Abstract
A four-channel sonic log of a 1-km section of oceanic crust at site 504B, Eastern Equatorial Pacific Ocean has been spectrally analyzed. The cross-spectrum of seismic waveform pairs was converted into integrated energy and peak-frequency at each shot point for both compressional and shear coda. These plus an ultrasonic borehole televiewer fracture count were combined into a “fracture spectrum log” over the bottom 500 m of the borehole. Cross correlation of P and S frequency and energy variations downhole revealed a natural resonance at wavelengths of 30–40 cm in the oceanic crust. This resonance appears as a low-pass filter of P energy and a high-pass filter of S energy. Because the energy attenuation is phase coherent with the fracture density, we interpret this natural resonance to be caused by venting of acoustic energy through intersecting fractures of an aspect ratio appropriate to the resonance wavelength. This is the first time such uniform fracturing has ever been observed in the oceanic crust. [Work supported by ONR.]
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