Seismic wave propagation using Micropolar theory
Our group in Münster applies micropolar theory to study elastic wave propagation phenomena in presence of microstructure. Micropolar theory is a generalization of the classical linear elastic theory, where each particle has an intrinsic rotational degree of freedom, called micro-rotation and/or spin, and which depends on the so-called Cosserat couple modulus that characterizes the micropolar medium. The theory assumes that this spin field is different from the continuum rotation.
Elastic wave propagation in crystal structures is described by two types of modes: the acoustic one and the optic one. In the acoustic type (longitudinal acoustic LA and/or transverse acoustic TA), all the atoms in the unit cell move in phase, this means that the atoms move coherently in the lattice, resulting in the deformation of the lattice (see Figure 1). In the optic type (longitudinal optic LO and/or transverse optic TO), the atoms move out of phase (see Figure 1). In micropolar theory, we relate the frequency at which the optic modes are observed in laboratory experiments to the cut-off frequency.