Automatic gain control

 Echos get weaker with time, though the information content is unrelated to the signal strength. Echos also vary in strength as different materials are encountered by the outgoing wave. Programs for echo analysis typically divide the data by a scaling factor that is a smoothed average of the signal strength. This practice is nearly universal, although it is fraught with hazards. An example of automatic gain control (AGC) is to compute the divisor by forming the absolute value of the signal strength and then smoothing with the program triangle() or the program leaky() . Pitfalls are the strange amplitude behavior surrounding the water bottom, and the overall loss of information contained in amplitudes. Personally, I have found that the gain function t² nearly always eliminates the need for AGC on raw field data, but I have no doubt that AGC is occasionally needed. (A theoretical explanation for t² is given in IEI.)