Seismic AVO analysis of methane hydrate structures

MODELING APPROACH

Using the estimated interval velocities, the effects of different impedance structures on the BSR AVO response were explored in an attempt to reproduce the seismic data. Several models were constructed which were constrained to preserve the average interval velocity of each macro layer. To avoid possible tuning effects in this first, basic modeling approach, all layers were assumed to be thicker than a quarter of a wavelength. Synthetic AVO amplitude responses were then estimated for the individual models using Zoeppritz equations and compared with the amplitude responses observed in the seismic data.

Figure 4 shows the initial P- and S-wave interval velocities. The P-wave velocity was inferred directly from the seismic data, while the S-wave velocity was determined by assuming a Poisson's ratio of 0.4 which is consistent with brine saturated sediments.

orig
Figure 4. Initial velocity model of the data.

A considerably high P-wave velocity of approximately 2.5 km/s was obtained above the BSR which appears to be underlain by a lower velocity of around 1.6 km/s. The P-velocity trend for normal brine-saturated sediments is indicated by the dotted line. Being higher than this trend above the BSR and lower below it, the measured P-wave velocities might be compatible with a model of hydrate-bearing sediment overlaying gas-saturated sediments.

In a first attempt to model the observed AVO amplitudes, a thin layer of high-velocity, hydrate-bearing sediments was assumed to overlay brine-saturated sediments. As the measured P-wave interval velocity of 2.5 km/s above the BSR has to be preserved, the hydrate layer can not be smaller than a certain thickness in order to obtain realistic velocity values for this model. The modeled P- and S-wave velocities above and below the BSR are shown in Figure 5. The initial model is given by the dotted line whenever the modeled velocities differ from the initial ones. The S-wave velocity was obtained by assuming that the Poisson's ratio in the hydrate-bearing sediments is comparable with that of brine sediments.

thin-hydrate
Figure 5. Interval velocities above and below the BSR for a thin-hydrate layer overlaying brine sediment. The dotted lines represent the velocities of the initial model, while the solid lines give the velocities of this model. The arrows indicate the direction of the velocity change.

Using Zoeppritz equations, the AVO trend corresponding to the modeled velocities at the transition from hydrate to brine-saturated sediments is determined and compared with the one observed in the data (Figure 6).

thin
Figure 6. AVO Curve obtained from the thin-hydrate model (solid line) compared with the one observed in the data (crosses).

A comparison of both curves yields that the thin-hydrate model not only failed in reproducing the near offset reflection coefficients, but also the general AVO trend, having slightly increasing amplitudes with increasing offset. Assuming negligibly small density contributions, the near offset amplitudes are mainly dependent on the P-wave velocity contrast at the reflector, while the AVO trend is characterized primarily by the S-wave velocity contrast. Thus, the AVO response resulting from the thin-hydrate model implies the use of both incorrect P- and S-wave velocities at the bottom simulating reflector. Further thinning of the hydrate layer would increase the P-wave velocity in this zone due to the required preservation of the measured interval velocity, and thus the P-wave velocity contrast at the BSR. This would result in an even more pronounced difference between the observed and modeled zero offset reflection amplitudes. Consequently, a thin-hydrate layer overlaying brine-saturated sediments is not sufficient to explain the seismic data.

Based on this result, the subsequent modeling attempted to decrease the P-wave velocity contrast at the BSR in order to recreate the observed zero offset reflection amplitudes. The required decrease was performed by thickening the hydrate layer, thus yielding a thick-hydrate over brine sediment model. An evaluation of the effects of several different velocity combinations on the reflection amplitudes resulted in the model shown in Figure 7. The estimated P-wave velocity in the hydrate corresponds to the measured interval velocity, yielding a considerable thickness of the hydrate zone. The S-wave velocity was again obtained using a Poisson's ratio of 0.4 and is thus the same as in the initial model. The AVO curve based on the transition from the hydrate to the brine-saturated sediments was determined by Zoeppritz modeling and is shown in Figure 8.

The comparison of the modeled AVO responses with those observed indicates that this model could successfully reproduce the zero offset data. This suggests that the modeled P-wave velocities of 2.5 km/s in the hydrate and 1.6 km/s in the underlaying sediments might resemble the actual conditions at the BSR. However, the obtained AVO trend is still contrary to the observed one, displaying increasingly positive amplitudes with increasing offset. Hence, a change in Poisson's ratio seems to be required at the transition from the hydrate-bearing sediments above the BSR to the sediments below the BSR.

Continuously changing the possible velocities in the hydrate zone and the characteristics of the underlaying sediments resulted finally in a hydrate layer characterized by a P-wave velocity of approximately 2.5 km/s and an anomalously low S-wave velocity of around 0.5 km/s yielding a Poisson's ratio of 0.47. The hydrates appear to be underlain by sediments having a P-wave velocity of 1.6 km/s and an S-wave velocity of 1.1 km/s, yielding a Poisson's ratio of 0.1 which is consistent with free gas. The final model can be seen in Figure 9. The initial model is given by the dotted line whenever the modeled velocities differ from the initial ones. Based on the determined interval velocities, the thickness of the hydrate layer was calculated to be approximately 190 meters and the one of the gas layer to be around 250 meters. Neglecting possible tuning effects, a thin gas layer was not a good model representation, as it required a decrease in P-wave velocity with respect to the hydrate layer to preserve the measured interval velocity below the BSR. Thus, it resulted in a significant deviation of the zero offset reflection amplitudes of the model and the true seismic data.

A comparison of the synthetic AVO curve obtained for the model shown in Figure 9 with the amplitude picks obtained from a representative CMP gather is shown in Figure 10. Both the synthetic and the real data AVO amplitude responses are in good agreement for near and far offsets. Thus, a significant increase in S-wave velocity and a simultaneous decrease in P-wave velocity at the transition from hydrate-bearing sediments to sediments containing free gas is required to explain the observed seismic data.

brine-vel
Figure 7. Interval velocities above and below the BSR for a thick-hydrate over brine sediment model. The modeled velocities are correspond to the initial interval velocities.

brine
Figure 8. AVO curve obtained from the thick-hydrate model (solid line) compared with the one observed in the data (crosses).

gas-vel
Figure 9. Interval velocities for hydrate-bearing sediments overlaying gas-saturated sediments. The dotted lines represent the initial velocities. The arrows indicate where the modeled velocities had to be increased or decreased to match the seismic data.

gas
Figure 10. Synthetic AVO curve of hydrates overlaying sediments saturated with free gas (solid line) compared with an observed one (crosses).

The final velocity model for the entire section is shown in Figure 11. The deviation from the initial interval velocities is indicated by the dotted line. While the initial P-wave interval velocities corresponded to the modeled velocities in the hydrate and gas sediments, the S-wave velocities had to be modified with regard to apparently different shear properties in the hydrates and the gas compared to the brine sediments.

Based on the modeled increase of the S-wave velocity at the bottom of the of the hydrate stability zone, a large positive S-impedance contrast can be predicted for the seismic data. On the other hand, a negative P-impedance contrast can be expected at the BSR due to the decrease in P-wave velocity at the transition from hydrate to gas. In order to determine the actual effect, we performed a prestack migration impedance inversion of the seismic data.

final
Figure 11. Final modeled interval velocity model. The dotted line indicates where the initial model differs from the final model. The arrows describe the direction the velocity had to be changed in order to fit the seismic data.

Seismic AVO analysis of methane hydrate structures