Ground-penetrating Radar for Geoarchaeology

Lawrence B. Conyers

• Publisher: Wiley-Blackwell
• Year: 2016
• ISBN:
  9781118950029
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Geophysics

There has long been a strong collaboration between geologists and archaeologists, and the sub-field of geoarchaeology is well developed as a discipline in its own right. This book now bridges the gap between those fields and the geophysical technique of ground-penetrating radar (GPR), which allows for three-dimensional analysis of the ground to visualize both geological and archaeological materials. This method has the ability to produce images of the ground that display complex packages of materials, and allows researchers to integrate sedimentary units, soils and associated archaeological features in ways not possible using standard excavation techniques. The ability of GPR to visualize all these buried units can help archaeologists place ancient people within the landscapes and environments of their time, and understand their burial and preservation phenomena in three-dimensions.

Readership: Advanced students in archaeology and geoarchaeology, as well as practicing archaeologists with an interest in GPS techniques.

1
Introduction to Ground-penetrating Radar in Geoarchaeology Studies

Abstract: Geology and archaeology have long been integrated as a way to understand site formation processes, place artifacts within an environmental context, and as a way to study ancient people within the landscapes where they worked and lived. An analysis of sedimentary environments has long been necessary in this endeavor, but is often constrained by a lack of excavations, exposures, and other data to study areas in a three-dimensional way. Ground-penetrating radar (GPR) has unique three-dimensional abilities to place ancient people into an environmental context by integrating both archaeological and geological information within the buried context of a site over wider areas that is usually possible. The GPR method can accomplish this because it is based on the analysis of reflections produced from the interfaces and layers of geological units in the ground that are then studied three dimensionally. When this is done, robust analyses of buried geological and archaeological materials can be done for subsurface areas not visible at the surface in order to generate more holistic analyses of geoarchaeological studies.

Keywords: environmental context, sedimentary environments, three-dimensional analysis, buried materials and strata, stratigraphy, reflection generation, environmental reconstruction

Introduction

There has been a long period of collaboration between geologists and archaeologists, as it is impossible to separate the geological record from archaeological materials preserved within sediments and soils. These cross-disciplinary geoarchaeology studies involved stratigraphic analysis, environmental reconstructions, site selection for future excavations, and an analysis of site preservation and postabandonment processes (Butzer 1971; Rapp and Hill 2006). More recently, these types of collaborative geological and archaeological studies have included landscape analysis that places people within an often complex and changing environment (Bruno and Thomas 2008; Constante et al. 2010; Stern 2008). The inclusion of geophysical analysis within geological and archaeological studies has occurred more recently and is beginning to make an impact in many research projects (Campana and Piro 2008; Kvamme 2003) as buried deposits can be studied and integrated with more limited excavations and exposures. These geophysical studies for the most part employ magnetics, electromagnetic induction and electrical resistivity, and ground-penetrating radar (GPR). The use of these types of geophysical methods allows a more complete and broader aerial analysis of complex buried (and otherwise invisible) archaeological and geological materials than was possible in the past (Johnson 2006).

This book is devoted to one of these geophysical methods, GPR, and especially the integration of its unique imaging properties to measure and display materials in the ground along with geological and archaeological data. The GPR method transmits radar (electromagnetic waves) energy into the ground and then measures the elapsed time and strength of reflected waves as they are received back at the ground surface (Figure 1.1). Many thousands or hundreds of thousands of reflected waves are collected along the transects of antennas as they are moved along the ground surface to produce reflection profiles of buried layers and features analogous to viewing profiles in excavation trenches (Figure 1.2). When many reflection profiles are collected in a grid, three-dimensional images of buried materials in the ground can be constructed (Conyers 2013, p. 166). Ground-penetrating radar therefore has the unique ability to not just produce images of both geological and archaeological units in the ground, but to do so in three dimensions (Conyers 2012, p. 20).

Figure 1.1 Collecting GPR profiles with a GSSI SIR-3000 control system and 270 MHz antennas.

