Abstract
Obtaining accurate shallow ultraslow formation S-wave (shear) slowness is crucial for seismic exploration and hydrocarbon reserve assessment. Dipole acoustic logging is an effective method for estimating slow formation shear slowness. However, for the large borehole and unconsolidated soft sediments of the shallow marine environment, conventional dipole logging is often ineffective. To address this issue, we develop tool eccentricity for dipole S-wave logging. This study provides the theoretical justification of our method. The dipole acoustic wavefield of an eccentric tool is studied using a dual-cylindrical coordinate system in connection with the translational addition theorem for cylindrical Bessel functions, which provide analytical solutions to analyze the effects of tool eccentricity and formation elastic property. The results indicate that tool eccentricity in such an environment can substantially increase the dipole-wave amplitude for estimating the formation shear slowness. The S-wave logging can even be conducted by positioning the dipole tool against the borehole wall. This logging configuration is tested in a shallow test well with an oversized borehole. The measurement results are compared with their centralized counterparts. The measured data indicate that compared with the centralized tool measurement, eccentering the dipole tool can greatly enhance the measured S-wave amplitude and improve the accuracy of the formation shear slowness measurement, thereby providing an effective method for obtaining formation shear slowness in shallow soft sediments.
Paper Information:
Rao B, Su Y D, Lin J Q, Liu Y, Li S Q, Tang X M. 2025. Ultraslow formation shear slowness estimation in a large borehole using an eccentric dipole tool. Geophysics, 90(6): D111-D122, https://library.seg.org/doi/10.1190/geo2024-0675.1.
