Ground Penetrating Radar:

Overview and Applications in Archaeology

and Forensic Science with a Focus on

Florida

Kevin Gidusko

University of Central Florida

Anthropology Department

ANG 6918

Dr. John J. Schultz

July 25, 2014

ABSTRACT

Ground Penetrating Radar (GPR) is a geological prospection tool appropriated from

fields in the natural sciences into anthropological research that provides a relatively quick and

nondestructive method of investigating the near subsurface of a site. Originally utilized for

archaeological investigations, including the location and delimitation of historic cemeteries, GPR

has recently begun to play a larger role in forensic science. Ground Penetrating Radar can

provide the forensic investigator with a noninvasive method of locating clandestine burials. This

paper provides an overview of GPR and applications in the fields of archaeology and forensic

investigations. Special attention is paid to the utilization of GPR in the detection of cemetery

graves and further possible research in this area.

Key Words: Ground Penetrating Radar, Clandestine Burials, Geophysical Applications in

Anthropology, Historic Cemeteries

INTRODUCTION AND PURPOSE

Ground Penetrating Radar (GPR) is increasingly important to archaeological and forensic

investigations (Isaacson et al., 1999; Leckebusch, 2003; Ruffell and McKinley, 2005; Morgan

and Bull, 2007; Schultz, 2007; Schultz and Dupras, 2008; Wardlaw, 2009; Conyers and

Leckebusch, 2010; Pringle et al., 2012). Ground Penetrating Radar, as with other geophysical

applications in archaeology and forensic sciences, provides researchers with a relatively quick,

noninvasive, and increasingly more precise representation of anomalies at the near subsurface at

a given site. Relative to the manpower required to survey a site utilizing invasive methods such

as shovel surveys, GPR is more cost effective in terms of both the number of individuals and the

time required to carry out a site investigation (Johnson and Haley, 2006). Ground Penetrating

Radar has been used in a wide variety of archaeological sites and contexts; from investigations

into Roman era burial tombs in Egypt (Shaaban et al., 2009), to mass graves in North West

Ireland (Ruffell et al., 2009), to the remains of a Middle Archaic shell monument along the St.

Johns River in Florida (Sassaman et al., 2011). More recently, GPR has become an important

tool for forensic investigations as well, utilized often to detect clandestine burials (Unterberger,

1992; Mellett, 1992a; Miller, 1996; Nobes, 2000; Davenport, 2001; Ruffell and McKinley, 2004;

Morgan and Bull, 2007; Schultz, 2007; Schultz and Dupras, 2008; Billinger, 2009; Novo et al.

2011; Conklin et al.; Pringle et al., 2012). Because of the aforementioned factors and its

persistent integration into current research we may expect that GPR will continue to be an

integral aspect of archaeological and forensic investigations for the foreseeable future. As such,

it is worthwhile to become more aware of this technology and to determine what future avenues

of research this important tool will provide researchers. The purpose of this paper is to provide

an overview of GPR principles and methods. This will include overviews of GPR use in

archaeological and forensic contexts with a case study presented for illustration. A special focus

is paid to previous and possible future research in the state of Florida.

GPR HISTORY

Ground Penetrating Radar, as we know it today, has its inception in the radar technology

developed during WWII and was used largely for the detection of aircraft. As Conyers (2004)

notes, this technology was reevaluated in the 1950s as U.S. Air Force radar technicians noticed

that radar pulses penetrated glacial ice when flying over Greenland, suggesting its use for

subsurface prospection. An early GPR system was developed for use by NASA in 1972 and was

dispatched with the Apollo 17 mission in order to better understand the geology of the moon

(Conyers, 2006). The 1970s saw the growth of GPR as a geotechnical tool when researchers in

the fields of geophysics, geology and physics began to incorporate GPR into their investigations

(Leckebusch, 2003; Conyers, 2004; Conyers and Leckebusch, 2010).

The field of archaeology was quick to adapt GPR to site surveying with some initial,

albeit slow success. Early GPR units provided little in the way of immediate readability and data

storage; for example, in some models oscilloscopes provided a signal readout which had to be

photographed in order to be stored (Leckebusch, 2003). However, in 1975 one of the first GPR

applications to an archaeological investigation was conducted at Chaco Canyon, New Mexico.

Vickers and Dolphin (1975) demonstrated the utility of GPR for identifying anomalies at about

1m below surface. These anomalies, when ground-truthed, were found to be mud brick walls.

Early utilization of GPR in the field offered little more than the location of anomalies in the

subsurface which were often hard to distinguish between anthropogenic or archaeological

features and that of naturally occurring soil change (Leckebusch, 2003; Conyers 2004; Conyers

2006). Succeeding decades saw a refinement of GPR use within archaeology as researchers

focused on the functionality of this tool in a variety of conditions and locales (Sheets, 1985; Imai

et al., 1987; Goodman and Nishimura 1993; Conyers, 2004). Certainly a great impetus to the

increased use of GPR in archaeological surveys can be attributed to advancement of digital

technology over the last several decades which allowed for the manufacture of GPR components

providing real-time data displays as well as digital data collection (Leckebusch 2003; Conyers

2004). Advancements continue to be made in GPR processing and visualization, driven largely

by its increasing acceptance in the field of archaeology and especially due to its utilization in

cultural resource management (CRM) firms who take advantage of its relatively low cost (Table

1) and data collection speed (Grasmueck et al., 2004; Johnson and Haley, 2006; Lockhart and

Green, 2006; Conyers and Leckebusch, 2010; Conyers, 2012).

