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Remote Sensing

Archeologists use remote sensing to decode the past

From OE Reports Number 199 - July 2000
30 July 2000, SPIE Newsroom. DOI: 10.1117/2.6200007.0001

Figure 1. Queen Nerfertari (wife of Ramses the Great) depicted in a wall painting in her tomb within the Valley of the Queens, west of Luxar, Egypt. Remote sensing was used to investigate the cause of damage to the tomb's paintings.

Archeologists in the field have two big problems when considering where to excavate: first, they don't really know where to dig so it is a hit-or-miss proposition; and second, once they have dug, the area is destroyed. If the area is one that local people care about, destructive data gathering also makes the archeologists unwelcome. However, both problems are being addressed by remote-sensing technologies, ranging from satellite images to ground-based instruments.

Archeologists have used aerial photography since the turn of the century, and it continues to be useful today. But many other methods of remote sensing, including infrared and radar imaging, as well as ground-based methods called geophysical surveying, have been developed since the mid-1980s. These methods have not been universally applied, but they have provided some archeologists with information that could not have been found in other ways.

Some archeologists resist the change, insisting that the resolution from space-borne imagers is not sufficient for archeologic purposes, said Farouk El-Baz, director of the Center for Remote Sensing at Boston Univ. "The satellites showed pictures 120 m on a side," he said, "but the archeologist is only interested in a 10-m area." However, this discrepancy in scale has been changing with the ever-improving capabilities of satellite and Space Shuttle imagers. For example, the Landsat 7 satellite has a resolution of 10 m, and the IKONOS satellite can resolve objects with a dimension as small as 1 m.

Even if these are not useful for investigating a specific site, the images are useful for putting a site into context. Remote sensors with lower resolution are still useful for detecting and locating larger features, such as waterways, roads, and the location of ancient settlements. One can use remote sensing to look for anomalies, and then choose to start digging at those sites.

There are two different kinds of remote sensing used in archeology: long-distance (airborne or space-borne imaging) and short-distance (geophysical surveying) methods. Airborne methods do not necessarily involve airplanes. For example, El-Baz has used a tethered blimp, which he tied to a jeep and loaded with visible, IR, thermal IR, and radar imagers.

Imaging

Imaging from the air or space is attractive because it gathers a lot of data quickly. Multispectral scanners are especially useful, as they provide even more information than film photography.


Figure 2. CIR photo shows an ancient Maya causeway. Image courtesy of NASA.

Visible photography and color infrared photography have been used for decades, and skilled photo-interpreters can see details that aren't apparent to the untrained eye (Figure 1). Newer solid-state imagers, however, provide data that can be digitized, manipulated, and integrated using geographic information systems (GIS).

Archeologist and GIS expert Kenneth Kvamme at the Univ. of Arkansas said GIS links information to a space. The ability to link information from different sources -- such as radar, visible photography, ground topography, magnetometers and resistometers etc. -- allows users to visualize composite pictures, which may show relationships between the different sources. For example, remotely sensed images in different spectral bands, displayed in false colors, show the location of seasonally flooded swamps in northern Guatemala (see Remote sensing unravels archeological mystery, this page).

Multispectral

Many multispectral imagers have been developed recently and used for archeology. Due to attenuation in the atmosphere, UV is seldom used. Radar is attractive because atmospheric effects are minimized. Thermal IR can detect different kinds of vegetation, which in turn can indicate the presence of disturbed soil, such as buried walls, middens, roadways, etc.

Satellite and aircraft-mounted sensors include the Landsat Thematic Mapper, the SPOT satellite imager, Thermal Infrared Multispectral Scanner, Calibrated Airborne Multispectral Scanner, Airborne Terrestrial Applications Sensor, and Airborne Oceanographic Lidar. Additional low-cost sensors include an IR imager from Inframetrics that collects a broad thermal IR band and allows temperature measurements accurate to 0.1 deg. C.

As always, cost is a consideration when deciding what instruments to use. Images gathered from a plane may provide better resolution, but is generally more expensive (given that it's most likely a chartered flight) than satellite images.


Figure 3. A radar image strip, gathered by the Space Shuttle, is superimposed on a Landsat color image of the eastern Sahara. The radar penetrated the dry sand, revealing ancient river beds. The feature on the right edge is 12 miles across. Image courtesy of Boston Univ.

