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doi:10.2204/iodp.proc.336.107.2012

Methods and materials

Navigation

Navigation of the ship was achieved with differential GPS, resulting in an absolute accuracy of the ship position on the order of ±5 m. During station work, especially during heat flow measurements, the ship uses its dynamic positioning system together with the propulsion of two azimuth thrusters and a pump-jet, which makes navigation easy and very precise. The exact position of the heat probe was measured with a transponder and a Posidonia system, strapped ~50 m above the probe onto the wire.

Acoustic surveys

The goal of the acoustic surveys was (1) to map the bathymetry of the pond and the adjacent areas, (2) to investigate the upper 50–100 m of the sediment fill in detail, and (3) to map the sediment/​basement interface. Since these acoustic surveys use different source frequency, they provide geological information of different depth with different resolution.

Bathymetric survey

For deep-sea bathymetric surveys, the ship provides a hull-mounted Kongsberg EM 120 system with 191 beams and a main operating frequency of 12 kHz. Opening angles of the emission beam can be up to 130° across-track, whereas it is fixed to 2° along-track. Spacing of the beams can be set up either equiangular or equidistant. The absolute water depth was calculated with a sound velocity profile (obtained by a conductivity, temperature, and depth measurement) and the two-way traveltime for each beam. Significant accuracy of the measurement was achieved by using a combination of phase for the central beams and amplitude for the lateral beams.

Processing of the data such as outlier elimination and interpolation of the depth data on a grid was done using Neptune software (Kongsberg). Generic mapping tools (GMT) (gmt.soest.hawaii.edu/) were used to produce a map of the area by combining the new bathymetric data with that already existing, which has a much lower spatial resolution. This “old” data set was obtained during Cruise Conrad 30-01 in 1989 (Co-Chief Scientists R. Detrick and J. Mutter) and is available from www.marine-geo.org/​tools/​search/​entry.php?id=RC3001 (2 February 2012).

Parasound profiles

For subbottom profiling, the permanently installed hull-mounted Atlas Parasound P70 system (www.ifm.zmaw.de/​fileadmin/​files/​leitstelle/​merian/​MSM_HandbuchParasound.pdf, 2 February 2012) was used to map the upper sections of the sediment body. These surveys were very important in finding suitable sites for sediment sampling with a gravity corer and deployments of the heat flow lance. The system uses a combination of a low-frequency sediment echo sounder and a high-frequency narrow beam sounder for detection of water depth. It is operated in parametric mode with a secondary frequency of 3.8 kHz. The advantage of using a parametric system is that its footprint size is only 7% of the water depth, which is much smaller compared to conventional sediment echo sounding systems. The Atlas Parasound P70 system can reach penetration depths of up to 200 m. It was operated at intervals of 400 ms between pulses, resulting in a spatial resolution of 2–3 m at a ship speed of 5 kt. An echogram was recorded every second.

Postprocessing of these data was done using Atlas Parastore software for replays and the seismic data processing package Vista Seismic Processing 7.0. First processing procedures were conducted on board with a wide bandpass filter to improve the signal-to-noise ratio and amplification of the deeper and weaker reflections by normalization to a constant value. Further processing and description of the results was summarized in a Bachelor thesis by U. Beckert (2009) from the University of Bremen.

Seismic survey

To profile the deeper subbottom and to detect the interface between sediment and basement, a generator-injector (GI) gun and a streamer with hydrophones were used. During the cruise, operations were conducted in “true GI mode,” where the generator’s original volume of 1.7 L (105 in³) is reduced to 0.7 L (45 in³), which is the optimal configuration for totally suppressing bubble oscillations. The overall air consumption therefore amounted to 2.4 L (150 in³). Compressed air was provided by a mobile LMF (Leobersdorfer Maschinenfabrik AG, Austria) compressor with an actual pressure at the gun port of 200 bar.

The trigger signal was supplied to the generator and the injector from the homemade “TriBo” triggerbox system containing a high-precision quartz time base. The air gun was towed 10–15 m behind the vessel at a water depth of 5–6 m. The best results for this source depth were achieved using a delay time of 35 ms between generator and injector.

The seismic signals were received with a 101 m active length Teledyne streamer with 16 channels towed ~150 m behind the ship. Each channel (group) consists of eight hydrophones with one group forming a 6.25 m long unit. Analog to digital (A/D) conversion and acquisition of the data were achieved with a combination of the National Instruments NI SCXI-100 A/D converter system and a homemade seismic recording system built at the Department of Geosciences, University of Bremen. The data were recorded at a sample interval of 0.25 ms, a record length of 5 s, and a water delay of 3 s. An antialiasing filter at 2500 Hz filtered the incoming data. On board the ship, preliminary processing was accomplished with GEDCO VISTA VW Processing 3D (version 7.029) software. The 16 channels were stacked and time-migrated with a constant velocity of 1500 m/s. An Ormsby bandpass filter (frequencies = 20/25–200/220) was applied to remove noise. The WinGeoapp software (created by H. Keil, Department of Geosciences, University of Bremen) used information from the header data (recording time, shotpoint number) and on the geometry of the system streamer–gun–GPS antenna to calculate locations for the common midpoints of the shot gather. Also, the amplitude scale was adjusted manually, and automatic gain control scaling and exponential gain were set. Sediment thicknesses, determined from the interface between sediment and basement, were picked using VISTA.