Present research in GPR continues to focus on the advancement of processing techniques

and the testing of GPR in areas previously deemed unlikely to produce valid results

(Leckebusch, 2003; Schultz, 2008; Schultz et al, 2006; Schultz and Martin, 2011; Lualdi and

Lombardi, 2014; Teixidó et al., 2014). The ability to render subsurface data collected with GPR

in 3D models, while not new, continues to be a focal point of archaeological and forensic

investigations as digital processing technology advances. Increasingly, data collected by GPR

can be readily integrated into various other mapping and visualization software packages that

provide access to global positioning systems (GPS) information of the site and accurate

subsurface 3D models of anomalies that can be transferred into geographic information systems

(GIS). These applications allow for a noninvasive manner in which to effectively study sites as

well as a method of long-term digital preservation (Lualdi and Lombardi, 2014; Teixidó et al.,

2014; Catapano, et al., 2014).

Table 1. One of the driving forces for the inclusion of GPR in

archaeological investigations is its cost effectiveness. In the table below

a hypothetical cultural resource management survey is outlined. The top

portion of the table shows the cost of the survey using remote sensing

equipment and compares this to the cost of a traditional survey below it.

As can be seen, remote sensing surveys are far more cost effective than

traditional, invasive surveys (Johnson and Haley, 2006).

Operation of GPR

GPR units today often consist of just a few components that are capable of being

operated by a single user: The antenna, the control unit, and the display, all powered by a battery.

Often, these components can be loaded onto a cart or backpack for mobility. Many antennas

used in GPR research today operate in bistatic mode; two antennas are operated simultaneously

with one acting as a transmitter and the other as a receiver. Another version of antenna type,

known as monostatic, operates as both the transmitter and receiver; after a pulse of radar energy

the antenna automatically switches to act as a receiver. The display allows for real-time

visualization, setting adjustment, and data storage. Survey wheels may also be attached to GPR

units in a cart and can be set to collect location data to be integrated with GIS software (Conyers,

2004; Wardlaw, 2009; Gontz et al., 2011).

GPR works by transmitting an electromagnetic pulse from the surface antenna into the

ground which then propagates as a wave form. This wave will encounter variability in the

electrical and magnetic subsurface matrix and so the physical and chemical properties of this

matrix determine the speed at which the propagated wave is registered by the receiver as it

returns to the surface. This two-way travel time is recorded in nanoseconds (ns), or one billionth

of a second. Some of this energy will not return to the receiver due to certain chemical or

physical properties in the subsurface. This is known as signal attenuation (Conyers, 2004;

Conyers, 2012). The GPR unit will continue to emit radar energy and collect data as it is moved

along a transect. This data will be displayed on the screen in real time depicting anomalies as

they are encountered. These anomalies often appear as hyperbolas due to their detection by the

receiver upon approach, while immediately over them, and upon moving beyond them (Figure

1). These 2-D representations of a single transect are referred to as reflection profiles (Tischler,

2003; Schultz, 2003; Conyers, 2004).

Figure 1. A reflection profile showing GPR data collected in 2D along a single transect. Note

the two readily apparent downward facing hyperbolas which are representative of anomalies.

Electromagnetic waves are defined by their wavelength which is, in turn, determined by

the oscillating force producing them. This frequency is measured in units of hertz (Leckebusch,

2003; Conyers, 2004). Most GPR units operate with frequencies ranging from 10 to 1500

megahertz which is within the frequency band of many common communication devices. These

can possibly cause interference during data collection and should be accounted for in post-

processing of data (Schultz, 2003; Conyers, 2004).

One of the most important aspects for a GPR survey is the selection of the antenna which

depends largely on two variables: depth required and resolution needed (Table 2). An increase in

antenna frequency provides greater resolution but a decrease in depth of wave propagation.

Conversely, a decrease in antenna frequency provides less resolution but an increase in depth of

wave propagation (Tischler, 2003; Conyers, 2004; Schultz and Martin, 2011; Conyers, 2012).

Determination of the required antenna frequency is tied to the estimation of the relative dielectric

permittivity (RDP) of the subsurface media at a site. The RDP measures the capability of

propagated electromagnetic waves to travel through a given medium and the likelihood of either

a signal return or attenuation. For most archaeological investigations this is synonymous with

the velocity of the propagated radar pulse, a factor used to determine relative depth. Radar

energy will move slower through material with a high RDP and conversely radar energy will

move faster through material with a low RDP (Smith and Jol, 1995; Schultz, 2003, Tischler,

2003; Conyers 2004; Conyers 2012).

Table 2. Relative depth of penetration and

resolution of various antennas, based on a

scale of 1 to 10, adapted from (Tischler,

2003).

Antenna Frequency Depth Resolution

100 MHz 10 1

200 MHz 9 2

300 MHz 8 3

400 MHz 7 4

500 MHz 6 5

600 MHz 5 6

700 MHz 4 7

800 MHz 3 8

900 MHz 2 9

1000 MHz 1 10

Once site factors are taken into account it is possible to choose with relative surety the

best antenna frequency to operate at a site. Generally, many of the antenna frequencies available

on the market are impractical for use in archeological or forensic investigations. Very high and

very low frequencies provide either too much resolution and not enough depth, as in the former,

or too much depth with too little resolution, as in the latter. For the purposes of current

archaeological or forensic investigations an antenna frequency range of between 200 MHz and

500 MHz is sufficient for most surveys (Table 3).

Table 3. A selection of GPR surveys from within the last ten years showing a variety of site survey

types and the range of antennas used. Most archaeological or forensic surveys are conducted with

antennas falling between 200-500 MHz.

