Seismic Inversion Model Research Articles

Scattering attenuation as a function of depth in the Upper Oceanic Crust

Abstract 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 V p 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 = Q 0 −1 e βφ , where Q 0 −1 = 0.004, β= 25, that may be applicable at other oceanic crust locations and useful for constraining seismic inversion models.

Lateral variations in upper mantle thermal structure inferred from three‐dimensional seismic inversion models

Global shear wave velocity variations in the upper mantle (Woodhouse and Dziewonski, 1984) are employed as a basis for inferring lateral heterogeneities in temperature or composition at a depth of 150 km. Using experimentally determined elastic moduli for the olivine solid solution and Hashin‐Shtrikman bound theory, Vs and its partial derivatives with respect to temperature or composition (iron content) are calculated for the depth of interest. Based on these results, the observed seismic Vs variations are interpreted as temperature and compositional deviations relative to a spherically symmetric reference model.If it is assumed that the Vs variations primarily represent lateral temperature changes, then the present results can be used to construct a global surface heat flow map from the geotherm families of Pollack and Chapman (1977). Our inferred heat flow patterns, calculated independently of actual heat flow data, show clear agreement with the surface heat flow maps proposed by Chapman and Pollack (1975) and Chapman (1985). This result is consistent with and supports thermal structure as the primary governing factor in the generation of upper mantle lateral velocity variations. Compositional changes appear to correlate with temperature, but affect velocity structure to a lesser extent.

Use of Inversion for Reservoir Delineation Within a Fluvial Depositional Environment, Eromanga Basin, Queensland, Australia: ABSTRACT

Reservoir delineation within fluvial deposits has been a source of concern and disappointment to both exploration and development groups. Seismic methods have greatly enhanced the ability to predict the location of reservoir units within clastic sequences. Conventional seismic processing and displays of data still fail to adequately define these subtle reservoirs in many cases. Within a complex stratigraphic sequence, detailed information concerning the nature and distribution of reservoir units can be obtained through reprocessing and inversion of seismic data. The reprocessing requires frequency enhancement, detailed velocity analyses, and phase adjustment (using well data as a standard). The inversion process involves quantification, calibration, and careful selection of display parameters. Some well information is essential for modeling and calibration. Color displays of the well data modeling are called Earthlogs. This paper will discuss the application of seismic data inversion and Earthlog models within the Jurassic clastic sequence located in the Eromanga basin, Queensland, Australia. One of the main reservoir units (Hutton formation) was deposited within a braided-stream depositional environment.