Seismic velocity modeling of the Southern California Coastal Basins:Methods and Applications

M. Peter Suess¹, Carlos Rivero², John H. Shaw³, and Jeroen Tromp^4
1University of Tuebingen, Department of Geosciences, [email protected]
²Chevron Corp., Houston, [email protected]
³Harvard University, Department of Earth and Planetary Sciences, [email protected]
⁴Division of Geological and Planetary Sciences, California Institute of Technology, [email protected]

Proper velocity model building is a key to integrated reservoir and basin modeling, and errors in the velocity models can pose severe consequences for subsequent quantitative basin evaluations. Here we will review different methods for constructing velocity models and their applications in the Southern California Coastal Basins and Borderlands. Seismic velocities are measured in various ways, ranging from direct measurements in the lab, measurement in wells, check-shots and seismic reflection or refraction experiments. When working with observational data, several steps must be taken in generating a petrophysical property model including initial quality control, upscaling, interpolation and calibration. Using modern basin modeling tools, it is then possible to generate advanced velocity models integrating analytical functions, local and regional measurements as well as results from stratigraphic forward simulations. Furthermore, the velocity fields of complicated structures like thrust-wedges or salt-bodies, which are generally difficult to represent in classic layered-pie type models, can be addressed by modern approaches. Velocity models are primarily used for seismic reflection processing and time/depth conversion, thereby aiding to reduce the uncertainty of reservoir and basin models. Moreover, velocity is an important physical property that constrains the propagation of seismic waves. Hence besides their conventional application many other fields of uses of these models have developed. Our field of applications ranges from the relocation of seismic events to the the simulation of strong ground motions. The latter offers a unique opportunity to independently assess the quality of the models, through comparisons of observed and sythetic waveforms. The same techniques may also be used for higher frequency 3D-seimic forward simulations. New wave-form tomographic inversion methods further enhance the quality of our models.

We will demonstrate the application of these techniques on a lithosphere scale velocity model developed for the Southern California Earthquake Centre (SCEC). This integrated structural and petrophysical model of the seismic velocity and density structure of southern California sedimentary basins extends from Santa Barbara to Baja California, including the Salton Trough and offshore basins of the Inner Borderlands. This model serves as the basis for seismic hazard assessment through crustal-scale strong ground motion simulations of Southern California. Equivalent to industry-style reservoir models, the model consist of a structural model of the sedimentary basin and a volumetric model of its petrophysical content. The structural model is based on a broad range of sources, ranging from cross-sections, maps to high-resolution datasets from 2D and 3D industry reflection surveys. This model is also characterized by a faulted basement surface that is compatible with offsets and positions of the major fault systems as represented in the Community Fault Model of SCEC, thus serving as part of a unified structural representation of southern California. The volumetric geophysical model is generated from geophysical well measurements, and analytic functions derived from well and refraction seismic data. It defines compressional wave velocities (Vp) and calibrated relations between Vp, shear wave velocity (Vs), and density. The model includes precise structural descriptions of the Los Angels basin, Salton trough, Ventura basin and adjacent parts of the Santa Barbara basin, and the Inner California Borderlands basins. The basin velocity structures are embedded in an updated regional tomographic model of Hauksson (2000). The complete model now consists of one high-resolution volume, as well as surfaces used to define the volume of sediments (the top of the pre-cretaceous basement and topography/bathymetry). The model is now available through the SCEC/CME Velocity Model Server and the CVM-H website ..

 

AAPG Search and Discover Article #90066©2007 AAPG Hedberg Conference, The Hague, The Netherlands