LIRmeter: A new tool for rapid assessment of sea floor parameters. Bridging the gap between free-fall instruments and frame-based CPT

Physical and geotechnical parameters of marine sediments are of vital interest to fields like foundation-planning of offshore structures, surveying of cable routes, sediment dynamics, sediment manipulation (dredging, plowing), ground-truthing of acoustical surveys, risk assessment and mine burial predictions. Therefore, characterization of geotechnical in-situ parameters with dynamic penetrometers is of interest for research, consulting and the offshore industry, because in-situ methods, and especially dynamic tests, are generally less time consuming than static tests or measurements on samples. In addition, recovery of sediment samples from the seafloor may alter the sediment characteristics (pressure decrease, temperature change) and limits the information value. To date, the determination of fundamental parameters like shear strength, bearing capacity or grain size is predominantly done ex-situ. However, in-situ assessment of these parameters leads to a better characterization of marine sediments due to direct measurement under field conditions. In this paper, we present the technical specifications and performance of a newly developed dynamic penetrometer. The penetrometer, named Lance Insertion Retardation Meter (LIRmeter), is a winch-lowered device which can be used in a pogo-style fashion to make multi-penetration measurements. The observed parameter is the deceleration during penetration. The probe consists of a lance of four meter length equipped with electronics to record the deceleration during penetration, and is aimed to bridge the gap between lightweight free-falling systems and sophisticated static CPT. The LIRmeter weighs about 400 kg in water and can achieve penetration depths up to four meters in cohesive sediments in its current configuration. Signals from analog MEMS-acceleration sensors of different ranges of 1.7, 3.5 and 6 g are converted using a 16-bit/16-channel ADC. Data is sampled at rates of typically 500 Hz per channel assuring a high da- a-density even at short penetrations. Ambient pressure, inclination and temperature are also recorded as secondary parameters. Power supply is provided by a 20 Ah Lithium-Polymer battery, allowing an operation time of 14 h. Charging of the battery takes about four hours. Data acquisition, as well as communication is controlled by a rugged, low-power industrial PC/104-system running GNU/Linux. Charging and communication can be done without needing to open the pressure case. Data can be downloaded via ethernet-interfaces. The recorded time-series of deceleration during sea-floor penetrations can be integrated once and twice to obtain time series of velocity and penetration depth. Thus, a relationship between deceleration and penetration depth can be established. The justification for this approach is provided by a common and well-accepted premise that stiff to hard sediments cause a high deceleration during penetration, whereas soft sediments lead to lower peak deceleration of a probe during penetration. By using state-of-the-art methods, geotechnical parameters can be deducted from this relationship, and estimates on grain-size can be made. Calculating sediment-physical and geotechnical parameters from deceleration measurements is not yet common, but represents an up-and-coming field of research. Different approaches (momentum-based, empirical relations and numerical experiments) exist and have mainly been adopted for lightweight free-falling penetrometers in the context of sediment mobilization and burial predictions. Some of the theoretical approaches appear to be suitable for penetrometers like the LIRmeter. Here, we show results of an adaptation of these approaches. Data were collected in (a) the eastern part of the German Bight (North Sea) and (b) the western part of the German Bight. The variety of sediment types ranges from muddy sediments to sandy sediments with different grain sizes and a varying amount of mud. The deceleration of the probe is in the range