Complete the requirements for the module, ``Listening to the Earth: Controlled Source Seismology,'' by Richard G. Montgomery, UMAP 292-293. This module develops the elementary theory of wave reflection and refraction and applies it to a model of the earths subsurface. The model shows how information on layer depth and sound velocity may be obtained to provide data on width, density, and composition of the subsurface. This module is a good introduction to controlled seismic methods and requires no previous knowledge of either physics or geology.