1. ReferencesInfo Elog Strata Geoview Fundamentals Open Seismic Wavelet Estimation Horizon Picks Low Frequency Model Inversion Near Offset Far Offset Open Seismic Wavelet Estimation Low Frequency Model Inversion Save Project Hampson-Russell The fundamental model on which bandlimited inversion is based is the 1D convolution model where the multiples are assumed to be negligible: where:- the zero offset reflectivity of the earth expressed as a time series - the seismic wavelet, assumed to be constant - additive measurement noise The reflectivity is related to impedance of a series of layers in the earth by the equation above, where ‘I’ denotes impedance. This can be any appropriate parameter, corresponding to the stacked amplitudes to be inverted; for instance acoustic or elastic impedance corresponding to respectively near and far offset stacks. The bandlimited inversion scheme starts with building a low frequency model by picking the major reflectors on the seismic data and providing detailed velocity information from well logs. The well log values are extrapolated to produce a velocity value for each sample for every trace to be inverted. The picked reflectors are guides to extrapolation. By now, the low-frequency model represents a full bandwidth, but is supposed to represent only the lower frequnecies absent on the seismic data. The model is smoothed in order to construct this band. The seismic traces are inverted using the equation above, which is a recursive inversion of the reflectivity. This gives a middle frequency band, approximately 10-60 Hz, of the acoustic impedance. The combination of the frequency band of the low frequency model and the recursive inversion should in turn be comparable to well data. Fundamentals

2. ReferencesInfo Elog Strata Geoview Fundamentals Open Seismic Wavelet Estimation Horizon Picks Low Frequency Model Inversion Near Offset Far Offset Open Seismic Wavelet Estimation Low Frequency Model Inversion Save Project Hampson-Russell Open Seismic Start the Strata module by clicking the ‘Strata’ button in the Geoview window. We want to invert acoustic impedance from the near offset stack, and later invert elastic impedance from the far offset stack. In the Strata main window, select the ‘Open Seismic from Project’ option from the ‘Seismic’ menu. Select the ‘near.vol’ in the following window and assign Well_1 to CDP 37 in the ‘Well to Seismic Map Menu’.

3. ReferencesInfo Elog Strata Geoview Fundamentals Open Seismic Wavelet Estimation Horizon Picks Low Frequency Model Inversion Near Offset Far Offset Open Seismic Wavelet Estimation Low Frequency Model Inversion Save Project Hampson-Russell Open Seismic Many useful tools are available under the ‘Process’ menu. The amplitude spectrum can be obtained in Strata by selecting the option as shown. As previously mentioned, typically lower frequencies of the amplitude spectrum of the seismic traces are nearly absent. This band will also be absent on the impedance section inverted from the seismic traces. By combining the low-frequency trend of the well logs with the inverted traces will produce reliable impedance information. Amplitude spectrum of near offset stack Nearly absent low frequency band Low frequency trend from well log

4. ReferencesInfo Elog Strata Geoview Fundamentals Open Seismic Wavelet Estimation Horizon Picks Low Frequency Model Inversion Near Offset Far Offset Open Seismic Wavelet Estimation Low Frequency Model Inversion Save Project Hampson-Russell Wavelet Estimation Because the logs were correlated after the last wavelet extraction in the Elog module, we need to extract a new wavelet for the inversion scheme. This can be done in Strata from the ‘Wavelet’ menu.Elog Make sure that ‘density’ and the P- wave correlated to the near offset stack are set to be active, as show. This can be obtained from wavelet extraction menu in Strata. Continue on with the wavelet estimation as before. When done: it is wise to rename the wavelet intended to invert the near offset stack so something more intuitive than the default name. Select ‘Display Wavelet Tool’ from the Wavelet menu in Strata. Mark the wavelet to rename and choose ‘Rename Wavelet’ as shown.

5. ReferencesInfo Elog Strata Geoview Fundamentals Open Seismic Wavelet Estimation Horizon Picks Low Frequency Model Inversion Near Offset Far Offset Open Seismic Wavelet Estimation Low Frequency Model Inversion Save Project Hampson-Russell Horizons Picks Select ‘Pick Horizons’ as shown. Horizon 5 corresponds to ‘Top5’ on the well. Also pick the other four horizons. This might take a while. Tip: Start the correlation mode in Elog to find where the formation tops are located on synthetic trace and the seismic. Elog

