Instantaneous AGC

Seismic Data Analysis

Series	Investigations in Geophysics
Author	Öz Yilmaz
DOI	http://dx.doi.org/10.1190/1.9781560801580
ISBN	ISBN 978-1-56080-094-1
Store	SEG Online Store

Instantaneous AGC is one of the most common gain types used. This gain function is computed as follows. First, the mean absolute value of trace amplitudes is computed within a specified time gate. Second, the ratio of the desired rms level to this mean value is assigned as the value of the gain function. Unlike the rms amplitude AGC, this value is assigned to any desired time sample of the gain function within the time gate, say the nth sample of the trace, rather than to the sample at the center of the gate. The next step is to move the time gate one sample down the trace and compute the value of the gain function for the (n + 1)th time sample, and so on. No interpolation is therefore required to define this gain function. Hence, the scaling function g(t) at the gate center is given by

$g(t)={\frac {\text{desired rms}}{{\frac {1}{N}}\sum \nolimits _{i=1}^{N}{\left|{x_{i}}\right|}}},$

(11)

where x_i is the trace amplitude and N is the number of samples within the gate.

Figure 1.4-11  A portion of a CMP stack before and after application of five different instantaneous AGC functions. The numbers on top indicate gain window sizes in milliseconds used in computing the AGC gain function described by equation (11).

Figure 1.4-11 shows the ungained data and four instantaneous AGC-gained sections. Gate lengths are indicated on top of each panel. Very small time gates cause a significant can loss of signal character by boosting zones that contain small amplitudes. This occurs with the 64-ms AGC output. In processing, this is called a fast AGC. In the other extreme, if a large time gate is selected, then the effectiveness of the AGC process is lessened. In practice, AGC time gates commonly are specified between 200 and 500 ms.

See also

• Geometric spreading correction
• Programmed gain control
• Rms amplitude AGC
• Relative trace balancing

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