Effect of local high-velocity body

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Problem 9.24

The horizon velocity analysis for horizon A in Figure 9.24a indicates higher stacking velocities on opposite sides of the salt pillows B and C and low stacking velocities immediately below the salt pillow D. Does this have geological significance? The streamer was about 4 km long. Note that the upper 2 s of the section have been cut off.

Figure 9.24a.  (i) Seismic section; (ii) horizon velocity analysis along event .


A horizon velocity analysis is a more-or-less continuous series of velocity analyses along a single reflection event or along a series of more-or-less parallel events lying within a narrow time window that contains an event.

A salt pillow is a small dome formed by salt flowing from the surrounding region. Often the growth of a salt pillow is cut off when the salt supply (mother salt) is cut off.


With a 4-km streamer, the long-offset traces of CMP gathers immediately to the left and right of the salt pillows B and C are apt to have one leg of their long-offset travel paths through the salt whereas their short-offset traces may not involve salt travel. Because the salt is higher velocity than the sediments, this will flatten the gather hyperbolas and thus they will indicate fictitious high velocity. Over the center of the salt dome (D) the opposite will be the case, indicating fictitious low velocity.

The uplift in reflection , which is below the pillow, is probably a velocity pull-up because of the overlying high-velocity salt. The first 2.5+ km to the right probably involve little salt and show fairly uniform and good quality velocity data probably not involving salt travel, so that the average velocity above horizon , about 3 km/s, is probably a good average for the overlying sediments. The data to the left of the dome are not as clear, indicating other complications, perhaps more salt in this region. Horizon may be faulted under the center of the dome, which may explain why the dome developed here.

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