Geophysics

Introduction

In hidden chambers beneath the Great Pyramids of Giza, there lie mummified pharaohs for over four millennia. On the other side of the planet, a bullet train hurtles through Tokyo at 300 kmph, only to halt seconds before the ground starts to tremble. In chasms a thousand fathoms deep, lava writhes inside volcanoes, whispering slowly before it erupts. All of these are tied by the thread of Geophysics – the science that uncovers what lies beneath the surface. 

This discipline, which lies at the crossroads of geology and physics, shapes everything from archaeological discoveries to seismic alerts. Conventionally regarded as a niche pursuit in India, it has been introduced to undergraduates as a B.S. programme in Applied Geophysics at IIT Bombay. Through this article, we take a closer look at the world of geophysics.

What is Geophysics?

“Earth is ancient now, but all knowledge is stored up in her. She keeps a record of everything that has happened since time began. Of time before time, she says little, and in a language that no one has yet understood.”

Geophysics is often confused with geology; however, there’s a significant distinction. Geophysics is an extension of physics, applying techniques to find these properties in a non-intrusivemethods that assess the subsurface without physically altering the ground and to understand the language of the Earth.  Geology primarily deals with the history of the Earth and attempts to explain the properties and formation of rocks.

Geophysics encompasses a large range of applications. Applications such as global seismic networks to monitor the planet, ensuring no nuclear tests are conducted. Mars missions are dependent on geophysical instruments to detect Marsquakes and study subsurface structures. By bridging multiple disciplines, geophysics connects discoveries in one area to innovative solutions in another. The following sections will dive deeper into two fascinating examples illustrating the importance of Geophysics.

Pyramids

As the interiors of the Pyramids still remain largely elusive, understanding their structure, locating passages, and hypothesising their construction has been a herculean task. Over the past few decades, however, some passageways have been discovered, and there’s always a chance some are still hidden. The problem at hand was to locate the remaining chambers without damaging the structure. An unexpected discovery led to a serendipitous solution.
In 1936, American physicists Carl D. Anderson and Seth Neddermeyer, while observing cosmic rays, discovered particles with a negative charge that did not follow the typical path of any known particle in a magnetic field, leading to the discovery of new particles, heavier than electrons but lighter than protons. These particles are now known as muons (https://www.energy.gov/science/doe-explainsmuons). It was observed that muons have a high penetrating power, i.e., the ability to pass through materials.

A cutaway view of the Pyramid of Khufu shows the location of the “Big Void” as well as a corridor close to the pyramid’s north face. (ScanPyramids Illustration)

20 years later, an Australian physicist, E.P. George, realised that the high penetration power of muons can be used to quantify the density or presence of void. Able to travel through hundreds of meters of stone, muons lose energy gradually depending on the density and thickness of the material. He used this to measure the density of a rock. This imaging technique came to be known as muon tomography.

In 1965, American physicist Luis Alvarez proposed muon tomography as a non-intrusive method for detecting chambers. Alvarez and his team, which consisted of both physicists and archaeologists, placed muon detectors in a known chamber beneath the pyramid to detect incoming cosmic rays. These detectors measure the trajectories and flux of muons as they penetrate through the structure. The final image is created by the muons that pass through the chamber, while the others are absorbed in the structure. He and his colleagues also published a paper in the Science journal to that effect in February 1970. 

(Alvarez, L. W., Anderson, J. A., El Bedwei, F., Burkhard, J., Fakhry, A., Girgis, A., Goneid, A., Hassan, F., Iverson, D., Lynch, G., Miligy, Z., Moussa, A. H., Sharkawi, M., & Yazolino, L. (1970). Search for hidden chambers in the pyramids. Science, 167(3919), 832–839. https://doi.org/10.1126/science.167.3919.832)

In 2015, the ScanPyramids mission led by Cairo University and the French HIP Institute was launched. As the name suggests, the project aimed to scan the Old Kingdom Egyptian Pyramids, with the goal of locating internal voids and chambers. This effort culminated in a big discovery in 2017, where they uncovered a cavity above the Grand Gallery. This “Big Void” is roughly the size of a passenger jet and has remained hidden since the pyramid’s construction. 

