Some current research in P-S AVO and multicomponent OBS Jim Brown , Yan Yan , Alex Vant and Hongbo Zhang
Suppression of water-column multiples by wavefield separation and cross-correlation (with Yan Yan) Reverberations Air Water Seafloor sediment Downgoing waves
Receiver-side multiples Air Water Seafloor sediment Downgoing waves
Source-side multiples Air Water Seafloor sediment Upgoing waves
Dual-sensor method Upgoing Downgoing Hydrophone (W) Vertical geophone (Z) Upgoing Downgoing
In the dual-sensor method: Upgoing P waves register with same polarity on W and Z. Downgoing P waves register with opposite polarity on W and Z. We can extract the upgoing wavefield by summation. This can be considered as a kind of wavefield-separation technique. In fact, the wavefield-separation technique can be derived directly from the wave equation.
The wave equation as a 1st-order ordinary differential equation in stress and particle velocity (Aki and Richards, 1980): [p1 and p2 are horizontal slowness components.]
Inline Horizontal geophone Considering boundary conditions, the upgoing wavefield for every component can finally be expressed (after Amundsen and Reitan, 1995; Osen et al., 1999) as: Upgoing wavefield Hydrophone Vertical geophone Upgoing wavefield Inline Horizontal geophone Upgoing wavefield
After applying the wavefield-separation technique, the downgoing multiples in OBS data can be suppressed. However, source-side and peg-leg multiple energy remains as part of the upgoing wavefield. We have devised a method based on cross-correlation to further attenuate the source-side upgoing multiple energy.
Cross-correlation method In laterally homogeneous cases (uniform water depth), source-side multiples will have raypaths that are equivalent in length to those of corresponding receiver-side multiples on the vertical geophone records. Air Water Seafloor sediment Consolidated sediment The two types of multiples are of comparable energy but have opposite polarities on the vertical-component geophone records.
Model: Air Water Vp = 1500 m/s D = 500 m Hydrophone Geophones Seafloor sediment Consolidated sediment
Hydrophone component Modelled total wavefield Decomposed upgoing wavefield Upgoing wavefield after eliminating source-side multiple Direct Arrival 1st Reverberation 2nd Primary 1st Primary Source- and receiver-side multiple 2nd Reverberation 1st Primary 2nd Primary Source- and receiver-side multiple 2nd Primary 1st Primary
Vertical geophone component Modelled total wavefield Decomposed upgoing wavefield Upgoing wavefield after eliminating source-side multiple Direct Arrival 1st Reverberation 2nd Primary 1st Primary 2nd Reverberation Source- & receiver-side multiples 1st Primary 2nd Primary Source- & receiver-side multiple 2nd Primary 1st Primary
Horizontal geophone component Modelled total wavefield Decomposed upgoing wavefield Upgoing wavefield after eliminating source-side multiple Direct Arrival 1st Reverberation 2nd Primary 1st Primary 2nd Reverberation Source- & receiver-side multiples 1st Primary 2nd Primary Source- & receiver-side multiple 2nd Primary 1st Primary
Still ahead… Improving the cross-correlation technique for using the downgoing field to locate multiples in the upgoing field and to suppress them. Applying the entire multiple-suppression method to real OBS data, probably Mahogany. Future: Improving the robustness of the cross-correlation technique so that it applies also in the presence of dip.
A new approximation to the P-S reflection coefficient for small incidence angles (with Alexandru Vant) An approximation to the Zoeppritz equations for the converted-wave reflection coefficient, RPS Assumption: near-vertical incidence, i.e., small angles. No assumption of small changes in rock properties.
Zoeppritz expression for RPS: (Aki & Richards, 1980)
Model 1: The ‘normal’ situation, i1 = 5 1 = 2000 2 = 3500 (m/s) 1 = 800 2 = 1800 1 = 1900 2 = 2400 (kg/m3) Variation of RPS with 1 Variation of RPS with 2
Model 1: The ‘normal’ situation, i1 = 5 1 = 2000 2 = 3500 (m/s) 1 = 800 2 = 1800 1 = 1900 2 = 2400 (kg/m3) Variation of RPS with b1 Variation of RPS with r1
Model 2: Clastic over salt, i1 = 5 1 = 3600 2 = 4500 (m/s) 1 = 2400 2 = 2500 1 = 2600 2 = 2100 (kg/m3) Variation of RPS with 1 Variation of RPS with r1
Model 3: Shale over gas sand, i1 = 5 1 = 2150 2 = 1750 (m/s) 1 = 860 2 = 1500 1 = 2200 2 = 1950 (kg/m3) Variation of RPS with 1 Variation of RPS with b1
Model 3: Shale over gas sand, i1 = 10, 30 1 = 2150 2 = 1750 (m/s) 1 = 860 2 = 1500 1 = 2200 2 = 1950 (kg/m3) i1=10, variation of RPS with 1 i1=30, variation of RPS with 1
Model 3: Shale over gas sand, i1 = 10, 30 1 = 2150 2 = 1750 (m/s) 1 = 860 2 = 1500 1 = 2200 2 = 1950 (kg/m3) i1=10, variation of RPS with r1 i1=30, variation of RPS with r1
Model 1: The ‘normal’ situation, i1 = 0-90 1 = 2000 2 = 3500 (m/s) 1 = 800 2 = 1800 1 = 1900 2 = 2400 (kg/m3)
Model 2: Clastic over salt, i1 = 0-90 1 = 3600 2 = 4500 (m/s) 1 = 2400 2 = 2500 1 = 2600 2 = 2100 (kg/m3)
Model 3: Shale over gas sand, i1 = 0-90 1 = 2150 2 = 1750 (m/s) 1 = 860 2 = 1500 1 = 2200 2 = 1950 (kg/m3)
Model 1: Variation of the approximation error with i1
Model 2: Variation of the approximation error with i1
Model 3: Variation of the approximation error with i1
A review of AVO analysis (with Hongbo Zhang) Review of AVO from its early beginnings to P-S AVO A foundation for further research in P-S AVO and inversion About 55 papers referenced
Milestones in P-wave AVO Koefoed (1955) Bortfeld (1961) Aki and Richards (1980) Shuey (1985) Smith and Gidlow (1987) Hilterman (1989) Rutherford and Williams (1989) Fatti et al. (1994) Castagna and Swan (1997)
AVO classification Rutherford and Williams (1989) Castagna and Swan (1997)
More recent P & P-S AVO, anisotropy Goodway et al. (1997) Gray et al. (1999) Garotta and Granger (1987) Carcione and Tinivella, 2000) Rüger (1997, 1998) Kelly and Ford (2000a, b) Kelly et al. (2000) Larsen et al. (1999) Jin (1999) Jin and Michelena (2000) Ronen (2000)
P-S AVO Ronen et al. (2000) Jin and Michelena (2000)
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