Non-Invasive Archaeological Survey at Gyanvapi Mosque | 19 Aug 2023

Source: TH

Why in News?

Recently, 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.

What is the Purpose of the Survey?

• The petitioners argued that the mosque was built on the foundation of the temple and that there were several Hindu idols and structures hidden inside the mosque.
• The court directed the ASI to form a five-member committee of experts to conduct a comprehensive physical survey of the entire Gyanvapi compound using non-invasive techniques such as ground-penetrating radar (GPR) and carbon dating.
• The survey is expected to reveal whether there are any traces of a temple or other Hindu structures below or within the mosque and to establish the age and origin of the existing structures.
• The court also appointed an observer to monitor and supervise the survey process and report any irregularities or violations.

What are Non-Invasive Methods in Archaeological Prospecting?

• Non-invasive methods are used when investigations are undertaken inside a built structure and no excavation is permitted.
• Types of Methods:
  • Active Methods: Inject energy into the ground and measure the response. The methods provide an estimate of the ground’s material properties, such as density, electrical resistance, and wave velocity.
    • Seismic Techniques: Use shock waves to study subsurface structures.
    • Electromagnetic Methods: Measure electromagnetic responses after energy injection.
  • Passive Methods: Measure existing physical properties.
    • Magnetometry: Detect magnetic anomalies caused by buried structures.
    • Gravity Surveying: Measure gravitational force variations due to subsurface features.
  • Ground-Penetrating Radar (GPR):
    • ASI will use GPR to produce a 3-D model of buried archaeological features.
    • GPR operates by introducing a short radar impulse from a surface antenna and records time and magnitude of return signals from the subsoil.
    • Radar beam spreads like a cone, causing reflections before the antenna passes over the object.
    • Radar beams spread out in a cone, leading to reflections that may not directly correspond to physical dimensions, creating false images.
  • Carbon Dating:
    • Determine organic material age by measuring carbon content.

What are the Limitations of Various Methods in Archaeological Surveys?

• Similar physical properties of different materials can generate the same response, leading to ambiguity in identifying targets.
• Data collected is limited and contains measurement errors, making it challenging to accurately estimate the spatial distribution of properties.
• Archaeological structures are often made of heterogeneous materials with complex geometry, making data interpretation challenging.
• Geophysical tools might not accurately reconstruct target images, especially in complex scenarios.
• In cases like disputes over religious sites, emotional and political factors can influence interpretations and decisions.