An expendable penetrometer for rapid assessment of seafloor parameters

Information on the geoacoustic and geotechnical parameters of the seafloor under changing environmental conditions is important in sonar performance evaluation as well as in many engineering applications. Presently this knowledge is being gained through rather elaborate direct and/or remote measurements of sediment parameters. However, recently there has been an increase in the need for a simple system that can remotely measure some of these parameters from a moving ship or aircraft. The authors' work to date suggests that a freefall penetrometer, based on existing XBT technology and able to sense deceleration upon impact with the seafloor, would be a valuable tool in assessing these seafloor properties. When such a probe first impacts the bottom there is rapid deceleration controlled by the shear strength of the sediment, followed by a period of damped oscillation with frequency dependent on the geoacoustic properties of the sediment. By a complete analysis of the impact signature, including both the initial plastic penetration and the subsequent damped oscillation, the authors are able to make a first order classification of the sediment type, shear strength and geoacoustic properties which will be useful in a variety of different applications including sonar performance predictions, evaluation of off-shore foundation sites for drilling towers and other structures, and the assessment of bottom conditions relevant to dredging operations. At present a series of laboratory tests and field tests in the Baltic Sea and the Mediterranean are being carried out. Results of a portion of this work are presented in this paper.