sfagc implements Automatic Gain Control, a popular technique for normalizing signal amplitudes.
The algorithm is simple: divide the signal by its smoothed absolute value. The smoothing is controlled by rect#= and repeat= parameters, similar to the ones used by sfsmooth.
The following example from rsf/rsf/sfagc illustrates the application of sfagc in comparison with the application of sfpow, which applies a gain based on a power of time. The gains are applied to a shot gather from Alaska from the collection of shot gathers by Yilmaz and Cumro. A similar example appears on page 236 in Jon Claerbout’s Imaging the Earth’s Interior.

For a more accurate algorithm, try sfshapeagc, which computes the gain function using shaping regularization.