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This study presents the assessment of total cone resistance from in situ deceleration measurements using the Lance Insertion Retardation meter (LIRmeter) in the Southern North Sea. The penetrometer is equipped with a measurement lance that is up to 6 m in length. The aim was to validate LIRmeter data interpretation within the regional geological context by comparison with static velocity cone penetration testing (CPT) and sub-bottom profiles. In total, 13 datasets were taken, in addition to preexisting hydroacoustical and static velocity CPT datasets. The dynamically acquired data were processed and compared to the reference static velocity data. The validation encourages the use of acceleration-based dynamic penetration tests, since a high degree of agreement was demonstrated between independently acquired dynamic and static cone resistance data. Moreover, the results reveal evidence of two successive formations with different geotechnical properties, consistent with existing knowledge on the regional setting. Additionally, there is novel indication of an incised glacial valley with muddy low-permeability sediments extending much further than reported to date, which would necessitate updating of older maps. The main advantage of penetrometer-based deceleration measurements lies in the robustness of the method, and the reliability of the sensors. However, penetration depth is, for dimensioning reasons, limited to the order of a few meters. Additionally, data processing includes the dependency of knowledge about the soil type to correct the dynamic data. These limitations can be satisfactorily outweighed by combination with reference data from static velocity tests, as demonstrated by integrating these data into a soil classification scheme. [ABSTRACT FROM AUTHOR]

Within this paper we present the Lance Insertion Retardation meter (LIRmeter) as an instrument to determine the strength of marine sediments by a measurement of the deceleration of a probe during penetration into the seafloor. The instrument has been designed for the penetration of the upper 4 m of marine sediments and is therefore suitable for site investigation applications such as cable route surveys. The LIRmeter can be easily deployed from a floating platform in water depths of up to 4,500 m. The system is suitable for long lasting missions (more than 12 h) with pogo-style measurements due to a rugged design and a special selection of sensors and electronics. The LIRmeter provides a custom data acquisition software and a web interface for acquisition setup, data download and system administration. An adaptation of the instrument to specific problems (i.e. extremely soft sediments) is possible due to interchangeable tips and adjustable weights of the lance. The specifically developed user interface and the rugged design make the instrument very easy to handle and to maintain. The sensors and the data acquisition were tested in the laboratory as well as in the field. Field measurements took place in the North Sea, where numerous measurements were performed. This paper gives an extensive description of the design of the LIRmeter (mechanics, electronics and data acquisition) supplemented by a description of data analysis and results of field- and laboratory-tests. [ABSTRACT FROM AUTHOR]

The Gakkel Ridge in the Arctic Ocean with its adjacent Nansen and Amundsen Basins is a key region for the study of mantle melting and crustal generation at ultraslow spreading rates. We use free-air gravity anomalies in combination with seismic reflection and wide-angle data to compute 2-D crustal models for the Nansen and Amundsen Basins in the Arctic Ocean. Despite the permanent pack-ice cover two geophysical transects cross both entire basins. This means that the complete basin geometry of the world's slowest spreading system can be analysed in detail for the first time. Applying standard densities for the sediments and oceanic crystalline crust, the gravity models reveal an unexpected heterogeneous mantle with densities of 3.30 × 10, 3.20 × 10 and 3.10 × 10 kg/m near the Gakkel Ridge. We interpret that the upper mantle heterogeneity mainly results from serpentinisation and thermal effects. The thickness of the oceanic crust is highly variable throughout both transects. Crustal thickness of less than 1 km dominates in the oldest parts of both basins, increasing to a maximum value of 6 km near the Gakkel Ridge. Along-axis heat flow is highly variable and heat flow amplitudes resemble those observed at fast or intermediate spreading ridges. Unexpectedly, high heat flow along the Amundsen transect exceeds predicted values from global cooling curves by more than 100%. [ABSTRACT FROM AUTHOR]