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Site Characteristics and Remote Sensing

 

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The Nature of Archaeological Sites and the Relationship to Remote Sensing Methods:

Three-Dimensional Matrices of Materials Existing in the Present.

Archaeological sites in North America can be loosely divided into two principal categories, although the boundary between them has some fuzziness and overlap. Prehistoric sites are Native American sites occupied prior to European contact, where the date of contact varies across the continent.  North of Mexico they are generally devoid of metallic artifacts (with some important exceptions, mostly of copper) and stone was not generally employed for building.  Artifacts typically consist of stone, ceramic, or bone.  Settlements are often not well structured in their layouts. Most structural features are indicated only by changes in soils. 

Historic sites are more recent and typically reflect occupations of people who came to the continent after Columbus, or their descendents.  It is emphasized that many historic Native American sites also exist, but they tend to share the traditional characteristics of prehistoric sites with features common to Euroamerican and other historic sites.  Historic sites tend to contain metallic artifacts of various types, and glass is also common in addition to stone, bone, and ceramics (with china and glazed wares common).  Many structural features are of stone or brick and cellars are common, although many are also indicated only by more subtle changes in soils.  There tends to be a high level of geometric patterning associated with settlement layouts, for example, rectangular grids of blocks or streets.

 

The materials composing an archaeological site can be divided into three classes; artifacts, structural features, and sediments and soils. Artifacts are material objects modified by humans. Portable artifacts include smaller items that are easily moved, like tools employed in day-to-day activities (e.g., arrowheads, pots, knives). Non-portable artifacts include items not easily moved like cut posts, building timbers, shaped stones used in architectural constructions, and bricks.

 

 

 

Structural features result from the many types of human constructions, including places of human occupancy (e.g., buildings, houses, storage facilities, public structures), non-occupancy structures (e.g., exterior hearths, subterranean storage pits, wells, fortification ditches) and transportation facilities (e.g., roads, sidewalks). Many structural features are composed of multiple, robust non-portable artifacts like stone blocks (e.g., a building foundation made of many individual bricks). Most structural features are reflected only by more subtle changes in deposits, for example when ditches, house pits, storage pits are filled in with sediments, or buried posts and wooden structures decompose into soil.

 

 

Sediments and soils are the deposits within which artifacts and structural features lie. Most sediments and soils result from natural processes such as eolian or alluvial deposition. Many sediments and soils within archaeological sites are anthropogenic or are created or altered by human activity.  Such deposits have often been referred to as "midden." An additive deposit occurs where materials are accumulated such as places where refuse is dumped (typically rich in organic material like food waste, bones, discarded portable artifacts, ash from fires), or in mounding activity when soils are built up for burial mounds or in raised berms associated with ditch construction. Deposit subtraction occurs when parts of natural or cultural deposits are removed by human activity as occurs in the construction of ditches or cellars, ground incisions from foot traffic along lanes or trails. Many portable material objects not modified by humans are also deposited, such as unmodified animal bone resulting from food waste. Other deposits are altered by human activity. Intensive firing such as from a hearth or a burned structural feature (e.g., a house) profoundly increases soil magnetism. The simple act of human occupation subtly raises the magnetic susceptibility of the soils through the addition of organic material and the spreading of fired earths through a site area (e.g., the cleaning out of an earthen hearth).

 

Remote sensing in archaeology, at the current state of technology, can reveal certain characteristics of archaeological sites.

No remote sensing instrument is capable of detecting all subsurface characteristics of archaeological sites; most are sensitive to only particular kinds of physical characteristics.  Consequently, a mix of sensors, each sensitive to different kinds of physical phenomena, is necessary for successful archaeological detection. Small objects like portable artifacts cannot generally be remotely sensed. Large groups or masses of individual portable artifacts or other material objects might occasionally be remotely sensed as with a concentration of ceramic sherds that subtly raise the local magnetic field or a massive concentration of bone from a mass animal kill changes soil electrical properties or surface vegetation patterns above. Metallic artifacts of small size can be remotely sensed if not deeply buried using metal detectors, other EM methods, or magnetometers.

Theoretically a high frequency GPR antenna using high-density spatial sampling could detect individual non-metallic artifacts of small size (e.g., 2-40 cm) at shallow depths (e.g., < 50 cm) under proper soils conditions.  However, no work has been done in this area, as it would require demanding sampling densities and data processing.  Note: we are not proposing such an application in this project.  Current research is concentrating on structural feature identification with this relatively new technology and this will be the direction of use in the project.

Large non-portable artifacts like foundation or pavement stones frequently can be sensed. In general, only structural features can be remotely sensed because they are typically the only archaeological phenomena that are large enough to be detected at most sensor resolutions currently available. Human-created additive deposits are very unique in character. For example, middens, filled with refuse, possess different characteristics like more air voids, greater porosity, inclusions of solids like bone and discarded artifacts, and probably different levels of moisture retention, generally making them good resistivity/conductivity targets. Mound creation tends to employ build-ups of A-horizon (and can include B-horizon) material, general raising the local magnetic field. Removed topsoil for mound construction or through road/trail incisions tends to create local deflations in the magnetic field. Each deposit within a structural feature typically possesses physical properties different from the natural background or surrounding deposits; if the contrasts are sufficiently high features may be detectable with a remote sensing device. Burned structural features are particularly noticeable because they profoundly increase measured soil magnetism. Hearths are the most common feature where intensive firing occurs. Kilns and ovens also exhibit intensive firing. Burned structures like houses or other buildings are intensively fired. Long human occupations tend to subtly raise soil magnetism in the occupation deposits, largely through the introduction of fired materials and rich organics. Large structural features near the surface can impact vegetation growth patterns above, as when a buried stone pavement or wall retards growth or a moist sediment in a ditch fill advances growth.  The vegetation patterns may be detectable from the air or space using panchromatic or multispectral imaging methods. Variation in deposit materials, compaction, moisture retention and other factors affect absorption rates of electromagnetic radiation.  When this energy is re-radiated back into space as emitted energy, subtle variations (e.g., 0.1oC) can be detected and mapped by thermal scanners.

