Portable Free Fall Penetrometer Measurements: Shear Resistance versus Drag Resistance-Paper

Paper

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Portable free-fall penetrometers (FFPs) are lightweight, rapidly deployed instruments for characterizing the upper seabed, especially in environments where conventional methods are difficult to use. Their motion is commonly measured with accelerometers, and sediment properties are inferred from the resistance encountered during penetration. Interpreting that resistance is challenging because it may combine shear strength, fluid-like drag, buoyancy, side friction, added-mass effects, and forces transmitted through the deployment rope.<br /><br />This study reviews these contributions and considers their relevance to practical data analysis. In very soft, water-rich mud, the sediment may behave more like a non-Newtonian fluid than a conventional soil. Drag may therefore dominate near the seabed surface, with shear resistance becoming more important as penetration continues and the sediment flows less readily around the probe. The transition between these behaviors is not well understood. In sand, resistance is generally associated with friction and dilation, although drag or particle mobilization may also affect the shallowest penetration.<br /><br />The authors assess which forces can be estimated from routine measurements and device specifications, and discuss when soil density and buoyancy corrections may matter. They argue that some effects—such as rope forces during penetration and side friction for the particular probe considered—can often be neglected, while deeper penetrations may require better estimates of soil density.<br /><br />The paper concludes that FFP resistance may pass through drag-dominated, transitional, and shear-dominated phases. Better interpretation will require more paired penetrometer and soil data, further numerical modelling, and potentially automated analysis methods. In particular, the rheological behavior of very soft surface sediments warrants further investigation.

Keywords

free-fall penetrometer

seabed characterization

sediment resistance

shear strength

hydrodynamic drag

soft mud rheology

buoyancy correction

side friction

added-mass effects

automated data analysis