Time-frequency coherency analysis of three-component crosshole seismic data for arrival detection

A processing method for three-component wide-band crosshole seismic data is presented. It is based on the observed phenomena of a decrease in the cohe rency between the coda waves recorded by closely spaced sensors or excited by closely spaced seismic sources. We transfer signals to the time-frequenc y domain and compute the instantaneous coherency at each moment of time bet ween signals propagated on adjacent raypaths. As a result, we obtain the di stribution of the high-coherency time-frequency areas connected with cohere nt components of the signal. We can then see different modes of the recorde d seismic signal and identify them as direct P- and S-waves, reflected wave s, and converted tube waves. A reliable method of evaluation of the onset's time for different modes of the seismic signal is also proposed. The techn ique has good resolution and allows separation of even very closely spaced wavefield components.