6 Scientific Tools in Archaeological Exploration-I

V. N. Prabhakar

 

1. Introduction

 

The introduction to archaeological exploration and the various techniques used for locating archaeological sites was given in Module 6 on Exploration Techniques in Archaeology. An introduction on various traditional techniques like map reading, ground research on literature, old maps, travelogue accounts was also given.

 

A broad based understanding on the nature of sites can be obtained with the exploration techniques that were described in Module 6. In addition to the traditional techniques available, the application of sciences has enabled in better understanding of the landscape, sites and even buried features at a site. These scientific techniques and tools will be described in Modules 7 and 8.

 

2. Scientific Tools in Archaeological Exploration

 

Several scientific techniques and tools are nowadays used for the archaeological exploration. The scientific tools are helpful in not only locating the sites in a landscape but also investigating about the nature of individual sites and even buried features. In this regard, several scientific disciplines have helped archaeological investigations and in this case, exploration of sites.

 

The scientific tools that are used in archaeological exploration can be classified into the following categories:

 

2.1 Aerial Survey

 

2.2 Remote Sensing Survey

 

2.3 Geographical Information System

 

2.4 Subsurface Detection

 

2.4 Geophysical Methods

 

2.5 Underwater Archaeology

 

2.1 Aerial Survey

 

The aerial surveys and remote sensing surveys are useful tools in not only identification of potential areas for the location of archaeological sites, but also to document them to understand the changes through time (Renfrew & Bahn 2000). The history of aerial surveys, and more particularly, documenting complete sites and excavations, can be traced back to the World War I times and military reconnaissance (Schlitz 2004). The pioneering works of O.G.S. Crawford in England, Father Antoine Poidebard in Syria, Erich Schmidt in Iran, provided a beginning to this new field, which started with the principle of taking near vertical and oblique angled photographs in different light conditions from an aircraft, to discover, locate and document archaeological sites and monuments.

 

Two processes of photography are discussed by Renfrew & Bahn (2000), selective and unselective. The selective process consists of oblique photography, which is normally taken from a hand-held camera, while unselective process is of vertical photography. These two processes play a crucial role in the documentation of landscapes and terrains from low high kite or balloon photography to photography done using airplanes. While certain features like ditches and fortifications are observable through oblique photography taken during different periods of the day, the vertical photography aids in understanding plans of partially buried remains. The airborne photography was later modified to suit the documentation necessities of archaeologists and low altitude photography played a crucial role in the understanding of excavated structures and standing monuments. The photographic equipment can be mounted in a balloon, kite, ladder or a boom mast depending upon the necessity. Depending upon the availability of nature of equipment and area to be surveyed, different features like earthworks, soil marks or crop marks in a landscape are clearly observable in aerial photographs that may indicate burial features (Mick Monk 1989). Recently, the drone technology has enabled to not only document archaeological sites at different angles, but also to video document the terrain for better understanding and research.

 

 

2.1.1 Oblique Photography

 

One of the two types of photography taken from an aerial medium, which is taken using cameras mounted at a certain angle with respect to the surface. The oblique photographs are more suitable for having a pictorial effect and perspective of the surface and show the archaeological features in a better manner. The photographs can also be taken of known sites to understand the location of several components of fortification walls, streets, gateways, ditches, reservoirs, lakes, and others. The images, once taken have to be analyzed and interpreted by the archaeologists in addition to surface survey to have a better perspective.

 

2.1.2 Vertical Photography

 

Vertical photography is also taken from aircrafts or other mediums with cameras mounted perpendicular to the surface, thus giving a plan feature of the archaeological remains. Vertical photography is less preferred when compared to oblique photography, as the latter reveals the archaeological features in a better manner. However, vertical photography, taken with an objective to obtain stereoscopic vision, is a better tool for documentation of features from the air, which may also be helpful in having measurements. Photogrammetry comes into help here, where superimposing pictures with at least 60% overlapping are combined together using stereoscopic vision to obtain measurable photographs and interpretations of buried remains are possible.

