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Feature Article

Acquiring Thermal Conductivity Data From Shear-Resistant Sediments
FIELAX Modifies its HeatFlowProbe to Work With a Vibrocorer, Allowing In-Situ Measurements of Temperature, Thermal Conductivity

Dr. Melanie Dillon
Senior Scientist
Dr. Christian M'ller
Senior Scientist
Dr. Regina Usbeck
Managing Director
FIELAX Gesellschaft für wissenschaftliche
Datenverarbeitung GmbH
Bremerhaven, Germany

The offshore market has recently increased demand for thermal conductivity values in shallow-water regions, which are important for estimating the dissipation mechanisms of thermal energy and the environmental impact of power cables. Penetration depths of more than 3 meters are essential for optimal cable burial and obtaining geothermal gradient estimations independent of seasonally influenced sediment layers.

Thermal measurements taken with FIELAX GmbH's original heat-flow measuring system, the HeatFlowProbe, have been conducted in a wide range of deep-sea environments, with the device using its own weight and gravity to penetrate the seafloor. In environments where hard grounds prevail due to terrestrial sedimentation, however, this method encountered difficulties reaching the optimal penetration depth of 6 meters. Thus, penetration in shallow seas, coastal areas and continental shelf regions was limited, and temperature data in these areas were rare. Because of this need, a new method for obtaining thermal data had to be established.

FUGRO OSAE GmbH (Bremen, Germany) and FUGRO CONSULT GmbH (Berlin, Germany) contracted FIELAX to perform in-situ thermal conductivity measurements in shallow-water regions. Because the original HeatFlowProbe could not perform these measurements, FIELAX decided to use a vibrocorer as a 'carrier system' instead and adapted the HeatFlowProbe to Thomas Schmidt's VKG-6 Vibrocorer. FIELAX and Schmidt re-engineered the measuring part of the HeatFlowProbe's sensor string, the data acquisition unit and the power supply unit to match the VKG-6, developing a new device called VibroHeat.

Data Acquisition and Processing
VibroHeat's heat-flow measuring system records temperature data as a function of time. A full measuring cycle starts with the penetration process. The probe is lowered to the seafloor and penetrates the sediment, causing the temperature to rise due to frictional heating. This is followed by convergence toward sediment temperatures. As a result of the frictional heat and subsequent decay, relative sediment temperatures as a function of depth are derived, and the temperature gradient can be determined. After a certain period of stability time, a heat pulse (equivalent to about 20° C in air) with a duration of 20 seconds is fired and heats up the sensor tube and surrounding sediment, resulting in a sharp peak in temperature. The amount of energy dissipated from the heating wire is determined by voltage and current measurements. The probe remains in the sediment for another 10 minutes to measure the thermal decay of the heat, which allows the depth-dependent determination of the sediment's thermal conductivity.

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Dr. Melanie Dillon works as a senior scientist in FIELAX GmbH's department of marine geotechnology with an emphasis on temperature and thermal conductivity measurements. She holds a diploma in geophysics from the University of Bremen, where she also received her doctoral degree.

Dr. Christian Müller, senior scientist for FIELAX GmbH, holds a diploma in geophysics from the University of Kiel and received his doctoral degree from the University of Bremen. He has more than 20 years of experience in applied geophysics gathered from numerous marine, airborne and land-going expeditions, especially in polar regions.

Dr. Regina Usbeck, co-founder, managing director and senior scientist at FIELAX GmbH, is responsible for project coordination, system development and personnel management. She holds a degree in physics from the University of Oldenburg and received her doctoral degree from the University of Bremen.

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