Open Access
Review
Table 1
Comparison of S-wave birefringence detection methods.
| Alford-type method | Azimuthal DTS Rotation-Scan | |
|---|---|---|
| T-R ANI (Algorithm-1) | Array ANI (Algorithm-2) | |
| PRINCIPLE | Minimizing cross dipole energy, or minimizing the off-diagonal elements of the 4-term matrix of the Source(s) to Receiver(r) signals (XsXr, XsYr, YsXr, YsYr). | Scan of S-wave slowness/velocity over 180° azimuth range, to INDEPENDENTLY determine the azimuths of Vs-max and Vs-min. |
| ASSUMPTIONS | The propagation medium between Transmitter to the Receiver array is considered homogeneous, with same anisotropy axes, possibly stratified axisymmetric. | The propagation medium is considered homogeneous over the Receiver array ONLY. |
| The SHORT detection interval yields a higher depth resolution. | ||
| Emitted flexural S-wave particle motion remains LINEAR in the borehole formation located in the immediate source vicinity. | No polarization constraint: Emitted S-wave imparted into the formation can be in any form, LINEAR, ELLIPTICAL, CIRCULAR… | |
| Equal orthogonally emitted signals, in same shape, same amplitude. | Orthogonally emitted signals may be a bit different. | |
| For BOTH methods: | The borehole ruggedness and heterogeneous borehole altered zone over the detection depth interval may alter the accuracy of results. | |
| Differential attenuation (QDs) between the two principal S-wave modes is NOT considered. | When NO velocity anisotropy is detected, a scan of S-wave attenuation (Att-Rot-Scan) versus azimuth can be run to yield the S-wave attenuation anisotropy & the Differential attenuation (QDs). | |
| Observed RESULTS | Fast-S-wave Azimuth is searched FIRST, resulting in INACCURATE to FALSE principal S-wave principal azimuths when hypotheses are unsatisfied, or in case of weak S-wave anisotropy. | Fast-S-wave Azimuth is derived from the azimuthal STC slowness scan, resulting in higher azimuth accuracy of principal S-wave modes, and higher depth resolution. STC routine could be improved where the receiver array is located over a strong velocity contrast. |
| The borehole ruggedness and heterogeneous borehole altered zone over the detection depth interval may alter the accuracy of results. | ||
| The larger the detection interval, the larger potential bias… | ||
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