SEISMIC SURVEYS 153 Figure Schematic geometry for acquiring marine seismic reflection data Figure Acquisition geometry of traces sharing a common reflection point recorded data from different shots can be reordered to bring together traces corresponding to a single reflection point in the subsurface (Figure 5) Of course, the travel time increases as the source– receiver separation becomes larger (Figure 6), but this can be corrected so as to line up all the signal peaks at the same travel time They can then be added together (stacked) to create a signal with a much higher amplitude The correction required to align all the traces contains information about the average velocity of the seismic waves, which is useful in later processing However, the medium above a target reflector is usually strongly layered This means that signals can bounce back and forth between these shallow layers, and may perhaps arrive at the receiver at much the same time as a genuine reflection from a deeper layer (Figure 7) There are several ways to remove these ‘multiples’ Many of these methods depend on the difference in average velocity along the travel path between the primary and the multiples, caused by the general increase of velocity with depth due to compaction The multiples have spent more time at shallow depths, so their average velocity is lower than that of the deeper-penetrating primaries Correction for variable source–receiver distances will thus line up the primaries but not the multiples, which will be Figure Schematic plot of traces acquired with the geometry shown in Figure Figure Travel paths for primaries and multiples attenuated in the stacked result Various algorithms exploit this velocity difference to improve the discrimination against multiples further The same process of multiple bounces, on a smaller layer-thickness scale, acts to blur the crisp initial seismic signal on its passage through the Earth This combines with the effect of the absorption of seismic energy (which is more pronounced over a given distance for the higher frequencies) to reduce the content of high-frequency energy in deep reflections Commonly, reflections from a depth of 3000 m will have peak energy at a frequency of 25–30 Hz As we shall see, this reduces the resolution that can be achieved