Figure 1.2 Comparison of a 400 MHz reflection profile collected within a 50 ns time window to a 3 m thick outcrop of cross-bedded aeolian dune sands with a burned house floor. Reflection energy spreads from the surface transmission antenna, creating an average of reflections received back at the ground surface from subsurface interfaces.

From Conyers (2012). © Left Coast Press, Inc.

Ground-penetrating radar’s ability to produce two- and three-dimensional images of soils and sediments within depths that are usually of importance for archaeology (a few centimeters to 3–4 m burial at most) means that complex images of geological materials associated with archaeological deposits is possible. While some archaeological thinking views the geological matrix of a site as a volume of material that must be removed and discarded to get to the important artifacts and features, most recognize that there is important information to be gained by studying it (Davidson and Shackley 1976; Waters 1992, p. 15). It is this appreciation that geology cannot be divorced from archaeological research that forms the basis for the field of geoarchaeology. This cross-disciplinary focus can become even more important when GPR is integrated with the other datasets to project important information from the visible areas in outcrops or excavations into the invisible and still buried areas of a site.

Often much of what can be seen in GPR profiles and three-dimensional amplitude maps is more geological than archaeological, and there can often be confusion as to what is anthropogenic in origin, or instead the geological matrix (Conyers 2012, p. 19). Successful differentiation of the two, and an interpretation of radar reflections derived from all the units in the ground, is therefore crucial. As most archaeological sites are the result of burial and preservation by geological forces and processes, the various features in the ground that have been modified and altered by physical and chemical forces must be understood. This can be difficult even when exposures are visible to the human eye, but especially challenging when various buried features are visible but not necessarily understood in GPR images. The application of GPR to both geological and archaeological features and their interpretation within standard GPR-processed images is the goal of this book.

Scales and Applications of Geoarchaeological Studies with GPR

Geoarchaeological studies range in scale from very small scale analysis of micromorphology of soils and sediments using the microscope to large landscapes covering huge tracts of land (Goldberg et al. 2001; Rapp and Hill 2006). The GPR method of acquisition and data processing methods has very specific resolutions at measurable depths, which necessitates that it be employed within a middle-range of the usual standard geoarchaeology studies. These scales of study typically involve a few hectares aerial extent at most, with depth of analysis of 3–4 m and feature resolution usually larger than about 20 cm in the maximum extent. There are some notable examples of very large data sets recently collected by multiple array systems towed by motorized vehicles that can study many tens or even hundreds of hectares (Gaffney et al. 2012; Trinks et al. 2010) but these are still relatively rare. Within the scope of most geoarchaeological applications (French 2003, p. 6), and with most of the examples presented here, the study area may be on the order of a few hundreds of square meters in dimension to depths of about 6–7 m.

Geological analysis within the context of archaeology, which can be expanded on and broadened using GPR, can be used to study landscape evolution (Ricklis and Blum 1997) where settlement changes are a function of environmental fluctuations. Specifically, GPR datasets can define fluvial units that are the product of erosion and redeposition (Behrensmeyer and Hill 1998) and associated soil units, which are a function of landscape stability over many centuries (Birkeland 1999; Ferring 2001; Holliday 2001). An analysis of these geological units using an integration of stratigraphic units (Shackley 1975) with GPR datasets (Conyers 2009), within a dynamic landscape will also allow for the study of site formation processes (Schiffer 1972).

Studies that are expanded beyond site formation processes can show the effects of humans on a landscape and their adaptation to environmental change over time (Campana and Piro 2008). This is done by focusing on the geological matrix of a site first, defining depositional environments and changes in those environments laterally and vertically over time. The archaeological record is then placed within this context to understand human adaptation to and modification of their environment. This definition and understanding of environments is one of the key foci in GPR integration with geoarchaeology. This book will provide examples of various common environments discernable in GPR data sets, and then place human activities within those contexts.

The important geological packages of sediments and associated geological units that can be studied and analyzed with GPR are most of the terrestrial depositional environments (such as rivers, floodplains, sand dunes, beaches and other coastal environments), bedrock features that were part of an erosional landscape and later buried, and soil horizons that were living surfaces providing some degree of stability in the past. These types of...