7 Scientific Tools in Archaeological Exploration-II
V. N. Prabhakar
1. Introduction
In Module 07, the various scientific tools used in the archaeological exploration was introduced, and three components, viz., (i) aerial reconnaissance, (ii) satellite based remote sensing survey and (iii) geographical information system, were dealt with. In this module, the remaining three important components of scientific tools, viz., (i) subsurface detection, (ii) geophysical investigations and (iii) tools used in underwater archaeology will be studied.
2. Scientific tools used in archaeological exploration
The site specific surveys follow the large area based surveys and preliminary understanding of a particular region or geographical zone. Once the potential sites are discovered, visited and their significance understood, the prominent and most significant ones can be taken up for detailed investigations. The surface study of an archaeological site again combines traditional foot walking for identifying the various cultural material, to detailed sampling strategies as discussed in Module 05. The same principles of gridding and linear walking can also be adopted for sites for better understanding and interpretation of various working areas and zones of a given archaeological sites. Once this preliminary understanding of the site is completed, before carrying out detailed excavations, scientific tools can be used to detect buried archaeological features of certain prominent or important areas from within the archaeological site, that could have been understood based on the surface studies and sampling strategies. This is a very useful step as excavation is an irreversible operation and destruction caused to the archaeological deposits, and hence any investigation towards the understanding the potentiality of burial archaeological features is always welcome.
There are various categories of equipment used in the detection of archaeological features buried underneath.
2.1 Subsurface detection
The detection of the nature of subsurface archaeological remains using various simple tools had been in practice since a long time. The nature of theseequipment is very simple and not complicated and could be used very easily. It is also a practice to simply tap the surface to understand whether the subsurface is hollow or solid, depending upon the sound made while tapping. The various simple mechanisms available for subsurface probing and detection are as follows:
2.1.1 T – Probes
A simple mechanism to understand the nature of subsurface archaeological or buried remains is a steel rod in the form of ‘T’, with a pointed tip. This rod is inserted on the surface and pressure applied to drive inside the surface, and depending upon the solid or hollow buried remains, the pressure can be felt and those points can be marked on a map to understand relative hollow and solid portions of the archaeological remains. However, in this method, sampling from various depths cannot be obtained.
2.1.2 Augur Survey
This is an extension of the probes mentioned in 2.1.1, with an addition of a corkscrew shaft, which can bring the soil and other samples from the subsurface that can be examined and the nature and context of archaeological remains can be understood. The augur, however, cannot be driven into deeper depths and hence has a limited application. In a further improvement, hollow pipes can be driven into the surface, as has been done by geologists to sample the geological samples, to obtain samples from various depths. This will allow not only understanding the buried archaeological remains, but also the nature of geology at a site at various depths, and all natural deposits. At Kalibangan, adjacent to the Harappan site and on the dry bed of River Ghagger borings were carried out to understand the various episodes related to the river.
2.1.3 Other devices
Probing into the archaeological remains can also be done using various other devices. For example, probing of an unopened portion of a heritage building can be carried out using endoscope technique. Similarly, ultrasensitive sensors can be used to detect hollowness beneath wall surfaces of structures to understand their behavior, which can be mapped.
2.2 Geophysical Surveys of Archaeological Sites
The role of scientific aids in archaeological reconnaissance is undeniable, be it in the form of a simple handheld GPS for recording locational details of artefacts to structures, but also sophisticated equipment like drone and laser scanners. The geophysical aids in reconnaissance is a fast developing field which has conjured much interest among the archaeologists for its non-invasive and non-destructive methodology to probe into the subsurface arena much before the excavation. One of the advantages of geophysical tools is that it can cover larger areas of landscape for detecting the buried architectural features (Bevan 2006). The geophysical tools help in the “detection, imaging, and mapping of subsurface features over large areas in potentially great detail”(Kvamme 2003). There are disadvantages as well as of the geophysical tools, depending upon the nature of soil conditions, and loosely buried features like stone debris and compact soil conditions mixed with archaeological remains (Bevan 2006). Further, the role of geophysical tools is one among the multi-methodological approach to be adopted and not the only aid in archaeological exploration.
