# Detecting seismic waves using Software Defined Radio

I understand about physic and waves, but I am a totally rookie in this topic. I was wondering which are the typical frequency ranges of the S-waves, P-waves and surface waves.

My idea would be to detect any seismic event using Software Defined Radio, but I am not sure at all If any peripheral could detect these kind of waves properly. For example, a HackRF peripheral works between 1 MHz to 6 GHz. And, as far as I know, lower frequencies below 3MHz travel efficiently as ground waves. Because of that I was wondering If I could detect anything through Medium Frequencies (0.3 to 3 MHz). For example, a HackRF would detect between 1 MHz to 6 GHz operating frequency, so it may detect something, but I don't know "what to look for".

If MF is too high, would there be any chance to detect anything through their harmonics?

I hope I understand your question right, I think there are some confusion about frequency range and wave-types.

Seismic waves are acceleration of matter that transfer kinetic energy from a seismic source (e.g. an earthquake, an explosion or ambient noise) to heat.

It's similar to sound waves, vibrations from the surface of e.g. a music instrument set the air in motion and compress the molecules of the air to transfer a wave that eventually reach e.g the eardrum. However, to add to the complexity, in solid matter some additional elastic properties are in play, as you notice. Waves can progress not only as pressure waves (P) but also as shear waves (S) and at boundaries special surface waves can form, often of high amplitude. The progression of the seismic waves depends on the elastic properties of the Earth.

A large part of the terminology used for seismic waves are similar to how we describe electromagnetic waves. E.g. frequency, damping and resonance are relevant for seismic waves as well. However, the ranges are very different and electromagnetic waves and acoustic waves behaves different. The earth is a efficient filter, and even transient energy, that contains high frequencies, is limited to low frequencies as it progress through the Earth. Ground wave propagation, as know from radio signals, are different from the properties of seismic surface waves.

An interesting coincidence is that wavelengths can be in the same magnitude, but that is because radio waves progress about 100.000 times faster than crustal seismic waves.

A figure, prepared by Professor Charles J. Ammon, shows the ranges of frequency and amplitude. Source here. The amplitude and period have a large range, that's why prefixes as milli or micro can be used and a logarithmic scale is used for amplitude.

In shallow active reflection seismic surveys, we sometimes can use frequencies up to maybe 200Hz, but in the range the penetration depth is very small. For very local events, e.g. in mines and infrastructure frequency range up to 500-1000Hz might be used.

Most energy for P and S waves used in teleseismics are in the range of a few Hertz and maybe up to 10-20Hz.

Surface waves have the interesting property that the velocity depends on the frequency because low frequency content is affected by deeper elastic properties whiles high frequency content is more affected by shallow geological features. This is used for lithosphere tomography with rather good vertical resolution. The frequency range is 0.005Hz and up to 0.5Hz. This is the frequencies that you can actually see in ground motions caused by earthquakes, if you take resonance in consideration.

Normal modes are often in the range 0.3 - 1mHz. (m, as in milli, not M, as in Mega). That is frequencies about one billiard times lower than some radio frequency bands.

Another thing to remember in the context of Software defined radio, is that there is no carrier wave for seismicity. When seismologists talk about frequency, we mean energy content of a signal. We are looking at the convoluted wavelet, not continuous signals.

If you are interested in working with earthquake detection and to improve algorithms of signal processing of seismic waves, I'd suggest you to take a look at the Obspy package, there are some nice examples and tutorials that can get you started. There are also some project where people are constructing open architecture seismometers, eg the Raspberry Shake. For less than \$100 or so you can build a seismograph and start to monitor local seismic events. Most geophones work by transfering ground acceleration to an induced electric current in a coil.

It's very possible that inspiration from other fields of signal processing, as in Software defined radio can improve seismic techniques in future. It is always great to get inspiration from other fields of research.