Rms amplitude 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

The rms amplitude AGC gain function is based on the rms amplitude within a specified time gate on an input trace. This gain function is computed as follows. The input trace is subdivided into fixed time gates. First, the amplitude of each sample in a gate is squared. Second, the mean of these values is computed and its square root is taken. This is the rms amplitude over that gate. The ratio of a desired rms amplitude (say 2000) to the actual rms value is assigned as the value of the gain function at the center of the gate. Hence, the scaling function g(t) at the gate center is given by

$g(t)={\frac {\text{desired rms}}{\sqrt {{\frac {1}{N}}\sum \nolimits _{i=1}^{N}{x_{i}^{2}}}}},$

(10)

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

Figure 1.4-10  A portion of a CMP stack before and after application of two different rms AGC functions. Numbers on the top indicate the window sizes in milliseconds used in computing the AGC gain function described by equation (10).

Typically, we start out with a certain gate length at the shallow part of the trace. Gate length can be kept either constant or it can be increased systematically down the trace. At each gate center, the value of the gain function is computed as described above. Function g(t) then is interpolated between the gate centers. Note that the specified time gates are stationary — they do not slide down the trace.

Figure 1.4-10 shows the ungained data and two rms-gained sections. The gate lengths are indicated at the top of each panel. When the gate used in the computation is kept small, say 64 ms, then strong reflections become less distinct.

See also

• Geometric spreading correction
• Programmed gain control
• Instantaneous AGC
• Relative trace balancing

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