 

2.1.3 Earthworks

 

The large features like ditches, fortifications, or even archaeological mounds slightly raising above the surrounding plain areas can be detected by a feature known as ‘shadow marks’, which helps in identifying large ‘earthworks’. These earthworks are principally man made features and can be identifiable at a suitable height whose features may not be truly appreciated from the surface. The shadow marks depends upon the period of time during the day in which the photographs are taken from the air, as the photographs taken during the mornings and evenings are more suitable to bring out the surface contours more sharply. The earthworks may also be revealed due to differentiation of vegetation on the surface, depending upon its growth either on a ditch or on surfaces that have buried structures.

 

2.1.4 Soil Marks

 

The soil marks are surface features, often viewed on barren land when it is not under cultivation. The ploughing activities bring the earth from sub-surface and if they contain archaeological remains of different composition and content, they comes into the surface and mixed with the top layer. Thus colour modifications and changes occur on the surface, which may be clearly identifiable from the air through photographs. Often, it has also been observed that the difference in retention of moisture by the buried features and topsoil may also lead to various patterns that can be detectable from the air.

 

2.1.5 Crop Marks

 

The buried archaeological features control the growth of vegetation above them, depending upon their properties. A buried ditch or moat, or even mud-brick fortification supports abundant growth, while a brick or stone wall decreases the growth level, due to the variations in the depth from the topsoil and also their properties. Thus, if a vast archaeological feature is buried underneath an agricultural field, the differential variations in the growth patterns of crops indicates the nature of buried feature, as stunted or overgrown vegetation can be correlated with nature of archaeological feature. However, Renfrew & Bahn (2000) points out that photographs from multiple seasons and years are necessary to have a better understanding of the features, in addition to the local knowledge of the land use in a particular area.

2.1.6 Examples in Aerial Photography

 

In the Indian context, the aerial photographs from the sites of Tughlaqabad (Waddington 1946), Sisupalgarh (Lal 1949), are very good examples in understanding the role of such techniques in the documentation of archaeological sites. It is interesting to note the significant changes in the case of Sisupalgarh when the image taken in 1948 is compared with the recent Google Earth image. The aerial photograph taken in 1948 shows considerable vacant areas inside the fort and all around. The recent Google Earth image clearly shows the extensive development and encroachment of areas very close to the site. However, an interesting feature in both these images is the presence of water bodies on the four corners of the fort, and another water body to the northwest of the fortification.

 

As mentioned in Module 06, Google Earth is a latest and simple tool in not only understanding various aspects related to terrain and geographical locations, but also helpful in locating archaeological sites, and in some to observe more features that are not very easily observable on ground (Thakuria et al 2013). Thakuria et al (2013) demonstrated the use of Google Earth to understand the nature of construction at the site of Talpada, while at Lathi, a potential archaeological site was discovered. Tools like Google Earth combined with GIS also helps in predicting modeling of potential archaeological sites, as demonstrated by Gillespie et al in the case of edicts of Mauryan ruler Asoka (Gillespie 2016). Gillespie et al (2016) uses an algorithm known as Maxent to understand the relationship between known Asokan edicts and predicting potential areas of edicts based on similar habitat.

 

2.1.6 Advancements in Aerial Photography and Reconnaissance

 

Considerable advancements have been in the aerial photographic techniques with the development of drone technology or unmanned aerial vehicles, which can also be used to take vertical as well as oblique photographs. Further, the drones can also be fitted with different sensors like thermal, infrared as well as Light Detection and Ranging (LiDAR) to enhance the images to even detect subsurface features like ditches and moats. Techniques like photogrammetric interpretations of archived stereo-aerial photographs combined with laser scanned images of landscapes and digital maps helps in digital surface models of archaeological sites for various perspectives including heritage management (Papworth et al 2016).

