Dig Diary: Cutting-Edge Geophysics Tools

by Meg Watters

Fort James was the one site where Time Team America was working on the archaeology from scratch. Although Dr. Mike Fosha knows a lot about the site and led our investigations, most of the known information for Fort James is from historic documents and features that are visible on the ground surface. So this time, instead of working from a previously established site survey system and record keeping method, we initiated the site survey control and methodology ourselves.

Fort James presented itself as the perfect site to showcase some of the more advanced technologies that are increasingly being used in archaeology. This is the first time in the U.S. that the Foerster magnetometer and Terravision GPR systems were used to map an archaeological site. Both pieces of equipment use multiple sensors which allow us to collect large amounts of data at a higher resolution and a faster pace than conventional survey methods. The Foerster magnetometer array, generously provided by Foerster USA, had three fluxgate gradiometers mounted on a 2-wheeled cart. With two operators, the magnetometry survey covered 1.23 hectares (3.07 acres). The Terravision GPR system, provided and operated by a team from Underground Imaging Technologies, covered the same area by towing a 14-antenna GPR array behind a golf cart-like vehicle.

Both systems are also equipped with an integrated "differential GPS" (DGPS) which uses a dedicated GPS antenna to record the geographic position of the equipment at the same time it is collecting the geophysics data. The GPS data from the geophysics can then be combined with other survey data collected on site to create highly accurate, integrated survey maps.

Good survey practice is fundamental to accurately record of all the team's activities, such as the position of excavation units, archaeological features, small finds and geophysical surveys. Accurate survey methods enable all of the different archaeological work that we do to be related together in order to help answer archaeological questions. Just as importantly, it allows the incorporation of information from historic documents and other types of evidence (environmental, soil cores etc.) into Time Team America's findings and enables archaeologists to return to the site in the future to continue investigations.

At Fort James we had two surveyors (a volunteer from Leica joined Steve Wilkes, our Time Team Amercica surveyor)Êon site working with DGPS, which enabled fast and accurate mapping of not only site topography and structural features but also precise mapping of excavation units and archaeological features. Working as a team with integrated mapping technology, we were able to use the data collected by the Foerster magnetometer and Terravision GPR systems to produce excellent geophysical survey maps that helped provide insight to the extent of the fort as well as the character of some of the structures. Without the integrated DGPS, the geophysical surveys would have had to follow more conventional survey methods which would have probably cut the total coverage area in half.

All of the survey results are assembled in a GIS (geographic information system) project that helps us examine and interpret the geophysical survey results in relation to the existing site features and topography. We are currently working on integrating the final site report results into the GIS for a complete, integrated record of Time Team America's work at Fort James.

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I would like to personally thank all of the survey team members. The thought of using these technologies was very exciting but slightly intimidating as I am fully aware of the many issues that may occur when working with new systems in the field. Additionally, being eight months pregnant at the time, I was not able to run around as normal with equipment collecting data. The UIT [Underground Imaging Technologies] group were most impressive, two collecting data constantly in the field with occasional downloads and two others processing data throughout the day and all four processing into the night. The magnetic survey team walked back and forth across the site taking a break only for lunch (with downloading and processing) and processing in the evenings. Being able to direct these two teams allowed me to really work hand-in-hand with the archaeologists at Ft. James. I was always up to date on the latest information from the geophysical surveys and able to contribute real-time, which is the ideal Time Team America setup!

You can read more about the geophysics work done at Fort James and take a look at the data in the Incident Room.