6. ReferencesInfo Elog Strata Geoview Fundamentals Open Seismic Wavelet Estimation Horizon Picks Low Frequency Model Inversion Near Offset Far Offset Open Seismic Wavelet Estimation Low Frequency Model Inversion Save Project Hampson-Russell Horizons Picks After the horizons are picked, the section should look something like below. Optionally, the horizons can also be loaded from the file ‘horizons’ by using the load option from the Horizon menu

7. ReferencesInfo Elog Strata Geoview Fundamentals Open Seismic Wavelet Estimation Horizon Picks Low Frequency Model Inversion Near Offset Far Offset Open Seismic Wavelet Estimation Low Frequency Model Inversion Save Project Hampson-Russell Low Frequency Model Select ‘Build/Rebuild a Model’ from the ‘Model’ menu in Strata. Toggle the ‘Create a blank new model’ and ‘Typical setup of P- Impedance…’ options. Also type in an appropriate name for the model. Click ‘Next’ when done. Select Well_1 in the next window and click ‘Next’.

8. ReferencesInfo Elog Strata Geoview Fundamentals Open Seismic Wavelet Estimation Horizon Picks Low Frequency Model Inversion Near Offset Far Offset Open Seismic Wavelet Estimation Low Frequency Model Inversion Save Project Hampson-Russell Low Frequency Model Select the logs to use in the next window. Make sure to use the P-wave correlated to the near offset stack and the ‘density’. Also make sure to select the ‘ ’ option for ‘P- impedance’. Probably due to a bug in the code, the other options yields no result. Select all horizons except for the one corresponding ‘Top4’ on the well log (Horizon 4). Since ‘Horizon 4’ represents the reservoir zone, excluding this zone yields a low frequency model representing the stratigraphic column except for the reservoir sand. Any signs of the reservoir sand on the final result can therefore with certainty be tracked back to the seismic data alone, and not the low frequency model.

9. ReferencesInfo Elog Strata Geoview Fundamentals Open Seismic Wavelet Estimation Horizon Picks Low Frequency Model Inversion Near Offset Far Offset Open Seismic Wavelet Estimation Low Frequency Model Inversion Save Project Hampson-Russell Low Frequency Model The model will appear in the Strata window. Change the display options as shown. Also insert the P-impedance log instead of the synthetic trace.

10. ReferencesInfo Elog Strata Geoview Fundamentals Open Seismic Wavelet Estimation Horizon Picks Low Frequency Model Inversion Near Offset Far Offset Open Seismic Wavelet Estimation Low Frequency Model Inversion Save Project Hampson-Russell Low Frequency Model The model needs to be smoothed. Select the ‘Set Model Parameters’ option and go to the ‘Advanced’ folder and apply a running aveage. Instead of using 42 samples, 27 is also a good magical number.

11. ReferencesInfo Elog Strata Geoview Fundamentals Open Seismic Wavelet Estimation Horizon Picks Low Frequency Model Inversion Near Offset Far Offset Open Seismic Wavelet Estimation Low Frequency Model Inversion Save Project Hampson-Russell Inversion – Near Offset Select the ‘Bandlimited’ option from the ‘Invert’ menu and select the ‘near’ volume as seismic volume. Also select the P-impedance volume from the low- frequency model as impedance volume. Set the constant high-cut freqnecy to be 7 Hz. This is the frequnecy which the model and the inverted traces are merged. The processing sample rate should equal the sample rate of the seismic data, which in this case is 4 ms. Other values will yield re-sampling of the data volumes.

12. ReferencesInfo Elog Strata Geoview Fundamentals Open Seismic Wavelet Estimation Horizon Picks Low Frequency Model Inversion Near Offset Far Offset Open Seismic Wavelet Estimation Low Frequency Model Inversion Save Project Hampson-Russell Inversion – Near Offset On the inverted section, the reservoir zone shows up better, but the top reservoir is still not easily determined. This was also expected from examining the acoustic impedance log. The elastic impedance log at 36  indicates that inverting the far offset stack for this parameter will yield a better result. Reservoir sand?

13. ReferencesInfo Elog Strata Geoview Fundamentals Open Seismic Wavelet Estimation Horizon Picks Low Frequency Model Inversion Near Offset Far Offset Open Seismic Wavelet Estimation Low Frequency Model Inversion Save Project Hampson-Russell Save Project It is always a good habit to store the project and have a cop of coffee. Re-open the window where the AI model was created. If this window isn’t already opened, select the ‘Data Menu’ from the ‘Project’ menu. Select the volumes as shown and click the ‘Show’ button.

