What are seismic waves?
Seismic waves are elastic waves propagating through the ground. These waves are produced by an earthquake or some other physical energy source such as a hammer or weight-drop. The speed and path of the waves is determined by the density and elasticity of the ground.
There are three main types of seismic waves:
P-waves, more formally Primary or Pressure waves. This is the fastest wave as the wave motion and energy transfer are in the same direction. Particles are compressed and returned to their original shape in the direction of propagation. In an earthquake the P-wave arrives first, but is typically heard more than felt, due to the limited physical movement.
S-waves, also called Secondary or Shear wave. S-waves are slower as the wave motion and is at right angles to the direction of travel; particles are displaced laterally. In an earthquake the S-waves will be the first tremors.
Surface waves are slow travelling with a complex motion in several different directions, made up of a ‘swaying’ component and a ‘rolling’ component (termed Love and Rayleigh waves, respectively). Surface waves both deform and displace particles. In an earthquake these are the waves that do the most damage.
Seismic waves for survey The characteristics of the different waves can be used to our advantage in seismic surveys. P-waves are used in refraction surveys and can provide information about compressional strength. S-waves and Surface waves can give us an indication of shear strength and the stability of materials. Typically, S-waves are used in reflection survey while Surface wave survey has its own methodology. Surface wave survey brings advantages including the ability to work in ‘noisier’ environments, due to the larger signals associated with surface motion, but also because we can use that ‘noise’ as our seismic source in some cases.
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