The Seismic Wavefield provides a guide to the understanding of seismograms in terms of physical propagation processes within the Earth. The focus is on the observation of earthquakes and man-made sources on all scales, for both body waves and surface waves. Volume I begins with a survey of the structure of the Earth and the nature of seismic wave propagation using examples of observed seismograms. The second part provides a full development of the theoretical background for seismic waves. Volume II (to be published later) will cover local and regional seismic events, global wave propagation, and the three-dimensional earth.