Sensitivity of coda wave interferometry to fluid migration through rock.

The sound speed of a porous medium changes with fluid substitution when the fluids have different acoustic properties. The authors demonstrate that coda wave interferometry is capable of sensing subtle local sound speed changes associated with minute fluid displacements, Δh. In fact the resolution on fluid motion is given by a simple scaling relationship, Δh/λ∼te, where t is the waveform time, λ is the wavelength, γ is a constant that varies based on the nature of the acoustic propagation, and α is a system specific acoustic attenuation coefficient. In contrast to the conventional notion that later arrivals (further into the coda) give greater sensitivity to fluid movement, this scaling relationship suggests that there is a temporal optimum in sensitivity to Δh. This is the case even though later arrivals exhibit signal intensities well above the noise floor. The authors elucidate the physical basis for determining the waveform time at which the sensitivity is optimal.