IASPEI - Picture Gallery

IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy
(S) - IASPEI - International Association of Seismology and Physics of the Earth's
Interior
JSS014 Poster presentation 2289
A Interactive and integraged modeling tool for the crustal analysis using
OBS-control sources
Dr. Junzo Kasahara
Advisory Department Japan Continental Shelf Survey, Japan IASPEI
Gou Fujie, Kei Murase, Ryuji Kubota, Eiichiro Nishiyama, Tomoyuki Tanaka,
Kayoko Tsuruga, Shigeharu Mizohata, Shigeki Kawamura, Azusa Nishizawa,
Kentaro Kaneda
In order to obtain on accurate best crustal velocity structure model, it is very important to make the
best fit between the major seismic phases identified in the OBS wide-angle reflection and refraction
survey data, and the MCS reflection sections. Recently, we have developed a new interactive software
module for the Modeling-Pasteup crustal structure analysis tool, originally developed by Fujie et
al.(2002), for OBS survey data. The improved interactive modeling module has two major functions:
Modeling and Pasteup. Modeling and Pasteup can be run under X-Window circumstance of Linux OS.
Under such computer circumstances, we can do the crustal structure analysis by a PC. Modeling is
forward analysis software to carry out 1) crustal structure modeling, 2) travel time calculation for
refracted first and later arrivals, reflected arrivals, arrivals of P-S converted waves, and headwaves, 3)
raypath calculations, 4) depth to time conversion of layer boundaries and 5) gravity modeling. We
define the crustal structure by several layers with velocity grids on the layer interface. When a
discontinuous layer boundary is required, we can define two different velocities for the above and the
below the particular layer boundary. We can compare traveltime pickings based on observed records
and theoretical traveltimes on the Pasteup window. We also compute travel times of wide-angle
reflections and superpose on the OBS seismic record sections. A part of software module can compute
the two-way travel times of normal incident waves from the analyzed crustal structure model. The
software module also superposes the time contours of the layers on the crustal structure model over the
migrated time section of MCS using the Pasteup. Using a huge volume of real data, we confirmed that
strong reflections from layers in the crust and the Moho seen in the OBS wide-angle reflection records
are fairly consistent to the reflectors in the MCS section. By use of forward analysis, we also evaluate
kind of waves for later arrivals. It is great help to the crustal modeling to use full wave information on
seismic records. Synthetic waveforms calculated by FDM are also directly compared to observed ones.
Gravity data are also used to evaluate the correctness of the resultant model. Through the interactive
analysis, we can obtain the best fit model for observed data. Through above two processes, we can
confirm the correctness of the resultant crustal structure model. We use the result of forward modeling
as the input of traveltime inversion to minimize non-linear effect of traveltime inversion. The average
rms misfit is approximately 30-40ms for 100 OBS data along 500km distance if arrivals are clear
enough. In the actual processing, we used over 200 OBSs and every 200m airgun shots for the longest
survey line.
Keywords: crustal structure, obs, inteructive analysis