by Rich Estabrook and Jason Moser

Archaeology has changed a lot through the years. In the early days, it was mostly accomplished with pick axes, shovels, wheel barrows, and a lot of manual labor. Many of the earliest “archaeologists” were antiquarians–more interested in high value artifacts than in the information that could be obtained from the whole collection. They were also more interested in the artifacts, than in the information that could be determined from subtle changes in soil. The most sophisticated technologies that were used during this period–included transits for mapping sites, index cards for cataloging artifacts and typewriters for publishing–when they were published. Later, as the discipline developed more sophisticated techniques, they exchanged pick axes and wheel barrows for trowels, dust pans, screens, and even early punch card computers. Archaeology continued to rely on very basic tools for the last 50 years. With the increasing speed of technological innovation over the last several decades, new technological innovations have also played an increasing role in how we recover information about the past. These technologies have made it easier for archaeologists to do their jobs without undertaking large scale excavations. One useful technology that was introduced in archaeology about 20-30 years ago was Ground Penetrating Radar (GPR). GPR has now become relatively common in the field of archaeology and its results are widely accepted.

Ground Penetrating Radar is similar to the type of radar used by airports to track planes in the sky. A GPR unit consists of two components—an antenna, and a receiver. The antenna works by sending pulses of radio waves downward into the ground. The receiver detects and measures the delay of signals from reflected subsurface features. This technology won’t help archaeologists to find individual artifacts, but it will help them to locate buried walls, pits, shell middens, wells, and cemeteries. The GPR can often locate the stratigraphy of the soil layers to identify portions of the archaeological sites that might be intact. The technology is now widely used in many sciences and business applications.

Student pushes GPR at Mound J

The Florida Public Archaeology Network (FPAN) uses GPR both as a scientific investigative tool and to promote outreach in local communities. Most frequently, we use the GPR to locate unmarked graves in cemeteries. These were graves in which permanent headstones were never installed, or in cases in which the headstones had been removed. We have also used the GPR on archaeological sites to collect data about the internal structure of a site. Understanding this internal structure assists in the interpretation of the site. It can also help archaeologists to target specific areas to investigate with more traditional methods. This year FPAN Central had the chance to assist the University of South Florida (USF) and Ohio State field schools during several days of investigation. In the next segment, FPAN Central Director, Rich Estabrook, provides more detail about the GPR, and his efforts to understand the sequence of mound construction at Crystal River Archaeological State Park.

Preliminary map of recent GPR work conducted at Kirkland Cemetery.

I never thought that remote sensing could be so much work! It took a very dedicated group of students all day to drag two Ground-Penetrating Radar (GPR) units across Mound H at the Crystal River Archaeological Site. One hundred-twenty times the GPR equipment got pulled, pushed, dragged, and yanked over the mound in order to collect the data that will hopefully tell us more about the construction and function of this massive shell and sand structure.

Dr. Victor Thompson (Ohio State Univ.) pushing the GPR near mounds J and K.

Our GPR equipment is made by Geophysical Survey Systems, Inc. (GSSI) in New Hampshire, a leading expert in GPR and remote-sensing technology. The GPR can be configured in a variety of ways. The most popular is mounted in the “high-tech” jogger-stroller cart, but it can just as easily be dragged around with a handle and an attached survey wheel, or “distance-measuring” wheel that measures how far the GPR travels. The smaller 400 MHz antenna is configured this way. The larger 200 MHz antenna had to be physically dragged, and on occasion pushed and pulled over the mound. Whether being pushed along or dragged about, the GPR antennas can collect huge amounts of data that are both displayed on a video screen for immediate viewing and also stored in memory for later processing and image enhancement. A GPR collects data in much the same way that you cut or slice into a birthday cake. It makes a thin vertical cut from the top to the bottom of the deposit using microwaves. Special software is then used to turn the vertical “slices” recorded by the GPR into horizontal “layers” that are both easier to understand and can be used to measure the distances to and between the anomalies or targets identified by the GPR.

GPR Screen shot as seen by operator.

We’ve decided to use the two different antennas on the same mound in order to reach different depths. The smaller 400 MHz antenna can reach to roughly 10 ft (3 m) below the ground surface, whereas the 200 MHz “Big Box” antenna can reach depths of more than 21 ft (7 m), but at a much lower resolution. It was hoped that that two antennas used together would provide an image deep into the mound that would provide us with some clues about the earliest construction.

So what about the results of the investigation? Well, you’re going to have to wait until the data has been interpreted. Hopefully, the by early next year, the results of the GPR and Geoprobe investigations at the Archaeology Park will form centerpiece of new interpretations of the people that lived and worshiped at the mound complex at Crystal River.

To see more about FPAN, Crystal River Archaeological State Park, and GPR activities check out the following: