Fig. 8.3 (a, b, c) Variation of sound speed with temperature and salinity in the central Pacific,
where a sound speed minimum occurs at 650 m. d Variation of sound speed in the equatorial
Atlantic. Due to different properties of the respective water columns, the sound speed profiles
differ between the world’s oceans (modified from Jones 1999, by permission of Wiley)
Fig. 8.2 Wave fronts of a plane wave (top left) and of a spherical wave (top right); k is the wave
(propagation) vector with unit (1/m) and r is the position vector with unit m. In a plane wave, the
wave fronts are perpendicular to the propagation vector, while in the spherical wave, the wave
fronts are surfaces of concentric spheres around the source at r = 0. Propagation of wave fronts
according to Huygen’s principle for a plane wave (bottom left) and a spherical wave (bottom
right). Any point of a wave front serves as starting point of an elementary wave. The wave front
at a later instant is the envelope of the superposition of all elementary waves from a given wave
front
8 Acoustic Methods Overview
199