Automatic Gain Correction

Use the Grid and Image > Filters > Automatic Gain Correction menu option (GRIDAGC GX) to apply automatic gain compensation to a grid. After isolating signal and background components in the input grid for a given window size, a correction is applied to the signal component in order to equalize its amplitude over the grid.

Automatic Gain Correction dialog options

Input grid file	Select file name of the Input Grid. (.grd file) Script Parameter: GRIDAGC.GRD
New resultant grid	Select file name of New Grid. (.grd file) Script Parameter: GRIDAGC.NEW
Filter size	Size of filter used to isolate signal from background. Script Parameter: GRIDAGC.WIDTH
Maximum gain correction	Maximum gain correction to apply at any position. Script Parameter: GRIDAGC.MAXGAIN
Select method	Select method, "Full Amplitude" or "Local Amplitude". Script Parameter: GRIDAGC.METHOD

Application Notes

The local gain is estimated using a moving window. (The window Size is actually the closest odd number less than or equal to the input value so that it may be centered over each position). In this window the average RMS signal amplitude is calculated, and is taken to be an estimate of the local signal gain. During the initial pass over the data, the maximum "gain" value is recorded. In the second pass, all points are multiplied by the ratio of the maximum gain to the local gain. A maximum gain correction is specified to prevent the operation from "blowing up" in regions with little or no signal.

Using Local Amplitude

In this case the method first isolates a "signal" component from a "background" component. Inside the filter window, the best-fitting plane (that which minimizes the RMS misfit with the data) is calculated. The average RMS difference between the data values and the plane values in the neighborhood is considered to be a measure of the local signal gain, while the difference between the position's value and the best-fitting plane is considered to be the signal itself.

The first pass over the grid determines the "signal" and the "signal gain" at every position, and remembers the largest signal gain encountered. In the second pass, the signal at each position is multiplied by the ratio of the maximum signal gain to the local signal gain. The new "signal" is then added back to the original background value to obtain the final value.

NOTE: Selecting Local Amplitude does NOT remove the background from the final solution; just from the gain-estimation phase.

Choosing the Size is similar to choosing the corner frequency of a high-pass filter; those features with wavelengths longer than Width are relatively unaffected by the AGC, while features with wavelengths comparable to, or smaller than Size will be most strongly amplified. The second method tends to preserve more faithfully the longer-wavelength portions of the data.

Reference