RAW DATA AND RESULTS

Site 1253 was logged from 564 mbsf up to 413 mbsf with 1272 shots and a vertical resolution of 6 in corresponding to the receiver spacing. We present acoustic data from ~80 m of the lower igneous section, 30 m of sediment above it, and 17 m of the sill (Fig. F2). We show records of monopole modes (P&S and Stoneley) and concentrate on the results of semblance processing for the Stoneley wave in the lower igneous section.

Influence of Borehole Diameter on Waveform Records

Borehole geometry, essential for the interpretation of acoustic data, was determined using caliper data from one run of the triple combo logging string and two passes of the FMS-sonic tool string. Because only two opposing hydraulic arms out of four on the FMS tool worked, the second logging pass had a 90° rotation from the orientation of the first pass, thus providing a more complete picture of the borehole shape. Figure F5 shows the caliper measurements that reveal the boundaries from 10 in casing to the 14 in open hole at 413 mbsf and the narrowing to a 9 in hole at 423 mbsf. The sediment section from 430 to 460 mbsf is strongly washed out, so the caliper arms lost wall contact. The borehole has some smooth intervals in the lower igneous section, such as the one at ~465–475 mbsf, but generally shows numerous breakouts ranging up to 4 in. Figure F5 indicates that caliper measurements could not be unambiguously reproduced in the three runs (for example, in the breakout zone at 510 mbsf). The borehole seems to have a complex shape with changing ellipticity, which complicates Stoneley wave interpretation and influences acoustic data recorded in P&S and Stoneley modes. These so-called iso-offset sections (Fig. F5) show the full waveform records of one specific receiver (here the near receiver) with depth.

In P&S mode, data quality is generally good (Fig. F5, left); the first arrivals, the P head waves, are clearly visible and mimic borehole geometry. The S-wave, following the P-wave, cannot be identified in this illustration. The direct fluid wave is the strong onset at 1.8 ms. Arrivals are delayed, such as in the breakout zone at ~510–515 mbsf. A series of enlargements measured from 525 to 535 mbsf is only poorly portrayed in acoustic data records, and we suspect that they are disturbed by the strongly varying diameter or by a nonconstant tool movement.

Acoustic data recorded in Stoneley mode (Fig. F5, right) show a broad band of Stoneley amplitudes at ~2 ms but also a higher amount of noise. In the sediment section, the Stoneley arrival is strongly distorted, and perhaps the Stoneley mode could not develop. As in the P&S mode, Stoneley waveforms are displaced and disturbed by borehole irregularities at 510–515 and 525–535 mbsf.

Compressional and Shear Velocities

After bandpass filtering following Harrison et al. (1990), we calculated velocities and energies from raw unnormalized data. The parameters used in our semblance analysis are summarized in Table T2 for P&S and Stoneley modes.

S-wave velocities (Fig. F6) differ from ODP results even in a smooth section at ~465–475 mbsf, although data quality is good. Part of the deviation is explained by the fact that the shipboard processing calculated S-wave velocities from dipole measurements while we processed the monopole data. Dipole mode is usually only used in unconsolidated formations to derive S-wave velocity. Another explanation might simply be that our algorithm "sticks" at one semblance maximum at the lowest limit of velocity, which is water velocity. This occurs, for example, from 410 to 460 and 490 to 510 mbsf.

In contrast, the picked P-wave velocities agree well throughout the profile, with the exception of the sediment section and from 495 to 510 mbsf. In the sediment section, P-waves might be difficult to pick because of geometric disturbances (cf. Fig. F5). We suspect that from 495 to 510 mbsf, the algorithm picks the S-wave instead of the P-wave in the labeling process. However, good correlation in a great part of the borehole indicates that our algorithm works reliably.

Stoneley Velocity and Energy Estimates

Stoneley wave data quality was significantly improved by bandpass filtering, and our processing yields almost exactly the shipboard velocities in the majority of the logged igneous sections. Figure F7 shows that, from 460 to 490 mbsf, deviations between the two data sets are mainly <20 m/s and increase to 40 m/s below 490 mbsf. Greater differences in Stoneley wave velocities are in the wide sediment section, where hole conditions and data quality are obviously bad. Figure F7 also shows the loss of energy in decibels compared to the location of highest energy in the profile which is at 405 mbsf in the cased section. Without the possibility of comparing our results to the output of different processing methods, it is difficult to estimate the reliability of these data. However, because the variations in energy correlate, as expected, with variations in velocity (which were shown to be consistent with shipboard results), we assume a similar robust result.

Both energy and velocity variations correlate inversely with borehole diameter. The trend shows higher velocity and energy in smooth and narrow borehole sections, such as the one at ~465 mbsf, and lower velocity and energy in wider sections or breakout zones, such as the one at ~495 mbsf.

This correlation agrees with our theory but prohibits a clear identification of possible permeable zones. Because geometry and permeability can both attenuate and slow Stoneley waves, an interpretation of permeability distribution from these measurements alone is not possible. For this reason, we concentrate our discussion below on smooth borehole sections and assume that geometric effects therein are minimal. Figure F8 is a compilation of borehole diameter, Stoneley velocity, and energy from 460 to 540 mbsf and shows the smooth borehole sections marked by the light blue shading. These five zones are all within the lower igneous unit; the washed-out sediment section and the irregular borehole in the sill are not taken into account for further discussion.

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