Survey Survey type Average Depth Antenna Frequency

Teixidó et al., 2014 Monumental Platform

(Spain)

2 meters 400 MHz

Ruffell et al., 2009 Historic Graves

(Ireland)

2 meters 200 MHz

Doolittle and Bellatoni,

2010

Historic Graves (United

States)

2 meters 400 MHz

Sarris et al., 2007 Ancient Graves

(Greece)

2 meters 225 MHz and 450 MHz

Shaaban et al., 2007 Ancient Graves (Egypt) Up to 8 meters, most at

4 meters

200 MHz

Schultz and Martin,

2012

Forensic proxy burial

(United States)

0.5 to 1 meter 250 MHz and 500 MHz

While one benefit of GPR surveys is an immediate in-field estimation of anomalies, post

processing of reflection profiles is still necessary. Much of this processing is dedicated to

reducing background noise in reflection profiles that may have been caused by devices operating

at or near the antenna frequency in the area or, as is more often the case, to de-emphasize known

noise-causing agents such as tree roots or animal burrows. As much of the hardware and

software produced for GPR is focused more on industrial or geological prospection, processing

can be a difficult and time consuming process. Considerable effort has been put into researching

best practices for the application of these processing steps to archaeological sites, but there is no

one way to process raw data and indeed much of the processing is determined by the site, soil

profiles, and even the weather at varying times of the year (Leckebusch, 2003, Conyers, 2004;

Conyers and Leckebusch, 2010; Conyers, 2012). Conyers (2004) suggests three basic post-

processing steps that should happen in all cases: 1) prior to all other processing steps the profile

should be corrected spatially for horizontal and vertical dimensions 2) background removal, a

noise filtering process and 3) range gaining to enhance important reflections. As each site and

survey is unique, post processing steps beyond these basics may change, but once a system of

processing is established for a site it is recommended to maintain that processing order.

One of the latest advancements to aid in GPR surveys is the introduction of 3-D imaging

software (Leckebusch, 2003; Doolittle and Bellatoni, 2010; Conyers, 2012). By collecting data

on both the x and the y axes of a survey grid, this software can digitally interpolate information

for a 3-D image of anomalies encountered during a survey. This software also provides the

ability to produce amplitude slice maps that allow for a better estimation of size and depth of

subsurface cultural features (Figure 2). This software has proven to be a boon to GPR research

both in archaeological settings (Conyers and Leckebusch, 2010) and for forensic investigations

(Schultz, 2006; Schultz, 2010).

Figure 2. Amplitude slice maps showing the change in the register of an anomaly at different

depths. Ground penetrating data is collected on both the x and y axes of a survey grid and

combined during post-processing. This combined data produces 3D imagery and also allows for

sectional slicing, as is seen here.

Soil and GPR

Soils and the different properties of soils are important to understand in order to benefit

from a GPR survey. Soils encompass the mineral, organic, gaseous, and liquid material that

encompasses most surface land on earth. The type and distribution of soils is dependent on a

number of geological and environmental factors which change over time. Soils are characterized

by their chemical composition as well as their horizons, or layers. Horizons are formed by the

accumulation, transformation, or loss of material at a given location. Soil horizons, or layers of

soil, do not have homogenous electrical or chemical properties with layers above or below.

These differing properties register as differing amplitudes by the antenna’s receiver during a

GPR survey. It is this function of GPR that allows for the location of anomalies; given a site

with a known soil profile, the GPR registers that which is outside the norm for the profile (Soil

Survey Staff, 1999; Doolittle and Butnor, 2009; Soil Survey Staff, 2010).

One of the greatest hindrances to the feasibility of a GPR survey is the type and condition

of soil at the site (Doolittle and Collins, 1995; Conyers, 2004; Schultz et al., 2006; Schultz et al,

2008; Conyers, 2012). The electromagnetic pulses from GPR antennas are susceptible to signal

attenuation in certain soil and weather conditions. As anomalies are the representation of

electrical or chemical differences detected in the soil matrix, signal attenuation due to soil

characteristics renders GPR surveying difficult or even unviable at certain sites and at certain

times of the year. As such, a basic knowledge of soil characteristics and limiting factors should

be taken into account prior to a GPR field survey. In the United States this can usually be

ascertained through an examination of soil taxonomy information provided by the USDA (Soil

Survey Staff, 2010) or state and local soil surveys which are largely generated and refined

utilizing GPR (Doolittle and Collins, 1995).

In soils with high conductivities the dissipation of radar energy will occur quickly and

provide loss to both resolution and depth. Doolittle and Collins (1995) note four principle

factors that influence the conductivity of soils: 1) porosity and water saturation 2) amount and

type of salts in solution 3) amount and type of clay and 4) scattering (Table 4).

Table 4. Factors that influence the conductivity of soils (Doolittle and Collins, 1995).

Factor Effect

1.Porosity and degree of water saturation Generally, soils have lower signal attenuation

when dry than when wet

2. Amount and types of salt in solution Higher levels dependent on presence of clay

minerals, pH of soil solution, and degree of

water saturation

3. Amount and type of clay In clayey soils ions absorbed on clay particles

undergo exchange reactions with ions in the

soil solution and contribute to electrical

conductivity of the soil.

4. Scattering A subsurface plane sloping away from the

antenna or having a convex upward surface

will dissipate radar energy return.

While these factors may inhibit GPR surveys they do not necessarily disallow them all

together. The use of GPR in an archaeological context is often used to map larger areas; a

portion of a site versus an individual test unit, for example. In the published literature these GPR

surveys are often focused on locating and mapping the foundations of structures (Bevan, 2007;

Conyers, 2010; Conyers and Leckebusch, 2010; Pettinelli et al., 2012) or large, consistently used

burial sites (Chilton, 2007; Wardlaw, 2009; Doolittle and Bellantoni, 2010). Both of these

situations provide ample opportunity for testing and re-testing utilizing different antenna

frequencies or allowing for a change in weather. Soil conditions and weather may provide a

more daunting situation for forensic GPR surveys, however, as these surveys are often under

time and location constraints (Schultz, 2007; Schultz and Dupras, 2008; Schultz, 2012).

