Luckily, it has been discovered that certain functions of and K can be reliably determined and mapped. The velocity v and the acoustic impedance R are given by the equations
The velocity v is seen through a much smaller window. Observation of it involves studying travel time as shot-to-geophone offset varies and will be described in chapter . With this second window it is hard to discern sixteen independent velocity measurements on a 4-second reflection time axis. So this window goes from zero to about 2 Hz, as depicted in Figure 13.
Note that there is an information gap from 2-10 Hz. Even presuming that rock physics can supply us with a relationship between and K, the gap seriously interferes with the ability of a seismologist to predict a well log before the well is drilled. What seismologists can do somewhat reliably is predict a filtered log.
The observational situation described above has led reflection seismologists to a specialized use of the word velocity. To a reflection seismologist, velocity means the low spatial frequency part of ``real velocity.'' The high-frequency part of the ``real velocity'' isn't called velocity: it is called reflectivity. Density is generally disregarded as being almost unmeasurable by surface reflection seismology.