Piro (2009)and Thompson et al(2009)classifies the geophysical methods into two ‘main groups’, viz., (i) passive and (ii) active methods. The passive methods measure the “amplitude of nearly steady magnetic, gravitational and electrical perturbation fields, generated by buried features, are measured at the sensing device” while in the active methods,“artificial seismic, electrical and electromagnetic (inductive and impulsive) signals are emitted by the device, which then senses the return signals, more or less altered by the typical responses of the subsurface features”(Piro 2009). Under the passive method, magnetic prospecting (magnetometry) (Von Ferse 1984), gravitational surveying (Piro 2009) and Self-Potential (Wynn 1984) (SP) techniques are used for detecting buried features. Among these three techniques, the SP is considered to be the least expensive geophysical method in detecting archaeological features.
The techniques under active methods (Piro 2009; Weymouth 1986) are (i) seismic or acoustic, (ii) electromagnetic, (iii) resistivity or galvanic and (iv) ground penetrating radar.
2.2.1 Seismic or Acoustic Method
The seismic method involves transmission of sound waves and measuring the time of reflected ones based on density variations of buried features. The acoustic method is a similar technique, which is based on higher frequency sound “reflected by voids or objects of higher density than that of the surrounding soils”(Weymouth 1986). The relative movement of sound waves through harder and softer archaeological deposits can be measured with the aid of sensors and mapped to understand the nature of buried remains. The sound waves move faster through harder materials and slower through deposits like clay or softer materials. Echo sounding techniques or equipment using the sonar waves is also a technique used in detecting the nature of buried archaeological remains as they are more sensitive.
2.2.2 Electromagnetic Method
The second technique is also categorized as non-conducting electromagnetic (EM) or induction method(Piro 2009)which “ranges from simple metal detectors to sophisticated soil conductivity metres”(Weymouth 1986). Simple metal detector is an effective tool for archaeologists, when they suspect buried metal remains from the archaeological sites. The metal detectors works on the principle of both conductivity and resistivity and can detect both electrical conductivity of metals and magnetic susceptibility of ferrous metals. A metal detector normally consists of a handheld unit and a sensor probe that can be moved over the surface to find out the location of buried metal remains. The advantage of metal detectors is that they can give quick results and also useful as a probing method prior to excavations and to locate areas where metal remains are buried underneath.
The magnetic surveys are particularly suitable for buried fired clay structures like pottery clusters, brick walls, brick debris, furnaces, pottery kilns, hearths, pits and ditches. The magnetic surveys work on the principle that minute quantities of grains of iron oxides are present in the clay and soil, that are randomly oriented in unfired condition, and when fired to temperatures above 7000 C, they become permanently fixed aligning with the Earth’s magnetic field, the magnetic field created by such an object and groups of objects can be detectable using suitable sensors. The different instruments used in the magnetic surveys are (i) proton magnetometer, (ii) fluxgate magnetometer and fluxgate gradiometer, and (iii) caesium magnetometer. Their main features are given below (Renfrew &Bahn 2000):
2.2.3 Resistivity or Galvanic Method
The galvanic or soil conduction electrical methods(Piro 2009) works on the principle of conduction / non-conduction of soil, rocks and buried features (particularly stone), and a resistivity profile is created with the help of multiplexed electrode arrangements. The conduction of electricity also depends upon the moisture content in the soil, the damper the soil, more conductivity. The nature of buried archaeological remains is indicated by conduction / non-conduction of electricity, by injecting current through the surface with the aid of electrodes, and the potential difference between the two electrodes are measured arrive at a mapping of the subsurface features. The distance between the two electrodes is gradually increased to obtain the profile of greater depths. This method can be used to measure the buried stratigraphy, archaeological remains and to obtain a map of the structures, geological features, landfills, and other deposits. The electrical resistivity method is effective over other methods as it can be operated even in urban areas and close to power lines and metals (Renfrew &Bahn 2000). One distinct drawback is that the technique is very slow and continuous contact with electrical source is necessary.