Soils are classified in the United States according to the United States Department

of Agriculture (USDA). The USDA’s Soil Taxonomy (1999) provides information on the twelve

recognized soil orders found in the United States: Alfisols, Andisols, Aridisols, Entisols,

Gelisols, Histosols, Inceptisols, Mollisols, Oxisols, Spodosols, Ultisols, and Vertisols. This soil

information provides some of the most basic data necessary to determine the efficacy of a GPR

survey at a given location. In 1954 (Fine) an early map outlining the relative soil conductivity of

the United States was produced. This map provided general information based on state soil

surveys, but had issues with its scale (the scale was far too small to effectively differentiate GPR

applicability at the local level) and its sample size was far too few to be truly effective (Doolittle,

2012). More recently, a more precise GPR soil suitability map was produced by the USDA with

the aid of state soil surveys and a far more defined delineation of soil orders at the local level

(Doolittle and Butnor, 2009; Doolittle, 2012).

Soil in Florida (Figure 3) falls into seven of the twelve soil orders as outlined by the

USDA’s Soil Taxonomy (1999). The soil order is the highest hierarchical level for soil

definition. Collins (1985) states that soil orders, “are distinguished in relation to the five soil-

forming factors: (1) climate and (2) living organisms acting on (3) parent materials over (4) time

as conditioned by (5) relief.” Soil orders are broken down into more defined sub-units dependent

on formation processes, presence or absence of minerals, and even utilization by humans. For

the purposes of this paper, however, it suffices to contain the discussion to the seven soil orders

found in Florida, their overarching characteristics, and what effects these characteristics have on

the potential use of GPR in a field survey.

Figure 3. Distribution of Florida soil orders

(http://waterquality.ifas.ufl.edu/GIS/Soil%20images.htm)

The seven soil orders of Florida are the Alfisols, Entisols, Histosols, Inceptisols,

Mollisols, Spodosols and Ultisols (Table 4). Of these, only the Entisols and Histosols are devoid

of either clayey substances or medium to high soil cation exchange. This does not preclude the

applicability of GPR surveys in other locations, nor necessarily suggest that these are the only

soil orders available for GPR research. Indeed, Histosols are representative of excessively wet

environments where the upper layer of soil is composed of partially decomposed plant remains;

essentially muck (Collins, 1985). Moreover, some recent studies have shown the efficacy of

GPR in clayey soils despite previous evidence otherwise (Schultz et al., 2006; Pringle et al.,

2013; Lowe et al., 2013) as will be discussed later. The important factor for GPR surveying

appears to be a sound knowledge of both the equipment and the environment as well as an

understanding of proper post-processing procedures.

A basic understanding of soil science is necessary to the proper use of GPR in a field

survey. Failing to note properties in soil that may cause a rapid attenuation of radar energy has

the ability to make a survey fruitless. By garnering as much information of a site’s formational

processes and understanding the capabilities of the GPR equipment and post-processing

procedures, it may be possible to expand the role of GPR in future surveys.

Table 4. The seven soil orders of Florida, their characteristics, and percentage of land cover.

Adapted from (Collins, 1985).

Soil Order Description Percentage of

Land Cover

Alfisols Argillic horizon, medium to high mount of bases in soil,

water generally available to plants, ochric epipedon.

4.6 million acres

Entisols Does not reflect major set of soil-forming processes, form in

inert parent materials such as quartz sand or limestone.

Absence of distinct pedogenic horizons except for ochric

epipedon, albic horizon, and a spodic or argillic diagnostic

subsurface horizon below 80 inches.

7.5 million acres

Histosols Very high organic carbon in upper 32 inches. Most formed

from partially decomposed plant remains accumulated in

water. Peat or muck.

4.0 million acres

Inceptisols Moderate to high cation exchange capacity in the clay

fraction. Dark, thick, and low base saturation surface

horizon.

1.0 million acres

Mollisols Dark, brown surface horizon, high amount of calcium

versus other extractable cations present in the soil, and clay

minerals of moderate or high cation-exchange capacity.

Poorly drained.

1.0 million acres

Spodosols Have undergone soil process that translocate organic matter,

aluminum, and iron as amorphous materials. Spodic

horizon, accumulation of black or reddish amorphous

material having a high cation-exchange capacity. Poorly

drained.

8.4 million acres

Ultisols Argillic horizon, but more leached than Alfisols. Low

supply of bases and usually enough moisture for crops

yearly.

6.9 million acres

GPR Survey, an Overview

The basic format of a GPR survey is fundamentally similar across the wide variety of

situations it may be used in. While post-processing, as discussed above, is an integral aspect of

the GPR survey, it is meaningless without the successful detection of anomalies in the field in a

manner that provides the best possible information on subsurface anomalies. For the purposes of

presenting the process of a GPR survey, an example of a recent survey undertaken in part by the

author will be used (Schultz and Gidusko, 2014).

This survey was conducted at the Fountain of Youth Archaeological Park in St.

Augustine, Florida (FOY). The park is operated as a tourist destination and sits alongside the

Matanzas River, which periodically floods the park grounds. The site has been the location of

extensive archaeological work and, in fact, this survey was initiated to attempt to locate previous

excavation units from the middle of the last century. The location of past test units were sought

in order to better map in previous work conducted at the site to an updated grid system that had

been emplaced which was tied into GPS and GIS data. A GPR survey could thus inform future

excavation at the site and save valuable field hours.

