Forensic Archaeology

Closely linked to anthropology, forensic archaeology is the application of archaeological techniques and methods to legal investigations. Employers will often be police and other government agencies who hire the archaeologists to assist in locating potential gravesites through geological and geophysical surveying methods, carrying out assessments, documenting and mapping scenes, excavating artefacts and remains, and reporting the findings.

Primarily dealt with will be sites of clandestine burials or other buried artefacts relevant to a criminal matter, such as personal effects of a victim or murder weapons. Forensic archaeological techniques are particularly utilised in the excavation of mass graves, generally under investigation by the UN or a similar organisation, aiming to both identify victims and gather evidence for war crime indictments.

A vast array of detection techniques are available to aid in locating sites of interest, as discussed further on in the detection methods section. Archaeologists will particularly look for any indicators of clandestine graves, including visible skeletal remains, decomposition odours, clothes and items on the surface, signs of animal scavenging, soil and vegetation disturbances, and alterations in soil compaction. Following the identification of a site of potential importance, an excavation will be conducted.

Excavations
Any archaeological excavation is conducted slowly and painstakingly, with every stage being recorded scrupulously. The proper excavation of a grave can provide information regarding the time and circumstances of burial, the manner of death, and the tools and techniques used throughout. The excavation aims to carefully uncover bodies and any artefacts, artefacts being the likes of weapons, jewellery, clothing and other personal effects.

Initially a datum is established, which is a particular easily-identifiable fixed point close to the scene. Further sub-datums may then be placed closer to the site of buried remains. The area to be excavated is generally divided into a grid system, allowing for sections to be examined individually and the exact location of objects to be exactly noted. The surface of the grid is first gently exposed, removing any irrelevant debris such as leaves and vegetation. Small and precise tools should be used throughout the excavation so as to prevent any damage occurring to remains or artefacts. When any item of potential evidential value is located and recovered, its exact location must be documented, including its latitude, longitude and depth in the ground.
Before vacating the site after remains and artefacts have been collected, a final examination and cleanup will be conducted to check for any remaining evidence.

Soils are often divided into naturally-occurring stratigraphic layers, also known as strata, distinguished by their colour, texture, grain size, and components. The uppermost layer of soil, known as the topsoil, often contains a larger amount of organic matter, giving it a darker appearance compared to the lower subsoils. During an archaeological excavation, a single strata is removed at a time, ensuring that evidence from different levels are not mixed. The general rule would state that objects within the same strata are of the same relative age, therefore combining artefacts found at different levels could compromise the establishing of a timeline. By studying these stratigraphic layers closely, it may be possible to determine if a site has been disturbed, such as if an older layer has been mixed with a more recent layer. A number of occurrences can result in these layers being mixed, including animal activities, uprooted vegetation, campfires and, most importantly, burials.

Soil and debris is often filtered through a fine wire mesh to separate any potential items of trace evidence, whether through hand-held screens or larger rocking screens. The colour and state of the soil itself may be useful to the investigation, such as if bodily fluids seep from a discarded body into the soil. Due to this, samples are often taken of soil from different layers of an excavation site. The archaeological site is mapped in intricate detail, including scale drawings and detailed descriptions of the state of the site. Like with any crime scene, the location of any individual piece of evidence is documented carefully.

Detection Methods
Archaeologists employ a wide variety of techniques in the detection and study of sites of interest, such as gravesites. Some of these methods may be fairly simple and cheap, whereas others are complex and expensive, used for examining both above and below ground.

Aerial Photography
This is a remote sensing technique which allows for large areas to be studied from the air. The method involves photographs being taken from a helicopter, aircraft or satellite, allowing archaeologists to study the images of the ground to search for any signs of burials or other areas of interest. The way in which the images are taken can be altered to suit the needs of the search. Vertical views can produce images which act like a map, whereas oblique views are ideal for uncovering more detail. Additionally, a continuous photo throughout the flight can be obtained using strip cameras, or stereo cameras can be used to obtain a 3D view of the area. Different filters can be used with the photography equipment to achieve certain effects. For example, ultraviolet filters can improve contrast for concrete, rock and metal, whereas infrared filters can improve plant variation contrast. Through studying the images produced, experts can look for signs of burial. Certain shadows may indicate height differences in the soil, perhaps due to recent burials or grave collapse. Mixing of topsoils and subsoils caused by digging may be seen in the images through soil colour variation, as with certain vegetation growth and disturbances. Aerial Photography may also be coupled with remote sensing, a technique utilising infrared and radiowaves.