2.2.4 Ground Penetrating Radar Method
The ground penetrating radar or GPR is considered as a better technique when compared to other three, the range of detection being from a few mm to several metres beneath the earth surface. The GPR works on the principle of transmitting different wavelengths of electromagneticwaves and then measuring the continuity / discontinuity of reflected signals depending upon the properties of soil conditions and buried features. The emitted electromagnetic wave penetrates through the surface and reflected back depending upon the nature and characteristics of the buried remains, thus, the intensity might be strong or weak, which are reflected in the form of anomalies, that can be analyzed and interpreted with respect to the depth.
A typical GPR equipment consists of antenna, flattish at the bottom that can be dragged or drawn on the surface area where investigations are to be conducted. This antenna emits short pulses of electromagnetic waves, which penetrates the surface, and depending upon the frequency of radar waves, the range varies. The higher frequency antennae can reach greater depths and the lower frequency antennae are used for detecting subsurface features very close to the surface like pipelines, nature of concrete pavements, etc. These waves hit the subsurface features whose properties vary due to their different electrical properties and then travels back to the surface, which is detected and measured as anomalies that are recorded in an onsite processing unit connected to the antenna. The data, thus recorded is transferred to a computer and processed for further results and interpretations.
The drawback of ground penetrating radar system is that it is very slow to operate as electrical contact with the soil is always necessary and considerable time is needed to cover a large area. However, if used in combination with other techniques like electrical resistivity and magnetometer, the results can be worthwhile.
2.2.5 Examples of Geophysical Surveys of Archaeological Sites
The use of electrical resistivity and magnetic gradiometry at Sisupalgarh by Smith and Mohanty (2007) helped in prospecting an area of around 13 acres, which revealed “presence of a 300-m long section of ancient road and its associated side streets and structures”. The efficiency of GPR and other geophysical methods in prospecting a large area, which otherwise is not possible by regular excavation methods, a slow and expensive methodology by its nature, is an advantage for archaeological sites, has been indicated effectively by Smith and Mohanty at Sisupalgarh.
At the early historic site of Ahichchhatra, Sravanthiet al(2012)carried out GPR in order to assess buried archaeological features spread across at least four sites of this vast city, two of them with high resolution. Ahichchhatra is an early historic site, one of the well preserved, datable from second millennium BCE to 14th c. CE, and witnessed several phases of developmental and expansion activities.
The GPR method seems to be the only cost effective, most suitable and less time consuming methodology for such a site when compared to excavation. Another interesting and important aspect of this survey is the testing of anomalies obtained from GPR profiling through excavation. In one of the areas subjected to GPR profiling (Grid 1), a trough indicated by anomaly was proven to be a collapsed and sunken structural phase (Sravanthiet al 2012). The other three profiles obtained through GPR indicate “closely spaced deserted walls which could be related to common dwelling (Grid 2)”, “single wall which is having more intersections and also forms the part of dwelling (Grid 3)” and compacted surfaces that could have been used as walkways (Grid 4) (Sravanthiet al 2012).
The LothalRevisitation Project (Frenez 2014) is an excellent example of ‘geo-archaeological project, combined with remote sensing and field activities’, in which various non-invasive geophycial techniques were used with the objectives, “to detect different natural and artificial subsoil features, complemented by series of core drillings to determine the shifting of palaeo-channels and shorelines”, “…to reconstruct the palaeogeography around Lothal during the Late Mid-Holocene and the hydraulic structures that interfaced the site with the surrounding environment”(Frenez 2014). Lothal is an important Harappan site with evidences of occupation during second half of third millennium BCE, twin-fold habitation surrounded by fortification, craft activities, warehouse, maritime trade and external contacts, cemetery, and a ‘dockyard’. The techniques used in the investigations include(Frenez 2014), ‘geomorphological remote-sensing analysis of the area round Lothal’, ‘3D Digital Elevation Model…using relative kinetic GPS’, ‘magnetic survey of the non-excavated archaeological area using cesium magnetometer’.