Choice of antenna is largely affected by desired depth and resolution needed. These are

factors of the soil type and condition as well as the type of feature being investigated. That is, a

building foundation will require a different approach from an historic grave site; one is large and

requires relatively little resolution and the other is small and requires higher resolution. As this

survey sought large (several meters long), but relatively shallow (>1 m) archaeological test units,

there was little need for extreme depth or resolution. A 250 MHz antenna was decided on for the

survey.

Ground Penetrating Radar surveys should occur on a grid system that is either placed by

the surveyors or works within an existing grid system put in place by a site’s principal

investigator. This allows for a careful control of data collection. Within the grid system

individual transects for the GPR survey can and should be adjusted dependent on subsurface

resolution desired (Goodman et al., 2009). As the GPR unit pulses radar energy into the ground

the wave is propagated in a roughly conical shape that spreads wider as it travels away from the

antenna. For a large subsurface feature, such as a building foundation, a wider spacing will

provide reasonable results but would miss smaller-scaled anomalies. For this reason, it is best to

keep data collection to shorter transect intervals in order to provide enough radar energy overlap.

This provides a clearer depiction of subsurface anomalies, limits the possibility of missing these

anomalies, and provides better data for 3D modeling software. For the survey here, intervals

were set at .50 m. Grid lines should also be set in such a way that the GPR is kept away from

noise-inducing items such as metal demarcation nails or pin flags (Goodman et al., 2009; Annan,

2009).

Before a survey can commence at a site it is also necessary to set the GPR’s velocity,

which in newer GPR models is synonymous with depth. Velocity refers to the propagation of

the radar energy from the antenna and is changed on a site to site basis according to several

factors such as clay content of the soils, moisture in the soil, salinity in the moisture, and the

relative dielectric permittivity of a known soil order. An excellent method of attuning the GPR

unit to the velocity needed at a site is to either 1) collect data over a known subsurface feature

that can be probed for precise depth or 2) bury a highly reflective, conductive material (rebar

posts were used in this survey) at a set depth, then collect data over this area (Figure 4). This

allows for a far more precise method of setting a GPR’s velocity at a given site than to apply

preset relative dielectric permittivity settings.

Figure 4. Calibration of velocity for GPR can be achieved at a site by burying highly reflective

material at a known depth and then adjusting velocity (depth) as the GPR unit is moved over the

buried item. Here, rebar is buried 50 centimeters below surface in a previously backfilled test

unit at the site of a GPR survey. This allows for a good approximation of the type of soil the

survey will encounter (Schultz and Gidusko, 2014).

Once prepared, data is collected at set intervals along a transect and anomalies are noted

on a log sheet as they are seen in real time. Careful information should also be kept about the

presence of trees along transect lines, weather conditions, or anything else that may promote

confusing noise in the reflection profiles during processing. For this survey, data was collected

along 25 transects. After post-processing, anomalies were mapped onto the section of the site

where the GPR survey took place. Excavations at the site continued based on information

gathered during the GPR investigation.

Regardless of type of site or situation which calls for a GPR investigation, these same

steps should be adhered to: Information about the site’s formation and use should be examined

prior to survey; the proper antenna frequency must be used based on information of possible

anomaly types to be encountered; the survey should be conducted on a grid system with transect

intervals kept close; information about possible noise-inducing agents should be recorded; and

finally, data must be processed.

GPR in Archaeology with a Focus on Florida

It was not long after initial attempts to utilize GPR for archaeological surveys began that

its worth became apparent to researchers in the archaeological community, though perhaps ease

of operability lagged behind utility by a few decades (Vickers and Dolphin, 1975; Imai et al.,

1987; Conyers, 2004). Much of this intervening time has seen the creation of new, more readily

accessible GPR units with a wider array of antenna frequencies, visualization capabilities, data

storage, and post-processing options. These advances in the technology associated with GPR

helped secure its role in future archaeological investigations for the foreseeable future; becoming

a mainstay of research agendas, conference presentations, and peer-reviewed journal articles.

Ground Penetrating Radar, along with other archaeogeophysical technologies, is now

regularly used in the CRM industry (Lockhart and Green, 2006; Johnson and Haley, 2006). This

is likely due to its cost-effectiveness and consistently reliable results. Such extensive use in the

realm of CRM, however, exposes both benefits to the continued use of GPR as well as some

research deficits. Most archaeological research in the United States today, as it has for the past

three decades, occurs in the CRM sector. This is due to several legislative acts, but especially

due to the National Historic Preservation Act of 1966 and the Archaeological and Historic

Preservation Act of 1974. This legislation was largely due to a growing recognition of cultural

resources as finite and necessitating active preservation. Currently, more than two-thirds of the

archaeologists conducting work in the United States represent CRM entities (Weymouth, 1986;

Green and Doershuk, 1998; Lockhart and Green, 2006). The growth of CRM firms and the use

of GPR are concurrent and highly intertwined. While this has meant the abundant collection of

data utilizing GPR it also creates the unique situation in which much of this data is represented

only in grey literature, not easily accessible or perhaps even often utilized after the completion of

a project. While it is important to note the presence of this accumulated data and its obvious

potential worth, this paper focuses primarily on research-driven and/or published examples of

GPR use.

The following examples provide a look at GPR applications in archaeology. The first

example (Imai et al., 1987) provides a look at the early application of GPR; its efficiency and

capability at a variety of sites in Japan are described as is a basic overview of the operation of

GPR. The next two articles (Thompson and Pluckhahn, 2010; Thompson et al., 2014) show

current applications of GPR to archaeological research. Contrasted with Imai’s research, these

examples show the growth of GPR and its integration into the wider field methods used during

field investigations.

