For many years, seismic exploration techniques have been used by geophysicists for imaging underground structures (identifying their general shapes) and, consequently, locating oil and gas reservoirs (hydrocarbon-bearing formations). This exploration was achieved first in two dimensions, using acoustic sensor arrays, and evolved to three dimensions (3D seismic) in the 1980s as computing power increased. Over the past 15 years, 3D seismic data have been used to characterize hydrocarbon-bearing formations in terms of thickness, porosity, fluid content, and so on. In the late 1990s, a new technique for reservoir monitoring and management called time-lapse seismic or 4D seismic was developed (the four dimensions being the three geometrical dimensions plus time). By mid-2001, about 70 time-lapse seismic operations were under way throughout the world.