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P and S waves are body waves and expand within the main rock mass. Other waves, known as Love waves, are generated at interfaces, while particles at the Earth’s surface can follow elliptical paths to create Rayleigh waves. Love and Rayleigh waves may carry a considerable proportion of the source energy but travel very slowly. In many surveys they are simply lumped together as the ground roll. 3.1.2 Seismic velocities The ‘seismic velocities’ of rocks are the velocities at which wave motions travel through them. They are quite distinct from the continually varying velocities of the individual oscillating rock particles. Any elastic-wave velocity (V) can be expressed as the square root of an elastic modulus divided by the square root of density (ρ). For P waves the elongational elasticity, j is appropriate, for S waves the shear modulus, μ. The equations:
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