10:00 〜 10:15
[SSS04-05] Seismic investigation of internal structures of submarine volcanos in shallow water by full waveform inversion
キーワード：Full Waveform Inversion (FWI), First Arrival Travel Time Tomography (FATT), Magma Chambers, Numerical Simulation
A magma chamber contains plenty of liquid rock beneath the surface. They are generally shaped as a chamber beneath the surface and less dense than the surrounding rocks. If the magma finds a path through the surface, the result will finalize as volcanic eruption. Magma chambers are difficult to detect beneath the surface. Plenty of them are located close to the surface, commonly between 1 km and 10 km down. Magma chambers have low velocity zones which makes a strong distortion effect on the seismic sections. Because of that reason, it makes difficult to image and interpret these complex structures. To overcome these difficulties and to understand better how to detect and image the magma chambers below the shallow seafloor, we need an accurate velocity model which describes these structures well. For this purpose, in this study, we used the Full Waveform Inversion (FWI) to obtain an accurate velocity model to detect those complex structures.
FWI is a challenging inversion method that estimates the velocity model by reducing the residual errors, which also known as misfit function, between the observed and modeled of waveforms. It leads us to obtain an accurate velocity model which describes the structures below the shallow seafloor well enough. In this study, we used a synthetic data, which corresponds from shallow water to land coastal area, generated by finite difference method with fixed spread acquisition. The synthetic data includes 1200 receivers with 25 m interval and also contains 300 shots by 100 m intervals along the 30 km survey. The synthetic data also contains the three circular shape of magma chambers, which are located beneath the shallow seafloor (~300 m) with a radius of ~2 km wide at ~2, ~5, and ~8 km depths in the middle of the seismic section. First, we need to obtain an initial velocity model which describes the first arrivals on the synthetic data well to start the FWI. For this reason, we have applied first arrival travel time tomography (FATT) to obtain an initial velocity model for FWI. FATT is also an inversion method that estimates the velocity model by reducing the residual errors, between the observed and modeled of the traveltimes. It uses the first arrivals that have been picked form the data, and generates a velocity model at the end of the inversion process. After that, we have applied frequency domain FWI to obtain an accurate velocity model by using TOY2DAC software which is published by SEISCOPE Consortium. The process flow of the FWI could be seen in the Figure 1.
The velocity model that has been acquired from the FATT, describes the first arrivals on the synthetic data well and used as an initial model for FWI. After obtaining an accurate velocity model from the FWI, we have successfully obtained the two circular shape magma chambers below the shallow seafloor. The magma chambers have been located in their correct locations and shapes. However, the aim of the study is to image the targeted three circular shape magma chambers, so imaging the first two magma chambers are not enough to be successful. To achieve the aim of this study, we are still doing some tests based on chancing some key parameters, especially; the offset ranges, damping values, and maybe the most important one, the initial velocity model. Since there isn’t any study about investigating the internal structures of submarine volcanos in shallow water setting by FWI, this study could be an important study to present, how to tackle with the difficulties generated by magma chambers, and also how to detect and image them below the shallow seafloor.
FWI is a challenging inversion method that estimates the velocity model by reducing the residual errors, which also known as misfit function, between the observed and modeled of waveforms. It leads us to obtain an accurate velocity model which describes the structures below the shallow seafloor well enough. In this study, we used a synthetic data, which corresponds from shallow water to land coastal area, generated by finite difference method with fixed spread acquisition. The synthetic data includes 1200 receivers with 25 m interval and also contains 300 shots by 100 m intervals along the 30 km survey. The synthetic data also contains the three circular shape of magma chambers, which are located beneath the shallow seafloor (~300 m) with a radius of ~2 km wide at ~2, ~5, and ~8 km depths in the middle of the seismic section. First, we need to obtain an initial velocity model which describes the first arrivals on the synthetic data well to start the FWI. For this reason, we have applied first arrival travel time tomography (FATT) to obtain an initial velocity model for FWI. FATT is also an inversion method that estimates the velocity model by reducing the residual errors, between the observed and modeled of the traveltimes. It uses the first arrivals that have been picked form the data, and generates a velocity model at the end of the inversion process. After that, we have applied frequency domain FWI to obtain an accurate velocity model by using TOY2DAC software which is published by SEISCOPE Consortium. The process flow of the FWI could be seen in the Figure 1.
The velocity model that has been acquired from the FATT, describes the first arrivals on the synthetic data well and used as an initial model for FWI. After obtaining an accurate velocity model from the FWI, we have successfully obtained the two circular shape magma chambers below the shallow seafloor. The magma chambers have been located in their correct locations and shapes. However, the aim of the study is to image the targeted three circular shape magma chambers, so imaging the first two magma chambers are not enough to be successful. To achieve the aim of this study, we are still doing some tests based on chancing some key parameters, especially; the offset ranges, damping values, and maybe the most important one, the initial velocity model. Since there isn’t any study about investigating the internal structures of submarine volcanos in shallow water setting by FWI, this study could be an important study to present, how to tackle with the difficulties generated by magma chambers, and also how to detect and image them below the shallow seafloor.