Slowness (seismology) - AbsoluteAstronomy.com

Slowness is a quantity introduced in Seismology

Seismology

Seismology is the scientific study of earthquakes and the propagation of elastic waves through the Earth or through other planet-like bodies. The field also includes studies of earthquake effects, such as tsunamis as well as diverse seismic sources such as volcanic, tectonic, oceanic,...

which is the reciprocal

Multiplicative inverse

In mathematics, a multiplicative inverse or reciprocal for a number x, denoted by 1/x or x−1, is a number which when multiplied by x yields the multiplicative identity, 1. The multiplicative inverse of a fraction a/b is b/a. For the multiplicative inverse of a real number, divide 1 by the...

of velocity

Velocity

In physics, velocity is speed in a given direction. Speed describes only how fast an object is moving, whereas velocity gives both the speed and direction of the object's motion. To have a constant velocity, an object must have a constant speed and motion in a constant direction. Constant ...

. Thus travel time of a wave is nothing but the distance that the wave travels times the slowness of the medium ( in seismology, it refers to different layers of Earth

Earth

Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

exhibiting different densities

Density

The mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...

)
Thus,

Slowness = 1/Velocity

If we divide the path followed by the ray into N segments numbered i = 1, 2 .. N the total travel time of the wave

Wave

In physics, a wave is a disturbance that travels through space and time, accompanied by the transfer of energy.Waves travel and the wave motion transfers energy from one point to another, often with no permanent displacement of the particles of the medium—that is, with little or no associated mass...

is

Γ= ΣsiΔxi
Therefore, the residual travel

Travel

Travel is the movement of people or objects between relatively distant geographical locations. 'Travel' can also include relatively short stays between successive movements.-Etymology:...

time is due to departures in slowness experienced by the wave along the raypath.

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