Nonlinear seismic phenomena as recorded by distributed acoustic sensors

Due to their granular nature and presence of fluids, elastic moduli of most crustal rocks show a strong stress dependency. This means that the relationship between stress and strain is nonlinear, which should cause nonlinear wave phenomena. In particular, interaction of seismic waves of different frequencies should generate higher harmonics and combinational frequencies. Analysis of these effects in field data can potentially help find areas of anomalous nonlinear properties, such as fractured zones, mixed saturation or overpressure. To better understand the potential of nonlinear seismology, we observed and analyzed nonlinear seismic effects in field and laboratory experiments. The field experiment was performed using two seismic vibrators generating monochromatic signals of different frequencies. The wavefield was recorded with a fiber optic distributed acoustic sensing (DAS) cable cemented in a 900 m deep borehole. The signals recorded both on the surface and in the borehole show combinational frequencies, harmonics, and other intermodulation products of the fundamental frequencies. The laboratory experiment, which was designed to replicate the setup of the field experiment, shows similar nonlinear products of the fundamental frequencies. Furthermore, the nonlinear effects show a dependency on the saturating fluid. These tests confirm that nonlinear components of the wavefield propagate in a form of body waves, are likely to be generated in rock formations, and have the potential for reservoir fluid characterization.