Geology 554 -Wilson Environmental and Exploration Geophysics II Computer Lab - Seismic Resolution I

Transcription

1 Geology 554 -Wilson Environmental and Exploration Geophysics II Computer Lab - Seismic Resolution I In this lab we will examine the response of a thinning layer and use it to develop an understanding of seismic resolution and detection limits. Traditionally seismic data were used primarily to evaluate the structural configuration of an area. With improvements in seismic acquisition and processing technology, seismic applications are now very heavily focused on the resolution and interpretation of fine detail in the earth's seismic response. One may have an interest, for example, to obtain information about facies changes within a given stratigraphic unit, to locate perched aquifers, to map coal seam discontinuities, to determine fracture intensity, porosity, or fluid content within a reservoir or aquifer, etc. By the time you start this lab, you will already have the basic information needed to evaluate the resolution limits of a particular data set. You will be familiar with constructive and destructive interference effects between reflections from closely spaced layers and the effect of bandwidth on the duration of a seismic wavelet. You have also been introduced to the concept of wavelet phase, which, basically refers to the shape of the wavelet. Along with this assignment I have included an example of a seismic resolution study conducted for the exploration seismic data set. One of the target intervals in that study is the Big Injun sandstone. The data collected in that study have a bandwidth of approximately 20 to 90 hertz. We will model a layer that thins from 100 feet on the right to zero feet on the left. The interval velocity of the thinning layer is f/s. The velocities above and below the layer are greater than f/s so that reflection coefficient across the top of the layer is positive and that across the base, negative. The purpose of the model is to examine how measured interval transit times through the thinning layer and the reflection amplitude differences between the top and bottom of the layer vary as thickness changes. Compare this study to the concepts discussed in the paper by Sheriff (handed out previously). Note the location of the tuning point in Figure 3 (attached discussion). The tuning point is the point of maximum constructive interference between reflections from the top and base of the layer. In Figure 4 note the transition from a 1-to-1 relationship between actual and apparent travel time through the layer to one which has zero slope. This curve is plotted again in the lower part of the figure so that one can compare apparent travel time to changes of layer thickness. What has happened here is that once you reach the resolution limit, time differences between the negative cycle associated with the top of the Big Injun and the positive cycle associated with the base of the Big Injun decrease no further. Instead, there is a gradual decrease of the amplitude of the negative and positive cycles as they begin to interfere destructively or gradually cancel each other out. 1

2 Assignment: The following exercise is designed to give you some familiarity with the different variables affecting seismic resolution. The main purpose of this exercise is to get you to begin thinking about how the duration of the seismic wavelet affects seismic resolution. Follow the procedures listed below and answer questions A, B, and C. Procedures 1. Construct the thin-layer model defined over the range specified in the Struct model parameters window. As mentioned above, we ll input a layer that thickens from feet. The following model parameters are suggested as an example: 2

3 Note that perhaps the easiest and most accurate way to do this is to type in the X and Z coordinates at the bottom of the screen (see below). Place interface at 2100 ft Enter coordinates directly END then input base of the layer. Terminate the lower interface 1000 feet from either end of the line. END. Your model should look like that shown below. 3

4 FINISH your model and assign velocities of 15000fps to layer 1, fps to layer 2 and 15000fps to layer 3. Remember Layer 2 can be though of as the Big Injun sandstone. Your finished model should look something like that shown below. 15,000 fps 11,000 fps 15,000 fps Seismic Wavelet We've mentioned the seismic wavelet many times and understand that it represents the shape of the mechanical disturbance that is generated by the seismic source and that it propagates through the subsurface. As you might expect the wavelet's shape can change as it propagates through the earth. Energy is absorbed (converted from mechanical energy to heat energy) preferentially at higher frequencies. Wavelets tend to get longer as the higher frequencies are removed. Frequency content is decreased as the wavelet travels farther and farther beneath the surface and thus lengthening increases with travel time. In this exercise you will derive a wavelet directly from the seismic data you are interpreting in the exploration project. This will make your estimation of resolution limits more relevant to the data you are interpreting. 4

5 First, copy the file Line3.sgy from the H:\Drive to your G:\drive Next, open up the GeoGraphix Seismic Modeling module, click on utilities, and then select the Wavelet Editor from the utilities manager(see below). 5

6 When the Wavelet Editor comes up click on the Wavelet From Trace button (see below). This will bring up a file-selection menu. Navigate to your G:\Drive and select Line3.sgy Wavelet from Trace Fill in the menu as shown below. I picked trace number 150. Estimate the wavelet from a 200 to 600 millisecond window, which spans the Big Injun reflection event. A 10% taper length should be fine. 6

7 Click OK. The following screen should appear. Amplitude Spectrum The phase is zero Seismic Wavelet Now save your wavelet. Go to File - Save and give your wavelet a name, for example, ResoWav (see below). The wavelet is automatically saved to GeoGraphix s Modeling folder. 7

8 Now return to the Structural Modeler click on Synthetic, then Normal Incidence. Under wavelet select file wavelet and then select the wavelet file Whatever you named it.wav (below). Open and then generate trace. Once the ray trace and seismic trace plots appear select and maximize the trace plot window. Go into the plot setup menu and uncheck the title, plotting parameters and general comments boxes. Then open up the color palette, click on Palette Selection and select All White from the list of palettes (see below). 8

9 Your trace plot should look something like this. Note that the response of the thinning layer consists of a large amplitude negative swing followed by a large amplitude positive swing in the trace. Given the velocities we used to build this model we know that the reflection coefficient across the top will have negative value and that across the base will be of equal magnitude but opposite in sign. 9

10 Note that you can zoom in to obtain a better view of these waveform variations (below). Negative swing Positive swing To obtain detailed time marks for analysis you could set the time scales similar to that shown below. This will give you 5 millisecond tick marks and time lines every 25 milliseconds. With a detailed plot like that shown below you should be able to make the detailed measurements needed to construct the resolution plots. 10

11 Lab Activities and Report Content Questions 1. Measure and plot actual travel time through the layer versus the apparent (or measured) travel time through the layer. Remember the apparent travel time can be thought of as the time difference between the peak negative amplitude associated with the negative cycle from the top of the layer to the peak positive cycle associated with the reflection from the base of the layer. Trough Apparent Travel Time Trough-to-Peak Peak 11

12 The actual travel times through the layer can be easily measured from your Cross Section - Time file. From the Plot Structural Model top menu bar open file and select Cross Section - Time. With adjustments to plot scales and labeling your time model plot should look similar to that shown below. 12

13 2. Measure and plot the amplitude difference between the reflection from the top and base of the thinning layer. Peak-to-trough amplitude 3. Present both plots and discuss. Include the following Figures: Depth model (Figure 1) Trace plot (Figure 2) Time Plot (cross section in time) (Figure 3) Do not show all plots you generated to measure the time and amplitude differences Describe the appearance of your resolution plots (Figure 4 and 5) and discuss what they tell you about the resolution limits of the data collected over Granny Creek (Line 3 in this instance). *Due Tuesday, Dec. 3 rd,