What Causes Time Line Reflections, and What Do They Tell Us About the Petrophysics of Rocks? Posters
Presiding: H Tono, Department of Earth and Ocean Sciences, Duke University; G Karner, Lamont-Doherty Earth Observatory
T43A-01 1330h
Seismic Reflections in a Materially Uniform Sedimentary System
Following the rules of sequence stratigraphy, seismic profile interpreters regularly encounter reflections that are associated with sequence timeline boundaries, whereas from a physical standpoint they represent acoustic boundaries. What produces reflections from materially uniform sections where no contrasts in bulk material impedances exist? In an attempt to understand whether grain size sorting associated with sediment deposition can create physical conditions resulting in discrete changes in acoustic impedance, we undertook a laboratory investigation of seismic reflections from time-separated and grain size controlled sediment deposits. Two-layer deposits were created in several separate tanks by introducing two, time separated, pulses of sediment. The bulk properties of the sediments were identical, including their grain size distributions. In sediments with narrow distributions only weak reflectors are observed. In sediments with increasing broader distributions of grain sizes, increasingly stronger reflections were observed, with the distribution width correlating with the reflection amplitude. Evidently, the more heterogeneous in terms of grain size an otherwise uniform package of sediments is, the more likely it is to yield timeline associated reflections, indicating that sedimentary processes can create events that are both timelines and acoustic boundaries.
T43A-02 1330h
Relationship Between Strong Seismic Reflectors In Young Igneous Oceanic Crust And Borehole/Core Measurements Of Physical Properties
Multichannel seismic data from a tectonic exposure of young oceanic crust, 30° N on the western flank of the Mid-Atlantic Ridge, display a prominent reflector at 0.2-0.5 s two-way travel-time. The reflector is continuous over several km and has a gently domal shape that is reminiscent, although not directly bottom-simulating, of the domal morphology of the Atlantis Massif. Expeditions 304 and 305 of the Integrated Ocean Drilling Program have drilled past 1000 m at this site. Physical properties measurements on core samples and in-situ density and velocity measurements provided by downhole logging provide a basis for interpreting the relationship between the local reflectivity at the site and physical properties of the section. Onboard density and compressional velocity measurements of core sample show a relationship to borehole logging estimates of seismic velocity with depth. Lithologically, the recovered section is essentially of near-constant density gabbros. These data can be used to define acoustic impedance and hence offer an explanation for reflectors in oceanic crust. In particular, laboratory-measured velocity measurements are systematically higher than logging sonic velocities from the seafloor to ~220 meters below seafloor (mbsf). From 220-780 mbsf, the laboratory and logging velocities are coincident. The pattern in the upper ~220 m can be explained by the closure of fractures, the rocks of which are not sampled by coring. Preliminary analysis suggests there may be a correspondence between the degree of alteration, which is related to variation in olivine content within gabbroic layers, the degree of fracture closing, and the velocity and density of the oceanic crust. The progressive closing of fractures within the upper few 100's of meters is consistent with caliper data. The velocity distribution with depth might be explained in terms of the closing of macro fractures shallower than 220 mbsf, the systematic closure of micro-cracks between 220-780 mbsf, followed by a systematic decrease below 780 mbsf in rock volume determined by Poisson's ratio and overburden stress. Derived synthetics underscore the importance of crack closure and potential effects of alteration in controlling seismic reflectivity.
T43A-03 1330h
Pore structure effects on elastic moduli - porosity relationships in carbonate rocks
One characteristic of carbonate rocks is the lack of correlation between velocity and porosity. This is often due to diagenetic changes in the pore structure. Understanding the effect of rock texture on acoustic properties of carbonate rocks is vital for reliable porosity determination from seismic signatures. In carbonates, the pore type and porosity mainly controls the velocity and herewith the impedance contrast. The carbonate pore type combines information about the pore structure geometry and grain to grain contacts. In this study we investigate systematically the effect of pore types on velocity on a dataset of 127 samples from 3 different pure carbonate formations. Their porosity covers a range from 3% to 50%, the compressional velocity span from 2900 m/s to 6300 m/s. Experimental results show that theoretically impedance contrasts can be solely caused by changes in pore types, without the need for changes in porosity or mineralogy. In order to highlight the importance of pore structure, the data is presented in normalized bulk moduli-porosity space, hereby eliminating effect of differing calcite and dolomite velocities. Samples of the same pore types shows distinctive moduli-porosity best-fit curves. Empirical relationships between moduli and porosity have been derived for carbonates with high amount of microporosity and vuggy dolostones. Vuggy Miocene dolostones from ODP Leg194 show a considerably stiffer rock frame then their microporous counterparts. Vuggy dolostones show non-linear moduli-porosity relationship, with bulk modulus stiffness outside the Nur's modified Voigt boundary. Qualitative image analyses objectively characterize the pore structure and reduce the uncertainties present in velocity scatter. We measured macroporosity from digital image analysis and related it to total plug porosity to derive microporosity. Image macropores are all pores visible in thin section, generally pores bigger than 30 microns in diameter. The microporosity gives a good correlation to the velocity and constrains the microporosity uncertainty. Furthermore, the velocity-microporosity best-fit curve is in close vicinity of the Wyllie's time average trend line. Image macroporosity shows a positive correlation with velocity deviation calculated from velocities of Wyllie's time average equation. Pore space stiffness calculated from the poroelastic properties alone, uniquely quantify the velocity deviation at a given porosity. Recrystallized dolomites with secondary vuggy porosity show distinctive higher pore space stiffness values of around 0.2 compared to high microporosity limestones with lower pore space stiffness (0.1). These results indicate that there is hope for pore structure inversion in carbonate rocks.