NORDIC ASSOCIATION FOR APPLIED GEOPHYSICS 6 7

Two-dimensional synthetics - a tool for simulating seismic reflection surveys

B.O. Rosland a, O.A. Sandvin a, J.E. Lie ° and E.S. Husebye b aBergen Scientific Centre, Bergen, Norway

bDepartment of Geology, University of Oslo, Oslo, Norway

Deep seismic reflection surveys are impor- tant but expensive means for crustal structural mapping. In this presentation we explore the use of seismic synthetics as a planning tool for real surveys but also for guidance in interpre- tation of complex record sections. The starting point is the numerical solution of the 2D acoustic wave equation, including an absorb- ing boundary operator for minimizing reflec- tions from model edges.

The geological models being considered comprise scattering bodies and associated re- fractors, dipping multilayer reflectors, steeply

dipping faults and classical end-refractors. The profiling acquisition parameters used were typical for these of the Mobil Search survey in the Skagerrak, albeit the time length was lim- ited to 4s TWT. The synthetic data were pro- cessed commercially. We have for a reasonable extent been able to extract by conventional interpretation the original model bodies.

Further developments would aim at recon- structing entire seismic sections based on de- rived (interpreted) models and realistic phys- ical structural parameters.

Geometrical optics, point-to-point raytracing and amplitudes in a two-dimensional, isotropic, seismic model with a piecewise

constant velocity gradient

Peder Hedebol Nielsen University of Aarhus, Department of Earth Sciences, DK-8000 Aarhus C, Denmark

A library of FORTRAN-77 routines has been written for raytracing: the simple 2D model is divided into quadrilaterals by vertical lines, each with the same number of nodes, and "ho- rizons" of piecewise linear segments connect- ing nodes on consecutive vertical lines. Each node has a depth and a seismic velocity. To

avoid anisotropy each quadrilateral is divided into 4 triangles by the 4 corners and the mid- point of the quadrilateral. Each triangle gets a seismic velocity field with a constant gradient. The velocity field, but not its derivatives, is continuous from triangle to triangle.

Raytracing in the model is simple. In each