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doi:10.2204/iodp.proc.335.202.2015

Results

Figure F5 summarizes the results of shear wave anisotropy analysis for Hole 1256D. The tool azimuth profile indicates that the tool was not rotating consistently inside the borehole over most of the logged interval. At least three depth intervals, however, exhibit a consistent tool rotation (~400–500 and 625–760 mbsf and deeper than 980 mbsf). Caliper logs show large washouts (borehole enlargements) throughout the hole, but the caliper is nearly in gauge at 490–520, 600–625, 700–780, and 850–910 mbsf and deeper than 1080 mbsf. With poor borehole conditions, obtaining reliable anisotropy parameters is not feasible. Waveform processing was unable to achieve consistent near-zero values for the minimum cross-energy, so fast and slow components of the shear wave field are poorly distinguished. The best results from the processing show several intervals where this parameter is close to zero: ~500, ~710, ~820, and ~990 mbsf and deeper than 1080 mbsf (Fig. F5). However, even in these zones, the maximum cross-energy is not very high, indicating low energy and little shear splitting. Consequently, there is almost no difference between the fast and slow shear wave slownesses at these depths, suggesting that either the formation is isotropic in a horizontal plane or that anisotropy could not be detected or both (the slowness resolution of the DSI is 2 ?s/ft [i.e., 1%–2%] of the measured slowness range). The computed slowness anisotropy is 1% of average V_S (Fig. F5) and is quite variable. The computed time anisotropy is much greater, up to 10% in certain intervals, but these values are likely overestimated because of the poor borehole conditions. As expected, there is no dominant orientation of the fast V_S azimuth in isotropic intervals (Fig. F5). Fast shear azimuth is variable and averages 90°, the median value of the range of all possible orientations (0°–180°); therefore, it does not help to differentiate a fast V_S azimuth. Hole conditions toward the bottom of the logged interval are consistent, however, and the computation of anisotropy parameters is reliable. Over this interval, our sonic anisotropy analysis suggests that the lower oceanic crust is isotropic in a horizontal plane.

Figure F6 illustrates the sonic waveform data and the anisotropy computation over the lower interval of Hole 1256D (1140–1190 mbsf). The interval is characterized by a relatively large difference between the minimum and maximum cross-energy values (Fig. F5) and relatively high coherence of the fast and slow components (Fig. F6). The fast and shear slowness curves can be reliably determined and are nearly identical. The estimated anisotropy is close to zero, which suggests that the formation is isotropic in the plane perpendicular to the borehole (i.e., in the horizontal plane). This interval corresponds to the sheeted dike section of the oceanic crust (Fig. F2); given the vertically oriented morphologies in this formation that are similar in all azimuthal directions, it is anticipated to be isotropic in a horizontal plane unless a strong stress imbalance or preferred crack orientation is present. Low estimates of anisotropy, however, are also computed at shallower depths in intervals where the borehole conditions are good and sonic logging waveforms have high coherence (i.e., 310–340, 470–530, and 970–1005 mbsf and deeper than 1060 mbsf). Within all of these intervals, dispersion curves show overlapping curves for fast and shear slownesses at all frequencies (Fig. F7). From these consistent analyses of sonic anisotropy over select intervals in Hole 1256D, our results clearly suggest that at sonic logging frequencies these sections of upper oceanic crust are characterized by isotropic shear wave velocity in a horizontal plane.

In summary, poor borehole conditions and insufficient tool rotation presented significant challenges for sonic anisotropy analysis in Hole 1256D. Anisotropy parameters could be reliably determined from the logging data over limited depth intervals. They clearly suggest that the upper oceanic crust is nearly isotropic in a horizontal plane, with V_S anisotropy estimates between 0%–1% of the average horizontal V_S, and/or that weak shear-wave anisotropy is below the detection limit of the DSI used to acquire the data.

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