Many professional archaeologists claim that the greatest advancement in archaeology since the pick and shovel is Ground Penetrating Radar (GPR), otherwise known as remote sensing. GPR equips the specialist excavator with a tool that permits an ability to ‘observe’ the archaeological record without disturbing or excavating it.
This type of data gathering can occur either from the air or the ground. See the other articles on Aerial Archaeology and Geophysical Surveys for more information. Walking over the archaeological site with hand held GPR will provide a rough picture of the sub-surface and be of some assistance to specialists who are trained at decoding the acquired data. Likewise Diffraction Tomography achieves similar results by sending shock waves into the ground and reading the reflected waves as they return to the surface and aerial photography and mapping can also be useful as all these methods are non-invasive and helpful.
Images From the Sky
The most spectacular remote sensing devices are those that produce images of the earth from outer space.
The human eye is not well tuned for differentiating rock type or terrain. Our eyes are limited to naturally see only in the visible spectrum. Longer or shorter wavelengths of electromagnetic energy reveal a further rainbow of colours that, when understood, can provide valuable insight to an archaeological expedition.
NASA Launches Landsat
The first multi-spectral imaging satellite was launched in 1972. It was science’s attempt to ‘see’ what we could not. Landsat-1 read visible and infrared bands of the spectrum. The satellite’s images were full of distorted, psychedelic colours. Rivers and lakes were black. Vegetation was red, yet dense forest appeared gold. Once the colour shifts were understood, scientists could isolate and detect rock and soil types, earthquake fault-lines, and other geological features.
Sunlight Verses Radar
Archaeologists were not particularly assisted by this early technology as it only provided images of the surface using the sun’s reflected light. However, archaeologists appreciated the wide potential of outer space imaging when NASA launched SIR-A. SIR-A was additionally equipped with an active radar sending system. The satellite could pulse out radar signals and then record the returned echo from the earth as an image.
This technology enables archaeologists to penetrate the ground level and look beneath the surface of vast areas of land without any excavation or prospecting.
Rivers in the Sahara
One of the first examples of its excellent use in archaeology was revealed in images taken over the Sahara Desert. This desert is the driest in the world and rapidly expanding. Radar penetration images revealed a sub-surface full of old riverbeds, mountains, valleys, and even expansive lakes. In fact, what lay under the roasting sand was once a landscape typical of Europe or North America.
Archaeologists were quick to cease the opportunity of grid mapping these exciting subterranean features as they know that where there was once water there would likely be material relics of man. Along the beds of these ancient but now hidden rivers were excellent sites for archaeological excavation. Numerous expeditions set out in the harsh Saharan climate and their endeavours yielded a horde of lithic tools and other artefacts.
The satellite imaging systems, although a powerful tool in the archaeologist’s kit, still has its limitations. For instance, although the pulse can be sent from hundreds of kilometres above the earth, it can only penetrate three to five metres into the ground. And, to achieve maximum penetration, the ground conditions must be very dry and have fine-grained soil type such as sand.
Satellite imaging technology continues to advance and today there are a host of imaging satellites that regularly orbit the earth. As innovative as they are they are not the golden trowels of archaeology. For most practical excavations the pick, shovel, trowel and keen human eye, at ground level, will continue to be the mainstay of archaeological discoveries.