How Neutrino Beams Could Reveal Cavities Inside Earth

Neutrinos are peculiar particles. They have little mass, no charge and come in three flavours. These flavours are not fixed. The strange thing about neutrinos is that once created, they change from one flavour to another as they travel. 

For a long time, that puzzled physicists. A neutrino’s variety determines how it interacts with matter. Physicists built experiments to detect  the flavour coming out of the Sun only to find far fewer than they expected. 

In 2001, that mystery was solved when they discovered that the missing neutrinos had flipped, or oscillated from one flavour to another, during their journey from the Sun to the Earth.  

Since then physicists have scrambled to understand neutrino oscillations in more detail. It turns out that the effect is sensitive to the distance that the neutrinos have travelled and also to the amount of matter the particles have passed through.  

That’s given Carlos Arguelles and pals at the Pontifical Catholic University of Peru in Lima an idea. These guys say that  neutrino oscillations ought to be sensitive to changes in the density of the Earth. 

So the oscillations in a beam of neutrinos created at one point on the Earth and beamed through the crust to another point, ought to reveal information about any change in density along the way.  

These guys aren’t the first to suggest that a neutrinos beam can effectively x-ray the Earth. But they are the first to explore the size and shape of the density changes that ought to be visible using this method. 

They say the technique ought to be able to spot cavities some 200 km across or larger filled with water,  iron-based minerals or even regions of charge accumulations. They suggest that this might take as little as 3 months.

That’s interesting because some seismologists suggest that earthquakes lead to the accumulation of charge in specific volumes of rock, so the technique might be useful for studying this. 

But there’s a more significant factor driving interest in this work. This technique could also reveal geological formations likely to contain oil and so could attract considerable commercial investment.   

One important question, however, is whether Arguelles and co have made realistic assumptions in their model. One problem they face is that the beam of neutrinos must be intense enough to produce a result in a reasonable period of time–certainly less than 18 months. 

To achieve this, Arguelle and co have to assume that it is possible to create beams at a rate some 5000 times higher than is achievable today.

Since it’s not at all clear how this could be done, that’s a big fly in the ointment. 

So while this technique looks possible in theory, this kind of assumption places a big question mark over whether it will be possible in practice in the foreseeable future.

Ref: arxiv.org/abs/1201.6080 : Searching For Cavities Of Various Densities In The Earth’s Crust With A Low-Energy ν¯e β-Beam