Azimuthal variation inP-wave signatures due to fluid flow

Abstract

The favored dominant mechanism for attenuation in the upper crust at seismic frequencies is intracrack fluid flow. In cracked media, the azimuthal attenuation of the P-wave amplitudes arising from such flow is predicted to be quite substantial. The consequence of this variation in azimuth is a modification in the amplitude behavior of the base event from a cracked reservoir due to transmission through the attenuative layer. Indeed, the effect is of sufficient strength to exacerbate, diminish, or reverse variations that arise solely due to the reflectivity coefficient. Thus, although this attenuation is always greatest perpendicular to the crack strike, the direction of the dimming or brightening of the base reservoir event will depend upon the exact attenuation law and the crack properties. The combination of these factors contributes to a wave behavior that can provide a more adequate discrimination between conditions of brine and oil fill than an interpretation assuming the reflection coefficient alone. Unfortunately, regions of commonality do remain, and the distinction between fills is still only apparent for particular reservoir and crack conditions. The attenuated amplitudes are large enough to be seen at small offsets and may possibly account for the significant azimuthal variations in the P-wave signatures observed in field data.