|What is GPR?|
Geophysics in archaeology involves a method of data collection that allows field archaeologists to discover and map buried archaeological features in ways not possible using traditional field methods. Using a variety of instruments, physical and chemical changes in the ground related to the presence or absence of buried materials of interest can be measured and mapped. When these changes can be related to certain aspects of archaeological sites such as architecture, use areas, or other associated cultural features, high definition maps and images of buried remains can be produced. Their maps act as primary data that can be used to guide the placement of excavations, or to define sensitive areas containing cultural remains to avoid. Some archaeological geophysicists have used geophysical mapping as a way to place archaeological sites within a broader environmental context as a way to study human interaction with and adaptation to ancient landscapes. Most importantly, geophysical methods can gather a great deal of information about the near-surface in a totally non-destructive way, allowing large areas with buried remains to be studied efficiently and accurately, while at the same time preserving and protecting them.
Sara Gale prospecting for the foundation of a colonial church in a parking lot in Santa Fe, New Mexico.
Ground-penetrating radar (GPR) is one of the near-surface geophysical methods that is gaining acceptance as a viable means of field study in archaeology. It involves the transmission of high frequency radar pulses from a surface antenna into the ground. The elapsed time between when this energy is transmitted, reflected from buried materials or sediment and soil changes in the ground, and then received back at the surface is then measured. When many thousands of radar reflections are measured and recorded as antennas are moved along transects within a grid, a three-dimensional picture of soil, sediment, and feature changes can be created.
The primary goal of most GPR investigations in archaeology is to differentiate subsurface interfaces. A series of reflection traces collected along a transect that are produced from a buried layer will produce a horizontal or sub-horizontal line (either dark or light in gray scale reflection profiles) that is referred to as a “reflection" (see below). These types of distinct reflections are usually generated from a subsurface boundary such as a stratigraphic boundary or some other physical discontinuity such as the water table, a buried soil horizon or a horizontal feature of archaeological interest.
GPR Reflection profile. Distance along the profile is measured in meters and two-way radar travel time, measured in nanoseconds, is converted to depth below the surface. This profile consists of 305 individual, sequentially stacked, reflection traces. This profile was collected over a pit house floor near Alamagordo, New Mexico, USA, in a small area within a large pit-house village.
All sedimentary layers and other buried materials in the ground have particular physical and chemical properties that affect the velocity of electromagnetic energy propagation, the most important of which are electrical conductivity and magnetic permeability.
The reflectivity of radar energy that occurs at a buried interface is primarily a function of the magnitude of velocity changes at a buried interface. This is measured by differences in the relative dielectric permittivity (RDP) at that interface, which are measuring velocity changes. The greater the change in velocity at an interface, the higher the amplitude of the reflected wave.
The highest amplitude radar reflections usually occur at an interface of two relatively thick layers that have greatly varying properties. For instance, a difference of this sort might be between a compacted clay floor of a buried pit-house and overlying sand or gravel layer that covers it (see above). If the target archaeological features are composed of almost exactly the same material as the matrix, or have the same physical and chemical properties, there will no variation RDP between them, and therefore little or no reflection will occur at their interface.
In the future GPR's ability to non-invasively map not only buried structures and other cultural features in real depth, but also reconstruct the ancient landscape of a site and human interaction with it, will become increasingly important.