Seismic wave velocities
This brief chapter is included prior to embarking on the central theme so as to remind us of the physical rock properties which ultimately govern all seismic wave propagation.
Seismic wave velocity pertains to the speed of a seismic disturbance propagating through a material medium; it is a physical property of the medium. Particle velocity, on the other hand, refers to the actual motion of a local portion of the medium; it is a function of the disturbance of the medium rather than a property of the medium.
Whereas seismic wave velocities have dimensions of thousands of meters per second, particle velocities are typically in the order of millionths of a meter per second. We will not discuss particle velocity further in this work as it relates more to the dynamics than kinematics (i.e., amplitude rather than traveltime) of wave propagation.
Many field, laboratory, and theoretical studies have been devoted to the problem of determining seismic wave velocities and establishing empirical and analytical laws between these velocities and the various parameters that influence them (Faust, 1951; Gassmann, 1951; Wyllie et al, 1956, 1958; Press, 1966; Gardner et al, 1974a; Domenico, 1974; Toksoz et al, 1976; and Timur, 1977).
Temperature and pressure (which are dependent mainly on depth), as well as lithology, grain packing, porosity, and cementation all affect seismic wave velocity. Variations in lithology and the fluid and gas content of a porous rock can be important sources of strong velocity variation. Likewise, microfracturing can cause major reductions in material
Figures & Tables
Interval Velocities from Seismic Reflection Time Measurements
“Over the years, ray theory has furnished the exploration geophysicist with most of the working tools for understanding and interpreting events observed on reflection seismic sections. Even today, notwithstanding the pace at which the more powerful acoustic wave theory is introducing its new tools, ray theory, in the hands of the authors, retains its preeminence for providing insights into fundamental problems in reflection seismology. Professor Krey's earlier contributions are part of ray theory's rich heritage. Alongside C. Hewitt Dix and Hans Durbaum, he elucidated relationships between interval velocity and observed reflection moveout.”