Difference between revisions of "Rms amplitude AGC"

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where ''x<sub>i</sub>'' is the trace amplitude and ''N'' is the number of samples within the gate.
 
where ''x<sub>i</sub>'' is the trace amplitude and ''N'' is the number of samples within the gate.
  
<gallery>file:ch01_fig4-7.png|{{figure number|1.4-7}} (a) A field record from a marine survey, (b) after muting guided waves, and (c) after the geometric spreading correction using ''t''<sub>2</sub> as the scaling function. Shown on top are the amplitude spectra averaged over the shot record, and at the bottom are the autocorrelograms.
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[[file:ch01_fig4-10.png|thumb|left|{{figure number|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 ({{EquationNote|10}}).]]
file:ch01_fig4-8.png|{{figure number|1.4-8}} Gain is a time-variant scaling defined by a function, ''g''(''t''). Based on some criteria, this function is defined at the time samples (shown by solid circles) that are usually at the center of specified time gates along the trace as indicated by 1, 2, 3, and 4. Gain application simply involves multiplying ''g''(''t'') by the input trace amplitudes.
 
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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.
 
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.
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*[[Instantaneous AGC]]
 
*[[Instantaneous AGC]]
 
*[[Relative trace balancing]]
 
*[[Relative trace balancing]]
*[[Gain applications]]
 
  
 
==External links==
 
==External links==
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[[Category:Fundamentals of Signal Processing]]
 
[[Category:Fundamentals of Signal Processing]]
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[[Category:Gain applications]]

Latest revision as of 16:12, 28 August 2014

Seismic Data Analysis
Seismic-data-analysis.jpg
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


(10)

where xi 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

External links

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Rms amplitude AGC
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