Ocean Drilling Program (ODP) Hole 990A was drilled along the Southeast Greenland margin during Leg 163 in a water depth of 542 m. The hole was drilled to a total depth of 343 meters below seafloor (mbsf), including 212 m of sediment overlying 131 m of igneous basement comprising 13 basaltic lava flow units (Duncan et al., 1996). Core recovery within the basement was ~80%. The lava flows, which were subaerially erupted from a volcanic rift similar to the modern-day Iceland rift zone, have been dated to ~56 Ma (Tegner and Duncan, Chap. 6, this volume), the approximate time of the initial opening of the North Atlantic Ocean. (It is presumed that a hot spot existed at this site and time and caused extensive volcanism.)
Downhole sonic logs from similar geologic sections (e.g., Hole 642E on the Outer Vĝring Plateau and Hole 917A on the Southeast Greenland margin) have shown the P-wave velocity to vary within subaerially erupted, basalt flow units (e.g., Shipboard Scientific Party, 1987; Barton et al., 1989; Shipboard Scientific Party, 1994; Planke, 1994). The P-wave velocity is lowest at the flow top and bottom (typically 2-5 km s-1) and is higher in the massive, central region of the flow unit (typically 5-6.5 km s-1) (e.g., Planke, 1994; Duncan et al., 1996; Planke and Cambray, 1998; Planke et al., Chap. 2, this volume). The variation of P-wave velocity is caused by variations in the moduli of the solid and variations in porosity and pore shape distributions (assuming that the pore fluid is constant). Generally, there is a greater percentage of low-modulus alteration products (e.g., clays) and pore space in the flow tops and bottoms than in the massive central regions (Shipboard Scientific Party, 1996; Planke and Cambray, 1998). This difference may cause the observed variation of seismic velocities.
Voids, in the form of vesicles and cracks that occur in subaerially erupted basalts, are the result of degassing of the lava and of fracturing and alteration. The amounts and types of voids affect the seismic velocities of rocks (e.g., Cheng and Toksöz, 1979; Berryman, 1980; Wilkens et al., 1991; Berge et al., 1992). Seismic velocities in basalt can be readily measured both in situ and in the laboratory. Thus, by using appropriate crack theories, we should be able to model void types and distributions within igneous flow units from high-resolution in situ and laboratory seismic data. The primary purpose of this research is to use the theory of Kuster and Toksöz (1974) to assess the effects of crack variations and apparent grain moduli on the seismic velocities of subaerially erupted basalt recovered from Hole 990A. The principal results are that (1) there is a strong relationship between P-wave velocity and porosity for this suite of samples, suggesting a similarity of pore shape distributions among the samples, (2) a simple, average, normalized aspect ratio distribution explains the velocity data from the massive central region of the basaltic flow units to within a root-mean-square (RMS) misfit of 0.050 km s-1, (3) a greater percentage of higher aspect ratio (more equant) voids is present in the more altered samples, and (4) grain densities (an alteration index) and apparent grain moduli of the basaltic samples are directly related.