INTRODUCTION

LOOKING INTO THE EARTH'S INTERIOR

This book describes the study of magnetic fields originating in the crust and, possibly, the upper portion of the mantle of the earth, as measured at satellite altitudes, roughly 150–800 kilometers (km) above the earth's surface. Questions addressed include the significance of measuring magnetic fields for lithospheric investigations, the contributions made by satellite altitude measurements, and the procedures for reducing the magnetic field measurements and isolating and interpreting the lithospheric component. Background is provided for the measurement and analysis of lithospheric fields at satellite altitude, and the results of those measurements over the past few decades are summarized.

Except for information derived from mines and drill holes, knowledge of what is beneath the earth's surface is dependent upon indirect measurements made at or above the earth's surface. Our deepest penetrations into the earth have been little more than 10 km, or roughly 0.2% of the earth's radius, and that depth has been reached in only few locations. Several methods have been developed in the effort to probe the interior of the earth, including laboratory replications of how the earth's increasing temperature and pressure with increasing depth are likely to affect the materials believed to exist at great depths, imaging of the interior with seismic waves from natural or man-made mechanical forces, and measurement and interpretation of gravity, heat flow, and magnetic and electrical fields.