2-D Impedance Estimation

The 2-D impedance inversion is performed on the preprocessed, migrated, and amplitude-calibrated gathers and reflection angles described in Chapter 2. Since the angles of incidence for the Blake Outer Ridge data are restricted to values of less than $35^{^{\circ}}$, I do not include the density contrast $\rm D$ (see equation ) in the inversion. Therefore, the linearized Zoeppritz equations are only solved for relative changes in P- and S-impedance using the method described in the previous section (3.2.1).

The resulting P-impedance contrast section is displayed in Figure . It clearly shows that the seafloor and the BSR have P-impedance contrasts of opposite polarity, but approximately the same magnitude. The BSR contrast is strongest between 44 km and 50 km lateral distance, and strongly discontinuous between 36 km and 44 km. The BSR's negative P-impedance contrast suggests a velocity decrease at the transition from hydrate-bearing sediment to the sediment underneath. This observation matches the velocity information as described in Chapter 2, section 2.3. The flat reflector below the BSR gives a strong impedance contrast of the same polarity as the seafloor.

The S-impedance contrast section is displayed in Figure . The seafloor and the BSR appear to be characterized by S-impedance contrasts of the same, positive polarity. This indicates an S-impedance contrast across the BSR of opposite polarity than the P-impedance contrast. Underneath the BSR, the S-impedance contrast enhances the structure dipping against the BSR. The flat reflector, which was strongly visible in the P-impedance contrast map, has nearly disappeared in the S-impedance contrast map, suggesting that this transition zone has a very small S-impedance contrast.

 

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Figure 3 P-impedance contrast section.

 

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Figure 4 S-impedance contrast section.

The windowed P- and S-impedance contrast sections are shown in Figure . As described before, the figure clearly displays the differences in the polarity of the P- and S-impedance contrasts across the BSR. While the P-impedance contrast appears to be negative, the data show a positive S-impedance contrast. Furthermore, the S-impedance contrast section appears to enhance structural interference across the BSR from underlying features.

 

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Figure 5 Windowed P-impedance contrast (left panel) and S-impedance contrast (right panel).

A simple multiplication of the P- and S-impedance contrast sections produces a P*S anomaly map shown in Figure . P- and S-impedance contrasts of the same sign are plotted as black, while contrasts of opposite sign are plotted white. The anomaly map shows the same contrasts at the seafloor. Since the transition from water to sediments is characterized by an increase in P-wave velocity, S-wave velocity and density, both a positive P-wave impedance contrast and a positive S-wave impedance contrast can be expected. In a small section above the BSR, there is a ``quiet'' zone where no diffractions or reflections are visible that might be due either to the presence of disseminated methane hydrate in the sediments Lee et al. (1994) or to the naturally low reflectance of a uniform sedimentary section at the Blake Outer Ridge Holbrook et al. (1996). The BSR reflection displays mostly opposite P and S impedance polarity. The velocity analysis in Chapter 2 has shown that the BSR is characterized by a significant decrease in velocity, and thus a negative P-impedance contrast. P- and S-contrasts of the opposite polarity would indicate a positive S-impedance contrast at the base of the hydrate zone. The S-impedance contrast is the sum of the relative changes in S-wave velocity across an interface and the corresponding density changes. Assuming small changes in density across the BSR, the positive S-impedance contrast would be the result of a positive S-wave velocity contrast, which is clearly opposite to the present negative P-wave velocity contrast. A hydrate-gas transition zone would explain such P- and S-impedances, whereas a hydrate-brine transition zone could be expected to have both negative P- and S-impedances (see Figure ). This seems to indicate the presence of free gas beneath the BSR. Moreover, the strong reflector beneath the BSR, which is characterized by a strong, positive P-impedance contrast, seems to have nearly no S-impedance contrast. This small change in shear impedance suggests the transition from gas- to brine-saturated sediment.

 

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Figure 6 P*S impedance contrast map.

The structure beneath the BSR as seen in Figure and can interfere with the actual BSR amplitudes and cause strong distortions in both waveform and amplitude, and invalidate the inverted P-and S-impedance contrasts. Furthermore, possible uncorrected residual moveouts in the data can add to errors since the 2-D inversion depends upon perfectly flat reflectors. Therefore, I evaluate the BSR amplitude responses locally, both to obtain an insight into the effect of the structure under the BSR, and to minimize possible residual moveout errors.