A Deterministic Evaluation of eismic Fidelity using Velocity Modeling ...

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At Vinton Dome, where salt approaches the surface to within a few hundred feet, P-wave seismic velocities can double in the salt compared to the adjacent sediment. Strong lateral velocity changes in a seismic survey require prestack depth migration (PSDM) (Yilmaz, 1987),. illustrating the coupling of geology and processing. I used PSDM to achieve improved fidelity and applied a basic pre-migration processing flow to the data that was structured to maintain a broadband characteristic to the data. Data were PSDM using a Kirchhoff migration. In any PSDM the accuracy of the image depends on the velocity model used in the migration, so I designed an innovative approach to velocity modeling and compared it to the conventional method of velocity modeling. Attribute analysis, velocity analysis, and the use of well data as quality control (QC) helped to insure achievement of the goal of this study. The goal of this study was to assess and improve the spatial fidelity of seismic imaging through a deterministic evaluation of surface seismic data with velocity modeling and attribute analysis. 1.2 Overview of the Study The study is divided into seven chapters, an introduction, a discussion of the geology of Vinton Dome, a description of the pre-migration processing, velocity modeling and migration, attribute analysis used for QC, a chapter on interpretation and conclusions and discussion. 2
1.2.1 Geology Vinton Dome is a piercement salt dome located in the upper continental margin of the Gulf of Mexico. Sediments surrounding the dome range in age from Oligocene to Miocene. Salt tectonics and deposition have influenced the structural characteristics surrounding the dome. Issues of interest are the depositional environments, salt tectonics and structure associated with the dome. Even though this is a mature basin with over one hundred years of geologic investigation, there are a number of geologic issues to be resolved. 1.2.2 Processing An examination of shot gathers from the field data revealed the need for pre-migration processing. The most profound problem was high amplitude low frequency noise. To focus the study on velocity modeling and depth migration I used a simplistic approach to pre-migration processing. My main objective in processing was to maintain as high a bandwidth as possible and eliminate the noise. Testing and filtering data provided a data set to apply deconvolution and to calculate a residual static. Data were common midpoint (CMP) sorted and I used stacking velocities calculated using the velocity field from the PSTM. I gained CMP sorted data using a spherical divergence gain control and had both the datum static and residual static applied. Static corrected data were amplitude balanced using automatic gain control (AGC). These data then had a shot deconvolution applied and were filtered using a 5-6-70-80 filter. Filtered data had the first breaks muted and amplitude balanced again using AGC. These data were the input for the PSDM. 3
Page 1: Chapter 1 Introduction 1.1 Summary
Page 5 and 6: layer in land data that in extreme
Page 7 and 8: logs as being an effective approach
Page 9 and 10: Chapter 2 Geology of Vinton Dome 2.
Page 11 and 12: The Gulf of Mexico Basin contains U
Page 13 and 14: developed from repetitive episodes
Page 15 and 16: of the upper Frio. Overlying the lo
Page 17 and 18: structures form by the differential
Page 19 and 20: Dividing the Gulf of Mexico Basin i
Page 21 and 22: 2.6 Geologic Modeling Historically,
Page 23 and 24: 2.6.1 Vinton Geology Analysis of se
Page 25 and 26: surface and a number of small subma
Page 27 and 28: established the Mississippi delta a
Page 29 and 30: Figure 2. Outline of the extents of
Page 31 and 32: Figure 6. Salt sediment contacts us
Page 33 and 34: Figure 10. Fault pattern representa
Page 35 and 36: Figure 14. Stratigraphic column of
Page 37 and 38: Top Hackberry Sand Hackberry Base N
Page 39 and 40: Figure 18. Structure map contoured
Page 41 and 42: Gyroidina scalata Outer shelf Gyroi
Page 43 and 44: Upper Frio Sands A Sand base B Sand
Page 45 and 46: Top Anahuac Middle Shelf Heterosteg
Page 47 and 48: Miocene sands Figure 27. Spontaneou
Page 49 and 50: Vinton Dome Figure 30. Late Miocene
Page 51 and 52: 3.1 Introduction Chapter 3 Processi
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3.3 Processing My primary objective
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38-42). For QC I would brute stack
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Another significant issue was to id
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Table 2 Final processing flow Input
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Number of stacked traces 12.5km 12.
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TWT seconds a 12.5km TWT seconds b
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TWT seconds 12.5km Figure 38. Brute
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Chapter 4 Velocity Modeling and Pre
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quality motivated a reassessment of
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The layer-based approach is much mo
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of seismically derived velocities.
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To test differences between seismic
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ased data, indicating higher fideli
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4.5 Conclusions and Discussion I co
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By using sonic logs the velocities
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Figure 50. Velocity cube of the sed
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Velocity in ft/sec Figure 52. Prest
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Depth in kilo-feet a. b. Figure 56.
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Figure 59. Graphic user interface s
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Truncation of sediment onto salt a
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s 1 s 1 S e C 0 1 O W 34 M M 0 C a
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Prestack time migrated velocity mod
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Salt on PSTM Prestack time migrated
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Time in seconds Bright spot 12.5km
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PSTM prestack time well-based migra
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attributes also provided me with an
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types of coherence. The first type
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comparisons, those at the same dept
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Identifying reduced fidelity in the
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Faults Channel Figure 72. PSTM cohe
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Depth slice of coherence cube from
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Compartmentalized faults 5895 feet
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Well-based PSDM - faults traced fro
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Channel Radial faults 3985 feet a C
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Well-based PSDM Prestack time migra
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Well-based PSDM Faults imaged with
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structure and they must complement
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Imaged poorly was a fault identifie
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interpretation of the high angle gr
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Inline 1391 well-based PSDM Inline
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Figure 83. Interpretation showing t
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Channel nnen a Channel b Figure 85.
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yet there are several issues regard
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7.2.3 Attributes I used attributes
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seismic data provided a more detail
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educed the noise in the field data
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PSDM data to evaluate velocity mode
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10, Society of Exploration Geophysi
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Marfurt, K. J., Kirlin, R. L., Farm
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