Thermal Imaging
This method of detection is a technique based on utilising natural sources of infrared radiation, perhaps emitted from warm materials such as the human body. The radiation is detected using either a thermal scanner or a specialised film, allowing areas to be scanned in search of warmer materials. IR radiation detection is best performed at night. However a number of factors must be taken into consideration. Only the surface temperature is actually detected, though thermal imaging may be used to detect burials to a certain extent. For example, a decomposing body buried at about 1m is a few degrees warmer than surrounding soil, but greater depths may make detection more difficult. The amount of IR radiation emitted is also dependent on the material in question, so the technique may not always be as accurate in all situations. Environmental conditions, such as sunlight and moisture, can also have effects on detection.

Ground Penetrating Radar
Ground Penetrating Radar, or GPR, is a geophysical technique used to study the subsurface. A sound pulse is emitted through discharging a capacitor into the ground under investigation. This pulse is then reflected back, receiving antennas detecting this. The time taken for the reflected pulse to return to the device can provide information regarding the depth of any buried items. Similarly, the direction in which the pulse is reflected can give further details of what is buried. The GPR equipment is generally mounted on a small cart or in handheld form, so the method requires an individual to manually move the device over the area. Although this technique is used to study the area below the surface, its depth penetration is limited to a few metres due to rock and soil absorbing radio waves. Unfortunately GPR equipment is expensive and complex.

Magnetometry
This detection technique is based on the magnetic susceptibility of materials, which is essentially how magnetic a particular substance is. Iron is an extremely magnetic element and, as most rocks and soils contain different iron oxides, they are slightly magnetic. Magnetic fields, which are generated by the Earth, magnetic materials or current-carrying coils, vary across different regions, therefore making it useful in detection. As time passes, materials become magnetised along with the Earth’s field. Any disturbances to this, such as those caused by digging and burials, can be detected by magnetometry. The magnetometer is carried across the area under investigation along lines, data being recorded and plotted into a contour map. The accuracy is dependent on a number of factors, including size, depth and soil contrast, as well as any interfering background noise. There are three main types of magnetometer; proton, gas, and fluxgate.

Metal Detectors
Perhaps one of the better-known methods of detection is the use of metal detectors. The basic construction of a metal detector includes the stem, usually a pole, the control box, and the search head. Inside this search head is a transmitter coil which generates an alternating magnetic field. This field induces what is known as an eddy current in any metallic object encountered, which generates a secondary magnetic field. The search head, which contains a transmitter coil, sends a current and creates an alternative magnetic field which travels into the ground. If this encounters any metallic object, the induction in this object leads to a power loss or a secondary field. The receiver in the search head measures these effects and converts them into an audible signal for the user, indicating the presence of a metallic object.
Metal detectors have the obvious advantages of being portable and light-weight, comparatively cheap and easy to use. They can generally detect objects up to 40cm underground through soil, concrete and vegetation, depending on conditions and the metallic material. During forensic investigations and the search for clandestine graves, metal detectors are only useful in locating buried bodies if there is an associated metal object such as jewellery or dental fillings. There are three primary types of metal detector; Balanced Induction/Very Low Frequency, Beat Frequency Oscillator, and Pulsed Induction.

Resistivity Surveys
This technique is based on the resistivity of the ground, which is essentially a measure of its resisting power to an electric current. Resistivity surveys are beneficial in the detection of recent burials and resistivity changes can be caused by such disturbances. The technique is also useful in detecting and distinguishing between different metals, as metals have very low resistivity values. In order to measure the resistivity of an area, two electrodes are placed a certain distance apart and a current supplied. Both the voltage and current measured are used to calculate the resistivity. There are two primary methods. The two-terminal method is more ideal for use in forensic investigations, as the current and voltage are from the same terminals and so the method is generally faster. However the four-terminal method or Wenner array is better for calculating soil variation averages.

