Using Thunderquakes to X-ray earth

Why in News?

A new study has demonstrated that thunderquakes can be used to study the Earth’s shallow subsurface.

• The technique combines naturally occurring seismic energy from thunderstorms with fibre-optic cables.
• Thunderquakes – Lightning superheats the surrounding air, producing a shock wave heard as thunder.
• When part of this energy reaches the ground, it is converted into seismic waves, called as “Thunderquakes”.
• Technology – Distributed Acoustic Sensing (DAS) used to convert ordinary fibre-optic cables into thousands of vibration sensors.
• A laser-based interrogator sends light through the fibre and detects tiny changes caused by ground vibrations.
• The study used a 4+ km fibre-optic cable with more than 2,100 sensors. It recorded 458 clear thunderquakes over two years.

Thunderquakes ‘X-ray’ the Earth subsurface –

• Thunderquakes generate air-coupled Rayleigh waves.
  • These waves travel along the Earth's surface and contain information about subsurface structures.
• Through seismic dispersion, different frequencies travel at different speeds and depths.
  • Analysing these variations helps estimate seismic wave velocity and subsurface properties.
• The study could image the subsurface to about 100 metres depth, without drilling.
• Applications
  • Detecting groundwater movement
  • Identifying sinkholes and underground voids
  • Detecting fractured or weathered rocks
  • Monitoring environmental contamination
  • Assessing building sites and foundations
  • Monitoring ground subsidence
• Significance –
  • The study provides a low-cost and non-invasive method of subsurface mapping while using existing telecommunication fibre infrastructure.
  • It can enable continuous monitoring of urban subsurface conditions.
• Similar atmospheric shock waves may also generate useful seismic signals from Sonic booms, Volcanic eruptions, Meteor airbursts.

Karst Landscapes

• Karst terrain develops where soluble rocks such as limestone and dolomite are dissolved by groundwater.
  • It can form caves, fractures and sinkholes.
• Karst landscapes cover about 20% of the world's continental land area and affect nearly 25% of the global population.

Possible planetary application

• It can be applied beyond Earth. Titan, Saturn's largest moon, is a potential candidate because atmospheric disturbances and lightning may occur there.
• NASA's Dragonfly mission is expected to explore Titan.

Reference

The Hindu| Using Thunderquakes to X-ray earth