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Amplitude correction Important methods: Correction for spherical Divergence AGC - Automatic gain control Programmable Gain functions Trace balancing.

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Presentation on theme: "Amplitude correction Important methods: Correction for spherical Divergence AGC - Automatic gain control Programmable Gain functions Trace balancing."— Presentation transcript:

1 Amplitude correction Important methods: Correction for spherical Divergence AGC - Automatic gain control Programmable Gain functions Trace balancing

2 Reflection and transmission at an interface Geometrical spreading Absorption Receiver response Measurement system Loss of amplitude due to

3 Problem for Data processing  Individual large Amplitudes dominate the processing  Reflections are difficult to recognize  Strong amplitude contrasts influence the digital filtering (especially for large travel-times)

4 Common shot-gathers just after demultiplexing Yilmaz, 1987

5 Advantage: Physical base for amplitude correction Relative Amplitude difference remains equal Disadvantage: Velocity function not known beforehand Noise sources can still remain dominant Correction for spherical divergence Homogeneous space: Layered space:

6 Raw field records from land survey Rapid decay in amplitudes at late times

7 Corrected field records from a land survey Restored amplitudes at late times by correcting for geometrical spreading (unfortunately ambient noise also has been strengthened Yilmaz, 1987

8 Normalization of amplitude for a certain time sample in a certain time window (not for the whole trace) Advantage: Amplification of Amplitudes for larger travel times Disadvantage: Shadow effect Can lead to amplification of noise All traces are more equal which is needed for further processing (Stacking: summation of different traces) No physical base for amplification AGC - Automatic Gain Control

9 Different AGC functions Numbers on top indicate gain window sizes in milliseconds Small time gates: loss of signal character by boosting small amplitude zones

10 Compensation for losses and geometrical spreading: Advantage: Partly based on physics Known function: original data can be recovered Disadvantage: Results depend strongly on used gain function Programmable Gain function

11 Calculation of decay of amplitude and determine a Gain function programmed gain curve

12 Yilmaz, 1987

13 Four different PGC functions Scale factors are indicated by the circled numbers at the times of application

14 All traces are normalized using a certain amplitude: RMS Median value Maximum Value Advantage: All traces are more equal which is needed for further processing (Stacking: summation of different traces) Disadvantage: No physical base for amplification No equalisation of losses with time Large value in a trace can dominate Trace balancing

15 Common shot-gathers just after demultiplexing Corrected for wavefront divergence After trace balancing Yilmaz, 1987

16 Field record from marine survey Raw field record +Geometric spreading correction +Trace balancing Yilmaz, 1987


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