Scattering attenuation as a function of depth in the upper oceanic crust

Author(s): Goldberg, D.; Sun, Y. F.
Author Affiliation(s): Primary:
Lamont-Doherty Earth Observatory, Palisades, NY, United States
Volume Title: Developments in petrophysics
Volume Author(s): Lovell, M. A., editor; Harvey, P. K.
Source: Geological Society Special Publications, Vol.122, p.367-375; Developments in petrophysics, London, United Kingdom, Sept. 1995, edited by M. A. Lovell and P. K. Harvey. Publisher: Geological Society of London, London, United Kingdom. ISSN: 0305-8719. ISBN: 1-897799-81-0
Note: In English. 50 refs.; illus., incl. 1 table, block diag.
Summary: The in situ attenuation is computed through 2.1 km of the upper oceanic crust in the vicinity of ODP Hole 504B on the southflank of the Costa Rica Rift in the eastern equatorial Pacific. 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 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 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 10.0 m wavelengths that are associated with fracturing and the 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 layer 3 has not been reached in Hole 504B. From the observed results, we derive an empirical relationship between attenuation and porosity Q-1 = Q10 eβφ, where Q-10 = 0.004, β = 25, that may be applicable at other oceanic crust locations and useful for constraining seismic inversion models.
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; Backscattering; Boundary conditions; Costa Rica Rift; Crust; DSDP Site 504; Deep Sea Drilling Project; Deep-sea environment; East Pacific; Fractures; Geophysical methods; Geophysical surveys; IPOD; Leg 111; Leg 137; Leg 140; Leg 148; Leg 69; Leg 70; Leg 83; Leg 92; Marine environment; Ocean Drilling Program; Ocean floors; Oceanic crust; Pacific Ocean; Seismic attributes; Seismic methods; Surveys; Upper crust; Velocity; Wave dispersion
Coordinates: N011335 N011338 W0834348 W0834357
N011203 N011400 W0834000 W0834500
N011338 N011338 W0834349 W0834349
N011338 N011338 W0834349 W0834349
N011300 N011340 W0834320 W0834400
N011000 N020000 W0834000 W0835000
N003000 N014000 W0834000 W0863000
N011337 N011338 W0834348 W0834349
S192706 N011400 W0834349 W1294614
Record ID: 1998039458
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