In November, Dr. Tim Horsley conducted preliminary geophysical survey of archaeological remains at the Weedon Island Preserve. Dr. Horsley accompanied Christina Sampson, a doctoral student at the University of Michigan, to the Preserve in order to find features to be excavated for her dissertation research. Geophysical survey involves locating and mapping underground features using techniques that measure physical properties of the ground, looking for variations – anomalies – that might indicate buried archaeological features like hearths, posts, pits, and middens. For this stage of the project, two geophysical techniques were used: magnetic susceptibility and magnetometry.
Processes associated with human habitation, such as burning and the decay of organic midden material, can cause naturally occurring iron oxides in the soil to become more magnetic, resulting in an enhancement in topsoil magnetic susceptibility around occupation areas. Measuring this property across an area can therefore help to indicate locations of past human activity, which can then be focused on with other methods, such as magnetometry. Magnetometers measure slight variations in the Earth’s magnetic field caused by variations in the magnetic properties of the ground, allowing buried features to be detected that cannot be seen on the surface.
An experienced geophysicist can use techniques like these to find underground features before archaeologists even start to dig, which helps to make excavations more efficient and accurate. Already, preliminary data collected over a few days at Weedon Island suggests the presence of buried occupational features close to a group of previously visible shell mounds. Along with excavation of some of these features, we can use information from geophysical survey to form a better picture of how early inhabitants of Weedon Island organized themselves on the landscape.