering) will cause reflections that are picked up by a receiver. The travel time of the reflected signal indicates the depth. Data may be plotted as profiles, or as planview maps isolating specific depths.
GPR can be a powerful tool in favorable conditions (uniform sandy soils are ideal). It is unique both in its ability to detect some spatially small objects at relatively great depths and in its ability to distinguish the depth of anomaly sources. The principal disadvantage of GPR is that it is severely limited by less-than-ideal conditions. The high electrical conductivity of fine-grained sediments (clays and silts) causes conductive losses of signal strength; rocky or heterogeneous sediments scatter the GPR signal. Another disadvantage is that data collection is relatively slow.
Common metal detectors do not create a logged data set, and thus cannot be used for directly creating maps. Irresponsible (and often illegal) use of metal detectors by artifact collectors or treasure hunters has resulted in extensive damage to the archaeological record, both by the unrecorded removal of artifacts and the destruction of their context by uncontrolled excavation. However when used responsibly and in a systematic manner, they can be a useful tool in archaeological research.
Data collection is broadly similar regardless of the particular sensing instrument. Geophysical survey usually involves walking with the instrument along closely spaced parallel traverses, taking readings at regular intervals. In most cases, the area to be surveyed is staked into a series of square or rectangular survey "grids" (terminology can vary). With the corners of the grids as known reference points, the instrument operator uses tapes or marked ropes as a guide when collecting data. In this way, positioning error can be kept to within a few centimeters for high-resolution mapping. Survey systems with integrated global positioning systems (GPS) have been developed, but under field conditions, currently available systems lack sufficient precision for high-resolution archaeological mapping. Geophysical instruments (notably metal detectors) may also used for less formally "scanning" areas of interest.
Data processing and imaging convert raw numeric data into interpretable maps. Data processing usually involves the removal of statistical outliers and noise, interpolation of data points. Statistical filters may be designed to enhance features of interest (based on size, strength, or other criteria), or suppress obscuring modern or natural phenomena. Processed data are typically rendered as images, as contour maps, or in false relief. For many geophysical problems the recovery of an image from measured data is mathematically an inverse problem.
Development of geophysical survey
The use of geophysical survey is well established in European archaeology, especially in Great Britain, where it was pioneered in the 1940?s and 1950?s. It is increasingly employed in other parts of the world, and with increasing success as techniques are adapted to unique regional conditions.
In early surveys, measurements were recorded individually and plotted by hand. Although useful results were sometimes obtained, practical applications were limited by the enormous amount of labor required. Data processing was minimal and sample densities were necessarily low.
Although the sensitivity of sensors has improved, and new methods have been developed, the most important developments have been automated data logging and computers to handle and process large amounts of data. Continuing improvements in survey equipment performance and automation have made it possible to rapidly survey large areas. Rapid data collection has also made it practical to achieve the high sample densities necessary to resolve small or subtle features. Advances in processing and imaging software have made it possible to detect, display, and interpret subtle archaeological patterning within the geophysical data.
Further reading
A general overview of geophysical methods in archaeology can be found in the following works:
- Clark, Anthony J. (1996). Seeing Beneath the Soil. Prospecting Methods in Archaeology. London, United Kingdom: B.T. Batsford Ltd..
- Gaffney, Chris; John Gater (2003). Revealing the Buried Past: Geophysics for Archaeologists. Stroud, United Kingdom: Tempus.
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