5:15 PM - 6:45 PM
[MIS22-P09] Seismological effective elements for causing acoustic blanking structure related with methane hydrate on seismic surveys
Keywords:methane hydrate, acoustic blanking structure, seismic simulation
Methane hydrate (i.e., MH) has been expected to be our new energy source to increase the self-sufficiency rate in a resource-poor Japan. Previous seismic studies showed several important characteristics like as both “bottom simulated reflection” (i.e., BSR) and “acoustic blanking zones” (here we call it ABZ) on the seismic reflection profiles as a subsea structure including MH. We know that BSR can be caused by a strong seismic reflection at the material boundary between a solid MH and gas-fluid zones even though ABZ has not been clearly explained its reason yet. Matsumoto et al. (2009) suggested it may be related with gas chimney in a shallow depth around the offshore of Niigata, Japan.
We have been investigating about the seismological effective elements for the particular subsea structure including such a methane hydrate zone by using a simulated seismic waveform with some kinds of models (e.g., Tsuruga et al.,2008; Maeda et al., 2022; Asakura et al., 2022). Maeda et al. (2022) has observed the characteristics of seismic wave records calculated with some simple structure models and suggested an acoustic blanking zone may be caused by the following three major elements: (1) high-velocity anomalies due to methane hydrate, (2) low-velocity and high attenuation anomalies due to free gas and/or (3) some branched faults.
In this study, we therefore evaluated the seismological effective elements for causing ABZ on the seismic reflection CMP profiles by using simulated seismic records. We calculated seismic waveforms by FDM wavefield calculations (Larsen, 2000) with several structural models assuming the above three possible elements: methane hydrate layers, free gas layers and (branching) faults. We also referred to much more information of subsea data such as borehole samples, acoustic reflection profiles in a very shallow depth by a sub-bottom-profiler and so on (e.g., Asakura et al., 2021).
We assumed three major structural models as follows: (Model-A) a reference model which consisted of two sedimentary layers (i.e., low-velocity sedimentary layers below the subsea) laid above an acoustic basement rock, (Model-B) a model with a MH layer above the gas-fluid zone in the 1st layer, and (Model-C) a model assuming with some branching faults running through all layers on Model-B. We focused two target P-P reflection phases on the stacking profiles for each model: one (i.e., 1st reflection phase) was the reflected waves at the 1st layer boundary between the unconsolidated sediment layer just below the subsea and a sedimentary rock layer, and the other (i.e., 2nd reflection phase) was the reflected waves at the 2nd layer boundary between the sedimentary rock layer and an acoustic basement rock layer.
It was found that the amplitude of both target reflection phases clearly decreased due to the existence of methane hydrate zone, gas-fluid zone, and/or some faults. Especially the amplitude of both target reflection phases extremely decreased down to less than 29 % on Model-C. It suggested that the existence of faults may play an important role as one of the seismological effective elements in ABZ, although in our simulation we used the simple physical properties for the structural models.
In the future study, we will discuss any other the seismological effects due to the small-scale heterogeneities and its bathymetric feature as well as the effect of the analyzing method for the interior of acoustic blanking zones using any other seismic phases such as refracted waves.
We have been investigating about the seismological effective elements for the particular subsea structure including such a methane hydrate zone by using a simulated seismic waveform with some kinds of models (e.g., Tsuruga et al.,2008; Maeda et al., 2022; Asakura et al., 2022). Maeda et al. (2022) has observed the characteristics of seismic wave records calculated with some simple structure models and suggested an acoustic blanking zone may be caused by the following three major elements: (1) high-velocity anomalies due to methane hydrate, (2) low-velocity and high attenuation anomalies due to free gas and/or (3) some branched faults.
In this study, we therefore evaluated the seismological effective elements for causing ABZ on the seismic reflection CMP profiles by using simulated seismic records. We calculated seismic waveforms by FDM wavefield calculations (Larsen, 2000) with several structural models assuming the above three possible elements: methane hydrate layers, free gas layers and (branching) faults. We also referred to much more information of subsea data such as borehole samples, acoustic reflection profiles in a very shallow depth by a sub-bottom-profiler and so on (e.g., Asakura et al., 2021).
We assumed three major structural models as follows: (Model-A) a reference model which consisted of two sedimentary layers (i.e., low-velocity sedimentary layers below the subsea) laid above an acoustic basement rock, (Model-B) a model with a MH layer above the gas-fluid zone in the 1st layer, and (Model-C) a model assuming with some branching faults running through all layers on Model-B. We focused two target P-P reflection phases on the stacking profiles for each model: one (i.e., 1st reflection phase) was the reflected waves at the 1st layer boundary between the unconsolidated sediment layer just below the subsea and a sedimentary rock layer, and the other (i.e., 2nd reflection phase) was the reflected waves at the 2nd layer boundary between the sedimentary rock layer and an acoustic basement rock layer.
It was found that the amplitude of both target reflection phases clearly decreased due to the existence of methane hydrate zone, gas-fluid zone, and/or some faults. Especially the amplitude of both target reflection phases extremely decreased down to less than 29 % on Model-C. It suggested that the existence of faults may play an important role as one of the seismological effective elements in ABZ, although in our simulation we used the simple physical properties for the structural models.
In the future study, we will discuss any other the seismological effects due to the small-scale heterogeneities and its bathymetric feature as well as the effect of the analyzing method for the interior of acoustic blanking zones using any other seismic phases such as refracted waves.