Changes

==Automatic gain control, trace balance, and filtering==

==Automatic gain control, trace balance, and filtering==

At display time, scaling and filtering parameters must be selected. Energy decays very rapidly as it penetrates the earth and is reflected back to a receiver. If the energy reflected from the deeper horizons were displayed with the same amplitude scale as that from the near surface, peaks and troughs from deeper events would be invisible to the eye. ''Automatic gain control'' (AGO is a method to normalize all amplitudes in a trace by looking at time windows over that trace and adjusting the amplitude of events within the window relative to a chosen standard.

At display time, scaling and filtering parameters must be selected. Energy decays very rapidly as it penetrates the earth and is reflected back to a receiver. If the energy reflected from the deeper horizons were displayed with the same amplitude scale as that from the near surface, peaks and troughs from deeper events would be invisible to the eye. ''Automatic gain control'' (AGO is a method to normalize all amplitudes in a trace by looking at time windows over that trace and adjusting the amplitude of events within the window relative to a chosen standard).

The choice of the design windows for the AGC affects the display. If the window is very short in time, the program tends to raise all amplitudes to the standard and very little differentiation between events is preserved. Generally, a window of about 250 msec is a good place to start. As the gate becomes longer, more of the original amplitude relationships are preserved. A single window trace equalization is sometimes desirable in processing data intended for relative amplitude studies, while a fast (short) AGC could be helpful in data having poor signal to noise ratio.

The choice of the design windows for the AGC affects the display. If the window is very short in time, the program tends to raise all amplitudes to the standard and very little differentiation between events is preserved. Generally, a window of about 250 msec is a good place to start. As the gate becomes longer, more of the original amplitude relationships are preserved. A single window trace equalization is sometimes desirable in processing data intended for relative amplitude studies, while a fast (short) AGC could be helpful in data having poor signal to noise ratio.

Another method of gaining the section is ''programmed gain control''. This method applies a predetermined gain function to the trace and is most commonly encountered in field displays.

Another method of gaining the section is '"programmed gain control'". This method applies a predetermined gain function to the trace and is most commonly encountered in field displays.

Since some form of gain is required to display the data at a usable scale, what are often called ''true amplitude'' sections are in reality relative true amplitude sections. The relative changes in amplitude are preserved by the method of gain used, and yet the entire data set is interpretable from the same display.

Since some form of gain is required to display the data at a usable scale, what are often called ''true amplitude'' sections are in reality relative true amplitude sections. The relative changes in amplitude are preserved by the method of gain used, and yet the entire data set is interpretable from the same display.