Seismic wave attenuation

Seismic waves propagating through the earth are attenuated by the conversion of some fraction of the elastic energy to heat. Using terminology analogous to that used for the elastic properties that control seismic velocities, attenuation properties are characterized as anelastic properties. Attenuation data complement other physical measurements for characterizing rock properties. In seismic studies, attenuation data can at least double the information obtained from velocities alone. An understanding of the attenuative properties of the earth has two major motivations. First, seismic wave amplitudes are reduced as waves propagate through an anelastic medium, and this reduction is generally frequency dependent. Second, attenuation characteristics reveal much information, such as lithology, physical state, and degree of saturation of rocks. The phenomenon of attenuation is much more complex than the elastic aspects of seismic wave propagation, Both laboratory and field measurements are difficult to make. The mechanisms contributing to attenuation are numerous, and small changes in some conditions can affect attenuation significantly. However, sensitivities to some parameters, su ch as fluid saturation, make the measurement and understanding of attenuation highly important for many applications. The realization of the need and promise for specific data and models has prompted astrong resurgence of interest and research concerning attenuation in the fields of both seismology and rock physics. Laboratory measurements of attenuation in rock samples under varying pressures, temperatures, strain amplitudes, frequencies, and saturation conditions are presently being carried out. Detailed theoretical modeling of processes that may be responsible for attenuation is being undertaken. Measurements of attenuation in the earth using direct and refracted compressional and shear waves, surface waves, reflection seismograms, vertical seismic profiling, and full-wave acoustic well logs are being explored intensively. The net result of these field, laboratory, and theoretical studies will be a rapid expansion of our knowledge concerning the attenuation of seismic waves in the earths crust. We have undertaken the editing of this volume to help the broad-range research effort gain abetter understanding of attenuation and its applications to seismic exploration problems.