The more proper term for remote sensing in archaeology is Geophysical Survey, which is a non-invasive, non-destructive method for identifying subsurface “anomalies” or disturbances that represent archaeological features.  When people live at a site, they deposit refuse (garbage), burn fires, dig holes, erect buildings and walls, bury people, build mounds, farm the land and reshape it. All these activities affect and are intrusive into the natural soil deposits as they are manipulated, combined, redistributed and mixed with residue of human occupation.

To detect where these activities have taken place, a variety of instrument types can be carried, drug, rolled, or pushed into the surface to measure variations in magnetic properties, how electrical currents are conducted or resisted as they pass through these soils and identify areas of disturbed soils or different types of soils, as well as texture and moisture differences,. These methods are also able to produce computerized or digitized maps of the “anomalies” they detect.

At Cahokia most houses were erected in basins dug one to three feet deep. After a house was dismantled or burned down, the house basin then became the neighborhood dumping ground. Storage pits dug in and around the houses eventually became refuse pits. Trenches dug to erect palisade walls cut through not only the natural deposits but also through former residential areas, and that mixture of soil and debris was the fill packed around the posts. Borrow pits dug for soil to build mounds often were filled in with feasting and community debris and the mounds themselves were built with soils from many sources that often included former village debris. These are the types of disturbances remote sensing can detect.

However, these tests do not usually tell you exactly what is there, just that there is something, and to confirm what it is usually requires what we call “ground truthing,” or excavation. But, once a representative anomaly is excavated, it suggests, with caution, that similar anomalies will be the same type or similar feature.  Large areas and sometimes entire sites can be surveyed in a few hours or several days. The remote sensing can indicate where to conduct future excavations that will help answer research questions and provide information about community planning and settlement patterns, as well as areas to avoid.

The following are simplified explanations of some of the most common land-based methods that have been used at Cahokia.  Each system has a variety of types of instruments with varying degrees of sensitivity and depth capability. Some work better in certain types of soils or moisture conditions, and the technology is improving every year.

Magnetometry systems detect magnetic variations in the soil that can be caused by the features mentioned above, which are measurably different than the undisturbed soils around them. Humans burn things and leave a lot of organic matter on the surface. When the surface is disturbed and redeposited by construction and digging, the degraded organic matter and associated bacteria, iron oxides in the soil, and burned matter, develop magnetic signatures that can be detected, measured, and plotted up to two meters deep

Electrical Resistivity uses electrode probes pushed into the soil at regular intervals that

measure how easily electrical currents pass through the soil between probes. Highly organic or wetter soils are more susceptible to the flow of electricity, and those types of soil are most commonly found in man-made features, disturbed soils, and burned deposits. The depth of detection is up to 1.5 meters (about 5 feet).  Another type of resistivity (tomography) can also create a vertical cross-section of the soil deposits to a much greater depth (up to 60 feet) and can detect voids, internal structures in mounds, perched water deposits, as well as ditches and walls.

Electromagnetic Con-ductivity measures contrasts in the electrical conductivity of subsurface soils and features and is most useful in detecting ditches, palisade trenches and walls or buried landforms such as ridges and swales, or even leveled and buried mounds. Also, probes into the soil are not required and instruments can be carried or drug across the surface.

Ground Penetrating Radar instruments are drug across the surface and transmit pulses of electromagnetic energy into the ground and the amount that is reflected back to the instrument.  The time it takes to reflect back provides an estimate of depth. They can take horizontal and vertical slices through the soil and even can be used over paved areas.  GPR is effective in locating some structures, graves, ditches and walls. However, at Cahokia it has not been very effective due to the soil conditions.

Seismic Sounding utilizes sound waves generated by small explosions at the surface, such as shotgun shells, or by hitting a metal plate with a sledgehammer. The acoustical waves travel below the surface and are reflected by changes in density to geophone sensors at the surface. Variations in speed or strength of the returning waves can reveal information about stratigraphy or presence of some archaeological features, particularly those of stone.

Lidar (Light Detection and Ranging) is an airborne remote sensing system, done from aircraft or drones, which emits pulsed light waves from a laser to the ground that bounce back to a sensor. With filtering, this method can remove trees and other vegetation to reveal the terrain and man-made features such as mounds..

USAGE AT CAHOKIA – All these methods have been used at Cahokia, some more than others. One of the first usages of magnetometry in the Ramey Field east of Monks Mound was by John Weymouth, a physicist from the University of Nebraska who pioneered remote sensing applications in archaeology. Most of the other instrument-based tests have been conducted at the site since the mid- to late-1980s. There is not enough space in this article to include all the projects at Cahokia, so a few will be highlighted here.

Numerous researchers have conducted projects at the site. Some of the most prolific were Rinita Dalan, Michael Hargrave and, Jarrod Burks, working on many projects, sometimes cooperatively. Other researchers who have done one or more projects include Casey Barrier, Tim Horsley, Brett Landsdell, Ed Henry, Bill Woods, Susan Lowry, Ken Williams, Richard Lundin, John Sexton, Duncan McKinnon, Doug Wiens, and Berle Clay. The National Park Service also held a Geophysical Workshop here in 2003 with a number of participants, including some of the above, and they tested portions of the Ramey Field and the Grand Plaza with a wide variety of instruments.

Many other areas have been tested, including Monks Mound, Mounds 36, 37, 41, 48, 49, 56, 60, 61, 62, 72,  to name a few, and numerous areas of the Ramey Fields east of Monks Mound, the Merrell Tract, Fingerhut Tract, Grand Plaza, Powell Tract, the Rouch Mound area, and the Spring Lake Tract.