GPR in Archaeology, an Early Example

An early and excellent example of the use of GPR in an archaeological context can be

found in the work conducted by Tsuneo Imai and others in1987 in Japan (Imai et al., 1987).

Imai notes that geophysical methods are regularly used at archaeological sites in Japan both

before excavation or construction and as a form of cultural resource documentation. Imai and

colleagues used a combination of GPR and resistivity surveys to investigate four sites, the first

three buried under successive layers of volcanic ash. Imai’s first three sites were located in the

Gumma Prefecture of the Kanto region. The first site was surveyed using a bistatic GPR antenna

and sought to discover foundation walls and dwelling floors located up to 70 centimeters below

surface. The second site was far more complex in size and variability. At this site Imai sought

to map and delimit the remains of a small town buried under up to 2m of pumice deposits from

an eruption in the sixth century A. D. Most notable at this location was the determination of a

large burial mound with a diameter of nearly 15m. Imai noticed a unique reflection profile from

within the mound that suggested an inner stone tomb, later confirmed through excavation. The

third site was used by Imai to test the ability of GPR to differentiate between what he terms

“culture layers,” essentially noting the cultural continuity associated with differing stratigraphic

layers (Imai, 1987). Imai noted that these layers were distinctive in detection up to nearly 3m

below surface and that GPR was thus fit to adequately map the distribution of culturally

significant stratigraphic layers in relation to each other. Imai’s fourth site showed the utilization

of resistivity and GPR surveys to detect a stone-lined waterway in a Nara-period town, which

was mapped and confirmed through excavation.

Imai and others conducting early archaeological work using geophysical methods set the

stage for further analyses and refinements of methods. This early paper on GPR prospecting in

an archaeological context illustrates the utility and purpose of future research into this field of

study.

GPR in Florida, Two Examples

Two recent examples from Florida will show the use of GPR in current archaeological

research. Both examples examine the prehistoric cultural landscape of two sites on Florida’s

west coast and demonstrate the utilization of GPR not only as a viable mapping and visualization

tool, but a capable addition to the methods of cultural interpretation at an archaeological site

(Thompson and Pluckhahn, 2010; Thompson et al., 2014).

The Crystal River site (8C11) is a large complex comprised of at least six mounds, the

largest of which is over 9m tall, several burial mounds, platform mounds and three stone

monuments (Thompson and Pluckhahn, 2010). The site’s construction began in Florida’s Late

Archaic period (ca 3000 to 1000 BC) but is mostly noted for its use during the latter Woodland

period (1000 BC to 1000 AD) which saw large populations of hunter-gatherers utilizing the site

(Sassaman, 2004; Thompson and Pluckhahn, 2010). At nearly 15 acres, the site’s excavation is a

large, on-going undertaking. Thompson and Pluckhahn (2010) conducted a geophysical survey

of the site to better discern the construction processes of the monumental architecture at the site.

While resistivity surveys were undertaken for area mapping, the GPR investigations focused on

the monumental architecture at the site. Surveys were conducted along grids at 50cm intervals

utilizing a 400 MHz antenna. This provided excellent resolution of the monument construction

process as depositional layers became apparent upon processing suggesting a purposeful,

communal construction. GPR surveys on Mound H at Crystal River also gave evidence of

limestone blocks, collapsed and buried atop depositional layers (Figure 5). This may point to

early occurrences of social stratification tied into mound construction and ideas of community in

succeeding archaeological periods (Sassaman, 2004).

Figure 5. Reflection profile of Mound H, Crystal River (Thompson and Pluckhahn, 2010).

A second example of GPR use in Florida archaeological investigations can be found in

the geophysical survey conducted at the Pineland Site Complex in south-west Florida

(Thompson et al., 2014). This site is associated with the Calusa peoples and represents one of

their largest permanent settlements. While the site’s last habitation dates into the seventeenth

century, there is evidence of consistent use for about 1700 years prior to that. Thompson and

colleagues (2014) note that one challenge in investigating sites associated with the Calusa is their

sheer size; many sites exceed 125 acres. Large-scale views of these sites are improbable with

limited time and resources, though a viable solution is the controlled geophysical assessment of

these sites. Pineland represents distinctive late Calusa infrastructure; canals, long mounds built

high for house platforms, plazas, and a built environment into the nearby shoreline. All of this

requires large populations and definite social complexity as such monumental tasks require much

upkeep, especially systems of canals. GPR investigations were utilized in a grid at several

locations using a 400 MHz antenna. On Citrus Ridge at the site the GPR was able to provide

information about the early construction of this mound through amplitude slice maps.

Interestingly, deeper anomalies were represented as smaller, circular structures with a change

over the next centuries to larger, multi-family structures (Figure 6). These culminated in the

long, multi-family houses atop mounds that were mentioned in Spanish accounts at the time of

contact (Thompson et al., 2014). GPR here provides invaluable information to the research at

this site: Traditional survey methods are unfeasible due to size, time, and landscape complexity

which leaves this geophysical survey as the best possible option to allow for a large-scale, site-

wide interpretation of past cultural periods. Through 3-D imaging of GPR data, shifts in

household economies may even be evaluated or utilized to form future research questions.

GPR and Clandestine Burials

The search for clandestine burials, whether in the context of an historic cemetery or a

forensic case, is an increasing part of current GPR research (Bevan, 1991; Unterberger, 1992;

Ruffell and McKinley, 2005; Conyers, 2006; Schultz and Dupras, 2008; Ruffell et al., 2009;

Schultz, 2012). The utility of several GPR antenna frequencies to noninvasively detect

anomalies 1-2m below the surface, while providing information on depth, and with quality

resolution makes this geophysical survey method preferable to most others.

