Comparison of Seismic Anisotropy Measurements From the ICDP Outokumpu, Finland Borehole With Theoretical Model Results
The 2.5 km deep ICDP (International Continental Scientific Drilling Program) borehole located in Outokumpu, Finland was drilled in order to gain a more complete understanding of the regional geology. Outokumpu is the site of a historic base metal mine, and large scale crustal seismic reflection surveys have previously been used to explore the geology of the area. Glacial deposits in the region overly a biotite-rich schist that extends to a depth of ~1300 m. The schist is expected to be strongly anisotropic as a result of lattice preferred orientation (LPO) of the biotite and of aligned fractures. In May 2006 the University of Alberta executed a multi-depth multi- azimuth walk-away VSP using the Outokumpu borehole in order to study the seismic anisotropy. In addition, the 2006 survey included a high resolution zero offset VSP, a far offset VSP and a reflection/refraction profile collected along the same seismic lines as the walk-away VSP. The walk-away VSPs included a three component receiver located at depths of 1000, 1750 and 2500 m, and were processed with static corrections, removal of powerline harmonics, polarization filtering and, finally, the application of a tau-p transform. The greatest anisotropy was observed in the southeastern walk-away VSPs, with the velocity ranging from ∼5330-5950 m/s between 50-1000 m in depth, and up to ∼6150 m/s between 1000-1750 m in depth. Laboratory measurements made by other researchers using schist core samples from the borehole indicate the schist has an intrinsic orthorhombic symmetry. Forward modeling of an orthorhombic media with fluid- filled cracks was done in an attempt to fully describe the velocity anisotropy of the schist observed between 50- 1000 m depth. Good agreement is found between the theoretical model and the walk-away VSP measured velocities. The theoretical model for this upper 1 km of the subsurface predicts the foliation plane of the schist to be nearly horizontal, which corresponds with the known geology of the area. The dip of the cracks in the theoretical model can be compared to the angles of fractures visible in borehole televiewer data, and the azimuthal orientation compared to the known geology of the region. Further, theoretical and walk-away VSP measured velocities can be compared to laboratory measurements made on a range of core samples from the depths of interest.
Seismic Modeling of Geological Heterogeneity: an Application to Gas Hydrate Reservoirs of ICDP Program
Natural gas hydrates, a type of inclusion compound or clathrate, are composed of gas molecules trapped within a cage of water molecules. The occurrence of gas hydrates in permafrost regions has been confirmed by core samples recovered from the Mallik gas hydrate research wells located within Mackenzie Delta in Northwest Territories of Canada. In hydrate-bearing sediment, compressional and shear wave velocities, as well as the attenuation of seismic signals, depend sensitively on the spatial variability of both the hydrate distribution and the subsurface lithology. Therefore, seismic wave velocities and energy attenuation have been used to estimate gas hydrate concentrations. For those estimates to be accurate, a model is required that can account for the impact of both large- and small-scale heterogeneity on the transmission of seismic energy. An algorithm based on stochastic medium theory is developed to delineate the subsurface hydrate distribution and to assess the attenuative effects of local heterogeneities. Using a modified Biot-Gassmann theory, the statistical parameters obtained from Mallik 5L-38 borehole logs, and the horizontal correlation length estimated from acoustic impedance inversion, we constructed a heterogeneous model based on borehole log data from the Mallik permafrost gas hydrate research site in the Canadian Northwest Territories. The model can be used to estimate the volume of hydrate in place and results suggest a range of total hydrate content between (528 ∼ 768)× 106 m3/km2, nearly an order of magnitude lower than early estimates in which small-scale heterogeneities were not accounted for. The heterogeneous models were further used with parallel 3-D viscoelastic Finite Difference modeling software to simulate a near offset Vertical Seismic Profile and cross borehole numerical surveys. The synthetic results indicate that the observed attenuation of seismic energy may not represent the intrinsic attenuation of hydrate-bearing sediment, but instead can largely be attributed to scattering from small-scale heterogeneities, and the highly-attenuative, "leaky mode" propagation of energy through larger-scale heterogeneities.
