Since the days of the very first reflection seismic line acquired for petroleum exploration, geologists and geophysicists have wanted better quality reflection data and more resolution of the subsurface. As technology improved from single-channel analog recorders to multi-channel digital recording, so did the ability of explorers to find more reserves in shorter periods of time. Borehole seismic surveys started two decades ago with vertical seismic profiles (VSPs). However, despite their success, geophysicists continue to demand more from borehole surveys. In the mid-1980s, several research teams within the major companies designed and tested various systems of borehole seismic receivers and sources. Such surveys promised to allow companies to shoot from one well to another, thereby providing what amounted to a seismic ?catscan? of the earth between the two wells. While potentially useful in exploration ventures, the real promise of these so-called cross-well surveys lies in their ability to image producing horizons within existing fields, thereby enabling production and injection strategies to be optimized. Unfortunately, cross-well surveys suffer from high data acquisition costs. Based on the experience of several operators active in the field, there is a direct correlation between the number of receivers one puts into the well bore and the cost of the survey. For cross well seismic to achieve its real potential, a major change in data-acquisition technology had to be found.