Glossary

Absorption - heat energy generated by distortion of the materials through which seismic waves propagate. This heat reduced the total energy of the wave.

Air wave - a compressional wave created by energy sources used near on the surface. It travels through the air to the geophones.

Amplitude - the maximum displacement associated with the particle motions that occur as the wave passes through the material.

Angle of incidence - the angle between a ray and a normal to the horizontal surface.

Angle of reflection -

Array - An ordered arrangement of seismometers or geophones, the data from which feeds into a central receiver.

Arrival - The appearance of seismic energy on a seismic record.

Arrival time - The time at which a particular wave phase arrives at a detector

Azimuth - The angle, measured clockwise from north, from the source to the receiver. Thus, it has a range of 0 - 360 degrees.

Backazimuth - The angle, measured clockwise from north, from the receiver to the source. Thus, it has a range of 0 - 360 degrees.

Critical angle - the angle required for a wave to propagate itself along a surface

Critical refraction - propagation of a wave along a wave front, when it hits a surface at the critical angle.

Diffraction - if a disturbance is generated at the surface and encounters a sudden change of curvature at a velocity discontinuity; additional waves are produced that are not predicted by drawing rays for reflection or refraction.

Elongation - the change in length of a line in its final or deformed state divided by its original length.

Fermat's Principle (principle of least time) - This principle states that in the propagation of waves, the wave path between any two fixed points is that one along which the time of travel is the least of all possible paths. Therefore, the wave path or ray must be a straight line in a medium of constant velocity.

Filtering - the removal of certain noise (Hz) from a reading by filtering certain ranges out.

Flat response - the geophone responds with the same sensitivity to all frequencies above the natural frequency (Earth's).

Frequency - the number of repetitions per unit of time

Geophones - cylindrical coils of fine wire suspended in a cylindrical cavity in a magnet. When ground motion occurs, the geophone moves, inducing relative motion between the magnet and the coil due to the inertia of the coil. This generates a voltage that is proportional to the amount of ground displacement.

Ground roll - Rayleigh waves

Head wave - produced by a wave that strikes an interface at its critical angle is refracted parallel to the interface. Head waves involve energy that enters a high-velocity medium (refractor) near the critical angle and travels in the high-velocity medium nearly parallel to the refractor surface

Hookean behavior - materials are elastic and exhibit an instantaneous linear relation between stress and strain. When a stress is applied to such materials, the resulting strain is immediate. When the stress is removed, the material returns to its original shape.

Microseism - A more or less continuous motion in the Earth that is unrelated to an earthquake and that has a period of 1.0 to 9.0 seconds. It is caused by a variety of natural and artificial agents.

P-waves (Longitudinal wave) - particle moves back and forth along the direction of propagation of the wave. Also known as a compressional wave due to particle compressions during their transport along the wave.

Period - the time it takes for two successive wave crests to pass said reference point, completing one cycle.

Rays - the direction of propagation perpendicular to the wavefront.

Rayleigh waves - waves that propagate by particle motion confined to a vertical plane. Particle displacements are greatest at the surface and decrease exponentially downward.

Refraction - energy flows through a material, and when density and therefore velocity changes, the wave is bent along the angle of refraction. If the critical angle is achieved, then the wave will propagate along the velocity change, then is bent back to the surface.

Total reflection - If the angle of incident increases beyond a 90-degree angle, then no refraction occurs.

S-waves (Transverse wave) - particle motions are perpendicular to the ray and transverse to the direction of movement of the wavefront. Cannot propagate through liquids.

SH-component - component of an S-wave, parallel to the ground surface.

SV-component - component of an S-wave lying in a vertical plane containing the incident ray.

Shot point - source of energy, i.e. a hammer blow.

Signal-to-noise ratio - The comparison between the amplitude of the seismic signal and the amplitude of noise caused by seismic unrest and (or) the seismic instruments.

Stress - measure of the magnitude and direction of deforming force

Strain - measure of the change in length, volume and shape in stressed material

Travel time - The time required for a wave train to travel from its source to a point of observation.

Uniaxial compression - a Hookean material will shorten in the direction of the applied stress but at the same time will lengthen in a direction at right angles to the compression.

Wavelength - the distance between two adjacent points on the wave that have the same phase or similar displacements.

Wavefront - the array of rays emanating from the source, in this case a shot point.

Sources: http://www.geo.arizona.edu/saso/Education/Qanda/glossary.html

Seismic Exploration: Fundamental Considerations: Chapter 2.

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