14. ReferencesInfo Elog Strata Geoview Fundamentals Open Seismic Wavelet Estimation Horizon Picks Low Frequency Model Inversion Near Offset Far Offset Open Seismic Wavelet Estimation Low Frequency Model Inversion Save Project Hampson-Russell Open Seismic Depending on which window you are working from, the AI model might still be opened. If so, close the AI model by selecting the close-option from the `Model` menu. Open the far offset stack volume, which was loaded in the Elog section.Elog

15. ReferencesInfo Elog Strata Geoview Fundamentals Open Seismic Wavelet Estimation Horizon Picks Low Frequency Model Inversion Near Offset Far Offset Open Seismic Wavelet Estimation Low Frequency Model Inversion Save Project Hampson-Russell Wavelet Estimation We need to extract a new wavelet for the far offset stack. Select the ‘Use Well’ option from the Wavelet menu as before. Remember to make sure that the P-wave correlated to the far offset stack is marked as active, and estimate a new wavelet as for the near offset stack. Rename the wavelet intended to invert the far offset stack to something more intuitive than the default name by accessing the wavelet tool.

16. ReferencesInfo Elog Strata Geoview Fundamentals Open Seismic Wavelet Estimation Horizon Picks Low Frequency Model Inversion Near Offset Far Offset Open Seismic Wavelet Estimation Low Frequency Model Inversion Save Project Hampson-Russell Low Frequency Model Create a low-frequency model for elastic impedance as shown. The process is similar to creation of AI model. Add the well as before. Select the P-wave correlated to far offset stack and the ‘Computed’ option as shown. Click the `Angle/Offset specification’ button as shown to specify input parameters for computation of elastic impedance.

17. ReferencesInfo Elog Strata Geoview Fundamentals Open Seismic Wavelet Estimation Horizon Picks Low Frequency Model Inversion Near Offset Far Offset Open Seismic Wavelet Estimation Low Frequency Model Inversion Save Project Hampson-Russell Low Frequency Model Enter the parameters as shown. The average angle of incidence and transmission is assumed to be 36  for the far offset stack. A factor of 0.5545 is applied for the the constant Vs/Vp term. This average value is derived from the reservoir zone. Select the horizons as shown below. Also this time, deselect horizon 4. Finish the model by clicking ‘Ok’.

18. ReferencesInfo Elog Strata Geoview Fundamentals Open Seismic Wavelet Estimation Horizon Picks Low Frequency Model Inversion Near Offset Far Offset Open Seismic Wavelet Estimation Low Frequency Model Inversion Save Project Hampson-Russell Low Frequency Model Go to ’Model’ once more and select ’Set Model Parameters’. Click the ’Advanced’ tab and smooth the model with a moving average of 27 samples. Click ’Ok’ when done.

19. ReferencesInfo Elog Strata Geoview Fundamentals Open Seismic Wavelet Estimation Horizon Picks Low Frequency Model Inversion Near Offset Far Offset Open Seismic Wavelet Estimation Low Frequency Model Inversion Save Project Hampson-Russell Inversion – Far Offset We should by now be ready to invert the far offset stack for elastic impedance at Select ’Bandlimited’ from the ’Invert’ menu. In the appearing window, select ’far’ as seismic volume and the elastic impedance model you just created as impedance volume. The constant high cut frequency is also here set to 7 Hz.

20. ReferencesInfo Elog Strata Geoview Fundamentals Open Seismic Wavelet Estimation Horizon Picks Low Frequency Model Inversion Near Offset Far Offset Open Seismic Wavelet Estimation Low Frequency Model Inversion Save Project Hampson-Russell Inversion – Far Offset The inverted volume shows up in a new window. Post the EI log on the section. As can be seen, the top reservoir zone got to some degree better imaged, but the termination is absent. If the resulting inversion is showing only green colors, that is, the color key is out of scale, click the ‘eye’ button and the ‘Seismic’ tab. Click the ‘data range’ button to adjust the scale. A default scan should be appropriate.

21. ReferencesInfo Elog Strata Geoview Fundamentals Open Seismic Wavelet Estimation Horizon Picks Low Frequency Model Inversion Near Offset Far Offset Open Seismic Wavelet Estimation Low Frequency Model Inversion Save Project Hampson-Russell References Connolly, P., 1999, Elastic impedance: The Leading Edge, 18, no. 4, 438-452. Duffaut, K., Alsos T., Landrø, M., Rognø H., Al-Najjar. N. F., 2000, Shear-wave elastic impedance: The Leading Edge, 19, no. 11, 1222-1229. Hampson-Russell 4.1.5 Reference Manual