Seismic interferometry and estimation of the Green's function using Gaussian beams

This study investigates seismic interferometry in which the Green＇s function is estimated between two receiv- ers by cross-correlation and integration over sources. For smoothly varying source strengths, the dominant contributions of the correlation integral come from the stationary phase directions in the forward and backward directions from the alignment of the two receivers. Gaussian beams can be used to evaluate the correlation integral and concentrate the amplitudes in a vicinity of the stationary phase regions instead of completely relying on phase interference. Several numerical examples are shown to illustrate how this process works. The use of Gaussian beams for the evaluation of the correlation integral results in stable estimates, and also provides physical insight into the estimation of the Green＇s function based on seismic interferometry.

Monitoring Media Velocity Variations with Coda Wave Interferometry

Multiply scattered waves are sensitive to media changes owing to the effect of repeated sampling,superposition and amplification. Based on this characteristic,small-medium changes could be detected by using coda wave interferometry. In recent years,coda wave interferometry has been widely used in estimating velocity variation with high precision in areas such as seismology and non-destructive testing. This paper systematically presents the principle and research status of coda wave interferometry,and especial focus is placed on the research of media velocity variations by using repeating earthquakes,artificial sources,and ambient noise. Applications of coda wave interferometry can contribute to the more subtle understanding of dynamic evolution process in the medium.

Study on virtual source method under complex overburden in horizontal well

In conventional seismic exploration,complex overburden prevents us from imaging the deeper underground structure,because near-surface velocity model cannot be efficiently simplified and accurately established. The virtual source method provides a promising solution,which images below a complex overburden without the knowledge of overburden velocities and near-surface changes. In this paper,the authors simulate the forward modeling under complex overburden with sources on the surface and receivers in the horizontal well,and reset the real sources into the horizontal well using the virtual source method,which subtly avoid the distortion effects of the complex overburden. Finally the underground structure can be imaged using Kirchhoff migration,so the feasibility and advantages of the virtual source method are verified.

Equipartitioning is not sufficient for Green's function extraction

The extraction of the Earth＇s Green＇s function from field fluctuations is a rapidly growing area of research. The principle of Green＇s function extraction is often related to the requirement of equipartitioning, which stipulates that the energy of field fluctuations is distributed evenly in some sense. We show the meaning of equipartitioning for a variety of different formulations for Green＇s function retrieval. We show that equipartitioning is not a sufficient condition, and provide several examples that illustrate this point. We discuss the implications of lack of equipartitioning for various schemes for the reconstruction of the Green＇s function in seismology. The theory for Green＇s function extraction is usually based on a statistical theory that relies on ensemble averages. Since there is only one Earth, one usually replaces the ensemble average with a time average. We show that such a replacement only makes sense when attenuation is taken into account, and show how the theory for Green＇s function extraction for oscillating systems can be extended to incorporate attenuation.