A Method for Resolving Small Temporal Variations of Effective Elastic Properties

A method for studying temporal variations of wave propagation properties in the earth is specifically useful for studying crustal dynamics, but also for studying reservoir geophysics. Medium anisotropy causes three phases to propagate along almost the same path. This allows a determination of the relative difference of shear-wave velocities δβ̂ in a fashion, which is remarkably insensitive to potential errors. Even instrumental timing errors should not affect the results. If there are nearly identical doublet sources, which often occur naturally, one can determine differences of δβ̂ with extreme accuracy. This allows resolving small temporal variations of wave propagation properties, which cannot usually be detected within the larger measurement uncertainty of δβ̂ . An application to data from a hydraulic fracturing experiment in a deep drilling borehole (KTB) showed that such temporal variations indeed exist, even at substantial depth levels in the crust. The relative difference of split shear-wave velocities decreased by about 2% during a 12-hour interval in the experiment. This can be explained only in terms of changing effective elastic properties of the medium. The method can be used with artificial and natural sources.