Understanding the Waveform

When an event occurs deep underground, energy is released into the surrounding rock and travels outwards in the form of seismic waves.  These seismic waves travel by causing elastic deformation, meaning that they can apply a force that stretches or deforms the medium they’re travelling through (i.e. rock), but then that medium returns to its original state once the force is removed.  The waves apply longitudinal stress (i.e. compression or dilatation) and shear stress (force applied parallel to the surface).  This stress causes elastic strain, and the material becomes temporarily deformed.  As the wave moves past a specific point in the earth, the stress is released and everything goes back to normal.

The type of stress and strain caused in the earth is dependent on the type of seismic wave that is propagating.  Waves that travel solely through the earth are called body waves and behave very differently from those that propagate on the surface (such as ripples along the surface of water or the destructive waves from a large earthquake).  Body waves are the type of waves that geophysicists are interested in. 

There are two types of body waves: P-waves and S-waves.

P-Wave

The P-wave or primary wave is a very fast traveling seismic wave. The P-wave can move through solid rock and fluids, like water or the fluid layers of the earth. It pushes and pulls the rock it moves through the medium.  Particles in the medium have vibrations that are parallel to the direction of wave propagation.

Seismic P-wave

S-Wave

S-waves or secondary waves travel much slower than P-waves and can only move through solid rock. This wave moves rock up and down, or side-to-side.  Particles in the medium have vibrations that are orthogonal to the direction of wave propagation.

The greater the separation between P-wave and S-wave arrivals, the greater the distance between the sensor and event source.

Seismic S-wave

Moveout

Moveout is a term that describes the effect of the distance between a seismic source and receivers, on the recorded arrival times.  Arrival times of waves will typically first be detected at the sensor which is nearest to the source.  A delay will be observed on all other receivers which represents the additional distance that is traveled from the source to receivers which are farther away. 

In the example below, arrival times recorded on individual sensors are plotted with respect to time.  The source occurred nearer to the centre sensor and a small delay in arrival times is observed on sensors which are farther from the source.  Notice also how P-wave arrivals are detected distinctly before S-wave arrivals.

Seismic Moveout

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