Transcript Strata
Fundamentals Hampson-Russell Geoview Elog Strata Fundamentals Near Offset Open Seismic Wavelet Estimation Horizon Picks Low Frequency Model The fundamental model on which bandlimited inversion is based is the 1D convolution model where the multiples are assumed to be negligible: t i r j wi j 1 ni j where: - the zero offset reflectivity of the earth expressed as a time series - the seismic wavelet, assumed to be constant - additive measurement noise rj I j I j 1 I j I j 1 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, Save Project corresponding to the stacked amplitudes to be inverted; for instance acoustic or elastic impedance corresponding to respectively near and far offset stacks. Far Offset The bandlimited inversion scheme starts with building a low frequency model Open Seismic by picking the major reflectors on the seismic data and providing detailed velocity Wavelet Estimation information from well logs. The well log values are extrapolated to produce a velocity Low Frequency value for each sample for every trace to be inverted. The picked reflectors are guides to Model 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 Inversion is smoothed in order to construct this band. Inversion 1 r j I j I j 1 1 r j 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. Info References Open Seismic Hampson-Russell Geoview Elog Strata Start the Strata module by clicking the ‘Strata’ button in the Geoview window. Fundamentals We want to invert acoustic impedance Open Seismic from the near offset stack, and later Wavelet Estimation invert elastic impedance from the far offset stack. Horizon Picks Near Offset Low Frequency Model Inversion Save Project In the Strata main window, select the ‘Open Seismic from Project’ option from the ‘Seismic’ menu. Far Offset Open Seismic Wavelet Estimation Low Frequency Model Select the ‘near.vol’ in the following window and assign Well_1 to CDP 37 in the ‘Well to Seismic Map Menu’. Inversion Info References Open Seismic Hampson-Russell Geoview Elog Strata Fundamentals Near Offset Open Seismic Wavelet Estimation Horizon Picks Low Frequency Model Inversion Save Project Far Offset Open Seismic Wavelet Estimation Low Frequency Model Inversion 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. Amplitude spectrum of near offset stack Nearly absent low frequency band By combining the low-frequency trend of the well logs with the inverted traces will produce reliable impedance information. Low frequency trend from well log Info References Wavelet Estimation Hampson-Russell Geoview Elog Strata Fundamentals Near Offset Open Seismic Wavelet Estimation Horizon Picks Low Frequency Model Inversion Save Project Far Offset Open Seismic Wavelet Estimation Low Frequency Model Inversion 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. Make sure that ‘density’ and the Pwave 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. Info References Horizons Picks Hampson-Russell Geoview Elog Strata Fundamentals Near Offset Open Seismic Wavelet Estimation 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 Horizon Picks Low Frequency Model Inversion Save Project Far Offset Open Seismic Wavelet Estimation Low Frequency Model Inversion Info References Horizons Picks Hampson-Russell Geoview Elog Strata 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 Fundamentals Near Offset Open Seismic Wavelet Estimation Horizon Picks Low Frequency Model Inversion Save Project Far Offset Open Seismic Wavelet Estimation Low Frequency Model Inversion Info References Low Frequency Model Hampson-Russell Geoview Elog Strata Select ‘Build/Rebuild a Model’ from the ‘Model’ menu in Strata. Fundamentals Toggle the ‘Create a blank new Open Seismic model’ and ‘Typical setup of PWavelet Estimation Impedance…’ options. Also type in an appropriate name for Horizon Picks the model. Click ‘Next’ when Low Frequency done. Model Near Offset Inversion Save Project Far Offset Open Seismic Wavelet Estimation Low Frequency Model Select Well_1 in the next window and click ‘Next’. Inversion Info References Low Frequency Model Hampson-Russell Geoview Elog Strata Fundamentals 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’. Near Offset Open Seismic Wavelet Estimation Horizon Picks Low Frequency Model Also make sure to select the ‘<computed>’ option for ‘Pimpedance’. Probably due to a bug in the code, the other options yields no result. Inversion Save Project Far Offset Open Seismic Wavelet Estimation Low Frequency Model Inversion 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. Info References Low Frequency Model Hampson-Russell Geoview Elog Strata 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. Fundamentals Near Offset Open Seismic Wavelet Estimation Horizon Picks Low Frequency Model Inversion Save Project Far Offset