Abstract
To establish an effective method for seismic exploration in investigating subsurface structures and physical properties beneath potentially active submarine volcanos in coastal area, we conducted a feasibility study using synthetic seismic data by numerical simulation. In this study, we assumed a 30-km-long seismic survey line across a shore line with a geological model of 10 km depth including virtual structures of past volcanic activities and present activity potential of a shallow submarine volcano. Because the virtual models contained only three low velocity bodies simulating magma chambers at multiple depths and no horizontally continuous reflectors in the crust, estimating velocities was difficult by general reflection-based velocity analyses. First, we generated synthetic data of first-arrival traveltime and waveform that simulated a seismic survey by numerical modeling of traveltime and elastic wave propagation respectively. Using the synthetic data, we estimated velocities by applying first-arrival traveltime tomography and full waveform inversion sequentially. Then, we applied reverse time migration for reflection imaging of subsurface structures using the inverted velocity models. We also tested the effects on the imaging results from different source-and-receiver layouts and shapes of the low velocity bodies. In our results, the general velocity structures to the bottom with two velocity bodies down to approximately 6 km were well inverted by the sequential inversion, and the geometry of the three low velocity bodies were clearly imaged by the reflection imaging in the depth domain. Although improvements with higher resolution and deeper targets should be necessary on the velocity estimation in further studies, our results suggest that the combination of velocity estimation by full waveform inversion and reflection imaging by reverse time migration is effective in complex geology where general reflection-based velocity analysis is challenging. The results also indicate the importance of pre-survey feasibility study using numerical simulation with the presumed models for designing seismic surveys, including optimal source-and-receiver layouts and effective data analysis methods.