 

2.1.7 LiDAR in Archaeological Explorations

 

LiDAR is a synonym for Light Detection and Ranging, which works on the principle of targeting subjects with pulsed laser light and a receiver detects the time taken for the reflected beam. LiDAR is particularly helpful in a wide array of applications as it is very fast and precise data (~5 – 20 cm). Further, it is unaffected by the terrain and dense vegetation cover. There are two major types of LiDAR, namely terrestrial and airborne. The terrestrial laser surveys are carried out with the help of a tripod mounted unit and data collected from a relatively small area resulting in a dense, precise point cloud. Most modern terrestrial laser units have 3600 coverage on the horizontal plane, which is useful to record as much data with one scan. The airborne LiDAR is mounted on an airplane and pulses are generated. The main advantage of an airborne LiDAR is that it can cover vast areas within a short period of time. The resultant output from a terrestrial or airborne laser survey is in the form of point cloud with X, Y and Z coordinates, which are georeferenced with the aid of a Global Positioning System (GPS). The laser beams falling on surfaces with thick vegetation or forest cover and the reflected signals can be classified based on the multiple pulse returns from reflection off multi-tiered vegetation.

The application of LiDAR in the field of archaeological exploration and investigations has increased worldwide recently. LiDAR is effective even in densely forested areas as indicated by successful examples from Angkor Wat (Ornes 2014, Evans et al 2013), Mayan Site of Caracol, Belize (Chase et al 2011). In Cambodia, the LiDAR survey under the Khmer Archaeology LiDAR Consortium (KALC) was carried out in an area approximately of 370 sq. km (Evans 2016). The survey period was decided during the shedding of deciduous forest and also between the dry and rain seasons so that maximum coverage could be obtained with minimum intervention from natural and manmade causes. The LiDAR survey enabled KALC to “…generate incredibly detailed and informative archaeological maps…for the understanding of socio-ecological dynamics over large scales of time and space” (Evans 2016).

 

In the case of Caracol, Belize, 200 sq. km. surveyed by LiDAR technique enabled in understanding and visualizing “…not only the topography of the landscape, but also, structures, causeways, and agricultural terraces (Chase et al 2011).” The 200 sq. km. area was surveyed using LiDAR mounted on an aircraft flying at a speed of 80 m / second, with a total laser scanning of 9.24 hours thereby collecting ‘laser shots per sq. m.’ The end result obtained was a point cloud image at multiple levels, starting from the canopy surface to the ground surface. The point cloud data could be then converted into a DEM and surface modeling for further interpretation.

 

2.2 Remote Sensing Survey

 

The Remote Sensing Survey from high altitudes is another medium in which very large areas can be investigated. In a broader sense, Remote Sensing also includes the aerial photography techniques, and all such survey techniques aimed at collecting the data remotely, or from a distance (Ebert 1984). However, for the sake of differentiation and clubbing all techniques under ‘remote sensing’, only the techniques using the high altitude imageries, particularly from the satellites, are discussed under remote sensing survey here. The imageries from the satellites that are orbiting the earth are of different kinds and nature, depending upon the sensors used in obtaining these imageries. It may also be noted that the airborne and satellite images have their own disadvantages in detecting sub-surface features. This can be overcome by remote sensing using, “…electromagnetic radiation in the visible, near infrared, short infrared and thermal infrared” of which thermal infrared is more suitable, “…for detecting surface anomalies correlated with subsoil surface” (Ben-Dor et al 1999). Remote Sensing works on the principle of “…detection and measurement of electromagnetic radiation, reflected from the surface of objects, and emitted primarily by the sun” (Ebert 1984). The spectrum of electromagnetic radiation, which also includes ultraviolet, visible, and near infrared depending upon its properties, are detected by different means of receivers or detectors, and therefore a multispectral scanners are the most preferred medium to “…record patterns and intensities of radiation in a number of narrow wavelength bands over a target area simultaneously” (Ebert 1984). The infrared imageries are particularly helpful in identifying buried archaeological features leading to variations in crops growth patterns that are easily detectable. Various kinds of satellite imageries are available based on the objectives of investigation. The https://earthexplorer.usgs.gov offers a wide arrange of imageries ranging from aerial imagery, commercial satellites like IKONOS, OrbView, SPOT, Digital Elevation, Digital Line Graphs, Shuttle Radar Topography Mission (SRTM), Landsat, ISRO. Imageries from Corona and Quickbird are other sources that can be used for various applications in archaeology.

