5:15 PM - 7:15 PM
[MIS20-P05] Acoustic blanking and heterogeneities under the seafloor in methane hydrate-bearing zone
Keywords:methane hydrate, acoustic blanking, seismic simulation, seismic reflection survey
Sub-seafloor “gas chimneys”, a characteristic geological feature of methane hydrate (MH)-bearing zones, are often observed as regions of significantly attenuated seismic reflectivity, referred to as acoustic blanking zones, in seismic reflection and sub-bottom profiler data (Matsumoto et al., 2024). This acoustic blanking phenomenon has the potential to provide valuable insights into the distribution of MH and the heterogeneity of physical properties beneath the seafloor. However, many previous studies have treated it merely as an indicator of MH and/or gas-fluid presence, overlooking its potential to offer quantitative information about MH and/or gas-fluid distribution as well as associated physical properties.
In our study, to clarify the quantitative characteristics of enigmatic sub-seafloor structures in MH-bearing areas, we attempted to simulate seismic reflection surveys using datasets generated from 16 sub-seafloor structural models. These models were designed based on relevant geoscientific characteristics of MH-bearing areas. We have simulated seismic survey cross-sections and summarized the major relationship between the acoustic blanking and the heterogeneity under sea floor.
We reported our method and results as follows: First, we created the base model with four-layered subseafloor structure based on geoscientific information from Muramoto et al. (2007) and Saeki et al. (2009). Then, we developed several additional models by incorporating heterogeneities, such as MH, gas and fluid into the base model. Reflection cross-sections were generated from wavefield datasets calculated using finite-difference method wavefield simulation (Larsen, 2000) through seismic reflection survey methods. We quantitatively evaluated the characteristics of the acoustic blanking using the amplitude reduction rate and other metrics by comparing the reflection cross-sections obtained from each sub-seafloor structural model.
Comparing the amplitude reduction among the models, we found that the amplitude reduction rate was approximately 20% for the relatively simple models with a horizontal MH layer. In contrast, it exceeded 60% and exhibited some acoustic blanking in more complex models with localized MH areas. Furthermore, in a highly complex model where small MH areas were distributed in a mosaic-like pattern, the amplitude decreased by more than 80%. To identify the factors contributing to the acoustic blanking phenomenon, we compared the mosaic MH model, which exhibited a strong acoustic blanking, with other models. As a result, it was revealed that of the observed amplitude reduction of about 80%, horizontal inhomogeneity accounted for about 40%, scattering due to complex structures accounted for about 20%, the physical properties of MH accounted for about 10%, and vertical inhomogeneity accounted for about 10%. These results suggest that the acoustic blanking phenomenon is primarily caused by horizontal inhomogeneities that lead to stacking defects.
Based on the above results, we concluded that acoustic blanking is significantly influenced by horizontal inhomogeneity and scattering caused by complex heterogeneities under the seafloor. It was also shown that utilizing the qualitative characteristics of acoustic blanking and quantitative evaluation data can be an effective approach for estimating the under the seafloor structure that causes the acoustic blanking.
We consider that advanced techniques, such as seismic migration method, may be effective in mitigating the effects of amplitude reduction caused by processing methods. On the other hand, it is expected that, in the future, the accumulation of a seismic reflection profile database will enable the quantitative evaluation of acoustic blanking on reflection profiles using artificial intelligence (AI). Additionally, methods based on the frequency response of blanking may allow for the estimation of the microscopic size of MH.
In our study, to clarify the quantitative characteristics of enigmatic sub-seafloor structures in MH-bearing areas, we attempted to simulate seismic reflection surveys using datasets generated from 16 sub-seafloor structural models. These models were designed based on relevant geoscientific characteristics of MH-bearing areas. We have simulated seismic survey cross-sections and summarized the major relationship between the acoustic blanking and the heterogeneity under sea floor.
We reported our method and results as follows: First, we created the base model with four-layered subseafloor structure based on geoscientific information from Muramoto et al. (2007) and Saeki et al. (2009). Then, we developed several additional models by incorporating heterogeneities, such as MH, gas and fluid into the base model. Reflection cross-sections were generated from wavefield datasets calculated using finite-difference method wavefield simulation (Larsen, 2000) through seismic reflection survey methods. We quantitatively evaluated the characteristics of the acoustic blanking using the amplitude reduction rate and other metrics by comparing the reflection cross-sections obtained from each sub-seafloor structural model.
Comparing the amplitude reduction among the models, we found that the amplitude reduction rate was approximately 20% for the relatively simple models with a horizontal MH layer. In contrast, it exceeded 60% and exhibited some acoustic blanking in more complex models with localized MH areas. Furthermore, in a highly complex model where small MH areas were distributed in a mosaic-like pattern, the amplitude decreased by more than 80%. To identify the factors contributing to the acoustic blanking phenomenon, we compared the mosaic MH model, which exhibited a strong acoustic blanking, with other models. As a result, it was revealed that of the observed amplitude reduction of about 80%, horizontal inhomogeneity accounted for about 40%, scattering due to complex structures accounted for about 20%, the physical properties of MH accounted for about 10%, and vertical inhomogeneity accounted for about 10%. These results suggest that the acoustic blanking phenomenon is primarily caused by horizontal inhomogeneities that lead to stacking defects.
Based on the above results, we concluded that acoustic blanking is significantly influenced by horizontal inhomogeneity and scattering caused by complex heterogeneities under the seafloor. It was also shown that utilizing the qualitative characteristics of acoustic blanking and quantitative evaluation data can be an effective approach for estimating the under the seafloor structure that causes the acoustic blanking.
We consider that advanced techniques, such as seismic migration method, may be effective in mitigating the effects of amplitude reduction caused by processing methods. On the other hand, it is expected that, in the future, the accumulation of a seismic reflection profile database will enable the quantitative evaluation of acoustic blanking on reflection profiles using artificial intelligence (AI). Additionally, methods based on the frequency response of blanking may allow for the estimation of the microscopic size of MH.