Until the twentieth century, archaeologists relied solely on observation, excavation, and cataloging to learn about historical sites. After World War II, new geophysical methods of measurement and examination were added to their tool boxes.
One of the earliest discoveries was that photographs taken from the air could highlight the remains of structures that would be completely overlooked from ground level. Structural remains that still protrude above the ground can often be best photographed when the sun is low in the sky and thus the angle of light emphasizes the structures.
Buried remains affect the level of moisture and nutrients in the soil and therefore the amount of vegetation that will grow over them. Aerial photographs show “cropmarks,” visible lines that reflect differences in vegetation growth and indicate the presence of archaeological remains. Some excellent photographic examples can be seen at this website from the Emporia State University in Kansas.
Ground penetrating radar (GPR) is used to send a radar signal through the ground and measure the transit time for sending and return. Compiling the results of many signals within a grid can give a three dimensional picture of what lies beneath the surface due to the variations caused by buried structures or voids like tombs. If the ground is relatively flat, the instrumentation can be mounted on a cart and many readings taken in a short period of time. If the area is rugged, readings may have to be taken individually, which requires considerably more time. Visit this website for an interesting article by Dr. Lawrence Conyers from the University of Denver about the use of GPR at the Petra site in Jordan.
Metal detectors can be useful to archaeologists for finding metallic remnants over large areas like battlefields. Although metal detectors do not provide any kind of record of their findings, and interference from power lines and rocks with high mineral content can limit their effectiveness, they are portable, reasonably lightweight, and relatively inexpensive. Unfortunately, the general public knows this, too, and some archaeological sites have been scavenged by unauthorized searchers using metal detectors.
A magnetometer measures slight variations in the earth’s magnetic field. Archaeologists can use magnetometers because certain types of archaeological remains like fired clay and brick affect the magnetic field and the magnetometer can provide a detailed record of the data. While these tests do not give a clear picture of what is below the surface, they can offer clues to a site’s composition.
The greatest advantage of these and other geophysical methods is that they are non-invasive. All these procedures can be done on a site before any excavation takes place. And they can usually be completed in a relatively short time. Geophysical methods can aid the archaeologist to develop a plan for excavation by leading him to the highest concentrations of artifacts.
Archaeological digs often take decades. Geophysical testing can be completed on potential building sites to give an indication of the presence or lack of archaeologically significant remains, which can help planners decide whether or not the building should go ahead. Also, not all known archaeological sites can be excavated, because there are often more modern structures, ones that are in daily use, sitting on top of historical sites.
As with every scientific arena, technology is changing the way archaeologists work. Geophysical methods provide additional ways for these scientists to examine the remains of early settlers and give us clues to our past.
