Environmental and Exploration Geophysics II

1. Environmental and Exploration Geophysics II Frequency and Bandwidth: their relationship to Seismic Resolution tom.h.wilson tom.wilson@mail.wvu.edu Department of Geology and Geography West Virginia University Morgantown, WV Tom Wilson, Department of Geology and Geography

2. Frequency content of seismic signals How can we build this wavelet? Tom Wilson, Department of Geology and Geography

3. The range of frequencies present in the wavelet controls its ability to resolve the top and bottom of a layer of given thickness. The wavelet or transient mechanical disturbance generated by the source can be thought of as a superposition or summation of sinusoids with varying frequency and amplitude. Tom Wilson, Department of Geology and Geography Hilterman, 1985

4. The examples below illustrate the effect of increasing the frequency range or bandwidth of the wavelet. O. Ilmaz, 1987 Tom Wilson, Department of Geology and Geography

5. The following simple example helps illustrate the concept of an amplitude spectrum. Below is a signal consisting of two sinusoids. Tom Wilson, Department of Geology and Geography

6. Each sinusoid is associated with a specific frequency. There are two frequency components. The 32 sample per cycle component has a frequency of 4 and the 8 samples per cycle component has a frequency of 16. The amplitude of the 32 sample/cycle component is twice that of the 8 sample/cycle component. Frequency Domain > Time Domain The Spectrum The frequency spectrum (above) of the “signal” at the top of the previous slide is an equivalent representation of the signal. Tom Wilson, Department of Geology and Geography

7. Time Domain>Frequency Domain Frequency domain Time domain O. Ilmaz, 1987 Tom Wilson, Department of Geology and Geography

8. Amplitude spectrum Phase spectrum Individual frequency components Time-domain wavelets Zero Phase Minimum Phase Hilterman, 1985 Tom Wilson, Department of Geology and Geography

9. Extracting information about wavelet frequency content from an isolated reflection event. The dominant period (c) of the response corresponds to the time from one peak to the next or from one trough to the next. The reciprocal of this dominant period is a measure of the dominant frequency (fc) of the signal or wavelet spectrum. The reciprocal of the half-width of the response-envelop (b) provides an estimate of the bandwidth (fb) of the signal spectrum. Tom Wilson, Department of Geology and Geography Hilterman, 1985

10. The dominant frequency and bandwidth measured from the time-domain representation of the signal wavelet can be used to provide a sketch of the wavelet spectrum. Just as importantly these measures can be related directly to the resolution properties of the seismic wavelet. Hilterman, 1985 Tom Wilson, Department of Geology and Geography

11. Let’s come back to this issue in a minute, but first let’s pull some ideas together to develop a basic understanding of how the seismic signal arises in terms of reflection coefficients and wavelets. Shape of up-going wave is reversed In space the leading positive cycle changes to a leading negative cycle Tom Wilson, Department of Geology and Geography Exxon in-house course notes

12. Shape of up-going wave is unchanged Exxon in-house course notes Tom Wilson, Department of Geology and Geography

13. Reflection interference positive negative Tom Wilson, Department of Geology and Geography Exxon in-house course notes

14. As the two layers move closer and closer together we get to a point where the second cycle in the wavelet reflected from the top of the layer overlaps with the arrival of the lead cycle in the wavelet reflected from the base of the layer. This occurs at two-way time equal to 1/2 the dominant period of the wavelet. Tom Wilson, Department of Geology and Geography Exxon in-house course notes

15. At this point there is maximum constructive interference between the reflections from the top and bottom of the layer. The composite reflection event (at right above) attains peak amplitude. Exxon in-house course notes Tom Wilson, Department of Geology and Geography

16. Dominant or peak frequency Zero Phase Minimum Phase The peak period of the wavelet can be determined using peak-to-trough times which can be thought of as corresponding to one half the dominant period of the wavelet. Multiply those times by two to get the dominant period. Tom Wilson, Department of Geology and Geography

17. Reflection Coefficients trough Side lobe peak Maximum constructive interference illustrated for the zero phase wavelet. The peak-to-trough time equals c/2, which also equals delay time between consecutive reflection events Tom Wilson, Department of Geology and Geography

18. Once the separation in time drops to less than half the dominant period of the wavelet destructive interference in the reflections from the top and bottom of the layer will occur. However, as the layer continues to thin, the dominant period of the composite reflection event does not drop below 1/c. However, the amplitudeof the composite continues to drop. But not the period. Tom Wilson, Department of Geology and Geography Exxon in-house course notes

19. Maximum Constructive Interference Seismic Wavelet trough Side lobe peak Two-way interval time separating reflection coefficients is c/2 The peak-to-trough time equals c/2. Tom Wilson, Department of Geology and Geography

20. These amplitude relationships are summarized below in the model seismic response of a thinning layer similar to that which you will generate in lab today. Tom Wilson, Department of Geology and Geography

