Part 1: Calculating amplitude spectra and seismic wavelets

Transcription

1 Geology 554 Environmental and Exploration Geophysics II Generating synthetic seismograms The simple in-class exercise undertaken last time illustrates the relationship between wavelet properties, interval velocity and seismic resolution. Wavelet duration and shape are a function of frequency content and phase relationships. In this short exercise, you will compute the amplitude spectra of the data you are working with and also generate some seismic wavelets. Today, we explore use of computer tools that allow us to calculate spectra and estimate the resolving power of a given seismic data set. In particular you will be interested to estimate the resolving power of your project data. The following illustrations show you how to extract amplitude spectra from your data and also generate calibration curves to provide a quantitative wavelet based assessment of inherent resolution limits of your seismic data. For the purpose of this exercise it is recommended that you use the Golden or BEG data sets. Once you ve run through the exercise, it will be easy for you to repeat the analysis on your own data set(s) Part 1: Calculating amplitude spectra and seismic wavelets Calculating Amplitude Spectra 1. Open your project in Kingdom Suite 2. Bring up a line for reference Figure 1: from a different part of the Stratton field showing roll-over in the Vicksburg. Seismic 1

2 3. From the main menu bar select Tools > TracePAK> Survey Spectrum > 4. Select your 3D survey from the survey list (should be only item in the list for those of you working with the Golden data set). 5. Take the default Seismic Data Type of Amplitude (time). 6. Click on subset and specify the Line and trace range. If you have an inline up select about 10 of those and you can take the default on the trace range. 7. To begin with examine the amplitude spectrum in the upper part of your data set (0.5 to 1.5 seconds, for example). 8. OK > Apply 2

3 Figure 2: Amplitude spectrum computed for the subset defined in step Make a screen capture of your spectrum and copy to your word file. 10. There are different ways to define bandwidth. In general the term defines the range of frequencies between half-power points. In this exercise we ll just use amplitude. Roughly identify the low and high frequencies where the amplitude of the spectrum drops to ½ its maximum value. In the above spectral plot this corresponds to a range of from about 20 to 70 Hz. 11. With your Survey Spectrum window still open, select a deeper time window: e.g., 1.5 to 2.2 seconds (see below). Just select Subset again, specify the new time range; click OK to exit the subset window and then click Apply to generate the new spectrum. 12. Apply to generate the new spectrum. 13. How do the range of frequencies in this time window compare to that observed at earlier travel times. The examples shown here are from the BEG data set. Those from the Golden data set will probably look different. 3

4 Figure 3: The spectrum for deeper portions of the data has a bandwidth that extends from approximately 10 to 50 Hz. This is a little less than the bandwidth o the shallower data. 12. Cancel to exit Footnote: Depending on the data set, the spectrum for the shallow 0.5 to 1.5 second data may have smaller bandwidth. 4

5 Next let s compute a wavelet 1. Go to Tools > TracePak > Wavelets 2. This brings up the wavelet wizard Let s generate a theoretical wavelet 3. Select the Butterworth wavelet (see next page). 5

6 5. Since we re interested to get a match to the shallower data let s use the information gained from the spectrum in the 0.5 to 1.5 second range of the data. Specify the low and high pass cutoff frequencies based on the results you obtained above in the computation of the spectrum. 6

7 Use slopes on the low and high side of 18 and 36 db/octave. Note that I ve also specified a sample interval of seconds or ½ millisecond and a wavelet length of 0.1 second. 5. Click Next 6. You can take the default name if you wish or give it a name like Butterworth Part 2: Constructing a synthetic seismogram Computing a synthetic seismogram (the convolutional model in action) 1. For this exercise, it will probably be best if you shift to the BEG or Golden data sets. If you really want to stick with your project data for this demo, then you ll want to make sure you have a well with a sonic log. Density can be used to approximate sonic, but relationships between sonic and density vary significantly from area to area. In some areas density simply cannot be used to approximate sonic or velocity. 2. Go to Project > SynPak 3. Select well #15 (BEG or Golden) 4. Click OK 5. Take the default T-D chart under the T-D Chart Folder Tab 7

8 6. Go to the Velocity Tab and select the RhoB log. This is a density log. In the Gulf Coast areas the Gardner relation is often used to approximate velocity from density. The Gardner relationship is stated as ( ) 4 ρ = av 0.25 or v = ρ where a ~ 0.23 when units of a velocity in ft/s are used. 7. Under the density tab select the RhoB log again 8. Under the reference log tab select the gamma ray curve 8

9 9. For wavelet, select the Butterworth wavelet generated above. Your wavelet will vary depending on the data set you ve been using. 10. For trace to compare your synthetic to, select Extract Trace under the Trace tab 9

10 There should be only one survey in the list to extract survey from. This will probably be something like 3DMigration. Pick from traces within a certain radius (in this case 200 feet). 10. Click next to get the following display. Figure 4: Extracted trace near well

11 11. You don t need to save the trace, just click next or Finish and select it from the wavelet list. After you ve defined all parameters click Finish and you should get a display with lots of panels like that shown below (Figure 5). Figure 5: Synthetic seismograms (on right half) are located on either side of the traces selected from the 3D survey. 11

