Rayleigh wave

Rayleigh waves are a type of surface acoustic wave that travels on solids. They are produced on the Earth by earthquakes, in which case they are also known as "ground roll", or by other sources of seismic energy such as ocean waves [1] an explosion or even a sledgehammer impact. They can also be produced in materials by many mechanisms, including by piezo-electric transducers, and are frequently used in non-destructive testing for detecting defects. When guided in layers they are referred to as Lamb waves, Rayleigh–Lamb waves, or generalized Rayleigh waves.

Contents

Characteristics

Rayleigh waves travel across surfaces - and are thus a type of surface wave. In perfectly homogenous, isotropic and infinite materials, Rayleigh waves would not be apparent. In seismology, Rayleigh waves (called "ground roll") are the most important type of surface wave.[2] Most obvious close to the surface of mediums, Rayleigh waves are made of longitudinal and traverse motion that decreases exponentially in amplitude as distance from the surface increases. There is a distinct phase difference between the component motion types.[2] In isotropic solids the surface particles move in ellipses in planes normal to the surface and parallel to the direction of propagation – the major axis of the ellipse is vertical. At the surface and at shallow depths this motion is retrograde, that is the in-plane motion of the wave at the peak is in the opposite direction to the propagation of the wave. Deeper into the material the motion amplitude decays and the eccentricity changes. At greater depths the particle motion becomes prograde. The depth of significant displacement in the solid is approximately equal to the acoustic wavelength. Rayleigh waves are distinct from other types of acoustic waves such as Love waves or Lamb waves, both being types of guided wave in a layer, or longitudinal and shear waves, that travel in the bulk.

Rayleigh waves have a speed less than S-waves, by a factor dependent on the elastic constant (σ) of the material near the surface.[2]

Since Rayleigh waves are confined near the surface, their in-plane amplitude when generated by a point source decays only as {1}/{\sqrt{r}}, where r is the radial distance. Surface waves therefore decay more slowly with distance than do bulk waves, which spread out in three dimensions from a point source. The speed of Rayleigh waves on bulk solids, of the order of 2–5 km/s, is slightly less than the shear velocity.

The existence of Rayleigh waves was predicted in 1885 by Lord Rayleigh, after whom they were named.

Rayleigh wave dispersion

The elastic constant often changes with depth, due to the properties of the material. This means the velocity of a Rayleigh wave is dependent on the wavelength (and therefore frequency). Waves affected dispersion have a different wave train shape.[2] Rayleigh waves on ideal, homogeneous and flat elastic solids show no dispersion. However, if a solid or structure has a density or sound velocity that varies with depth, Rayleigh waves become dispersive. One example is Rayleigh waves on the Earth's surface: those waves with a higher frequency travel more slowly than those with a lower frequency. This occurs because a Rayleigh wave of lower frequency has a relatively long wavelength. The displacement of long wavelength waves penetrates more deeply into the Earth than short wavelength waves. Since the speed of waves in the Earth increases with increasing depth, the longer wavelength (low frequency) waves can travel faster than the shorter wavelength (high frequency) waves. Rayleigh waves thus often appear spread out on seismograms recorded at distant earthquake recording stations. It is also possible to observe Rayleigh wave dispersion in thin films or multi-layered structures.

Rayleigh waves in non-destructive testing

Rayleigh waves are widely used for materials characterization, to discover the mechanical and structural properties of the object being testing – like the presence of cracking, and the related shear modulus. This is in common with other types of surface waves.[3] The Rayleigh waves used for this purpose are in the ultrasonic frequency range.

They are used at different length scales because they are easily generated and detected on the free surface of solid objects. Since they are confined in the vicinity of the free surface within a depth (~ the wavelength) linked to the frequency of the wave, different frequencies can be used for characterization at different length scales.

Rayleigh waves in geophysics

Rayleigh waves from earthquakes

Since Rayleigh waves are surface waves, the amplitude of such waves generated by an earthquake generally decreases exponentially with the depth of the hypocenter (focus). However, large earthquakes may generate Rayleigh waves that travel around the Earth several times before dissipating.

In seismology longitudinal and shear waves are known as P-waves and S-waves, respectively, and are termed body waves. Rayleigh waves are generated by the interaction of P- and S- waves at the surface of the earth, and travel with a velocity that is lower than the P-, S-, and Love wave velocities. Rayleigh waves emanating outward from the epicenter of an earthquake travel along the surface of the earth at about 10 times the speed of sound (0.340 km/s), in air, that is ~3 km/s.

Due to their higher speed, the P- and S-waves generated by an earthquake arrive before the surface waves. However, the particle motion of surface waves is larger than that of body waves, so the surface waves tend to cause more damage. In the case of Rayleigh waves, the motion is of a rolling nature, similar to an ocean surface wave. The intensity of Rayleigh wave shaking at a particular location is dependent on several factors:

Local geologic structure can serve to focus or defocus Rayleigh waves, leading to significant differences in shaking over short distances.

Rayleigh waves in seismology

Low frequency Rayleigh waves generated during earthquakes are used in seismology to characterise the Earth's interior. In intermediate ranges, Rayleigh waves are used in geophysics and geotechnical engineering for the characterisation of oil deposits. These applications are based on the geometric dispersion of Rayleigh waves and on the solution of an inverse problem on the basis of seismic data collected on the ground surface using active sources (falling weights, hammers or small explosions, for example) or by recording microtremors. Rayleigh ground waves are important also for environmental noise and vibration control since they make a major contribution to traffic-induced ground vibrations and the associated structure-borne noise in buildings.

Other manifestations

Animals

Low frequency (< 20 Hz) Rayleigh waves are inaudible, yet they can be detected by many mammals, birds, insects and spiders. Human beings should be able to detect such Rayleigh waves through their Pacinian corpuscles, which are in the joints, although people do not seem to consciously respond to the signals. Some animals seem to use Rayleigh waves to communicate. In particular, some biologists theorize that elephants may use vocalizations to generate Rayleigh waves. Since Rayleigh waves decay slowly, they should be detectable over long distances.[2] Note that these Rayleigh waves have a much higher frequency than Rayleigh waves generated by earthquakes.

After the 2004 Indian Ocean Earthquake, some people have speculated that Rayleigh waves served as a warning to animals to seek higher ground, allowing them to escape the more slowly-traveling tsunami. At this time, evidence for this is mostly anecdotal. Another animal early warning systems may rely on an ability to sense infrasonic waves traveling through the air. [3]

See also

Notes

  1. ^ Longuet-Higgins, Michael (1950). "A theory of the origin of microseisms". Philosophical Transaction of the Royal Society of London, series A 243: p. 1--35. 
  2. ^ a b c d Telford, William Murray; Geldart, L. P.; Robert E. Sheriff (1990). Applied geophysics. Cambridge University Press. p. 149. ISBN 9780521339384. http://books.google.com/books?id=oRP5fZYjhXMC&pg=PA149. Retrieved 8 June 2011. 
  3. ^ Thompson, Donald O.; Chimenti, Dale E. (1 June 1997). Review of progress in quantitative nondestructive evaluation. Springer. p. 161. ISBN 9780306455971. http://books.google.com/books?id=3etLzmHu6bQC&pg=PA161. Retrieved 8 June 2011. 

Further reading

External links