Finite amplitude wave studies in earth materials.

The highly nonlinear elastic behavior of rock may enable new means of interrogating earth structure, of measuring physical properties, and of modeling the seismic source. Compared to uncracked materials, rocks have a large nonlinear response because they contain numerous microcracks that readily compress under applied stress causing large changes of elastic moduli with pressure. Thus nonlinear effects in rocks can be two orders of magnitude greater than those of the uncracked materials typically studied in nonlinear acoustics. Several areas of nonlinear research are currently being undertaken in these laboratories. First, low‐frequency (0.1–100 Hz) attenuation studies using a torsional oscillator show that nonlinear coefficients can be greatly increased by inducing additional microcracks in Sierra White granite. Second, ultrasonic parametric array studies demonstrate that strong difference frequency signal generation can take place inside rock samples. Lastly, energy redistribution of finite amplitude waves may produce progressive changes in observed spectra with distance. If a significant amount of energy is redistributed as a function of distance, then source models (based on assuming linear elastic wave propagation) may be in error. This theoretical and experimental work demonstrates that energy redistribution does indeed take place in rock.