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To improve quantitative interpretation of seismic data, we analyze the effect of fluid substitution in a porous and fractured medium on elastic properties and reflection coefficients. This analysis uses closed-form expressions suitable for fluid substitution in transversely isotropic media with a horizontal symmetry axis (HTI). For the HTI medium, the effect of changing porosity and water saturation on (1) P-wave moduli, (2) horizontal and vertical velocities, (3) anisotropic parameters, and (4) reflection coefficients are examined. The effects of fracture density on these four parameters are also studied. For the model used in this study, a 35% increase in porosity lowers the value of P-wave moduli by maximum of 45%. Consistent with the reduction in P-wave moduli, P-wave velocities also decrease by maximum of 17% with a similar increment in porosity. The reduction is always larger for the horizontal P-wave modulus than for the vertical one and is nearly independent of fracture density. The magnitude of the anisotropic parameters of the fractured medium also changes with increased porosity depending on the changes in the value of P-wave moduli. The reflection coefficients at an interface of the fractured medium with an isotropic medium change in accordance with the above observations and lead to an increase in anisotropic amplitude variation with offset (AVO) gradient with porosity. Additionally, we observe a maximum increase in P-wave modulus and velocity by 30% and 8%, respectively, with a 100% increase in water saturation. Water saturation also changes the anisotropic parameters and reflection coefficients. Increase in water saturation considerably increases the magnitude of the anisotropic AVO gradient irrespective of fracture density. From this study, we conclude that porosity and water saturation have a significant impact on the four studied parameters and the impacts are seismically detectable.

SUMMARY For the purpose of transversely isotropic (TI) normal moveout (NMO) correction, we propose analysis of plane wave transformed azimuthal gathers, interactively using a single azimuth data at a time and a new delay time equation (developed in this paper), which is a function of two parameters at each azimuth. Results from independently estimated multi-azimuth gathers, then, can be combined to estimate stiffness or Thomsen coefficients. Azimuthal τ–p analysis also avoids numerical ray tracing, resulting in a rapid algorithm. We demonstrate the applicability of our method using a set of P-wave synthetic seismograms from a multilayered medium, consisting of isotropic and HTI layers. Azimuth-dependent anisotropy parameters are derived by delay time fitting and NMO correction. The reflections from the bottom interface of an isotropic layer with an anisotropic overburden show apparent anisotropic traveltime behaviour, which is easily accounted for by our layer-stripping based azimuthal NMO analysis. Unlike the previous approximate HTI NMO correction equation, this equation performs better NMO correction for the HTI medium and is also applicable to the VTI medium. Presence of only two reduced parameters in the equation helps the anisotropic parameter estimation become less ambiguous.

The Mw 9.3 great Sumatra-Andaman earthquake of December 26, 2004 induced water level changes in Fairbanks, Alaska, at an epicentral distance of 10,800 km. Spike like water level changes followed by a step of water level rise were observed in at least four wells. We modeled the timing and magnitude of the water level rise using a combination of a linear trend and a step function. We calculated the misfit between the observed water level rises and our model by systematically shifting the timing of occurrence of step in water level. The minimum value of cumulative misfit suggested the timing of occurrence of steps. A previous study showed persistent water level rises in all these wells from the 2002 Denali fault earthquake and it’s major aftershocks. From those observations, we developed an empirical relationship between water level changes, epicentral distances and earthquake magnitude. This relationship attributed water level changes in the wells to ground shaking by seismic waves. The estimated average water level changes due to the Sumatra earthquake using that relationship was in agreement with the observed water level changes. Thus we concluded that ground shaking in Fairbanks, induced by surface waves from the Sumatra earthquake was sufficient to change water levels.

We study the effect of fluid substitution in a porous fractured medium using explicit expressions developed for aligned fractured medium. We investigate the effect of porosity and water saturation on (1) P-wave moduli, (2) horizontal and vertical velocities, (3) anisotropic parameters, and (4) reflection coefficients. Effects of fracture density on these four parameters are also analyzed. The systematic variations of the moduli and reflection coefficients reported in this paper can thus be used in developing AVO with azimuth in a porous fractured reservoir.