The magnetometer survey show three prominent anomalies (i) baked-brick embankment canal running east-west and perpendicular to dockyard, probably connecting the nearby palaeo-river to the dockyard (Anomaly A), (ii) an architectural complex consisting of rooms along a narrow street and separated by lanes to the north-eastern area (Anomaly B) and (iii) a possible monumental gateway or large drainage outlet to the south-western corner of acropolis (Anomaly C). Frenez also carried out ‘test trenches’ in the areas of anomalies, which largely confirmed with the findings. Thus, the non-invasive techniques of unexcavated areas at Lothal had not only reduced the time spent in speculative excavations to find buried features but rather using the geophysical techniques to first understand the subsoil features and then proceeding to limited excavations with maximum output.
Another best example for geophysical investigations from a site that is already excavated that had revealed monumental architecture and water management systems is from the Harappan site of Dholavira, Gujarat. The excavations at Dholavira, which was carried out between 1980 and 2005 brought to light the genesis, rise and decline of a Harappan settlement on an isolated island in the Great Rann of Kachchha in Gujarat. Extensive excavations have virtually enabled to draw on plan the important features of the site. However, the GPR prospecting (Majumdar 2015; Agrawal 2015) carried out during three site visits by Indian Institute of Technology, Gandhinagar, helped in understanding the vast buried remains from most of the unexcavated portions of the site. A vast and flat area to the east of East Reservoir and to the north-northwest of River Manhar divided into four designated areas [Area A (171 X 135 m), Area B (117 X 132 m), Area C (36 X 72 m), Area D (36 X 18 m)] were subjected to GPR probing. The area was selected due to the similar surface morphology as that of presented by the present East and South Reservoirs before excavation and lying in between East Reservoir and River Manhar except Area C, which lies immediately to the south of Lower Town. The area was also considered for probing as it was covered with a sheet of water during flash floods and heavy rains. Area C is also flat and devoid of any prominent surface architectural features, and very similar to Areas A, B and D, and hence attracted investigations.
The GPR investigations brought to light ‘T’ shaped parallel walls with a spacing of 7 – 8 m between them in Area A; to the east of are a set of parallel walls in a north – south alignment along with spread of rubble, which may be fallen walls(Majumdar 2015; Agrawal 2015). The overall findings from the investigations indicate the presence of shallow and smaller reservoirs to the east of East Reservoir, possibility of check dams, and fallen walls forming rubble and scattered debris (Majumdar 2015; Agrawal 2015).
The ongoing discussions clearly indicate the necessity of a thorough investigation of entire landscapes through surface reconnaissance, involving traditional as well as scientific methodologies, to arrive at a broad based understanding of landscapes in general and archaeological sites in particular. This survey helps in identifying potential areas to be investigated, which can be carried out with various scientific tools including the geophysical tools available at present. The case studies and examples presented here clearly indicate the advantage of such surveys, which are inexpensive and less time consuming than the actual excavation, which otherwise cannot be carried out on a large scale with minimum time. The GPR investigations at Lothal and Dholavira are clear indications of several features, which were missed due to conventional excavation methodologies, and showing several possible areas for future investigations.
3. Techniques of Exploration in Underwater Archaeology
Underwater Archaeology is an area in which even though the principles of archaeology come into play in a major way, the working environment is completely different from land archaeology. Due to this, techniques that are normally employed in land archaeology cannot be used here and improvisations right from exploration to excavation have been made. Underwater archaeology made its humble beginnings during mid-1850s when lowered water levels are a few lakes in Switzerland exposed wooden pillars, posts, pottery and other material remains. During the earlier period, diving bells were used as tools for working underwater. With the advent of scuba diving technique, miniature submarines, submersible crafts, remotely operated vehicles (ROVs) have enabled better investigation of underwater archaeological remains. Further, geophysical tools that are useful in land archaeology have also been modified to investigate large swaths of area underwater.
There are various techniques and procedures involved in the search for underwater archaeological remains, even though many discoveries are made accidentally also. Once preliminary investigations and area of probing is decided, search methods like (i) towed diver search, (ii) swim line (freeline search), (iii) offset method consisting of positions in relation to jackstays and ground lines and (iv) jackstay (corridor) search are adopted. In the case of towed diver search, a diver towed with a small boat moves over a pre-decided area for search of underwater remains. The swim line (freeline search) consists of three or four divers moving parallel in predefined area, normally marked on the water surface by buoys. The jackstay (corridor) search consist again of a predefined area, with two divers moving close to each other in a straight line and searching for remains.
