Introduction

Anisotropy can be introduced into earth modeling in a useful and natural way by recognizing a correspondence between anisotropic complexity and the complexity of the fabric of an otherwise relatively undisturbed sedimentary basin. The correspondence I have in mind looks something like this:

isotropic

Uniform depositional environment. Sediments are not much altered by overburden.

transverse isotropic

Layering effects. Variable deposition. Alteration caused by a uniaxial (gravity) stress field. Horizontal cracks and fractures.

orthorhombic

Further alteration due to a vertical crack or fracture system.

monoclinic

A set of such vertical fracture systems with a variety of azimuths.

We could go on from there and describe more complex systems, but plane-layered models have a long and successful history in our discipline, and the monoclinic system with horizontal symmetry plane is a natural adjunct to these plane-layered models, leading to quite major simplifications in the algebra of modeling. With this context in mind, it is the purpose of this note to present a simplication in the description of monoclinic stiffness and compliance matrices.