12 Make sure you save your synthetic > Go to Synthetic > Save as > give it a name Next let s compute the calibration curve 1. If you need to return to SynPak, go to Project > SynPAK 2. From the well name list select the Stratton #15 well 3. Click on the Select Existing radio button and 4. Select the Synthetic (e.g. Well15) from the drop down list. 5. The multipanel display generated earlier will appear showing various parameters (interval velocity, density, acoustic impedance (AI), etc. along with a plot of the wavelet, the synthetic seismic response and actual traces near the well. 6. Go to Tools > Tuning Analysis and 7. Select your Butterworth wavelet from the drop down window 8. Examine the wavelet in the tuning analysis window. 9. Relate its properties to those illustrated in the tuning curves (or calibration curves) plot. The peak-to-trough time is about seconds Tuning Time Tuning time is ~0.016 seconds Tuning Time 10. At what actual interval transit time does the normalized peak-to-trough amplitude rise to a peak (the point of maximum tuning)? 12

13 Exercise: Pulling it all together 1. Calculate the amplitude spectrum for your data set. For the BEG and Golden 3D data sets the calculation window should be centered roughly on the Oligocene sand intervals (that 0.5 to 1.5 second range). Make a screen capture of the amplitude spectrum you calculated. Paste it into a word file. Indicate the range of traces, lines and the time interval for which you calculated the spectrum. Note your estimate of peak frequency and bandwidth. 2. Generate a wavelet. Define the parameters used to generate the wavelet. Measure off the peak frequency (reciprocal of the dominant period) directly from the wavelet. Estimate this by measuring the interval time between the two side lobes or troughs in the wavelet. 3. Gulf Coast sonic logs suggest that Δt s for in the Oligocene sands in this area vary between 85 and 135 μsec/foot. This corresponds to velocities of between approximately 11,800 and 7,400 feet/s. In a CO 2 injection pilot northeast of Houston, CO 2 injected into one of the C-sands produced a drop in velocity from 8500fps to 6000 fps. Based on your analysis of the tuning plots generated in this exercise calculate the minimum resolvable thicknesses for the layer before and after injection. Use the wavelet derived from the Golden or BEG data sets to do this. Remember that the tuning times are two way times. If you opt to do this with your own data set (Barrel or thesis data sets), the same question applies. You can conceptualize the problem in terms of minimum resolvable thickness for gas saturated and unsaturated reservoir intervals. 4. The minimum resolvable thickness of a given layer corresponds to the minimum resolvable one-way travel time through that layer multiplied by the interval velocity. The travel time varies with the interval velocity, so in this example the same layer has two different resolvable thicknesses. The velocity drop resulting from CO 2 injection (or gas saturation) increases the resolution: i.e., it decreases the minimum resolvable thickness. Check off list for presentation 1. Make a screen capture of amplitude spectrum. Note the inline, crossline and time window used to calculate this spectrum. 2. Generate a wavelet and describe the parameters you used to generate the wavelet. 3. Make a screen capture of the synthetic panel display generated in SynPak. 4. Make a screen capture of the tuning analysis plot. The time between side lobes is seconds The dominant frequency is Hz. 5. What is the minimum resolvable thickness for the water saturated C-sand? Show your work. 6. What is the minimum resolvable thickness for the CO2 saturated C-sand? Show your work. 7. Summarize your findings in a paragraph. Present your work in a separate word document. Label the parts of your presentation using items 1 through 7. Due date is 13

14 The description below is from the help window accessed on the Kingdom tuning plot Tuning Analysis Dialog In an active SynPAK display, click on Tools>Tuning Analysis to activate the Tuning Analysis dialog. Tuning Analysis generates a plot of the selected wavelet and a tuning thickness chart for the selected wavelet. The purpose of the tuning thickness chart is to analyze the vertical resolution of the seismic data. The Tuning Analysis dialog allows you to select a wavelet from the top window. The resulting tuning thickness chart shows three curves. The thin diagonal line represents perfect resolution, where the actual time thickness is always equal to the apparent time thickness. The wavelet shown at the top is convolved with reflection coefficients spaced at intervals corresponding to the actual time thickness to produce the apparent time thickness line (the bold, sinusoidal, black diagonal line that approaches the actual time thickess as thickness increases). Comparison of the thin and bold lines displays the resolution limitations imposed by the selected wavelet. The apparent time thickness line is normalized to produce the normalized peak-trough amplitude line (third curve - a bold red curve that starts at the origin and ends with a normalized peak to trough amplitude of 1). In the graphic, the maximum of the normalized peak-trough amplitude is at about seconds (actual time thickness). The amplitude maximum that occurs at the tuning thickness is about 1.3, meaning that the tuned amplitude resulting from the reinforcement from the top and the base of the bed will be about 50% greater than a single from either the top or the base of the bed with no reinforcement or interference. 14

15 Dialog items include: Select a Wavelet: allow you to select an existing wavelet. Click on the down arrow (q) and select a wavelet from the list. Additional dialog items include: Export allows you to export the values in the tuning chart to a file. A standard Windows Save As dialog is activated. Enter the file name and click on Save to complete the step and dismiss the Save As dialog. Note: The file will contain the wavelet times and amplitude for every sample, and the tuning chart values of Actual Time Thickness (sec), Apparent Time Thickness (sec), and Normalized Peak-Trough Amplitudes. Print allows you to print the tuning chart. A standard Windows Print dialog is activated. Set the print parameters and then click on OK to print the chart. Close dismisses the dialog. 15