Validation of the applicability of the PS logger borehole probe for diverse geotechnical scenarios

The PS logger is a low frequency dipole-based sonic logging probe which is used widely to measure P- and S- wave velocities of earth materials. It is of particular interest to offshore environments where the latter can be used to derive the small-strain stiffness G_max, an important measurement in offshore construction. The method relies on the indirect excitation of the borehole via a dipole pressure wave excited in the borehole fluid, allowing shear wave velocity (Vs) to be measured in both fast and slow formations. In this study, the theoretical basis for the PS logger is presented, alongside the results of recent field projects which provide answers to key questions regarding the viability of the tool for a range of situations, and to the interpretation of results for widely ranging ground conditions. These include a comparison of results from both uncased (open) and cased boreholes, and from a wide range of ground velocities (Vs ≈ 220m/s to 3760m/s), and for varying borehole sizes. Full waveform analysis of time-series data is undertaken and underlines the need for consideration of theoretical principles in data interpretation. Ultimately, the results demonstrate the wide applicability of the tool beyond the geotechnical limits of traditional logging tools, which is attributed largely to the low frequency nature of the PS logger.