Dynamic nonlinear probe utilizing traveling multiwave interaction

The aim of this study is to utilize the nonlinear action of two overlapping propagating waves to characterize parameters of rock samples that are sensitive to cracks and fluids at the intersection location. Our experimental technique utilizes a strong shear wave pump that changes (minutely) the elastic properties of the sample and a weaker longitudinal wave probe that senses those changes. The pump wave signal is directed perpendicular to the longitudinal wave and is slowly varying compared to that of the probe; we use a 500 kHz longitudinal probe wave and a 50 kHz shear wave pump. Particle velocities are measured on the surface of the sample with the laser vibrometer to calibrate their absolute values for estimation of the strain with the help of an elastic model. Measured changes in time modulation were fitted to a nonlinear Hooke´s law. Nonlinear parameters for Berea sandstone were β = -542 and δ = -3 × 10⁹. The nonresonant time modulation techniques described can quantify and identify regions of local nonlinearity.