SEISMIC STRUCTURE OF THE UPPER OCEANIC-CRUST REVEALED BY IN-SITU Q-LOGS

In situ seismic attenuation is computed through 2.1 km of the upper oc eanic crust in the vicinity of ODP Hole 504B. The results strongly tie crustal properties to seismic measurables and observed geological str uctures: we find that the attenuation can be used to define seismic la yer boundaries and is closely related to the intensity of vertical het erogeneity. The in situ attenuation Q(-1) consists of both intrinsic a nd scattering contributions, but is dominated by the scattering attenu ation unless porosities are near zero, when it approaches typical esti mates from seismic refraction studies. The attenuation is analytically modeled by multiple backscattering from heterogeneities observed in a sonic Vp log and is found to decrease step-wise in relatively homogen eous layers from Q = 25 to Q > 300 between the top of seismic laver 2A and a sharp discontinuity at 1.3 km depth. These changes correspond w ith heterogeneities at 1.0-1.3 m and at 5.6-10.0 m wavelengths that we interpret to be associated with fracturing and structure of pillow ba salts and lava flows in seismic layers 2A and 2B. Although seismic vel ocity studies suggest that the layer 2/3 boundary also occurs at about 1.3 km, the large variation in Q (140 to 460) below this depth indica tes that a seismically homogeneous and uniform layer 3 has not been re ached in Hole 504B. We derive an empirical relationship between attenu ation and porosity Q(-1) = Q(0)(-1) e(beta phi), where Q(0)(-1) = 0.00 4 and beta = 25, that may be applicable at other oceanic crust locatio ns and useful for constraining seismic inversion models.