  An anisotropic elasticity tensor can be approximated by the closest tensor
belonging to a higher symmetry class. The closeness of tensors depends on the
choice of a criterion. We compare the closest isotropic tensors obtained using
four approaches: the Frobenius 36-component norm, the Frobenius 21-component
norm, the operator norm and the L2 slowness-curve fit. We find that the
isotropic tensors are similar to each other within the range of expected
measurement errors.
