Near surface geophysical methods are a cost-effective, non-invasive and non-destructive means of identifying a variety of buried archaeological features such as foundations, paths, pipes, privies, pits, metal, fire hearths, cooking pits, middens, human burials and use areas. These tools provide an understanding of the entire archaeological site as opposed to small glimpses provided by traditional archaeological excavation. This enhanced understanding of the subsurface results in more effective, long-term management of archaeological heritage values and can be a critical tool for resolving research questions.
There are several different types of technologies utilised to identify subsurface archaeological deposits. This suite of techniques includes geoelectrical, conductivity, magnetometry, magnetic susceptibility and ground-penetrating radar. Each instrument measures different physical properties of subsurface materials and therefore, holds the potential to identify different features of archaeological interest. Conversely, many archaeological features can be measured by multiple geophysical techniques. In these cases, the redundancy between data sets from different instruments results in more reliable assessment and interpretation of the geophysical results. The benefit of using multiple methods to investigate an archaeological site is now well documented.
Below is a brief, plain language description of the various techniques. It has been our experience that most people are familiar with GPR but many remain uniformed about other methods of near surface geophysical investigation. Following these descriptions is a reading list for those interested in a more refined understanding of geophysical applications in archaeology. Last, a bibliography of Australian examples is located at the bottom of the page.
Geoelectrical techniques measure the resistance to electrical flow by actively inducing a current into the ground using direct contact with a series of metal probes. When the electrical current is forced to flow around a buried feature, an increase in resistivity is measured and a positive anomaly is mapped. The opposite is true when low resistivity features are encountered. In these instances, electrical flow occurs with ease, low resistivity is measured and a negative anomaly is mapped.
The resistivity of a material varies depending on factors including the distribution of water, porosity and chemistry though water content is the predominant factor. This variability allows for reliable mapping of buried archaeological features including walls, foundations, track ways, graves, ditches and pits.
Theoretically, conductivity is the inverse of resistivity. Conductivity, commonly referred to as electromagnetic induction (EM or EMI), measures the ability of a material to conduct electricity. Like geoelectric methods, EMI is an active survey method. An EMI instrument sends an alternating current through a transmitter coil, which introduces a magnetic field into the soil. When the magnetic field encounters conductive soil, an electric current is generated, which in turn, generates a second magnetic field. This secondary magnetic field creates a current in the receiver coil, which allows for the measurement of conductivity at individual survey locations. Contact between the instrument and the ground surface is not required as EMI relies upon indirect coupling.
Archaeological features detectable with EMI include ditches, pits, soil disturbances, track ways, hearths and metal. Due to the sensitivity of these instruments, metal objects can often mask subtle archaeological features, limiting the usefulness of this instrument where historic impacts have affected pre-historic sites. Of significant benefit is that EMI instruments can also measure magnetic susceptibility, doubling data return.
Magnetometry is a passive method that measures the strength of the Earth’s magnetic field. Localised differences in this field are generally associated with ferromagnetic substances with a strong remnant magnetisation but weaker magnetic signals from magnetite, hematite and maghemite can also be measured. While these weakly magnetic minerals of iron oxides possess remnant magnetisation, they also have an induced magnetisation when in the presence of a magnetic field. When magnetic minerals are exposed to sufficient heat (above their respective Curie temperature) and then cooled to normal air temperatures a thermoremanence is imparted on them. Magnetometers cannot differentiate between thermoremanent, remanent and induced magnetisation as they measure the sum value of all magnetism.
Today, it is common to use two vertically separated magnetometers, referred to as a gradiometer. In this configuration, both magnetometers measure the local magnetic field simultaneously. The value recorded by the upper magnetometer is subtracted from the value recorded by lower magnetometer resulting in a measure of the magnetic gradient at that location. Gradiometers offer a significant advantage over single magnetometers as they are capable of identifying closely spaced anomalies, and the background magnetic signal is removed from the resulting data making archaeological features easier to identify. Magnetometry is well suited for mapping buried hearths, ovens, track ways, ditches, pits and areas of anthropogenic activity.
Magnetic susceptibility is a measure of the ease with which a material can be magnetised while exposed to a weak magnetic field. It is the induced component of magnetic minerals discussed above. Thus, laboratory and field based magnetic susceptibility instruments can be used in concert with magnetometers to separate thermoremanent/remanent and induced magnetic forces.
There are five factors that contribute to enhanced magnetic susceptibility of soils: fire, both anthropogenic and naturally occurring; a ‘fermentation mechanism’; inorganic in situ formation of ultra-fine grained magnetite; bacterial microorganisms; and, detrital input from modern pollutants. The first two mechanisms warrant further discussion as they account for the majority of magnetic enhancement.
Extended human occupation often enhances magnetic susceptibility through added organic and fired materials. In these instances, it is possible to use magnetic susceptibility to: identify site boundaries, activity areas, features, buried soils and cultural layers; build and correlate stratigraphic sequences; understand site-formation and post-depositional processes; and, locate fire hearths and pits.
Ground-Penetrating Radar (GPR) is an active method of geophysical investigation. That is, the instrument actively emits radar waves into the soil and then measures returning waves that have reflected from buried objects or stratigraphic boundaries. The energy is reflected when it encounters a layer or object that has a sufficiently different conductivity also referred to as relative dielectric permittivity (RDP), from the material above or surrounding it. RDP is a measure of the ability of a material to hold and transmit an electromagnetic charge and is determined by the composition, moisture content, bulk density, porosity, physical structure and temperature of a material. Thus, the greater the difference in dielectric permittivity between adjacent materials the greater the reflection and easier it is to image subsurface features of interest. In addition to the complexity and density of data collected, GPR is differentiated from other geophysical methods in that it accurately records depth information.
GPR has successfully identified ditches, pits, foundations, track ways, graves, shell middens and pit house floors.
Aspinall, A., C. Gaffney and A. Schmidt 2008 Magnetometry for Archaeologists. Latham, Maryland: AltaMira Press.
Bevan, B. W. 1998 Geophysical Exploration for Archaeology: An Introduction to Geophysical Exploration, Volumes A, B and C. Lincoln, Nebraska: National Park Service, Midwest Archaeological Center.
Clark, A. 1996 Seeing Beneath the Soil: Prospecting Methods in Archaeology. London: B.T. Batsford Ltd.
Conyers, L. B. 2012 Interpreting Ground-Penetrating Radar for Archaeology. Walnut Creek, CA:Left Coast Press.
Conyers, L. B. 2013 Ground-Penetrating Radar for Archaelogists (3rd edition). Latham, Maryland: AltaMira Press.
Conyers, L. B. 2016 Ground-Penetrating Radar for Geoarchaeology. Hoboken, New Jersey:Wiley-Blackwell.
Dalan, R. A. and S. Banerjee 1998 Solving archaeological problems using techniques of soil magnetism. Geoarchaeology 13(1):3–36.
Gaffney, C. and J. Gater 2003 Revealing the Buried Past: Geophysics for Archaeologists. Stroud, Gloucestershire: Tempus Publishing Ltd.
Johnson, J.K. (ed.) 2006 Remote sensing in archaeology: An explicitly North American perspective. Tuscaloosa: University of Alabama Press.
Sarris, A. (ed.) 2015 Best Practices of GeoInformatic Technologies for the Mapping of Archaeolandscapes. Oxford: Archaeopress Publishing Ltd.
Scollar, I. 2009 Archaeological Prospecting and Remote Sensing (with A. Tabbagh, A. Hesse, I. Herzog). Cambridge: Cambridge University Press.
Witten, A.J. 2006 Handbook of Geophyics and Archaeology. London: Equinox Publishing.