June 27, 2011
James Holt, GIS and Computer Technician for the Osage Nation Historic Preservation Office, participated in a National Park Service (NPS) organized training seminar titled, “Current Archaeological Prospection Advances for Non-Destructive Investigations In the 21st Century.” The seminar was held at the site of Fort Brown on the Palo Alto National Battlefield site in Brownsville, TX from May 23-27, 2011.
The seminar focused on the principles and usage of current non-destructive sensing techniques currently used in the field of archaeology. The methods covered included aerial imagery, Resistivity Survey, Magnetic Survey, Electromagnetic Conductivity, Metal Detectors, Magnetic Susceptibility, LiDAR, and Ground Penetrating Radar (GPR). Additionally, the seminar provided demonstrations for the software needed to process data gathered and output that data into archaeologically useful displays of objects and structures that are under the ground’s surface.
As a subject, the seminar focused on finding any remaining portions of the Mexican-American War era Fort Brown that may be found beneath what is currently a golf course near the Rio Grande. A small portion of the fort remains above ground, including one of the bastions and some breastworks. The rest of the fort was destroyed by floods and the construction of the golf course. The afternoons of the seminar were spent using the methods discussed to locate any remaining subsurface portions of the fort, learning their use and adding data to what is presently known about the fort site.
Aerial imagery includes photography taken at a variety of wavelengths (visible light, infrared, ultraviolet) from airplanes and satellites. Aerial imagery can provide useful information about archaeological sites and structures that are both on the surface and immediately below the surface. The observation of shadows from elevated structures such as mounds, cairns, and middens can provide information about the location of archaeological sites. Also, differences in the color of soil or vegetation can provide information about subsurface archaeological sites.
Resistivity surveys measure the differences in electrical conduction of soil to provide a picture of subsurface structure. The instrument consists of two steel stakes placed in the ground and attached to a power source to input electricity into the soil, and two steel spikes attached to electrical receivers and carried by the user in a grid pattern to measure how much of the electricity reaches the receivers at each point in the grid. This information is then converted into a two-dimensional image of the area showing any subsurface differences that may be attributable to cultural activity. In the case of the seminar study, the resistivity survey provided excellent data and helped locate one of the buried bastions presently beneath the driving range of the golf course.
Magnetic survey measures any anomalies in the earth’s magnetic field caused by a variety of sources including buried objects, disturbed soils, and differences in soil moisture. Conductive metal objects provide the strongest targets for magnetic surveys, though soils that are either burned or highly organic soils also provide enough difference to be detected by magnetic surveys. In the case of the seminar study, a number of metallic targets were located. These targets could be objects remaining in place from the battle at the fort, cannon balls or other artifacts related to the fort.
Electromagnetic conductivity measures variations in the earth’s magnetic field generated by the instrument itself. Electromagnetic conductivity is capable of locating conductive metal objects, but is more specifically targeted at determining differences in magnetism of soils and substructure. Additionally, the electromagnetic conductivity survey machine connects to highly accurate GPS receivers to produce large scale data outputs. The survey of the fort site revealed the potential of buried breastworks near the area of known fort structures.
The usage of metal detectors are generally well known for finding metallic objects near the surface of the soil. By using a magnetic coil, metal detectors find conductive metals such as steel, iron, gold, and silver. Metal detectors are used frequently in battlefield and historic archaeology to locate potential sites and isolated artifacts. In the case of the seminar research, the metal detectors did not locate any artifacts related to the activity at the fort.
Magnetic susceptibility research uses probes to determine how susceptible a given soil is to taking a magnetic field. This susceptibility to magnetism can show past disturbance of soil, higher organic content, burned soil, or higher moisture content. The instrument is placed directly on the soil in question, a reading is taken, and the readings are input into programs that graphically display the spatial relationships and readings, usually as a density map of high and low susceptibility readings. The susceptibility readings were similar to the electromagnetic conductivity readings in the case of the fort.
LiDAR is an acronym that stands for Light Detection And Ranging, and works on a similar principle as RADAR. LiDAR studies use an airplane flying linear paths and directing a laser beam down onto the surface below, taking readings that are translated into topographic data. These studies are capable of seeing through vegetation and detecting very small variations in landform and elevation. These studies are used to locate above ground features as small as graves. They are often performed in preparation for large alterations in the land surface such as reservoir construction, strip mining, wind farm construction, and large natural gas fields.
Ground Penetrating Radar (GPR) uses microwave energy to detect underground anomalies. These anomalies include unmarked graves, contacts between soil disturbances, buried foundations, buried floors, and other solid subsurface objects. The output is presented as a three dimensional model of the land being studied. The GPR data provides spatial and depth information for objects and features in the study area, providing accurate information to guide excavation. The GPR data provided the location of buried breastworks and the moat that fronted the fort’s walls.
The Osage Nation Historic Preservation Office intends to begin wide-scale use of these technologies to pursue archaeological sites of relevance to the Osage people. With future funding and staff additions, the Historic Preservation Office seeks to acquire the equipment to perform these studies to locate sites, graves, and artifacts to begin filling in the blank spots in Osage history and prehistory. For more information about these techniques, contact the Osage Nation Historic Preservation Office at (918) 287-5328.