Introduction

Abstract

Applied geophysics deals with the geometry, location, and size of geological structures and/or physical parameters such as velocities, electrical conductivities, or density differences. Measurements are made of permanent fields that depend only on location (for example, gravity and magnetic measurements) or of reactions to excitation (i.e., processes dependent on both location and time, for instance, seismic reflection and refraction surveys as well as geoelectric and magnetotelluric surveys). The measured values are not always optimal for interpretation. For example, gravity and magnetic measurements represent the total field of several sources at different depths; separation of the contributions of each source is possible only under certain conditions. On the other hand, there is a relationship between the wavenumber spectrum of the field and the depth of the source that can often be used to estimate the depth. Numerous other effects in applied geophysics are also frequency dependent, for example, the depth of penetration of electromagnetic waves and the propagation of seismic waves. In such cases the spectral representation is often more useful for interpretation than the measurements themselves. Examples for this are the determination of absorption and dispersion in seismology and the use of spectral analysis for determining the elastic constants and density of the interior of the Earth from reflection seismic measurements (Harjes, 1979).