Archaeological Geophysics I
Presiding: J Maillol, University of Calgary; J Ortega Ramirez, National Institute of Anthropology and History
NS31A-01 08:30h
Near-surface Imaging of a Maya Plaza Complex using Ground-Penetrating Radar
The University of Calgary has conducted a number of ground-penetrating radar surveys at a Maya archaeological site. The purpose of the study is to discern the near-surface structure and stratigraphy of the plaza, and to assist the archaeologists in focusing their excavation efforts. The area of study is located in Belize, Central America at the ancient Maya site of Maax Na. Flanked by structures believed to be temples to the north and west, the archaeologists were interested in determining how many levels of plaza were built and if there was any discernable slope to the plaza. Over the last three years, both 2-D lines and 3-D grids were acquired at the plaza using a Sensors and Software Inc. Noggin Plus system at an antenna frequency of 250 MHz. The processing flow consisted of the application of gain, various filtering techniques and a diffraction stack migration using Reflexw. Interpolation of the gridded data was investigated using simple averaging, F-K migration, pre-stack migration and inversion techniques. As this study has evolved over different field seasons, measured velocities appear to change with the saturation level of the shallow section. Velocity measurements ranged from 0.058 - .106 m/ns during the wet conditions encountered in 2002 and 2004, while velocities of 1.22 - 1.40 m/ns were measured in the drought of 2003. The GPR images to date indicate continuous and interpretable images of the subsurface, showing evidence of structure, discontinuities and amplitude variations. A number of interesting anomalies have been identified, and prioritized for excavation.
NS31A-02 08:45h
Archaeological Applications of Rapid Multi-offset Ground Penetrating Radar Data
The most widely used method of ground penetrating radar (GPR) data acquisition is common-offset (CO) surveying, whereby transmitter and receiver antennas are held a fixed distance apart. The CO method's merits lie in its speed and ease of acquisition and processing. Multi-offset (MO) surveying, also known as wide aperture or multifold surveying is becoming more prevalent. The distance between the transmitter and receiver is progressively increased, resulting in traces from multiple offsets which are then stacked. The benefits of MO data include direct determination of the velocity structure and an increased signal to noise ratio. These advantages are paramount for successful investigations of subsurface archaeological material, which is often structurally complex and has minimal dielectric contrast with surrounding sediments. The main limitation of MO surveys is the onerous task of data acquisition, as the single transmitter and receiver must be painstakingly placed along the profile multiple times to achieve satisfactory subsurface coverage. For faster data acquisition, MO surveys can be automated by mounting one of the antennas on a sled and leaving the other antenna stationary. The traces are recorded while the sled antenna is in motion, and then later interpolated to discrete positions along the profile. This method of rapid multi-offset (RMO) profiling is basic in design and can be easily performed with a bistatic radar unit. RMO data collection is 3-5 times faster than "stationary" multi-offset profiling, while still maintaining comparable subsurface coverage. Two case studies highlight the usefulness of RMO data. The Roman bath-house of Tourega, located near Evora, Portugal, consists of highly structural and complex ruins. 200 MHz RMO data revealed crucial lower levels of occupation at a higher resolution and less noise than 200 MHz CO data. RMO data has also been used in at Fish Creek Park, Alberta, Canada. This site, located on a river terrace, contains Native American artifacts to depths greater than 2m and sedimentary layers of geoarchaeogical interest. CO 250 MHz GPR data was used for site reconnaissance and localization of large anomalies. Rapid multi-offset data was acquired for more accurate imaging of specific anomalies and for delineating different strata by velocity analysis.
NS31A-03 INVITED 09:00h
GPR and Geophysical Archaeometry
With advances in imaging software, the utility of ground penetrating radar (GPR) as a remote sensing tool for archaeological discovery has been greatly enhanced. Software has been the key to extracting subsurface information contained in (noisy) raw radargrams. Traditional time slice analysis, isosurface rendering, and "overlay analysis" are among several image analyses used to identify buried archaeology. Static corrections are developed for the first time which account for the tilt of GPR antenna at sites with topography. With the recent introduction of GPR-GPS surveying to facilitate and automate remote sensing, the accuracy in the 3D imaging of unmarked grave sites has been improved. Successful examples of GPR imaging and time/depth animations for the discovery of Roman sites, the internal design of Japanese tumulus mounds, and Native American Indian tribal grounds are shown
http://www.GPR-SURVEY.com
NS31A-04 INVITED 09:20h
Development Paths in Archaeological Surveying
Geophysical surveys of archaeological sites began in 1938, when an electrical survey was performed at the historical site of Williamsburg (Virginia, USA). Its full development, however, has been achieved by several European teams, which have continuously worked on it since the fifties. Geophysical survey is one step of archaeological site reconnaissance, which comprises many other non-invasive techniques such as document studies, field walking, air photo interpretation...Nevertheless solely geophysical techniques allow a direct exploration of the underground itself over a significant depth of investigation. Several physical properties can be measured to detect and map archaeological features and/or remains but electrical resistivity and magnetisation has been commonly used for fifty years and dielectric permittivity more recently. The major path of the technical evolution was to increase both the speed of the survey and the size of the area by using short measurement duration (less than 0.1 s) and to incorporate mechanical systems that allow the continuous pulling of the sensors on the field. Magnetic measurements are thus achieved either by fluxgate or optically pumped sensors, while electrical measurements are achieved by mobile multi-pole systems simultaneously over two or three different depths. In such surveys the mesh grid is 1 x 1 m or 0.5 x 0.5 m. Another aim is to limit the size of the surveyed area but to increase the geometrical resolution by using ground penetrating radars (GPR) with a very fine mesh (0.2 x 0.2 m) and by processing the data by `time slices' which allow to follow precisely the extension in depth of the different features. In addition for magnetic features, the simultaneous inversion of magnetic field and susceptibility (and soon viscosity) measurements using linear filtering allows the differentiation among the types of magnetization and allows for an improved determination of the depths of magnetic property contrasts. By considering the geological context, global magnetic susceptibility values in the field is also an indication of possible settlement and of the type of land use in the past.
NS31A-05 09:40h
Use of Ground Penetrating Radar and Gradiometry in Identifying Domestic Activity Areas Within the Kolomoki Mounds Archaeological Site, Georgia
The Kolomoki Mounds archaeological site (9ER1) in southwest Georgia appears to be one of the most important Woodland Period (ca. 1000 B.C. - A.D. 900) centers in southeastern United States. The site originally had at least eight mounds, exquisite ceramics and, seemingly, a year-round occupation. Due to an early archaeological misinterpretation, Kolomoki was, until recently, all but ignored in archaeological research. Consequently, the site and its occupation are not well understood. Today, the site is included in the National Register of Historic Places, and is not available for standard archaeological investigation, that is, extensive excavation. Therefore, non-destructive geophysical exploration provides an ideal means for investigating protected sites, such as Kolomoki, to obtain archaeologically relevant information. I will present the results of the first two geophysical surveys I conducted within the Kolomoki Mounds archaeological site in 2001. These surveys are part of my ongoing geophysical research with the purpose of better understanding intra-site settlement patterns at the Kolomoki Mounds archaeological site. The results of the ground-penetrating radar and gradiometry surveys indicate different activity areas at the site. There is a pronounced difference in appearance and density of anomalies between at least two areas, with one of these areas being a part of the habitation area.