[en] Waves, by way of their speed of propagation, interaction with boundaries, and energy loss mechanisms, yield information related to the physical properties of media through which they pass. Measurements of wave characteristics at the ground surface or in boreholes, therefore, provide a means by which to ascertain the geologic properties and internal geometry of a rock mass. The resolution of wave propagation techniques is shown to be theoretically limitless, depending only on instrument sensitivity and noise. Because of this, we recommend using techniques that can be applied in immediate proximity to candidate rock masses. Geotomography and wavefront reconstruction (holography) are considered two promising techniques for site investigation and characterization. Of the two, geotomography is treated in detail because of its lower sensitivity to noise, and ease of implementation. Algorithms are presented for both ray-optic geotomography and diffusion geotomography. Although the diffusion geotomography algorithm is more general and more satisfying from a physics standpoint, for large numbers of measurements (typical field situations) the mathematics of the inverse problem implicitly requires source frequencies that approach ray optic behavior. The ultimate conclusion is that, regardless of the algorithm used, one should probe using the highest source frequencies possible for given site conditions