Ambient Noise, External Forcing and Monitoring of Elastic Properties of the Crust

Abstract:

We present recent results concerning the origin of the ambient noise, natural or anthropic, and its properties deduced from array analysis. The noise records can be used to monitor changes of shallow elastic properties of the crust. We discuss why and how it is possible to perform high precision monitoring, even without a precise description of the noise. The temporal changes of the noise incident to the region of interest results in apparent changes of seismic speeds. A quantitative evaluation of these effects is presented. We emphasize the question of an anisotropic intensity with temporal variations of azimuthal distribution. Our analyses show that high precision monitoring (fractional seismic wave fluctuation of the order of 10^-4 can be achieved from noise analysis in presence of a fluctuating noise. We present examples showing how the elastic properties at depth change in response to external forcing such as precipitation, temperature variations, dynamic strain associated with strong long period motions, tides, and hydraulic load. We show that steady ambient noise of anthropic origin can be used efficiently for monitoring of changes related to meteorological processes. In addition, we present an application of noise based monitoring in the context of reservoir stimulation associated with a massive deep water injection in an urban area.