The ability of archaeological phenomena to be remotely sensed depends on a variety of factors in three domains; the nature of the sensor employed, specific environmental conditions at the time of data acquisition, and the nature of the archaeological site being remotely sensed. Important sensor characteristics include spatial and spectral resolution and the environmental variables that determine sensor effectiveness. The spatial resolution or density of the sampled measurements is most important to the success of remote sensing because of the small size of most structural features in archaeological sites.  Is the sensor on the ground, in the air, or in space?  Generally, the closer to the ground, the greater the spatial resolution and detail that can be achieved.  Precision of measurement (sometimes referred to as "spectral resolution") relates to the size of objects that can be detected. A magnetometer with precision at 0.05 nT can identify smaller, more subtle, and more deeply buried anomalies than one that measures at the 1 nT level, for example.

The kind of sensor employed is related to the environmental conditions such as season and time of day, soil type and condition, and site condition. Thermal remote sensing may be inappropriate at midday whereas multispectral work requires daylight. Low sunlight creates shadowing useful in aerial photography interpretation. Summer vegetation is necessary for detection of crop marking visible from the air or space. Soil resistivity/conductivity appears to be most productive during the moist soil conditions of spring and early summer. Surveys normally cannot be effectively conducted in frozen winter ground. Surface cover impacts the success of surveys. In general remote sensing is more successful in open fields with uniform ground cover like grasses. Forested or heavily vegetated areas negatively impact remote sensors. Magnetometry requires soil magnetism while soil conductivity requires soil moisture. Deposit character interacts with sensor type. Moist clays might preclude use of radar while dry sands can impede soil resistivity studies. Too much or too little soil moisture can negatively impact soil resistivity and conductivity surveys. Too much moisture can impede transmission of radar energy while thermal remote sensing is largely a function of ground moisture conditions. Noise is introduced by a wide range of disturbance factors. Rodent damage, density of historic (metal) litter on the surface, effects of agricultural practices (plowing), modern culture effects (e.g., power lines, automobile traffic in proximity), and biological effects (e.g., animal dens and tree throws) must be taken into account when selecting sensors.  On military installation the presence of military vehicle traffic and metallic debris can be common.

The effectiveness of a given sensor is a function of the nature of the archaeological site being investigated. More deeply buried structural features are more difficult to detect than shallowly buried ones.  Soils above the archaeological deposits act much like a low pass filter, degrading the signal of what lies beneath. They also introduce noise to the data. Structures of small size (e.g., pits, postholes sub-meter in size) have lower detection probabilities than large structures like houses, roads, or ditch systems. Strongly patterned anomalies consisting of lines, circles or rectangles are easier to recognize as archaeological than unpatterned anomalies that might also result from biological activity (e.g., tree throws, animal dens). Intensive occupations produce complex archaeological deposits with dense cultural stratigraphy or structural features that can "jumble" the signals that are remotely sensed, making anomaly identification and interpretation difficult. Sensor efficacy is also determined by the contrasts between cultural deposits and the natural background.  Anomalies, by definition, result from contrasts against "normal" background conditions. Successful remote sensing generally requires a high level of contrast. Low contrast features can often be detected, however, with the proper data density and use of filters and other image enhancement techniques.

Properties of Archaeological Sites and Associated Remote Sensing Requirements.

Because many archaeological sites are very large, cost effective remote sensing requires the use of sensors that can collect data at a high rate. Human settlements exhibit spatial organization. Recognition of culturally formed patterns like shape, size, arrangement, orientation, and context is required in data processing and analysis phases. Human-generated structural features within archaeological sites exhibit high spatial frequencies, because many features range in size from sub-meter to only a few meters in size, pointing to the need for high spatial resolutions.

The deposits and materials within archaeological sites possess different physical properties. Anthropogenic enrichment of topsoil, past removal or mounding of topsoil, burning, the importation of sediments (e.g., clays for floors) or rock (for floors/foundations), and the addition of ferrous artifacts beginning in proto-historic times alter magnetic susceptibilities of deposits and the cumulative magnetic field. Variations in material, porosity, particle size, salinity, moisture, and other characteristics of deposits impact their conductivity and dielectric properties.  Various geophysical survey instruments are ideally suited to detect and map these changes.

Soil and moisture changes within near-surface archaeological deposits influence surface vegetation patterns creating crop marks. Plants growing immediately over buried stone walls or pavements, for example, will tend to exhibit stunted growth or yellowing while other plants growing over wet, organically rich sediments that may have in-filled a ditch or house depression will tend to exhibit enhanced growth.  Aerial photography and aerial or satellite multispectral scanning are necessary to detect and map these changes. Soil and moisture changes within near-surface archaeological deposits together with surface vegetation patterns yield thermal variations recordable at the surface. Patterns in archaeological deposits can be expressed through minute temperature changes (e.g., 0.02o C) at the surface. Under proper conditions, airborne or ground-based thermal sensors can record these variations.