  This study focuses on the numerical modeling of wave propagation in
fractionally-dissipative media. These viscoelastic models are such that the
attenuation is frequency dependent and follows a power law with non-integer
exponent. As a prototypical example, the Andrade model is chosen for its
simplicity and its satisfactory fits of experimental flow laws in rocks and
metals. The corresponding constitutive equation features a fractional
derivative in time, a non-local term that can be expressed as a convolution
product which direct implementation bears substantial memory cost. To
circumvent this limitation, a diffusive representation approach is deployed,
replacing the convolution product by an integral of a function satisfying a
local time-domain ordinary differential equation. An associated quadrature
formula yields a local-in-time system of partial differential equations, which
is then proven to be well-posed. The properties of the resulting model are also
compared to those of the original Andrade model. The quadrature scheme
associated with the diffusive approximation, and constructed either from a
classical polynomial approach or from a constrained optimization method, is
investigated to finally highlight the benefits of using the latter approach.
Wave propagation simulations in homogeneous domains are performed within a
split formulation framework that yields an optimal stability condition and
which features a joint fourth-order time-marching scheme coupled with an exact
integration step. A set of numerical experiments is presented to assess the
efficiency of the diffusive approximation method for such wave propagation
problems.