Dating Methods
During archaeological studies, it may be necessary to estimate the age of any artefacts found. This is particularly vital in establishing whether an object found is relevant to the current investigation. Numerous methods of dating are available.

Radiocarbon Dating
This method of dating utilises the naturally occurring carbon isotope carbon-14 to estimate the ages of certain materials. As the Earth’s upper atmosphere is exposed to cosmic radiation, nitrogen in the atmosphere is broken down into carbon 14 (C-14). As this isotope is brought down through the atmosphere, it will react identically to other forms of carbon and so will be taken into plants via photosynthesis and ingested by animals. As long as an organism is alive, its level of C-14 will roughly remain constant. However when the organism dies, the amount of C-14 isotopes gradually decreases. C-14 has a half life of 5730 years, meaning that it will take 5730 years for half of the carbon-14 isotopes to decay by emitting an electron or beta particle. Dating using this method involves measuring the amount of carbon-14 present in a sample and calculating its age based on the atmospheric C-14 to C-12 ratio. Radiocarbon dating is particularly beneficial in that it can be used to date almost all organic materials, though it is not as accurate for recent samples.

Unpaired Electron Dating
Another method of dating is through the study of electrons within materials. Over time, certain materials will build up unpaired electrons trapped within them due to exposure to naturally-occurring high-energy ionising radiation. Over time, the concentration of these unpaired electrons will increase, thus allowing their measurement to be used to estimate the age of the material under investigation. Once the concentration of unpaired electrons has been measured, it can be determined how many unpaired electrons are produced by each unit dose of radiation. Using this, it is possible to calculate the paleodose, which is the total radiation exposure, and thus the age. Age is calculated by dividing the paleodose by the annual dose of radiation. Electron Spin Resonance (ESR) spectroscopy and Thermoluminescence (TL) dating are two methods.

During Electron Spin Resonance spectroscopy, the sample to be analysed is placed in a magnetic field, giving the unpaired electrons different energies depending on their spins. As the sample absorbs radiation, the spins of the unpaired electrons are flipped from low to high energy states. An ESR spectrum is recorded through scanning the magnetic field in order to measure the absorption of radiation at a particular fixed frequency. This spectrum intensity will ultimately give the concentration of unpaired electrons in the sample. One of the primary benefits of ESR spectroscopy is that the process does not release the unpaired electrons, so the measurement can be repeated if necessary. However it should be taken into consideration that some calibration procedures may involve extra irradiation, potentially changing the number of unpaired electrons and thus making the final estimate less accurate.

Thermoluminescence is an alternative method of dating by unpaired electron concentration. This technique involves heating the sample to around 500oC, which gives the electrons the sufficient activation energy and ultimately discharges any unpaired electrons trapped within the material. Any energy stored within the material from the original radiation will be released in the form of light. It is the intensity of this emitted light that is measured to determine the concentration of unpaired electrons.
However as the heating cycle discharges trapped electrons, the measurement can only be taken once, as a second heating cycle will reveal only incandescence. The high temperatures should not actually damage the item under investigation, though some materials may chemically decompose at such temperatures.

Careers & Education
A career in forensic archaeology will generally require at least an undergraduate degree in some scientific subject, such as forensics, biosciences or archaeology. This could then be followed by a masters or PhD in forensic archaeology or similar. Numerous universities and colleges offer forensic science courses, and some even specialise in forensic archaeology and anthropology. Experience is vital in this profession. Archaeological experience may involve volunteering on excavations, though some archaeology courses include such practical work. Forensic archaeologists are not generally employed full-time, but most work in museums, as regular archaeologists, or as lecturers at universities, taking on forensic work on a case-by-case basis.

Ellis, L., 2000. Archaeological Method & Theory. New York: Garland Publishing Inc.

Ikeya, M., 1993. New Applications of Electron Spin Resonance. Singapore: World Scientific Publishing Co.



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