Seafloor heat flow measurements

During the cruise we exclusively used the 6 m long Bremen heat flow probe, also called the Giant Heat Flow Probe. The mechanically robust heat probe is designed for operation in pogo-style mode, with a wide application ranging from 6000 m deep-sea trenches with mostly soft sediments to the upper continental slope, where sediments are often sandy and difficult to penetrate. Because of the 6 m length of its temperature sensor string, undisturbed temperature gradients can be determined even in shallow water where seasonal bottom water temperature variations are superimposed on the undisturbed temperature field close to the seafloor.

The heat probe (Fig. F4) is constructed in the classical “violin bow” design (Hyndman et al., 1979; Hartmann and Villinger, 2002), with 22 thermistors distributed over a total length of 6 m at 0.27 m intervals mounted inside an oil-filled hydraulic tube (outer diameter [OD] = 14 mm), which is attached to the strength member (OD = 130 mm). The sensor tube also contains a heater wire for the generation of high-energy heat pulses, typically >300 W for in situ thermal conductivity measurements (Lister, 1979). Only noncorrosive steel was used for the heat probe, with special high-strength noncorrosive steel for the strength member and the fins attaching the sensor tube to it.

The complete data acquisition unit, including power supply, is housed in a single 110 mm OD × 300 mm long titanium pressure case and mounted inside the probe’s weight stand. A second pressure case of the same size houses the batteries for heat pulses.

The signal of the temperature sensors is measured with a resolution of 20 bits at a sample interval of 1 s, resulting in a final temperature resolution of better than 1 mK at ambient seafloor temperatures. A calibrated PT-100 seawater sensor on top of the weight stand allows measuring the absolute bottom water temperature and checking the calibration of the sensor string in deep water. Inclination and acceleration of the probe are also measured at a 1 s sample interval to monitor the penetration process into the sediments and potential disturbances during the measurement period while the probe sits in the sediment. The complete data set is stored in the probe but is also transmitted via coax cable on board in real time, where the data are visualized and stored on a PC. In that way, the operator always has complete control of the instrument, allowing operational decisions during deployments of the probe. In addition, the heat probe can be operated in a completely autonomous mode with internal data storage and automated heat pulses if a coax cable is not available. The battery capacity allows for 3 days of continuous operation in pogo-style mode.

Winch speed during pay-out and retrieval is 1.0 m/s, which guarantees full penetration in the sediments of this working area. Time to equilibrate to in situ temperatures is assumed to be 7–8 min; time for heat pulse decay observation takes an additional 8 min. The mean duration of one measurement, including transit, is ~1–1.5 h per single point of measurement.

Penetration of the heat probe into the upper meters of the soft sediments generates a thermal disturbance due to frictional heating, and in addition, the sensor string has to come into thermal equilibrium with the sediments. This means that the probe stays in the sediment for ~10 min; however, it will not have fully equilibrated after this time. Therefore, the temperature decay has to be fitted to a theoretical decay model. In situ thermal conductivity is measured with the heat pulse method (Lister, 1979), where the sensor string is heated for typically 20–30 s and thermal conductivity is derived from the temperature decay. Both frictional and heat pulse decay can be described by the same mathematical model. The basic processing steps of heat flow measurements are outlined in Hyndman et al. (1979), which was a manual procedure based on the work of Lister (1970, 1979). The theoretical background for the analysis of heat flow measurements is discussed in Bullard (1954), Lister (1970), Hyndman et al. (1979), Villinger and Davis (1987), and Hartmann and Villinger (2002). To overcome deficiencies of the processing routine described in Villinger and Davis (1987) and to incorporate platform-independent plotting routines, a mathematically sound inversion scheme of observed temperature decays was implemented in a program.

Physical properties measurements on cores

During the cruise, we took gravity cores for pore water geochemistry sampling and porosity measurements on samples, but we also measured thermal conductivity and electrical resistivity on split cores. Thermal conductivity measurements were made on archive halves with a commercially available thermal conductivity instrument KD2PRO (www.decagon.com, 2 February 2012), which is based on the needle probe method. The needle used is 60 mm long with OD = 1 mm. According to specifications, the resulting thermal conductivity had an absolute accuracy of 5%. The split cores were measured after they equilibrated to ambient temperatures in the laboratory. If possible, measurements were made every 25 cm.

Measurements of porosity, thermal conductivity, and electrical resistivity were obtained onshore in the European Consortium for Ocean Research Drilling (ECORD) laboratories of the Center for Marine Environmental Sciences (MARUM; Bremen, Germany) with the multisensor core logger (MSCL; in Hellman, 2009).

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