3.1 Geophysical and Remote Sensing Methods in Underwater Exploration
In addition to the general exploration by divers, various scientific equipments are also used in the exploration, in particular if large areas have to be covered. The technique used in such geophysical surveys is based on acoustic principles. The equipments used are echo sounders, multi-beam swath systems, side scan sonars, sub-bottom profilers, bottom classification systems and integrated surveys.
3.1.1 Echosounders
The method is based on the sonar waves, which are emitted in pulses into the water, and are reflected back from the seabed or ocean bed and also remains on them. The time taken between the emission of pulse and reflected ones is measured, which enabled to measure the depth of water and also speed of sound in water. The relative differences between the time taken, if any, due to the undulations of seabed surface as well as remains like shipwrecks on the surface, are recorded and analysed for interpretations. The disadvantage of this system is that a single beam of pulse is emitted in a linear fashion at a time, due to which the time taken for probing a given area is enormous.
3.1.2 Multi-beam Swath Systems
This is an advanced version of echo sounders or single beam, wherein single beam pulse is replaced by emission of multi-beam pulses over a large area, and thus in a short time probing can be completed. The survey is also fast when compared with echo sounder and several repeat swaths can be carried out of the same area and multiple recordings are possible.
3.1.3 Side scan sonars
In this technique, the underwater imaging is carried out using a wide-angle pulse, similar to the multi-beam swath systems. In addition to multi-beam swath system, a tow-fish consisting of sonar equipment is dragged over the sea floor, at desired depths. The altitude of the tow-fish is altered to have a survey of wider or smaller area of sea floor.Further, instead of measuring the depth information from the returning pulses of sonar, the intensity of sound reflected back from the remains on the sea floor sediments and objects lying on the seabed are measured. This enables to have an undistorted imaging of sea floor in real time.
3.1.4 Sub-bottom profilers
Sub-bottom profiler is also similar to echo sounder. In this method, instead of mapping the sea floor, the sound waves penetrate the surface and reflected back depending upon the buried strata, debris, archaeological remains, rocks, and other remains. The equipment normally consist of two separate units, towed by a boat or ship, (i) source of sound waves and (ii) receiver to receive the reflected acoustic waves.
3.1.5 Integrated Surveys
The integrated surveys can host both acoustic and video imaging data collection devices towed at a single time to record multiple features depending upon the capabilities. The Remotely Operated Vehicles (ROV) and Autonomous Underwater Vehicles (AUV) are also examples of integrated surveys. Equipments like magnetometer, side scan sonar, sub-bottom profiler, ROV all can be towed by a single boat or ship to record both the sea floor and subsurface features. A real time mapping can be done of both the surfaces through the integrated survey. These surveys can perform multiple tasks of a diver and there is no limitation of time spent underwater. They can also perform survey, recording and photography simultaneously.
The surveys, both traditional and based on scientific equipment help in proper recording and visualization of the given survey area, which helps in planning for underwater excavation, which is both time consuming and expensive.
- Summary
In this module, introduction to the various scientific techniques helpful in archaeological exploration was attempted. This module also continued the techniques introduced in the previous module, i.e. Module 07. The scientific techniques include both simple ones like ‘T’ probes, augur survey to sophistication magnetometers and ground penetrating radars. The sophisticated equipments are both expensive and need lot of expertise to operate. Nowadays, scientific institutions have equipments like ground penetrating radar, which can be used by the archaeologists in collaboration with them. The data collection as well as analysis in a laboratory takes much time and specialized people working in this area interpret the anomalies in a better manner, with the help of a database.
The case studies explained in this module also indicate that a single system may not be suitable for all the archaeological remains, and hence a holistic approach is necessary by integrating several methods to have a good understanding of buried archaeological remains.
The underwater exploration is totally a new area wherein sophisticated equipment is available to only a few and again through collaboration with institutions like National Institute of Oceanography (NIO), Goa can enable such investigations.
Web links
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