Open Seismic Wavelet Estimation Low Frequency Model Inversion Info References Low Frequency Model Hampson-Russell Geoview Elog Strata Fundamentals Near Offset Open Seismic Wavelet Estimation 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. Horizon Picks Low Frequency Model Inversion Save Project Far Offset Open Seismic Wavelet Estimation Low Frequency Model Inversion Info References Inversion – Near Offset Hampson-Russell Geoview Elog Strata Fundamentals Near Offset Open Seismic Wavelet Estimation Horizon Picks Low Frequency Model Select the ‘Bandlimited’ option from the ‘Invert’ menu and select the ‘near’ volume as seismic volume. Also select the P-impedance volume from the lowfrequency model as impedance volume. Inversion Set the constant high-cut freqnecy to be 7 Hz. This is the Far Offset frequnecy which the model Open Seismic and the inverted traces are Wavelet Estimation merged. Save Project Low Frequency Model Inversion 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. Info References Inversion – Near Offset Hampson-Russell Geoview Elog Strata 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. Fundamentals Near Offset Reservoir sand? Open Seismic Wavelet Estimation Horizon Picks Low Frequency Model Inversion Save Project Far Offset Open Seismic Wavelet Estimation Low Frequency Model Inversion Info References Save Project Hampson-Russell Geoview Elog Strata Fundamentals It is always a good habit to store the project and have a cop of coffee. Near Offset Open Seismic Wavelet Estimation Horizon Picks Low Frequency Model Inversion Save Project 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. Far Offset Open Seismic Wavelet Estimation Select the volumes as shown and click the ‘Show’ button. Low Frequency Model Inversion Info References Open Seismic Hampson-Russell Geoview Elog Strata Fundamentals Near Offset Open Seismic Wavelet Estimation 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. Horizon Picks Low Frequency Model Inversion Save Project Far Offset Open Seismic Wavelet Estimation Open the far offset stack volume, which was loaded in the Elog section. Low Frequency Model Inversion Info References Wavelet Estimation Hampson-Russell Geoview Elog Strata Fundamentals Near Offset Open Seismic Wavelet Estimation Horizon Picks Low Frequency Model 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. Inversion Save Project Far Offset Open Seismic Wavelet Estimation Low Frequency Model Inversion Rename the wavelet intended to invert the far offset stack to something more intuitive than the default name by accessing the wavelet tool. Info References Low Frequency Model Hampson-Russell Geoview Elog Strata Create a low-frequency model for elastic impedance as shown. The process is similar to creation of AI model. Fundamentals Add the well as before. Select the P-wave correlated to far Open Seismic offset stack and the Wavelet Estimation ‘Computed’ option as shown. Near Offset Horizon Picks Low Frequency Model Inversion Save Project Far Offset Open Seismic Wavelet Estimation Click the `Angle/Offset specification’ button as shown to specify input parameters for computation of elastic impedance. Low Frequency Model Inversion Info References Low Frequency Model Hampson-Russell Geoview Elog Strata Fundamentals Near Offset Open Seismic Wavelet Estimation Horizon Picks Low Frequency Model Inversion Save Project Far Offset 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’. Open Seismic Wavelet Estimation Low Frequency Model Inversion Info References Low Frequency Model Hampson-Russell Geoview Elog Strata Fundamentals Near Offset Open Seismic Wavelet Estimation Horizon Picks 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. Inversion Save Project Far Offset Open Seismic Wavelet Estimation Low Frequency Model Inversion Info References Inversion – Far Offset Hampson-Russell Geoview Elog Strata Fundamentals Near Offset Open Seismic Wavelet Estimation Horizon Picks Low Frequency Model 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. Inversion Save Project Far Offset The constant high cut frequency is also here set to 7 Hz. Open Seismic Wavelet Estimation Low Frequency Model Inversion Info References Inversion – Far Offset Hampson-Russell Geoview Elog Strata Fundamentals Near Offset Open Seismic Wavelet Estimation 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. Horizon Picks Low Frequency Model Inversion Save Project Far Offset Open Seismic Wavelet Estimation Low Frequency Model Inversion Info References References Hampson-Russell Geoview Elog Strata Fundamentals Near Offset Open Seismic Wavelet Estimation Horizon Picks 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 Low Frequency Model Inversion Save Project Far Offset Open Seismic Wavelet Estimation Low Frequency Model Inversion Info References