SLIDE (Wellington): summary of borehole image and full-waveform acoustic processing and interpretation results

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Griffin AG, Kellett RL, Lawrence MJF. 2019. SLIDE (Wellington): summary of borehole image and full-waveform acoustic processing and interpretation results. Lower Hutt (NZ): GNS Science. 36 p. (GNS Science report; 2019/41). doi:10.21420/6QBV-C705.

This report provides a summary of data and results derived from eleven geotechnical boreholes that were drilled at six localities in and around the Wellington city area. The borehole localities are Orchy Crescent (Southgate), Priscilla Crescent (Kingston), St Gerard’s Monastery (Mt Victoria), Ngauranga Gorge, along with Dungarvan Road and Bendigo Grove (State Highway 2 near Newlands). Seismic velocity data (full-waveform acoustic logs or downhole seismic data) were acquired in all boreholes and acoustic and optical borehole image data were acquired in all but the Priscilla Crescent boreholes. Two seismic cone penetrometer tests were conducted at Priscilla Crescent. All data types were loaded into in house software packages where log quality control was undertaken before being reprocessed. The borehole image data was generally of moderate to good image quality but there were locally significant intervals of poor to bad image quality in which geological features were poorly resolved or not visible at all. Rare lithological layering was observed but displayed no consistent orientations in any boreholes. On acoustic borehole images fractures are dominantly of low acoustic amplitude (dark). High acoustic amplitude (bright) fractures are very rare. Both bright and dark fractures occur on optical borehole images, but bright fractures are dominant. Fracture appearance is most like consistent with a combination of fractures being clay smear, open and filled with drilling fluid, or mineralised. Fracture strike orientations are variable but appear consistent with the regional geology of the Wellington area. Fracture density is commonly high but can be highly variable. In some case fracture densities are affected by image quality. Very rare and low confidence in-situ stress features strike N-S to NE–SW. The full-waveform acoustic data were of variable quality. The compressional wave was resolved in most boreholes. Poor borehole conditions prevented the shear-wave from being picked in some boreholes. The derivation of the geomechanical properties relied on a combination of observed data and empirical relationships. The downhole seismic and the seismic cone penetrometer tests helped constrain the velocity model for the shallow sections of hole. Although there are some issues with the full-waveform data and the derived geomechanical values, the results from the geomechanical analysis match the fracture occurrence. (auth)