Theoretically, the inverse is of infinite duration, namely, . Since we know that the least- squares method abhors large errors and thus tends to equalize them, we should be able to guess the result. Since we cannot divide by zero, we should not try to divide by 1- Z n , which has zeros uniformly distributed on the unit circle. The method of least squares prevents disaster, but it cannot perform miracles. I consider ghosts to be a problem in search of a different solution.
Ghosts also arise when seismograms are recorded in a shallow borehole.
Figure 1 shows many seismograms that resemble each other but differ in the x location of the receiver.