 

Some of the applications of satellite remote sensing are in the areas of understanding human ecology and settlement patterns, both on individual and regional scale, also for predictive modeling for archaeological sites in a “…timely, quantifiable, cost-efficient and non-destructive manner” (Madry 1983). The satellite imageries often obtained through multispectral scanners, produce digital data, which can be classified based on the characteristic spectrums and properties, so that the thematic maps for “…water, forest, urban areas, agricultural land, pasture, etc…” can be produced which if used in consonance with the “…archaeological site locations and cultural affiliations, road networks…” for understanding “…regional scale settlement patterns and human ecology over time” (Madry 1983). Rowlands and Sarris (2007) discuss about the possibility of detecting the anomalies caused by buried archaeological remains on the topsoil, which can be detected by visible, near infrared and thermal imageries. However, they also point out the shortcomings in the recording of anomalies due to presence of vegetation, which needs to be taken care of while recording.

 

The use of “conventional film based approach of NIR aerial reconnaissance” to interpret crop marks in the identification of archaeological remains is also a recent attempt (Verhoeven 2012). A best example for the use of remote sensing data for archaeological purposes in the Indian context is the delineation of several palaeochannels of River Sarasvati in parts of Haryana, Rajasthan and parts of Pakistan by several scholars like Valdiya (2013, 2017), Ghose et al (1979), Gupta et al (2004), Bhadra et al (2009), Sharma (2009) and others. The presence of the palaeochannels in Haryana, Punjab, Rajasthan and Bahawalpur (Pakistan) is known since the late 19th century due to several field surveys by scholars like C.F. Oldham (1893), R.D. Oldham (1886) and archaeological sites of different periods have been traced on them by Stein (1942), Ghosh (1953) and others. The confirmation of these palaeochannels by remote sensing data is a classic example of reconfirming evidences in a better and scientific manner that substantiates several new features like continuity, multiple courses of palaeochannel, among others. The recent attempt by Rajani and Rajawat (2011) uses superimposition of archaeological sites on SRTM DEM combined with satellite data to create digital terrain modeling of the lost course of River Sarasvati and to understand relict levees vis-à-vis archaeological sites of Harappan civilization. This study is also crucial as it identifies news areas for exploration in the future based on the conclusions drawn from the analysis. Recently, Rajani (2016), using the satellite images of Nalanda (Bihar) and surrounding areas could identify natural and archaeological features/sites like water bodies surrounding Nalanda, more buried temples and monasteries, a large and buried ‘four-pointed’ structure towards the north. The attempt by Rajani is an example of application of satellite remote sensing on a micro scale and site-specific investigations.

 

Thus, it can been that the aerial and satellite remote sensing surveys have contributed in better understanding of the landscape as well as for reconnaissance purposes for archaeological sites.

 

2.3 Geographical Information System

 

The term Geographical Information System (GIS) was first coined by Roger Tomlinson in the early 1960s for the Government of Canada (Wright et al 1997). The definition for GIS by Burrough is well accepted: “a powerful set of tools for collecting, storing and retrieving at will, transforming and displaying spatial data from the real world for a particular set of purposes” (Freeman et al 1993). GIS is a powerful medium in which a database can be created and viewed in the form of a map, useful for interpretation and predictive modeling. In a GIS environment the location and attributes of a particular site or locality can be recorded. This is created in a georeferenced position on a map and can be overlayed into any environment, if they are also geotagged or georeferenced. The SRTM and satellite imageries are already geotagged and hence if any site or locality and its attributes are known then they can be easily plotted. This spatial data, which can be referenced, can be of three types, a point, a line or a polygon. Using these, several spatial data can be created connected to archaeological sites and related data. Each point, line or polygon, can also be interrelated to an attribute table and any desired information can be connected with that. Thus in a GIS environment each map (or layer) can be a combination of points, layers and polygons interconnected with their non-spatial attributes.