The availability of these different scanners allows researchers to examine sites in the spectral bands best suited to them. For example, NASA archaeologist Tom Sever and Univ. of Colorado researcher Payson Sheets used reflected IR thermal radiation to image compacted earth of ancient footpaths in humid Costa Rica.1 However, microwave radar is best for penetrating the dry sand of the Sahara to show underground geologic features (see image on right).

El-Baz used radar to locate the courses of ancient rivers that ran through the Sahara Desert 5000 years ago. Images from the Space Shuttle showed fault lines. In a present day application, the Egyptian government used this information to drill for water. El-Baz isn't sure how deep the radar can penetrate. "The deepest we dug was 16 ft.," he said. "In theory, it could penetrate 10 m."

Ground-penetrating radar

To investigate the site of an ancient fort in Northern Ireland, Elizabeth Ambos, a geophysicist at California State Univ. at Long Beach, used ground-penetrating radar (GPR). She said the GPR unit is placed on the ground and emits a fairly wide spectrum at low power. For the work at Navan Fort, the center wavelength was in the microwave range at 500 MHz, and the system could image objects 1 to 1.5 m deep and resolve objects as small as 5 cm.

The signal attenuates rapidly in the ground, but it reflects at interfaces of materials with different conductivity and dielectric properties. It can distinguish soil from bedrock or buried metal objects from the soil around them. This kind of radar can find buried walls, hardpacked clay floors, and can even distinguish between different soils that look the same to the eye, but which may have very different electrical response.

Ambos was able to define parts of the mound at Navan Fort that had not been looked at before, and she did it without excavating.

Farouk El-Baz was part of an expedition in 1998 in which ground-penetrating radar was vital. It was an extension of previous work done at Giza in Egypt, near the Great Pyramid (Figure 2). In 1954 a group found a 4600-year-old disassembled boat in an underground chamber. When the chamber was opened, one of the investigators reported smelling cedarwood, which suggests that the chamber had been hermetically sealed since soon after its completion.


Figure 4. Remote sensing methods were used to probe a sealed underground chamber located next to the Great Pyramid of Giza, Egypt. Image courtesy of Boston Univ.

The boat was placed in a custom-built museum at the site, but it began to shrink. Archeologists knew that another chamber existed near the first one, and hoped to gather environmental information from that chamber to help preserve the boat. This would require gathering information with as little perturbation of the chamber as possible.

El-Baz realized that if the second chamber was also hermetically sealed, it would also provide a sample of the atmosphere as it had been about 4600 years ago. The researchers mapped the chamber using ground-penetrating radar. In addition to discovering the shape of the chamber, the radar data provided the height of the contents. This was important because the group was to drill a small hole into the chamber, but didn't want to damage the contents. (The group also designed an airlock and a drill that could work without mixing air or fluids from one side to the other.)


Figure 5. A high-resolution camera captures an image of a disassembled wooden boat, one of the two oldest boats in the world, at Giza. The white patches are cementing material that fell off the roof of the chamber. Image courtesy of Boston Univ.

Once the borehole was finished, the researchers sucked out a sample of the atmosphere and sent it to the NOAO. They measured the temperature, pressure, and humidity inside the chamber, and used a high-resolution camera and an IR-filtered cold-light illuminator to examine the contents (Figure 3). Ironically, the second chamber was not hermetically sealed -- cracks had formed in the walls when construction equipment was brought in to build the museum.

Other tools

In addition to ground-penetrating radar, archeologists have other on-site tools for probing the soil beneath them. Researchers can measure variations in gravity at different sites, which allows imaging for large rocks and metals from heaths. Variations in the Earth's magnetic field can also be measured. In archeology, El-Baz said, this often is due to the presence of hearths: magnetic minerals near fires are heated up, and then change alignment before cooling off again. Electromagnetic sounders are devices very much like radar, which can operate at all kinds of frequencies and a variety of ranges, depending on local conditions.

A final common instrument is a resistivity meter, in which one runs current through the ground over a variety of paths, seeking anomalies. Sheets found resistivity to be an extremely useful tool for studying the site of an ancient village buried beneath 5 m of volcanic ash in El Salvador. Between resistance measurements and ground-penetrating radar, Sheets' group was able to image objects 5 m deep and 1 m in diameter.

Reference:

1. Payson Sheets and Tom Sever, High Tech Wizardry, Archeology, p. 28, Nov/Dec 1988.


Yvonne Carts-Powell
Yvonne Carts-Powell, based in Boston, writes about optoelectronics and the Internet.