Magnetostratigraphy and Environmental Magnetism of Laguna Potrok Aike: Preliminary Results from the ICDP Project PASADO
ICDP Lake Drilling at Laguna Potrok Aike, Argentina: a Progress Report of the PASADO Project
Offset Vertical Seismic Profiling in the Bosumtwi Impact Crater, Ghana (ICDP)
Impact cratering processes are known to have contributed the earth's evolution. In recent years, geoscientific studies of impact craters have helped in our knowledge of the composition and evolution of the earth's continental crust and in providing insights to the essential processes associated to impact cratering. The Bosumtwi impact crater, located in central Ghana, is the largest young and well preserved impact crater in the world. Seismic studies were conducted as a part of an integrated drilling, geologic and geophysical program in order to understand the structure of the impact crater whose target rocks are concealed by a lake (Lake Bosumtwi) and sediments. Offset Vertical Seismic Profiling (VSP) data was acquired in borehole LB08A to provide information on the in situ wavefield and physical properties of the impact crater. Results from analyzing first break attributes of data from the borehole sensors suggest that offset VSP acquisition geometry with a single sensor fixed at depth can be used as a quality control tool for shot positioning and seismic spectral content in a seismic survey. Estimates of geometric spreading within the sediments were also obtained. Such information can be valuable for improving seismic imaging of the crater by accordingly accounting for amplitude losses caused by geometric spreading in surface seismic data. References: D. R. Schmitt, B. Milkereit, T. Karp, C. Scholz, S. Danuor, D. Meillieux, and M. Welz, 2007, In situ seismic measurements in borehole LB-08A in the Bosumtwi impact structure, Ghana: Preliminary interpretation, Meteoritics and Planetary Science 42, 755-768 Scholz, C. A., Karp, T., Brooks, K. M., Milkereit, B., Amoako, P. Y. O., and Arko, J. A., 2002, Pronounced Central Uplift identified in the Bosumtwi impact Structure, Ghana, using multichannel seismic reflection data: Geology, Vol. 30, No. 10, P. 939-942.
34S/32S and 18O/16O ratios of dissolved sulfate from interstitial water samples above gas hydrate bearing sediments of IODP Expedition 311, Cascadia margin
Microbially mediated sulfate reduction affects the isotopic composition of dissolved and solid sulfur species in marine sediments. Although several details of the fractionation process remain controversial, the overall process is well understood and can be described as the sum of several mass dependent fractionations during the stepwise reduction of sulfate to sulfide. Experiments and field data show that the 18O/16O of sulfate is also modified in the presence of sulfate-reducing microorganisms. Here we use a reaction transport model to analyze these processes and to constrain the rates of organotrophic versus methanotrophic sulfate reduction. Our results show that even in cases where sulfate concentration decline in a linear fashion, up to 50% of all sulfate is consumed by organotrophic sulfate reduction.
Paleo-sea Surface Conditions in the NW North Atlantic During Recent Interglacials
Sites IODP-1305, ODP-646 and IODP 1302/1303 are ideally located for the documenting of Pleistocene climatic and glacial history over Greenland and eastern Canada, in addition to providing information on thermohaline conditions notably with regard to convection in the Labrador Sea (LS) and deep North Atlantic Water (NADW) sources (see also Hillaire-Marcel et al., same meeting). At site 1305 (Eirik Ridge), at the entry of the modern Greenland current, North-East Deep Atlantic Water and Denmark Strait Overflow Water into the LS, clay minerals and Nd-isotopes indicate that sedimentary supplies during marine isotope stages (MIS) 1, 5e, 7 and 9 differed from those of earlier interglacials, for which sedimentary supplies suggest soil development over volcanic rocks from partly deglaciated eastern Greenland. Pollen analyses also indicate denser vegetational cover than at present over southern Greenland during most interglacials prior to MIS 1, with specific features for each of them. For example, rich pollen assemblages dominated by spruce suggest the spreading of boreal forest vegetation over an ice-free southern Greenland during MIS 11, thus a significantly reduced ice-volume in comparison to those of more recent interglacials. Sheltered from direct influence of the western boundary undercurrent, IODP Site 1302/1303 (Orphan Knoll) provides a ca. 700 kyrs, uniform and continuous record of linkages between the northeastern Laurentide Ice-Sheet surge area (cf. Heinrich events) and the North Atlantic. The site also provides critical information about the Atlantic Meridional Overturning Circulation (AMOC) since all water masses contributing to the modern NADW still preserve their identity in the overlying water column. For example during MIS 5e, benthic foraminiferal assemblages, oxygen and carbon isotope records of Site 1302/1303, compared to those of site 1305, indicate distinct bottom water masses in the inner vs. outer basins of the LS, thus a distinct AMOC. Similarly, isotopic measurements in Globigerina bulloides suggest a warming trend in surface water toward the end of MIS 5e, in contrast to the present interglacial that shows an early thermal optimum. During most of MIS 5e, dinocyst data also indicate sea- surface temperatures much higher than present (+3°C in winter and +5°C in summer), too high for convection to occur in the LS. It is concluded that beside specific features for each interglacial, the Holocene stands clearly out, both for the intense convection in the LS from mid- to late Holocene, and the relative stability of the Greenland ice-sheet until the Present.