On August 4, the Supreme Court of India directed the Archaeological Survey of India (ASI) to conduct a detailed non-invasive survey of the Gyanvapi mosque in Varanasi, Uttar Pradesh to determine if the mosque was built atop a temple. As the matter has significant political implications, it is important to understand the working principles of the scientific methods used in such surveys, their abilities, and their limitations.
Archaeology has, due to the ever-increasing social and legal complexities that it found itself confronting, embraced modern scientific methods, including physics, molecular biology, geology, and anthropology. Archaeological investigations are normally performed in open spaces, along with excavation, where scientists have the option to use both ground and airborne systems to identify and test targets.
In the present case, since the investigation is being undertaken inside a built structure, and no excavation is permitted, experts — geophysicists in particular — must depend on non-invasive methods of earth-scanning.
The methods routinely used in archaeological prospecting are adapted from those applied in geophysical mapping. They may be active or passive. Active methods inject energy into the ground and measure the response of the buried target at the surface. They include seismic and electromagnetic techniques. Passive methods, such as magnetometry and gravity surveying, simply measure existing physical properties.
In both cases, the methods provide an estimate of the ground’s material properties, such as density, electrical resistance, and wave velocity. They are then interpreted in terms of the possible nature and geometry of the target. In the case of Gyanvapi, the scientists could be looking for the distinct physical properties of subsurface material constituting the structure.
Many earth materials could have the same physical property and generate the same response on the surface, leading to ambiguity in interpretation. In response, geophysicists use multiple methods and different physical properties of the earth’s materials to arrive at a reasonable characterisation of the target. It is expected of those involved in the survey to have considered this.
Media reports have suggested that the ASI will use ground-penetrating radar (GPR) to produce a 3-D model of buried archaeological features. GPR operates by introducing a short radar impulse from a surface antenna and recording both the time and magnitude of return signals reflected by the property contrasts in the subsoil. Significant improvements in instrumentation have allowed large amounts of digital data to be collected along closely spaced parallel paths for detailed analysis and imaging.
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Because the radar beam spreads out in a cone from the transmitter — similar to a smoke ring — the buried object will reflect part of the beam before the antenna passes directly over it. In such a situation, a part of the signal may bear little relation to the physical dimensions of the subsurface target and create false images.
An important aspect of the geophysical survey is to infer physical parameters from the complex and voluminous data acquired. This requires a good understanding of physical processes and powerful data analyses and modelling programs to generate reliable 3D images.
Since the archaeological object under investigation is made of heterogeneous materials with complex geometry, this object is simplified in the form of a representative model with well-defined and finite parameters.
In general, the laws of physics provide the means to compute some data given a computational model. Only in the ideal case does an exact theory exist that prescribes how the data should be transformed in order to reproduce the model. In most cases, including the survey being conducted by ASI, the mathematical framework assumes that infinite and noise-free data will be available.
A point to note
However, as the data will always be limited and have measurement errors, it may not be possible to estimate, in a unique and stable manner, the spatial distribution of physical property in the subsurface. As a result, supplementary information needs to be incorporated. This has the potential to produce meaningless results. This is well-recorded in the geophysical literature and graduate–level textbooks. Even data from sophisticated lunar penetrating radar systems analysed by different authors and published in different journals, have reported contradictory interpretations.
Despite its inability to reconstruct the images of targets in the best possible manner, geophysical tools have a high success rate in resource exploration. But in the case of a failure or partial success during the exploration of natural resources, the loss is merely financial. On the other hand, a ‘temple versus mosque’ problem is another matter entirely, involving emotional and sentimental issues and with long-term societal and political implications.
In such cases, nothing should be left to chance. GPR or any other geophysical method has limited abilities, and its findings must be interpreted within these contours. During data analysis, interpreting, and decision-making, experts as well as the people need to bear this in mind.
Shyam S. Rai is a former Professor and Chair in the Department of Earth and Climate Science, Indian Institute of Science Education and Research, Pune, where he continues to serve as an emeritus professor