Earth structure, global

Introduction

If this article were written some six years ago, it would be likely to contain only a description of the Earth as a spherically symmetric body (a body whose properties vary only with radius, or depth, and do not depend on geographical coordinates, the effects due to the Earth's ellipticity and rotation being implicitly accounted for). It is known, however, that there are lateral differences in physical properties at all levels within the Earth; indeed its dynamic behavior—mantle convection, for example—tells us that this must be so. Now, because of the availability of new sources of data, very large wavelength lateral variations in the deep interior of the Earth have been identified, and there is strong evidence that further improvement in the knowledge of the spherically symmetric properties is just part of the much larger and complex problem of determining the Earth's structure in three dimensions.

One way to look at the problem is as follows:

A spherically symmetric...