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Sea & Sun Technology A) A sketch of the Nimrod penetrometer. B) Nimrod deployment from RV Wega in the North Sea. C) Nimrod deployment using the robotic claw of the MIR 1 submersible in Lake Geneva. the water column, reaching velocities up to 11 meters per second, depending on water depth, type of tether and length, and hydrodynamic conditions, and penetrates the seafoor until its kinetic energy is dissipated. Penetration depths vary, depending on the sediment, from a few centimeters to 3 meters. It records deceleration, pressure and, optionally, temperature versus time. Velocity and penetration depth can be determined by singleand double-integration of deceleration, respectively. Assuming the monitored deceleration of the probe during sediment penetration results from sediment resistance only, the deceleration can represent sediment resistance force and, consequently, sediment strength. To account for the nonlinear backcoupling between decreasing penetration velocity and varying impact velocity, an approach has been presented to estimate an equivalent of quasistatic bearing capacity (QSBC). Nimrod���s steel tip and hull design ensure stable free-falls even in turbulent waters and under unstable deployment conditions (e.g., drifting kayak, jet ski, deployment by divers and submersibles), making it particularly suitable for investigating sediment remobilization processes. Its size (81 centimeters long and 11 centimeters in diameter) and mass (13 to 15 kilograms, depending on chosen tip geometry) allow deployment and recovery by hand, and do not require an engine-driven winch. The choice of three tip geometries���cone, hemisphere or cylinder���allows for increased sensitivity for very soft layers, such as fuid mud, by using the cylindrical tip, or increased penetration depth into harder sediments, such as fne sand, using the conical tip. Recently, an optional tail was developed in collaboration with the ��cole Polytechnique F��d��rale de Lausanne. For deployments from the Russian MIR submersibles in Lake Geneva, the original cross-fnned tail was replaced by a handle which could be held, released and grabbed by the MIR���s robotic claw. Potentially, this modifcation would also enable deployments from ROVs or other underwater vehicles in the future. Data acquisition for the Nimrod is designed for high-frequency, and thus, high-resolution profling. The customized data logger from Avisaro AG (Hannover, Germany) records 16 channels at 1 kilohertz, leading to a spatial resolution of less than 1 centimeter in the vertical profles. Five differently ranged microelectronic-mechanical systems acceleration sensors monitor the Nimrod���s acceleration or deceleration from 0.1 g, where g is the gravitational acceleration, up to 250 g with high resolution. The acceleration sensors, two of which are three axes, also give information about the tilt of the probe during deployment. Using the pressure transducer data, water depth can be determined, and pore pressure behavior (subhydrostatic or excess pore pressure) can be estimated. Sediment Mobilization Surveys More than 20 surveys with about 5,000 deployments were carried out with Nimrod in areas of sediment remobilization. Different sediment dynamic features were targeted: naturally evolved bedforms such as subaqueous dunes, shifting sandbars, sorted bedforms, mobilization in wave-affected areas, sediment mobilization in conjunction with marine engineering such as scouring at offshore wind energy converters and sediment deposition in dredged channels. In these studies, the authors focused on detecting and quantifying active sediment remobilization, identifying areas of sediment erosion and deposiwww.sea-technology.com FEBRUARY 2013 / st 55