Crust and Lithospheric Structure - Active Source Studies of Crust and Lithospheric Structure

Abstract

Controlled-source seismology methods are used to make high-resolution images of the seismic velocity and impedance structures in the Earth's upper crust, crust, and upper mantle over scale ranges from a few meters to a maximum of about a hundred kilometers in depth and a few tens of meters to hundreds of kilometers laterally. In this chapter, we review the principles behind the two most common controlled-source methods, reflection seismology and refraction or wide-angle seismology. The former is used to make images of the fine-scale seismic impedance structure of the Earth, which is often closely related to sedimentary layering and the large-scale features of structural geology. The latter is used to make lower-resolution images of the Earth's seismic velocity structure in which large structural geology features are observed and from which the constituent lithologies can often be inferred. The two methods complement one another, particularly since the velocity field developed from wide-angle data can be used in image formation with the reflection data. We discuss some of the standard data processing and analysis methods, particularly common midpoint processing and migration of reflection data, and travel time inversion of refraction data. We also describe some modem methods of analysis termed wavefield inversion and waveform tomography, which in some sense unifies the two different experimental approaches, as they can be applied to both types of data. Reflection and wide-angle seismic data were developed for and are still widely used in the industrial exploration for petroleum and minerals. They are now also being used for a variety of geotechnical investigations including fault and earthquake hazard assessment, groundwater resource evaluation, and environmental characterization studies for groundwater contaminant remediation. Interpretation of seismic reflection data has provided important information on global sea level rise and fall. In the past few decades, academic reflection/refraction surveys have been used to make dramatic images of the Earth's orogenic belts, continental margins, and cratonic cores, contributing greatly to our understanding of the Earth's evolution.