The discovery in 1987 that sunspots are strong absorbers of incident acoustic waves (p-modes) lead to a desire to develop and apply new methods of "local helioseismology" to probe local inhomogeneities like sunspots in the solar interior. Among these tools is what is termed "helioseismic holography" which is an efficient and versatile formalism for the production and analysis of diffraction-limited seismic images of the solar interior. I will trace the development of helioseismic holography and highlight recent applications in both "acoustic power holography" which concentrates on the detection of sources and sinks of acoustic waves, and "phase-sensitive holography" which is designed to probe wave-speed perturbations and mass flows. Helioseismic holography is perhaps best known for the ability to image active regions on the far side of the Sun, with which it is now routinely possible to predict the appearance of large active regions a week or more before they rotate to the side facing the Earth.
Students may wish to wish to review some of the general information links on the lecturer's home page: http://www.cora.nwra.com/~dbraun/ and download some of the publications available, particularly
Lindsey, C. and Braun, D. C. 2000, "Seismic Images of the Far Side of the Sun," Science, 287, 1799, http://www.cora.nwra.com/~dbraun/reprints/fsi_science.pdf
and
Braun, D.C., Birch, A., and Lindsey, C. 2004, "Local Helioseismology of Near-surface Flows," in Proceedings of SOHO 14 / GONG+ 2004. Helio- and Asteroseismology: Towards a Golden Future, http://www.cora.nwra.com/~dbraun/reprints/Braun_04.pdf