21. The amplitude difference - trough-to-peak remains constant for two-way travel times much greater than half the dominant period. As the top and bottom of the layers merge closer and closer together, the lead cycle in the reflection from the base of the layer overlaps with the follow-cycle in the reflection from the top and the amplitude of the composite reflection event begins to increase. Thickness =Vt/2 Tom Wilson, Department of Geology and Geography

22. Layer thickness is simply Vt/2, where t is the two-way interval transit time. Tuning occurs at two-way times equal to one-half the dominant period (c/2). If the interval velocity of the layer in question is known, the dominant period can be converted into the tuning thickness. Tom Wilson, Department of Geology and Geography

23. Difference of arrival time between the reflections from the top and bottom of the layer decreases abruptly at about 8 milliseconds. 8 milliseconds represents the two-way travel time through the layer; it is also the time at which tuning occurs and is half the dominant period of the seismic wavelet. 8 milliseconds is c/2 and the two way time through the layer. Thus, c/4 is the one-way time through the layer. Tom Wilson, Department of Geology and Geography

24. c/4, the one-way time through the layer, equals 4 milliseconds. The interval velocity in the layer is 11,300 f/s. Hence, the thickness of the layer at this point is ~45 feet. This is the tuning thickness or minimum resolvable thickness of the layer obtainable with the given seismic wavelet. Tom Wilson, Department of Geology and Geography

25. Back to this concept of an amplidude spectrum What is the amplitude spectrum of wavelet #5? Broader spectra produce sharper, shorter duration wavelets Ilmaz, 1987 Tom Wilson, Department of Geology and Geography

26. Spectral bandwidth, wavelet duration in the time domain and resolution. C is only one parameter that affects resolution. b is also an important parameter. Greatest Bandwidth Smallest Bandwidth Tom Wilson, Department of Geology and Geography Hilterman, 1985

27. The Convolutional Model Hilterman, 1985 Physical nature of the seismic response Tom Wilson, Department of Geology and Geography

28. The output is a superposition of reflections from all acoustic interfaces Tom Wilson, Department of Geology and Geography Exxon in-house course notes

29. Tom Wilson, Department of Geology and Geography Exxon in-house course notes

30. Tom Wilson, Department of Geology and Geography

31. One additional topic to consider in general is wavelet deconvolution and how wavelet shape can affect geologic interpretations …. Consider the following structural model Subsurface structure - North Sea Tom Wilson, Department of Geology and Geography Neidel, 1991

32. Below is the synthetic seismic response computed for the North Sea model. Potential hydrocarbon trap? Tom Wilson, Department of Geology and Geography Neidel, 1991

33. Consider the effect of wavelet shape on the geologic interpretation of seismic response. In the case shown below, the primary reflection from the base of the Jurassic shale crosses a side-lobe in the wavelet reflected from the overlying basal Cretaceous interval. Tom Wilson, Department of Geology and Geography Neidel, 1991

34. wavelet compression Deconvolution is a filter operation which compresses and simplifies the shape of the seismic wavelet. Deconvolution improves seismic resolution and simplifies interpretation. Tom Wilson, Department of Geology and Geography

35. Neidel, 1991 North Sea Seismic display after deconvolution. The geometrical interrelationships between reflectors are clearly portrayed. Tom Wilson, Department of Geology and Geography

36. Consider the following problem - You are given the seismic wavelet shown below. Using the estimation procedure discussed in class today measure the appropriate feature on the above seismic wavelet and answer the following questions: What is the minimum resolvable thickness of a layer having an interval velocity of 10,000fps? Show work on your handout What is the phase of the wavelet? Why do you say that? Tom Wilson, Department of Geology and Geography

37. footnote: Phase and resolution The minimum phase wavelet has its energy concentrated toward the front end of the wavelet. The amplitude of the disturbance decays exponentially. This wavelet is a causal wavelet and the location of the reflection coefficient is placed at the wavelet onset, which can be difficult for the interpreter to pick. Tom Wilson, Department of Geology and Geography

38. The zero phase wavelet is symmetrical. This wavelet is centered over the reflection coefficient. The zero phase wavelet is produced through data processing and is not generated naturally. It is non causal - half of the wavelet arrives before the reflector appears in time. It is easy for an interpreter to pick reflection times using the zero phase wavelet since highest amplitude occurs at the reflection boundary. Tom Wilson, Department of Geology and Geography

39. The zero-phase wavelet is also considered to have higher resolving power. It is generally more compact than the equivalent minimum phase wavelet and is, overall, easier to interpret. The exploration data is in a zero phase format. Tom Wilson, Department of Geology and Geography Hilterman, 1985

40. The default wavelet in Struct is the Ricker wavelet. The Ricker wavelet is zero phase. Tom Wilson, Department of Geology and Geography Hilterman, 1985

41. Questions? If you haven’t already, finish reading chapter 4. Tom Wilson, Department of Geology and Geography