Kazuya Shiraishi

Daylight seismic imaging from a buried point source

We applied acoustic daylight imaging technique to one buried point source problem. This method can synthesize shot records of the same number as receivers, with only one actual source explosion. The reflection response can be obtained by the cross-correlation of the transmission responses. In this paper, we show a numerical simulation and then we apply this method to the field data observed by an explosive response. We obtained subsurface images clear enough to estimate underground structures, and the obtained subsurface image is in good agreement with the section acquired by a conventional reflection survey.

Morpho-sedimentary features and sediment dispersal systems of the southwest end of the Ryukyu Trench: a source-to-sink approach

The southwestern Ryukyu Trench near Taiwan is an ideal place for source-to-sink studies because of the short sediment transport route between the terrestrial sediment source in Taiwan and the marine sink in the Ryukyu Trench. Bathymetric and seismic reflection data and core samples from the area around the southwestern Ryukyu Trench were used to identify features of the trench–arc system, including submarine canyons, the trench wedge, bathymetric ridges, and forearc basins, which together form two distinct sediment dispersal systems: a longitudinal (trench-parallel) system and a transverse (trench-normal) system. The longitudinal sediment dispersal system carries sediments eroded from the Taiwan orogenic belt eastward, primarily along the Hualien Canyon and a channel–terminal fan system at its mouth, and deposits them in the southwestern end of the Ryukyu Trench. The transverse sediment dispersal system carries sediments eroded from the Ryukyu Islands downslope and deposits them in the Hoping, Nanao, East Nanao, and Hateruma forearc basins, behind the barrier formed by the E–W-trending Yaeyama Ridge on the trench-slope break. The presence of pyrrhotite, a characteristic component of sediments sourced from Taiwan, in a seafloor sample from the Ryukyu Trench and its absence in a sample from the East Nanao forearc basin support the view that the southwestern Ryukyu Trench is longitudinally fed by sediment derived from Taiwan, whereas the trench-slope forearc basins receive sediment transported transversely downslope from the Ryukyu Islands.

Introduction to special section: Characterizing the subsurface with multiples and surface waves

Multiples and surface waves can give us extra views of the subsurface compared with just one view from primaries. As an example, surface waves in a layered medium are an interference pattern of up- and down-going multiple reflections, body-wave reflections and converted S-to-P events for Rayleigh

Seismic Wave Simulation based on Method of Characteristics

In this paper, we apply the CIP (Cubic Interpolated Profile) method to the simulation of elastic waves as a highly accurate and stable algorithm to solve first-order wave equations. The key idea of the CIP is that not only the physical value itself but also its first spatial derivative obeys the same equations. Using this property the solution is interpolated by cubic polynomials and interpolation coefficients can be evaluated arithmetically. We implemented this idea to the elastic wave simulation by derivation of the first-order wave equations from the basic equations of motion. The derived equations can be interpreted as the combined first-order wave equations for each mode of wave. Then, we define boundary conditions by using the merit of one-side propagation; free surface, solid-fluid boundary, irregular topography. From simulation study and the stability evaluation, we recognize the method of characteristics with the CIP is a very powerful simulation technique for the elastic wave propagation. Numerical dispersion is negligible, requiring about half the number of grid cells per wavelength than other solvers. This allows accurate, high-frequency, full-wavefield simulation in models with highly variable, random elastic contrasts with fluid-solid mixed media and complex topographic media.