In-situ geotechnical measurements with the dynamic penetrometer Nimrod on the continental shelf off the Coromandel Peninsula, New Zealand

We tested the ability of a small dynamic penetrometer, Nimrod, to infer geotechnical properties of sediment mixtures in the inner shelf. The penetrometer is light and easy to operate, and its operation by scuba divers ensures a greater degree of precision than ship-based penetrometer deployments. We have studied selected positions along a sorted bedform (~ 100 m wide) on the continental shelf off the Coromandel Peninsula close to Tairua, North Island of New Zealand, and additionally took sediment samples at the exact positions of penetrometer impact, also by scuba divers. The derived dynamic penetrometer signatures (i) measured deceleration of the probe and estimated quasi-static bearing capacity as a measure of sediment strength, (ii) reflected changes in grain-size distribution ranging from very fine to very coarse sands, and (iii) revealed the uppermost seafloor stratification (top layer 2-6 cm) potentially being an indicator for sediment dynamics. In this manner, the device proved to be suitable for spatially fine-scaled surveys using divers' support and might deliver complementary information about sediment dynamics, in this case sorted-bedform maintenance.

本研究测试了小型动态贯入仪（dynamic penetrometer）Nimrod在内陆架沉积物混合物岩土工程特性反演方面的性能。该贯入仪轻便易操作，由水肺潜水员开展操作相较船基部署方案，可实现更高的测量精度。研究团队在新西兰北岛泰鲁阿附近科罗曼德尔半岛外大陆架的一处宽度约100米的分选底形（sorted bedform）上选取了若干测点开展研究，同时由水肺潜水员在贯入仪触击的精确测点位置采集了沉积物样品。本次研究所得到的动态贯入仪响应特征包括：（i）测得探头减速度，并以准静态承载力作为沉积物强度的表征指标；（ii）反映了沉积物粒度分布从极细砂到极粗砂的变化；（iii）揭示了海底最上层2-6厘米地层的分层结构，该结构可作为沉积物动力过程的潜在指示指标。综上，该设备经证实可在潜水员辅助下开展空间精细尺度调查，并可为沉积物动力过程（本研究中为分选底形的维持过程）提供补充观测数据。