Seismic structure of the upper oceanic crust revealed by in situ Q logs

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doi: 10.1029/96GL02075
Author(s): Goldberg, D.; Sun, Y. F.
Author Affiliation(s): Primary:
Lamont-Doherty Earth Observatory, Palisades, NY, United States
Volume Title: Core-log seismic data integration
Volume Author(s): Goldberg, D., prefacer
Source: Core-log seismic data integration, prefaced by D. Goldberg. Geophysical Research Letters, 24(3), p.333-336. Publisher: American Geophysical Union, Washington, DC, United States. ISSN: 0094-8276 CODEN: GPRLAJ
Note: In English. 34 refs.; illus., incl. 1 table, block diag.
Summary: In situ seismic attenuation is computed through 2.1 km of the upper oceanic crust in the vicinity of ODP Hole 504B. The results strongly tie crustal properties to seismic measurables and observed geological structures: we find that the attenuation can be used to define seismic layer boundaries and is closely related to the intensity of vertical heterogeneity. The in situ attenuation Q-1 consists of both intrinsic and scattering contributions, but is dominated by the scattering attenuation unless porosities are near zero, when it approaches typical estimates 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 homogeneous layers from Q = 25 to Q > 300 between the top of seismic layer 2A and a sharp discontinuity at 1.3 km depth. These changes correspond with 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 basalts and lava flows in seismic layers 2A and 2B. Although seismic velocity 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 indicates that a seismically homogeneous and uniform layer 3 has not been reached in Hole 504B. We derive an empirical relationship between attenuation and porosity Q -1 = Q 0 -1 eβφ, where Q 0 -1 = 0.004 and β = 25, that may be applicable at other oceanic crust locations and useful for constraining seismic inversion models. Copyright 1997 by the American Geophysical Union.
Year of Publication: 1997
Research Program: DSDP Deep Sea Drilling Project
IPOD International Phase of Ocean Drilling
ODP Ocean Drilling Program
Key Words: 20 Geophysics, Applied; Attenuation; Crust; DSDP Site 504; Deep Sea Drilling Project; Downhole methods; East Pacific; Elastic waves; Equatorial Pacific; Geophysical methods; Geophysical surveys; IPOD; In situ; Leg 111; Leg 137; Leg 140; Leg 148; Leg 69; Leg 70; Leg 83; Leg 92; Ocean Drilling Program; Oceanic crust; Pacific Ocean; Physical properties; Q; Refraction methods; Seismic methods; Surveys; Upper crust
Coordinates: N011335 N011338 W0834348 W0834357
Record ID: 2001064193
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