In general, GPR does not detect the burial itself, but more so is detecting the change in

soil composition as a result of burial. Large burial vaults or even intact caskets may be detected

due to the void created by their structure which may be recognized as an anomaly.

Figure 6. GPR amplitude slice depicting initial Calusa settlement patterns of small, circular

structures. Note that the GPR survey is now tied into GPS and topographic data

(Thompson et al., 2014).

As with other GPR surveys, it is important to conduct a GPR survey of a clandestine

burial on a grid with transect intervals remaining under 1 meter (Bevan, 1991; Conyers, 2006).

Some more recent research suggests that best practices for a clandestine burial survey should

even stay below a half meter (Wardlaw, 2009). The grid should attempt to traverse the possible

burial at a right angle in order to best visualize the anomaly (Bevan, 1991).

GPR in Historic U.S. Cemeteries and an Example from Florida

There are a number of reasons that an historic cemetery may require a geophysical

search: written documentation about these sites may be missing or incorrect, thus providing

inaccurate information as to the site-limits; markers may have gone missing or have been moved;

encroachment from development threatens possible interments; the location may be lost due to

the overgrowth of the environment; human remains may need to be removed due to imminent

construction projects (Conyers, 2006b). Information on historic cemeteries also provides an

invaluable insight into community practices at known points in history; burial practices,

orientation of graves, gravestone iconography, and associated grave goods all provide a rich

depth of data to the investigation of community practices. GPR surveys thus provide an

excellent additional source of information to research questions associated with historic

cemeteries.

The ability of GPR to detect a burial in an historic cemetery is largely dependent on the

materials used in burial, the preservation of the body or coffin, and the condition of the soil

(Conyers, 2006b). Differing interment practices are associated with different time periods in

U.S. history. Early settlers often wrapped the dead in shrouds and buried them in wooden

caskets, which were most commonly used into the latter half of the 19th

century. This may prove

difficult for GPR surveys to detect as caskets collapse and deterioration over time, along with a

settling of grave shaft soil, may not provide enough of an amplitude difference for the GPR unit

to detect an anomaly. Metallic coffins became more widely available after mass production

began just prior to the Civil War and, if they are relatively intact, will produce high amplitude

signatures during a GPR survey. Currently, many burials in cemeteries are encased within a

liner or cement vault that is designed to preserve the ground stability. These provide large,

distinct targets for radar energy during a survey (Doolittle and Bellatoni, 2010). There are

several physical features that can be potentially imaged using GPR (Table 5), but each has to do

with the detection of differences in the subsurface media as detected by differing patterns of

amplitude as discussed previously.

Table 5. Features that have the potential to be imaged by GPR in an historical cemetery survey.

Adapted from (Conyers, 2006b).

1. Natural soil substrate below and surrounding grave shaft.

2. Buried coffin or body and associated artifacts.

3. Backfill used to fill in vertical shaft.

4. Surface layers of sediment that has accumulated over shaft after interment.

The following example provides a good overview of GPR field methods for the detection

of clandestine burials as well as provides an understanding for how this type of research may be

applied. Wardlaw (2009) in his research investigated an historic cemetery with many marked

graves which he was able to use as controls for the detection of unmarked graves. This research

has applicability to the forensic search for clandestine burials (Schultz, 2007; Schultz and

Dupras, 2008; Schultz, 2012) as well as providing a new dimension of data to archaeological

investigations (Bevan, 1991; Nobes, 1999; Conyers, 2006; Sutton et al., 2013).

As part of his thesis research at the University of Central Florida, Dennis Wardlaw

(2009) conducted a survey of Greenwood Cemetery in Orlando, Florida to determine the ability

of GPR surveys to detect burials at different points of interment. This granted an opportunity to

test the utility of GPR at an historic cemetery site that provided a wide range of burial dates that

may also be tested against current burials, as the cemetery is still in operation.

The Greenwood cemetery was incorporated in 1880 and purchased by the city of Orlando

in 1892. A local ordinance then required the removal of smaller cemeteries interspersed

throughout the Orlando area with the remains reinterred at Greenwood, which became the only

authorized burial site within city limits from this point forward. Greenwood is large, 68.7 acres,

and due to its size and age has a wide variety of interment types from different historical periods

represented (Wardlaw, 2009).

The survey was conducted on 29 out of 36 sections at Greenwood utilizing a 500 MHz

antenna. Transects crossed graves at a perpendicular angle at 50 cm intervals. As anomalies

associated with burials were encountered they were marked and noted for depth. This research

also compared the detection of anomalies with T-bar probe ground-truthing. The comparison of

these methods showed that utilizing the T-bar probe proved to be an effective method of

confirmation for GPR data collection in the field.

This GPR survey depicted the difficulty in detecting earlier graves by solely using GPR,

though the use of the T-bar probe in conjunction with a GPR survey was shown to be

worthwhile. Earlier graves likely proved harder to detect due to deterioration of the casket and

body, lack or loss of casket hardware that may have provided a significant amplitude signature,

and the settling of soil in the grave shaft over time. As GPR detects the difference in the

electrical or chemical properties of soil, compaction of the soil and weathering over time may

create a situation where there is little detectable difference in the substrate. By the 1960s,

however (Figure 7), the GPR unit readily detects both homogenized soils owing to the digging of

a grave shaft as well as the coffin or burial vault (Wardlaw, 2009).

