Geophysical Survey - Seeing Beneath the Earth's Surface

Geophysical Survey Lets Archaeologists Target and LImit Excavations

Ground-penetrating radar survey of the site of the Robert Given homestead
Ground-penetrating radar survey of the site of the Robert Given homestead, Pemaquid Falls, Maine, summer 2004. Neill De Paoli

Geophysical surveys involve the systematic collection of geophysical data of soil profiles for spatial studies. In archaeology, geophysical survey (sometimes called archaeo-geophysical survey) most often refers to the ground-based mapping of subsurface anomalies using a number of different sensing technologies. Using these techniques, data collected by mechanical means from the surface can be used for identifying and mapping subsurface archaeological features without excavation.

Archaeological excavation is expensive and destructive: excavating an archaeological site means painstakingly dismantling it piece by piece. The use of geophysical surveys to identify features ahead of time can guide and refine expensive excavation programs, thereby producing cost savings by making site explorations more efficient. Most importantly, these methods are nondestructive, so archaeologists can learn something of the site while allowing the resource to remain intact.

Types of Geophysical Survey: Magnetometry

Geophysical surveys leverage basic physical laws that distinguish the fundamentally different geochemical and magnetic differences between human and natural features. The most common types applied to archaeology are magnetometers, electrical resistance (or conductivity) meters, and ground penetrating radar.

  • Magnetometry is the most common form of geophysical survey used for archaeological purposes; it was first used on archaeological sites in the 1950s. The earliest surveys used proton-based magnetometers, but they weren't efficient for ground coverage. Today magnetometry uses fluxgate or optically-pumped gradiometer systems.

    Magnetometry measures variations from the naturally smooth magnetic field of the earth, a magnetic field that arises from the presence of natural minerals. In most types of natural soils, the concentration of magnetic mineral varies with depth, and is higher nearer the surface. Human alterations of the soil structures--pits and ditches, foundations, trenches, burials--all of that results in the mixing of soil horizons, interrupting the natural magnetic structure of the soil.

    Magnetometry maps those areas that vary.

    • Ground penetrating radar (often abbreviated as GPR) has had a military use since World War II, but it was first applied to archaeology in the mid-1970s. GPR is based on the fact that electromagnetic sound waves move in smooth patterns through the earth, patterns which can be disturbed by cultural features.

    GPR sends electromagnetic pulses to a transmitting antenna at the ground surface which produces a radio wave that travels through the subsurface. The speed of the wave depends on the ability for a given medium to transfer energy; when an approaching wave encounters a discontinuity, some of the wave front's energy is reflected back to the ground surface, and the time of that two-way travel is recorded by a receiver antenna. Culturally disturbed areas produce lower amplitude reflections or reflectionless anomalies in a background of higher amplitude reflections.

    • Soil resistivity, also known as electromagnetic induction or electromagnetic conductivity, relies on measuring the relative porosity of natural and culturally-disturbed soils. In a natural setting, porosity is dependent upon several soil characteristics. The human disturbance of those soils fills them with air: disturbed earth will be more loosely packed compared to undisturbed earth. The more porous a soil is, the more quickly an electric current will flow through it.

      Resistivity uses a conductivity meter made up of two coils, a transmitter and receiver coil. The transmitter coil produces a magnetic field perpendicular to the plane of the coil. That field induces an electric current in the subsurface that flows in a circular pattern and creates its own secondary magnetic field. The secondary field induces a current in the receiver coil at the other end of the conductivity meter, which measures the strength of the current in the receiving coil. coil.

      Issues and Improvements

      Since the first archaeological use of geophysical instruments in the mid-twentieth century, substantial improvements have been made. Most have an integral global positioning system (GPS) that provides real-time navigation and locational data. Early geophysical research required the archaeologist to physically grid off the surface before beginning the subsurface investigations: integrated GPS systems means locational mapping is part of the process.

      More importantly, instrumentation has been improved, such that the various methods can now identify deeper, more subtle variations. With better electronics, faster processing speeds, and data loggers, data can be returned in the field, rather than having to wait for an extensive data processing period. With the rise of cheap computing power, visualization techniques have also been improved; and specialist software has been developed to connect with other digital datasets.

      Ground-Truthing

      Ground-based geophysical remote-sensing surveys of sites have evolved from a revolutionary technique to a widespread method used to discover and map buried features without excavation. However, results vary by location due to differences in local soil properties and conditions, site formation processes and other factors. Despite their sophistication and sensitivity, geophysical methods do not identify what kind of anomaly has been detected. The only way to really know what types of features are being identified is to dig them up, called ground-truthing.

      One possible solution to support geophysical research without excavation is the use of soil chemistry. A researcher uses a comparatively small soil core to investigate anomalies. But, soil signatures are highly site-specific and difficult to interpret. Dirix and colleagues have had some success combining GPR with soil coring to identify subsurface anomalies as ceramic kilns, based on the presence of certain chemical concentrations.

      Sources and Recent Studies

      This article was originally written by Geoffrey Jones of ArchaeoPhysics, but has been substantially updated and edited by Kris Hirst.

      It is part of the About.com Dictionary of Archaeology

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      Hirst, K. Kris. "Geophysical Survey - Seeing Beneath the Earth's Surface." ThoughtCo, Dec. 28, 2016, thoughtco.com/what-is-a-geophysical-survey-171095. Hirst, K. Kris. (2016, December 28). Geophysical Survey - Seeing Beneath the Earth's Surface. Retrieved from https://www.thoughtco.com/what-is-a-geophysical-survey-171095 Hirst, K. Kris. "Geophysical Survey - Seeing Beneath the Earth's Surface." ThoughtCo. https://www.thoughtco.com/what-is-a-geophysical-survey-171095 (accessed December 17, 2017).