Lawrence B. Conyers

Ground-Penetrating Radar Techniques to Discover and Map Historic Graves

Ground-penetrating radar is a geophysical technique that can be used to identify and map features commonly associated with historic graves, including intact or partially collapsed coffins and vertical shafts. Data are collected by moving radar antennas that transmit pulses of energy into the ground along parallel transects within grids, recording reflections of those pulses from significant discontinuities within the ground. Visual analysis of radar reflection profiles can be used to identify both coffins and the vertical shaft features commonly associated with human burials. Spatial analysis of the reflection amplitudes within a grid consisting of many profiles (when converted to depth using site-specific velocities) produces three-dimensional maps of these burial features. The identification and mapping of graves can identify remains for possible excavation and study, and the results can also be used for statistical and spatial analysis when integrated with historical records. If identified by these methods, previously unidentified graves can be preserved in areas threatened by construction or erosion.

Ground-penetrating Radar Techniques and Three-dimensional Computer Mapping in the American Southwest

New techniques of ground-penetrating radar (GPR) acquisition and computer processing were tested at archaeological sites in the American Southwest and found to be highly effective in producing images of buried archaeological features. These new methods, especially amplitude slice-maps, were combined with more standard data processing and interpretation techniques and tested at sites with little or no surface expression. In southern Arizona, numerous pit structures buried in terrace alluvium were discovered and mapped. In the Four Corners region, a Chaco period great kiva and other pit structures and features were mapped by GPR and later confirmed through excavation. At some sites, GPR surveys did not successfully identify buried archaeological features. These failed surveys highlight both geological and methodological problems including soil conditions, surface disturbance, and equipment calibration that may be avoided or ameliorated in future GPR surveys.

Velocity Analysis in Archaeological Ground‐Penetrating Radar Studies

In ground-penetrating radar (GPR) investigations of archeological sites, the accurate conversion of two-way travel times to distance or depth can be made only if radar wave velocities are known. Because radar data are typically collected in a manner that does not facilitate the determination of velocity, special efforts must be given to measure the speed of radar waves in the subsurface. Field tests can be performed on objects at known depths where radar wave travel times can be measured directly. These tests can be integrated with the correlation of known stratigraphic units to radar reflections in order to confirm the velocity measurements and map subsurface interfaces. Transillumination tests can be performed in adjoining excavations in order to determine velocity variations with depth. Common mid-point type tests are helpful in estimating near-surface velocity changes but are less valuable in determining velocity at greater depth. Data from these tests can be used to derive velocity gradient curves and identify possible buried reflection surfaces. The integration of many velocity tests can assure accurate conversions of travel time to depth over a large study area. Mapping of surfaces or features in true depth as opposed to radar travel time can confirm the correlations of GPR reflections to known subsurface units and enhance the understanding of an archaeological site.

Analysis and interpretation of GPR datasets for integrated archaeological mapping

An integrated approach to ground-penetrating radar interpretation should include not only the standard amplitude slice maps and isosurface renderings but also an analysis of individual reflection traces and adjusted and processed reflection profiles. Only when all those basic datasets are interpreted can the plethora of reflection features at various depths and locations within a grid be understood, especially in complex geological and archaeological settings. Topographically adjusted profiles can provide important clues to changes in reflectivity along a transect, indicating why certain amplitude features are visible (or not) in slice maps. An integration of excavation and outcrop data with reflection profiles can often indicate what features are producing high-amplitude reflections and which are yielding no reflection at all. Even individual reflection traces can be studied for polarity changes, which can help in identifying the types of buried materials that are producing reflections. All these datasets, some of which are often overlooked, must be integrated during interpretation, especially in complicated ground conditions.

Discovery, mapping and interpretation of buried cultural resources non-invasively with ground-penetrating radar

Ground-penetrating radar is an extremely useful tool for the mapping and interpretation of buried cultural remains within 2–3 metres of the surface, especially when the stratigraphy is complex. Standard reflection profiles can be processed to correct for depth and distance, and also filtered and processed to make cultural features visible. When many profiles are collected in closely spaced transects in a grid, reflections can be re-sampled and displayed in amplitude slice-maps, and isosurface renderings to make buried features visible. Sometimes, however, the abundance and complexity of subsurface reflections is so complex that each individual profile must be interpreted manually, which necessitates an understanding of radar wave propagation, reflection, refraction and attenuation in the ground. In order to differentiate reflections from cultural features this understanding of radar energy must be merged with an understanding of the chemistry of the ground, soil and geological stratigraphy, and how those variables affect radar reflections. When taken as a package of visualization tools, GPR can be used as an effective tool for interpreting aspects of history and culture at buried sites in ways not possible using traditional archaeological methods.

The use of ground-penetrating radar in archaeology

This chapter focuses on the usage of ground-penetrating radar mapping method in archaeology. Ground-penetrating radar is a geophysical method that can accurately map buried archaeological features in three-dimensions. Data are collected when radar waves are transmitted from a surface antenna into the ground, and reflected off buried archaeological features and stratigraphic horizons. The reflected waves are recorded back at the surface and the transmission time is measured that can be converted to depth in the ground. In todays climate of rescue archaeology, cultural resource management, and the prevalent ethic of site conservation, non-invasive methods of subsurface exploration and mapping are becoming increasingly important. New computer enhanced geophysical methods, including ground-penetrating radar, are being developed for site identification, mapping and analysis, which can non-invasively gather massive amounts of data from buried sites without having to dig. Archaeologists who are only familiar with the traditional methods of gathering data by the shovel and trowel method are being increasingly marginalized in this changing environment. It concludes that ground-penetrating radar surveys can also be of tremendous value for the rapid, nondestructive determination of the number, character and orientation of subsurface features at archaeological sites. The GPR mapping method can be used to produce maps that are a far more complete picture of a site than is possible using excavation alone.