Not limited to muon tomography, multiple new techniques are being used to uncover the construction and information about Egyptian civilisation. Yet this is not the only way geophysics is helping, not just uncovering the past but to building a future.

https://www.axios.com/2017/12/15/hidden-void-found-in-egypts-great-pyramid-1513306624

While muon tomography helps us look silently into monuments built thousands of years ago, geophysics is equally essential for understanding the restless Earth today. The same ability to interpret hidden clues that reveal hidden chambers also helps deliver life-saving warnings when the ground begins to shake. This is most evident in Japan, where geophysics forms the backbone of its earthquake early warning system.

Earthquake Early Warning System

In the 1960s, the Shinkansen project, commonly referred to as the bullet train, was in jeopardy. Japan, lying at the convergence of four tectonic plates, makes the region prone to frequent earthquakes, which can potentially cause derailment. Recognising these issues, engineers installed seismometers in the 1960s to monitor ground movement along these routes. This gave the train operators a few seconds to slow down whenever an activity was detected by these seismometers, minimising the damage; this braking was automated later in the 1970s. 

https://www.pexels.com/photo/white-electric-train-2169286

(The M9.0 Tohoku, Earthquake: Short-Term Changes in Seismic Risk – Scientific Figure on ResearchGate. Available from: https://www.researchgate.net/figure/Japans-tectonic-setting-illustrating-the-three-subduction-zones-Nankai-Trough-Sagami_fig1_309173507 [accessed 25 Oct 2025] )

However, there was no way to measure these activities beforehand. Few attempts were made by geophysicists, such as the SCAN (Seismic Computerised Alert Network), proposed in 1985 by American geophysicist Thomas Heaton, which aimed to shut down critical sites (power grids, hospitals) through wireless communication. Although never implemented, it was a stepping stone. A few years later, Japan Railways introduced its own system known as “Urgent Earthquake Detection and Alarm System”, or UrEDAS.

(A Model for a Seismic Computerized Alert Network | Science)

When an earthquake begins deep in the Earth’s crust, seismic waves are created. There are mainly two types of waves created:
P (primary) waves, which are fast and mild, and S (secondary) waves, which are slower and more destructive. Japan’s UrEDAS system exploits the time gap between these two waves. By detecting the P-waves, geophysicists can calculate the earthquake’s location, magnitude, and potential impact before the destructive S-waves arrive.

With time, this system has been seamlessly integrated into daily life, transforming ordinary infrastructure into life-saving tools. A few seconds of warning can become a boon, enough to stop trains, open elevators, shut off gas valves, and pause delicate surgeries. Warnings are broadcast through smartphones, TV, radio, subway speakers, and even vending machines. AI-based methods are also being explored to filter noise, reduce false alarms, and more accurately recognise foreshock patterns.

This is not a foolproof system, though. One challenge arises for the places located near the epicentre. As the system is based on the difference in speed of the waves, the time interval between the arrival of the S waves and the P waves is so short near the epicentre that the alerts cannot be sent in time. Although not perfect, giving a warning a few seconds earlier has greatly reduced casualties and damage, saving countless lives.

India, too, has begun taking steps towards early warning systems. The Uttarakhand Bhookamp Alert app, developed by IIT Roorkee, can provide a warning 10-30 seconds in advance for earthquakes of magnitude greater than 5. There are other initiatives, such as INCOIS’s People Centred Tsunami Early Warning for India. These contain upcoming undersea sensor networks that are aiding scientific and operational purposes, including climate research. These examples represent a significant leap forward for geophysics in India.

Conclusion

Looking back, geophysics emerges as a discipline that spans continents and generations, helping to solve mysteries and protecting us by harnessing the power of physics. These stories only scratch the surface of a science that explores far more than pyramids and high-speed trains. From Eratosthenes’ ingenious calculation of the Earth’s radius two millennia ago, which is often considered the first geophysical experiment in human history, to modern advancements like earthquake early warning systems and the use of muon tomography, the true impact of geophysics is much larger than we perceive.

The launch of the new BS program in Applied Geophysics at IIT Bombay, along with other established programs at leading institutes, marks a step towards shaping the next generation of earth scientists. While geophysics remains relatively unexplored in India, it carries immense potential, which is driven by expanding research efforts and the rising academic focus to address urgent challenges like climate change, disaster management, and resource scarcity.