Geophysical Applications Seismic Reflection Processing

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Gladys Owens
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1 Seismic reflection data are routinely acquired for multiple purposes such as exploration, mining, or engineering problems. The seismic data are generally acquired in shot-gathers, i.e. the data is sorted (stored) by grouping data from all receivers for the same shot. However, each receiver detects seismic energy emitted at the source from different subsurface points: From Reynolds, 1997 EPSC

2 In order to image the same depth point (referred to as Common-Depth-Point, or CDP), sources and receivers are moved across the area to be imaged. Each CDP now receives energy from different angles of incidence. The originally shot-sorted seismic data now need to be re-sorted into CDP-gathers representing the same point in the subsurface: From Reynolds, 1997 EPSC

3 The same idea seen before holds for 3-D seismic data acquisition, where a grid of source- and receiver lines is laid out across the survey area, and traces representing the same bin, or subsurface rectangle area are grouped together. From Reynolds, D seismic data acquisition allows to fully represent complex geologic structures and rock-properties. EPSC

4 In the following, several screen-shots from the demonstration in the class are shown for several steps in the seismic data processing flow. The data used are marine, multi-channel streamer data from offshore South Korea, acquired in the Ulleung Basin in the East Sea (Sea of Japan). The institute who shots the seismic data is the Korea Institute of Geosciences and Mining and Materials (KIGAM). The commercial processing software is GLOBE Claritas, from the Institute of Nuclear Science in New Zealand. EPSC

5 A typical marine seismic data processing sequence consists of the following steps: (1) Geometry definition (location of shot/receiver, CDP) (2) Quality control, definition of frequency content of data (3) Bandpass-filter (time-domain) (4) Deconvolution (shaping of source-wavelet) [ not shown] (5) Velocity analyses and normal move-out (NMO) correction (6) Stack (7) Migration EPSC

6 D R M Display of first shot-gather in seismic line. The direct arrival (D) is clearly seen in the upper left corner. Reflections (R) arrive at ~2.9 s two-way time (TWT). A multiple (M) is seen at ~ 6 s TWT. EPSC

7 Example shotgather (splitspread) from the Rimouski-field trip Platformreflection EPSC

8 Definition of frequency content of data (first shot-gather, zoom). EPSC

9 Frequency-wave-number representation of same data set used for special design-filters (directionality) EPSC

10 Application of band-pass filter to remove high-frequency noise. EPSC

11 Velocity-analyses are carried out on an initial stack or brute-stack (left) with constant-veloctiy gathers (middle) and semblance plots (right). In this example a velocity of 1480 m/s flattens perfectly the top few layers near the seafloor, but deeper arrivals are still bending upwards velocity is higher. The semblance is a representation of the stack-power of an NMO-corrected gather. It is maximum if the reflection hyperbola has been completely flattened. EPSC

12 Velocity-analyses are carried out on an initial stack or brute-stack (left) with constant-veloctiy gathers (middle) and semblance plots (right). In this example a velocity of 1520 m/s flattens perfectly a layers at 3.5 s TWT. The velocity is too high for the seafloor reflections, thus they bend downward. EPSC

13 The seismic interpreter now defines a velocity-time function through the points of maximum semblance, but avoiding spurious events. EPSC

14 With these velocities defined along the entire profile at several locations (in this example a set of 10 velocities would suffice), the seismic data will all get NMOcorrected (undo the hyperbola) and the stacked to get a single trace per CDP only. This forms then a complete section showing an acoustic image of the subsurface lithology. EPSC

$Note that compared to the stack, all diffraction hyperbolas have been removed by$

15 The last step in the processing is MIGRATION. This step ensures that the seismic energy recorded in time, gets properly imaged into depth by using the determined velocities! Note that compared to the stack, all diffraction hyperbolas have been removed by migrating the energy to the correct subsurface points from where they originated. EPSC

16 3-D seismic data offer the possibility to explore subsurface in all dimensions and define even complex structures, such as the saltdome and basin to the right. Slicing of data cube in inline crossline and time-slices. From Yilmaz, 2001 EPSC

17 3-D seismic data reveal new features, invisible by using 2D data only. A common attribute to show seismic discontinuities (faults, channels) is coherency: From Brown, 1999 EPSC

18 From Brown, 1999 EPSC

19 From Brown, 1999 EPSC

20 From Brown, 1999 EPSC

21 From Brown, 1999 EPSC

22 Horizon-slice analyses Offer 3D perspective of topography of lithologic units and their relation to structure features From Yilmaz, 2001 EPSC

23 4-D seismic data: repeated 3-D over same area. Used to demonstrate production changes in subsurface (migration of oil, steaminjection, CO2-sequestration). EPSC

24 Time-lapse seismic data from (a) preand (b) postproduction times. [MacLeod et al., 1999] From Yilmaz, 2001 EPSC

25 Time-lapse 4-D seismic data (time-slices) from 6 individual 3-D surveys taken at various times around a steam-injection hole [from Lumley, 1995]. The red, near circular feature corresponds to the spatial extent of the injected steam. From Yilmaz, 2001 EPSC

PART A: Short-answer questions (50%; each worth 2%)

PART A: Short-answer questions (50%; each worth 2%) Your answers should be brief (just a few words) and may be written on these pages if you wish. Remember to hand these pages in with your other exam pages!