The following discussion concentrates on five smooth zones within the lower igneous unit. We assume that geometry influence is minimal in these regions, so local energy and velocity minima occurring within these zones can be regarded as mainly permeability induced.
Figure F8 shows borehole diameter together with Stoneley velocity and energy from 460 to 540 mbsf and highlights smooth borehole sections with blue shading. In spite of the uniform borehole geometry, we observe local minima of velocity and energy at 468, 492, 500, 508, and 518 mbsf (red arrows). We suppose that permeability changes occur at those locations and therefore correlate our observations to fracture observations on FMS images and core samples in the next section.
To test the hypothesis of nongeometry-induced energy variations in smooth borehole sections, we take a close look at fracture abundance in the above defined regions. Fractures were observed during Leg 205 on the recovered core material by a thorough visual inspection (Shipboard Scientific Party, 2003b) and interpretation of FMS logging data.
These data sets are compiled in Figure F9 together with borehole diameter and energy changes. Logging data from the FMS tool complement core investigations in areas of a low recovery, such as at 466–470 mbsf, where the common ODP curation practice attributes the whole recovered core material to the top of the cored section.
We found that, in contrast to our first expectations, breakout zones do not show significantly more fractures than smooth zones. Only a small increase in number of fractures with depth is noted, which might fit the observation that the lower igneous unit is more fractured below 514 mbsf.
Energy losses within smooth borehole intervals are again marked with red arrows in Figure F9 and might be linked to a locally increased fracture abundance in those intervals. At the uppermost level, 468 mbsf, the number of fractures from the FMS is at its locally maximal value of four fractures per meter. The local maximum in fractures observed on core material occurs at shallower depths from 464 to 466 mbsf. This difference might arise from low core recovery and the curation practice. Slightly below 490 mbsf, in the vicinity of the second energy minimum, we find a local high in fracture abundance on core material and a rising number from FMS observation. This correlation is much clearer at 500 mbsf where both observations show a high fracture abundance in a smooth borehole section. At 508 mbsf, the energy decrease might be tied to the maximum in fracture number from FMS data. The last of the five smooth zones shows minimal energy at ~518 mbsf, which coincides very well to the overall maximal fracture number in the FMS data and the relative high abundance of fractures in core observations.
Within each of the five sections of uniform borehole geometry, we can assign an increased number of fractures, observed on FMS images and on cores, to a minimum in Stoneley wave energy. Unfortunately, a further correlation of these observations with petrophysical findings, such as subsections of the lower igneous unit or degree of alteration, is not possible. Our analysis is much too limited to the smooth zones for such a purpose and prohibits a detailed comparison to the whole logged igneous section.