Abstract:
A free-fall penetrometer based on existing XBT technology and able to sense deceleration upon impact with the seafloor has been developed and tested at a wide variety of different sites containing sediments varying from very soft mud to compact sand. When the probe impacts the bottom, there is rapid deceleration controlled by the shear strength of the sediment followed by a period of damped oscillation at a frequency that is dependent on the geoacoustic properties of the near-bottom strata. Extensive laboratory and field testing carried out over the past 3 years has established a convincing relationship between the characteristics of the deceleration signature (maximum deceleration, rate of change of deceleration, depth of penetration, etc.) and the physical properties of the sediment. By deploying the penetrometer at a wide range of sites where ``ground truth'' was simultaneously determined using quasistatic cone penetrometer tests, sampling and in situ measurement of wave velocity, it has been possible to develop a variety of correlations relating the deceleration signature to geotechnical and geoacoustic parameters such as undrained shear strength, shear-wave velocity, and porosity. [Work supported by the ONR and the SACLANT Centre.]