This survey provided a worthwhile initial investigation of the relationship between time

and GPR detection capabilities. Further research in this vein may provide a refinement of burial

detection abilities, especially for burials dating before the middle part of the last century. This,

in turn, may prove worthwhile for historic cemetery location in other locations that have a

relatively high pH level in their soils as does Florida.

Figure 7. Bar graph showing total graves comparison from

Greenwood cemetery survey. Note the efficacy of GPR as tested

against the probe (Wardlaw, 2009).

GPR Applications in Forensic Investigations

One of the latest applications of GPR is in the field of forensic investigations (Bevan,

1991; Mellet, 1992; Unterberger, 1992; Ruffell and McKinley, 2005; Schultz, 2007; Schultz and

Dupras, 2008; Schultz, 2012). Both forensic anthropologists and forensic archaeologists

(Schultz and Dupras, 2008) often assist law enforcement agencies with the investigation of

homicide victims. For forensic archaeologists this assistance may take the form of participating

in the location and recovery of the victim. Schultz (2012) suggests a multidisciplinary approach

and that for certain locations GPR may be the most useful tool to apply to these searches.

Noninvasive forms of search should be attempted first and certain questions should be

considered before a GPR survey is instituted (Table 6). Much of the success of a GPR survey in

a forensic setting can be attributed to the nature of the soil matrix in which the body was buried

along with the time elapsed since burial. Factors that will limit the utility of a GPR survey are

soils that may attenuate the radar energy, a long time period between burial and survey, and

excessive vegetation growth. Current research seeks to refine the ability of GPR to detect burials

in soils that generally attenuate radar energy and to note the change though time of anomaly

detection of a given burial.

Table 6. Questions to ask prior to performing a GPR survey for a forensic investigation (Schultz

2012).

1. When did event occur?

2. How deep is body buried?

3. Was body wrapped in anything?

4. Was anything placed in grave, especially anything metallic?

5. Was anything placed over body to aid in concealment?

6. What are characteristics of site: topography, vegetation, soil, etc.?

7. How has area changed since burial of body?

Three studies from Florida display the current research conducted to better apply GPR to

the detection of buried homicide victims. These studies utilized pig cadavers in a variety of

burial scenarios to test the varying degree of GPR to detect anomalies in these situations through

time, at differing depths, and with different antenna frequencies. This information may serve as

a proxy for various types of burials associated with forensic investigations.

The first study discussed here, by Schultz et al. (2006), sought to test the applicability of

using GPR in a controlled setting to monitor the ability to assess anomalies in a proxy burial

situation over a length of time. The study also attempted to note to what extent depth or time of

burial affected the ability of GPR detection. A total of 12 graves were monitored, each

containing a fully grown pig as a proxy for the relative weight of a fully grown human. The

surveys were conducted with a 500 MHz antenna throughout the process and blank control

graves were included in the survey to account for a distinction between radar energy detecting

soil homogenization or an actual buried body.

This study found that soil had the greatest effect on GPR’s ability to detect burials over

time. For the full period of the study those cadavers buried in sandy, well-drained soils were

able to be detected even at the point of complete skeletonization. This could be detected in the

field and did not require significant post-processing. Alternatively, those bodies buried in clayey

soils became increasingly difficult to detect over time.

Adding to previous research, Schultz (2008) attempted a similar investigation as

previously discussed. 12 small pig carcasses were buried in sandy soil, six at a shallow depth

and six deeper. This study examined the burials over a 21 month period using a 500 MHz

antenna. As in the previous investigation, control units were dug to determine whether the GPR

detected an actual burial or simply the disruption of soil from burial. At the shallow depth, all

carcasses were detected at the end of the first year but became more difficult to detect towards

the last two months. Moreover, these burials were largely visible with minimal post-processing.

Smaller carcasses buried at a shallow depth were harder to detect due to skeletonization after a

period of time and the inability of the GPR to detect an anomaly in the soil matrix. Carcasses

buried deeper, however, tended to preserve longer and were thus able to be detected by GPR

survey. These also required minimal post-processing (Schultz, 2008).

A third example of research into the applicability of GPR in a forensic setting is found in

Schultz and Martin’s (2010) comparison of antenna frequencies in a mock burial survey

situation. A large pig carcass was recorded after six months of interment using both the 500

MHz and 250 MHz antennas. It was found that the 500 MHz provided more detail of the burial,

including soil disturbance than that of the 250 MHz antenna, but that both proved capable of

detecting the burial anomaly (Schultz and Martin, 2010).

These studies allow for the refinement of techniques used by forensic archaeologists

when conducting a GPR survey in support of law enforcement agencies. Any survey undertaken

has a number of conditions that may disallow the use of GPR and a sound knowledge of these

limitations is necessary in order to provide proper assistance. In Florida, the use of GPR may be

hindered by excessive soil saturation, soils high in clay content, and especially vegetation.

Future research may work to better off-set these limitations by refining the capabilities of GPR

surveys.

Conclusion

GPR is a powerful geophysical tool that has been increasingly incorporated into

archaeological research. More recently it has become a useful addition to the survey and

recovery methods of forensic archaeologists when providing support to law enforcement. GPR

works be emitting pulses of electromagnetic energy into the ground and by registering the

amplitude of the propagated waves as they interact with the chemical and physical subsurface

matrix. By understanding the basic media in which radar energy is to be propagated it is thus

possible to detect anomalies within that media. This capability makes GPR one of the most

important noninvasive survey tools available. Research over the last several decades has seen a

refinement of GPR applications in archaeological and forensic research that has been concurrent

with technological advances making GPR increasingly cost-effective and approachable. Current

research continues to test the applicability of GPR in a variety of settings and conditions. These

surveys are critical to the future integration of this powerful tool in cultural resource protection

and for forensic investigations.

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