Dipole Shear Logging In Soft Sediments: Shear Wave Phase Shifts Through Thin Beds

Dipole shear logging makes routine measurement of shear slowness possible in soft formations. Recently acquired data indicate that shear slowness can often reach 500 ?sec/ft or more in coastal sediments. With such slow shear velocity, these soft sediments can produce dipole waveform features not commonly seen before. One example is the sudden and large phase shift of the direct shear wave or flexural mode when a thin layer is embedded in the formation. In many cases, velocity analysis using an array of receivers may not be able to resolve the thin-bed feature shown in the waveforms. To investigate the dipole waveform features in soft sediments, we use finite difference synthetics simulating a dipole array tool crossing four different features: a thin bed, a bed boundary, a borehole cave-in and a borehole enlargement. We show shear wave phase shifts for each discontinuity and indicate how they relate to the transmitter-receiver configuration. The variation of the shear wave phase shift is determined by the shear slowness contrast, bed thickness and borehole size. The results suggest that features such as phase shifts observed in waveform displays are frequency dependent. In combination with caliper measurement, the waveform data can be used to further constrain and improve estimates of the slowness and thickness of soft sediments.