A monumental cemetery built by eastern Africa’s first herders near Lake Turkana, Kenya

Significance Archaeologists have long sought monumental architecture’s origins among societies that were becoming populous, sedentary, and territorial. In sub-Saharan Africa, however, dispersed pastoralists pioneered monumental construction. Eastern Africa’s earliest monumental site was built by the region’s first herders ∼5,000–4,300 y ago as the African Humid Period ended and Lake Turkana’s shoreline receded. Lothagam North Pillar Site was a massive communal cemetery with megalithic pillars, stone circles, cairns, and a mounded platform accommodating an estimated several hundred burials. Its mortuary cavity held individuals of mixed ages/sexes, with diverse adornments. Burial placement and ornamentation do not suggest social hierarchy. Amidst profound landscape changes and the socioeconomic uncertainties of a moving pastoral frontier, monumentality was an important unifying force for eastern Africa’s first herders. Monumental architecture is a prime indicator of social complexity, because it requires many people to build a conspicuous structure commemorating shared beliefs. Examining monumentality in different environmental and economic settings can reveal diverse reasons for people to form larger social units and express unity through architectural display. In multiple areas of Africa, monumentality developed as mobile herders created large cemeteries and practiced other forms of commemoration. The motives for such behavior in sparsely populated, unpredictable landscapes may differ from well-studied cases of monumentality in predictable environments with sedentary populations. Here we report excavations and ground-penetrating radar surveys at the earliest and most massive monumental site in eastern Africa. Lothagam North Pillar Site was a communal cemetery near Lake Turkana (northwest Kenya) constructed 5,000 years ago by eastern Africa’s earliest pastoralists. Inside a platform ringed by boulders, a 119.5-m2 mortuary cavity accommodated an estimated minimum of 580 individuals. People of diverse ages and both sexes were buried, and ornaments accompanied most individuals. There is no evidence for social stratification. The uncertainties of living on a “moving frontier” of early herding—exacerbated by dramatic environmental shifts—may have spurred people to strengthen social networks that could provide information and assistance. Lothagam North Pillar Site would have served as both an arena for interaction and a tangible reminder of shared identity.

Subsurface mapping of a buried paleoindian living surface, Lime Creek site, Nebraska, USA

Analysis of continuous core and drill cuttings from drill holes was compared and correlated to data obtained from bore hole geophysical logs to obtain subsurface stratigraphic information at the Lime Creek Paleoindian site, southern Nebraska. The analysis of these bore holes indicated that sedimentary layers and a significant buried soil horizon could be correlated throughout the preserved terrace fill at the site. Geophysical information obtained in well bores in 1993 was compared to lithologic and radiocarbon data from recent core holes and then integrated with archaeological profiles and artifacts collected between 1947 and 1950. A paleotopographic analysis of the soil horizon where the majority of the artifacts were discovered was then made. This ancient living surface was found to have developed on the banks of an abandoned channel, now deeply buried, that ran parallel to modern Lime Creek about 10,000 B.P. Paleoindian people likely camped on the banks of this channel, protected from cold northerly winds by a large bluff to the north of the site.

Ground-penetrating Radar for Archaeological Mapping

Ground-penetrating Radar (GPR) is considered one of the more complicated of near-surface geophysical techniques, but also one of the more precise, because of its ability to map buried archaeological features in three-dimensions. Data from many two-dimensional reflections profiles within a tightly spaced grid, can be processed to remove noise, migrate reflections to their correct subsurface location, and then enhance important reflections from subsurface interfaces of interest. Three-dimensional images can then be constructed that produce realistic isosurfaces and amplitude slice-maps of buried features. When GPR reflections are incorporated with information derived from standard archaeological methods, and corrected to depth in the ground using velocity analysis, GPR maps can be used to display a large amount of information from limited excavations to produce a great deal of knowledge from a very large area. At the Albany, New York, town sites, historical maps of the city were compared to GPR images to determine neighborhood changes over time and the changing cultural landscape of one city block from early settlement through the early 20th century. At two sites in California and Colorado no reflections recognizable as cultural or geological were identified in reflection profiles, but amplitude slice-maps delineated spatial patterns that were found to be highly significant. Complex stratigraphy associated with buried cultural features can also be mapped, as illustrated in reflection profiles from aeolian dunes in coastal Oregon.

Reading of Ground-Penetrating Radar (GPR) Images of Prehistoric Flint Mine; Case Study from Krzemionki Opatowskie Archaeological Site In Central Poland

Abstract Geophysical surveys conducted in order to map tunnels and vertical shafts at the Neolithic chert mining field Krzemionki used a ground-penetrating radar(GPR to test hypotheses regarding orientation, depth and subsurface complexity of these voids.Using two-dimensional reflection profiles the vertical shafts, now mostly filled with lithic debris, were easily visible. Amplitude mapping visualized debris at shaft margins as well as a collapsed material inside the voids. Some shallower horizontal tunnels were also visible as sub-horizontal planar reflections generated from both ceiling and floors of these void spaces. Extension of these interpretations to un-mapped areas of the ancient mining district and complexity of these prehistoric mining features could be examined to determine excavation intensity and exploitation techniques used during the Neolithic.