 

Multiple maps can be created in a GIS environment with the help of suitable software, with each layer corresponding to specific information or properties. For example, with a particular study area being kept as a constant, different maps showing drainage pattern, geology, soil pattern, rainfall, temperature, and others, corresponding to that area can be created, each in separate layer and can be superimposed and as when necessary by simply switching on and off the particular layer. The terrain data can also be understood by analyzing the basic layer and even visibility analysis corresponding to a group of sites can also be understood.

 

A GIS database can be used for various analysis and interpretations including statistical analysis of site settlement pattern and catchment analysis, which may be further utilized to predict the probable location of archaeological sites, based on the parameters obtained from the already located sites. The SRTM and other satellite imageries with elevation data embedded in it of a particular region containing archaeological sites can be analyzed using GIS software to determine the slope analysis, drainage pattern, terrain pattern, watershed analysis, to carry out the cost-surface analysis. As the elevation data of imageries are known, predictive modeling in the format of rise or fall in sea levels corresponding to any particular coastline can also be understood and accordingly the location of archaeological sites and its relation to a particular ancient environment may be understood.

 

Gaur et al (2013), using Digital Elevation Model (DEM) generated using ArcGIS software, have predicted the coastline of Rann of Kachchh area during third millennium BCE with a 3 m rise in sea level in comparison to the present sea level. This data is also supported by the geological studies, which indicate a regular siltation of Rann at an average rate of 2 mm per year. They conclude that during third millennium BCE, Rann was an extended gulf and was navigable even up to the early centuries of current era.

 

In another instance of use of GSI predictive modeling to predict possible locations of Asokan inscriptions, based on the attributes of all the present available locations of inscriptions, Gillespie et al (2016), has proposed at least 121 possible locations in the Indian subcontinent. The methodology adopted by Gillespie et al consisted of initially locating all the known places of Asokan inscriptions in Google Earth software, creating a kmz file, importing the file in ArcGIS and creating a shape file embedded with all the attributes. With the available imageries on geological data from the Internet, 38 geological substrates were identified and classified followed by data generation of ‘human-induced global land-use change over the past 12,000 years’, climatic and elevation datasets. Gillespie et al used Maxent, a maximum entropy algorithm, a model with the relationship between the presently available locations of Asokan inscriptions and climate, topography, geology and population. The output obtained from the study was used to predict the location of probable inscriptions.

  1. Summary

 

In this module, a few of the scientific tools used in archaeological exploration have been reviewed. As it has been stressed, no single tool is ideal for any environment and a combination of multiple tools is always adopted for better results. An outline of the aerial survey, satellite based remote sensing survey and geographical information system was discussed. While a few of the techniques discussed here are direct techniques, in the sense that sophisticated equipment are used and based on the variability of surface features and indications, archaeological sites are located and discovered. A few other tools are based on lab work, like the GIS software, but it is a useful tool in predictive modeling to predict location of various sites and their probable locations. Further, the GIS tools are more and more useful to create database incorporating various attributes and data of a particular geographical and archaeological zone, which can be readily retrieved and superimposed with satellite imageries for interpretations. The satellite-based remote sensing with multispectral sensors are useful in various applications to locate even buried archaeological remains to trace a vast swath of area to delineate dry river channels. Similarly, LiDAR is being used extensively for documentation large areas of land within a short span of time, to create various models and DEMs for interpretation and locating archaeological sites.

 

In the next module, the various geophysical investigations used in the archaeological exploration along with techniques used in underwater exploration will be discussed.

 

Web links

  • http://www.redarrowmaps.com/lidarportal/
  • http://www.opentopography.org/
  • http://www.oregongeology.org/dogamilidarviewer/
  • http://pugetsoundlidar.org/
  • http://www.idaholidar.org/
  • http://www.nanaimo.ca/ortho/
  • http://www.nanaimo.ca/ortho/
  • https://earthexplorer.usgs.gov