Retro Review - 2000 - Physics Applications in Archaeology

Retro Review - 2000 - Physics Applications in Archaeology

Saturday, March 1, 2008

Keywords: physics, dating techniques, remote sensing, geophysics, magnetism
Above: Me in 1999 at Grinnell College studying physics and archaeology, in particular the mechanics of atlatls. Tossing these spears with Professor John Whittaker and members of the first collegiate atlatl team (the Raging Cows) on the soccer field was one of the things that got me into archaeology.
I must have a stack of photocopies of this 2000 article from Physics World somewhere. I was a physics major in college, I was a teaching assistant in the Physics Department in Minnesota when I first came to graduate school, and I basically grew up in the Physics Department at Lawrence University, where my dad works -- I know a lot of physics professors. Most of them receive Physics World, and just about everyone (it seemed) sent me a copy of this article because they knew it was my cup of tea. I am happy they thought of me, but I have multiple copies of this overview of physics applications in archaeology. It concentrations on dating and prospection techniques. As one might expect, this article starts with radiocarbon dating (which, for the most part, is so routine now that it has become a poor example of interdisciplinary cooperation). Thermoluminescence dating is next, discussing how a crystal lattice traps electrons over time. This phenomenon may be used establish when certain types of artifacts were last heated above a specific temperature. Prospection is the next topic, and the authors focus on resistivity and magnetic surveys as well as ground-penetrating radar, techniques common in geophysics. The article ends with a "Future Prospects" section, but little is predicted other than "computers with get faster and better" -- hardly going out on a limb there! No ground-breaking research is presented, but this article offers a decent overview or introduction for archaeologists and physicists about how their fields intersect.
Excerpts from Physics World:
Physics and Archaeology
By Andrew David and Neil Linford
Physics-based techniques play a crucial role in helping archaeologists to unravel the history of our ancestors' lives and reveal previously undiscovered sites without the need to excavate. . . Archaeology has developed into a flourishing professional discipline, greatly empowered by science. . . Physics has played a critical part in dating objects and in discovering new archaeological sites. . .
Radiocarbon Dating
Perhaps the single most important development for archaeology in the 20th century was the discovery of radiometric dating -- and of radiocarbon dating in particular. In 1946 Willard Libby of the University of Chicago predicted that all living plants and animals absorb the weakly radioactive carbon-14 isotope from the atmosphere. The process stops when the plant or animal dies, and the carbon-14 nuclei start to decay at a known rate with a half-life of 5730 years. Measurements of the residual radioactivity of a sample thus provide an estimate of its age, provided it is less than 50,000 years old. Libby and his colleagues were the first to estimate the age of archaeological samples in this way, work that earned Libby the Nobel Prize for Chemistry in 1960. . .
Luminescence, Spin and Magnetism
Other dating methods that depend indirectly on radioactivity are "luminescence dating" and "electron spin resonance". Luminescence dating relies on measuring the energy of electrons that are trapped in the crystal structure of the archaeological object. These electrons have been ionized by radiation from radioactive elements in the sample, or its surroundings, and then trapped by defects in the crystal lattice. The electrons are released from the traps by heating the sample -- for example when a pot is fired or a flint is burnt -- or by exposing it to light. In other words, heating resets the "thermoluminescence clock" to zero. The number of trapped electrons then accumulates according to a rate controlled by the local radiation. . .
Archaeological Prospecting
Physics-based methods also play a vital role in helping archaeologists to discover new archaeological sites, and help to define and interpret sites that are not completely understood. Tony Clark, a pioneer of archaeological geophysics, describes the subject as the ability to "see beneath the soil" -- surely one of the most cherished desires of archaeologists. . .
The geophysical methods that are now routinely used in archaeological prospecting are adapted and scaled-down versions of those applied in geological mapping, mineral exploration, civil engineering and environmental geophysics. These methods may be categorized as either active or passive. Active methods inject energy into the ground, like the crowbar, and measure a response at the surface. They include seismic prospecting, electromagnetic techniques and earth-resistance surveying. Passive methods, such as magnetometry and gravity surveying, simply measure existing physical properties. . .
Geophysics with Magnetism
Magnetic surveying is a passive technique that can measure minute variations in the magnitude or gradient of the Earth's magnetic field. Indeed, it can often detect such variations or "anomalies" over 50,000 times weaker than the ambient field strength. Magnetic surveys were initially used to locate burnt archaeological structures, such as Roman pottery kilns. However, the technique soon demonstrated its sensitivity to other features, such as ditches, rubbish pits and even individual holes into which timber posts were sunk.
The anomalies arise due to variations in the magnetic susceptibility of buried features, which occur when iron-rich minerals in the soil form more strongly ferrimagnetic materials such as magnetite and maghemite. This magnetic enhancement is usually related to burning, although more subtle inorganic and bacterially controlled mechanisms may take place under suitable soil conditions. Such conditions occur naturally in most topsoils, providing a source of magnetically enhanced material that becomes embedded into archaeological features and so produces almost indelible magnetic anomalies. . .
Ground-Penetrating Radar
The ultimate aim of geophysics is to produce a complete 3-D model of buried archaeological features that goes beyond the 2-D plans provided by traditional earth-resistance and magnetic surveys. Although these techniques are capable of providing some depth-related information, the length of time it takes to collect the data can be prohibitive and considerably reduces the area that can be covered. Recently, the combination of ground-penetrating radar and powerful computers to visualize the data has provided a new tool for archaeologists to address this problem.
Ground-penetrating radar operates by introducing a short impulse of electromagnetic energy from a surface antenna and recording both the time and magnitude of return signals reflected by dielectric contrasts in the subsoil. A graphical trace is then produced that shows the magnitude of reflection against the time taken for the pulse to travel from the transmitter to the target and back to the receiver (figure 4). This impulse signal covers a wide range of frequencies, but is generally tuned to a centre frequency between 100-1000 MHz, depending on the antenna chosen for the survey. The imaging resolution and penetration depth depend on both the chosen centre frequency and the average dielectric permittivity of the subsoil. Although a high-frequency, short wavelength impulse covers a smaller area, it is capable of resolving smaller objects. . .

To read the rest of the article, visit the Physics World website:
http://physicsworld.com/cws/article/print/654

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