1. Seismic Reflection Data Processing and Interpretation A Workshop in Cairo 28 Oct. – 9 Nov. 2006 Cairo University, Egypt Dr. Sherif Mohamed Hanafy Lecturer Title: Time Domain Filters

2. Time Domain Filters These are filters that applied on the seismic data at time domain. They includes:- Mean / median Filter Moving mean / median filters Subtract Mean (dewow) Cross-/Auto-Correlation Resampling Time Gain

3. Mean Filters For a pre-selected window, the amplitude values are summed and divided by the window length, the resulted value is addressed to the central point. The same process is repeated for the next group of data. The selected window must be an odd positive number.

4. Mean Filter

5. Median Filters For a pre-selected window, the median value is calculated, the resulted value is addressed to the central point. The same process is repeated for the next group of data. The selected window must be an odd positive number.

6. How to find the median 59185735918573 13557891355789 For a selected number of data:- Re-arrange them The central value is the median 5

7. Moving Mean/Median Filters For a pre-selected window, the amplitude values are summed and divided by the window length, the resulted value is addressed to the central point. The window is shifted one step down then the same process is repeated. The selected window must be an odd positive number.

9. Field Example Original Data Data after mean filter

10. Field Example Original Data Data after median filter

11. Subtract mean filter (dewow)

12. Time Gain Filter AGC AGC Linear Time Gain Linear Time Gain Exponential Time Gain Exponential Time Gain e power Time gain e power Time gain User Time Gain User Time Gain

13. AGC Automatic Gain Control AGC is applied on data to bring up weak signals. Automatic Gain Control AGC is applied on data to bring up weak signals. It must be used with care, a small AGC window makes strong reflections indistinguishable from weak reflections. It must be used with care, a small AGC window makes strong reflections indistinguishable from weak reflections. There are several types of AGC, such as RMS Amplitude AGC, Instantaneous AGC, …. There are several types of AGC, such as RMS Amplitude AGC, Instantaneous AGC, ….

14. RMS Amplitude AGC The input trace is subdivided into fixed time gates. The input trace is subdivided into fixed time gates. The amplitude of each sample in a gate is squared. The amplitude of each sample in a gate is squared. The mean of these values are computed and their square roots are taken. The mean of these values are computed and their square roots are taken. This is the rms amplitude over that gate. This is the rms amplitude over that gate. The ratio of the 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. The ratio of the 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. Gain functions between calculated points are linearly interpolated. Gain functions between calculated points are linearly interpolated.

15. Instantaneous AGC The mean absolute value of trace amplitude is computed within a specific time gate. The mean absolute value of trace amplitude is computed within a specific time gate. The ration of the desired rms level to this mean value is assigned as the value of the gain function. The ration of the desired rms level to this mean value is assigned as the value of the gain function. This gain function value is assigned to any point within the time gate. This gain function value is assigned to any point within the time gate. Move the gate one sample down the trace and compute a new gain function value. Move the gate one sample down the trace and compute a new gain function value. No interpolation is required. No interpolation is required.

16. AGC

17. AGC Window length = 200 samples

18. AGC Window length = 5 samples

19. Time Gain Time gain has two components; Time gain has two components; A linear components, which responsible for increasing shallow reflector amplitudes. A linear components, which responsible for increasing shallow reflector amplitudes. An exponential components, which responsible for increasing deep reflector amplitudes. An exponential components, which responsible for increasing deep reflector amplitudes. G(t) is the gain function, a is the linear coefficient, b is the exponential coefficient, and t is the time

20. Linear Component Effect

21. Exponential Component Effect