Reflectivity Imaging and Impedance Inversion of Seismic Data from the Outer-Rise Area of the Japan Trench

The geological structure of the Japan Trench outer-rise is complex due to the deformation of the subducting oceanic crust. This region underwent structural modification due to subduction-related plate flexure, which facilitated the development of bending-related faults and the formation of petit-spot volcanoes. These tectono-magmatic processes increased fracturing, hydration, and porosity and decreased the continuity of reflectors, all of which adversely affect seismic wave propagation. Additionally, the emplacement of these magmatic features, such as dikes and sills, introduces structural heterogeneities that scatter seismic energy, making the seismic imaging of this geological setting difficult.

In this study, we investigate the upper crustal structure of the oceanic plate at the Japan Trench outer-rise using two-dimensional multichannel seismic (MCS) reflection data acquired by JAMSTEC during cruise KR15-07 with a 6 km long, 444-channel streamer. The study area has a water depth of approximately 6 km, and this deep-water setting, combined with the limited streamer length, restricted offset coverage, and posed significant challenges for seismic reflector imaging in the upper oceanic crust.

The first goal of this study is to determine the depth of the sedimentary layers and to identify normal faults associated with the subduction of the oceanic plate. To achieve this, we first apply standard processing steps and prestack time migration, followed by the prestack depth migration to obtain the final reflectivity model.

As a second objective, we estimate an acoustic impedance model from the migrated reflectivity section using a regularized inversion framework. Acoustic impedance is known as an identifier of the subsurface properties that are related to lithology, porosity, pore filling, and other factors that characterize the subsurface. The problem of estimating acoustic impedance using reflection series data can be expressed as an inverse problem. In our case, the inversion incorporates a combined Tikhonov–Total Variation (TV) regularization scheme, optimized for reconstructing piecewise-smooth models. This formulation decomposes the impedance model into a smooth component, constrained by Tikhonov regularization, and a blocky component, constrained by the TV regularization. This hybrid approach mitigated the limitations of individual regularization methods.

According to the obtained results from reflectivity and the acoustic impedance models, bending-related normal faulting and petit-spot volcanism significantly modify the upper crust, producing strong lateral heterogeneities within the sedimentary section.