Instrumentation Overview
Conductive heat flow at the seafloor is generally calculated as the product of the vertical thermal gradient and the thermal conductivity. Heat flow is a gradient measurement and is therefore sensitive to environmental conditions. Modern heat flow surveys are typically made along carefully-navigated transects in combination with swath map and seismic data. Two kinds of instruments are currently available for measuring thermal gradients as part of the U.S. Marine Heat Flow Capability: multipenetration probes, and single-deployment outriggers used while gravity or piston coring. Both systems are capable of high accuracy (individual sensor resolution of 1 mK and absolute accuracy of a few mK) and are robust having the ability to survive operations at sea, including launch and recovery in rough conditions. Both systems make their measurements in sediments. Currently this capability does not support heat flow measurements in hard rock environments.
With both multipenetration and outrigger (coring) instrumentation, probes are inserted rapidly into the sediment and left in place for a few minutes, to allow partial equilibration following the frictional heating of penetration. In situ temperatures are extrapolated to equilibrium conditions using an asymptotic solution of the thermal decay curve for the appropriate tool geometry [Blackwell, 1954; Hyndman et al., 1979; Villinger and Davis, 1987].
Multipenetration surveys are typically used where closely spaced or multiple heat flow measurements along transects are required. Autonomous outrigger sensors can be deployed on gravity- and piston-core barrels when there is a need to make just a few measurements, often operating in a reconnaissance mode, when exploring a new area of the seafloor or when it is otherwise difficult to justify a full, multipenetration program. Our multipenetration system includes in situ thermal conductivity capability, whereas outrigger probes require separate thermal conductivity measurements. These two systems can be operated in concert as part of joint geothermal-geochemical (coring) surveys, which has proven useful to study of specific seafloor features.
Multipenetration heat flow instrumentation
The multipenetration “violin-bow” system (Fig. 1) is on long-term lease from the Pacific Geoscience Center, Geologic Survey of Canada. This system allows multiple measurements on a single instrument transit to the seafloor. Flexibility in the length of the sensor lance, data telemetry in real time, and the ability to operate continuously for 12-48 hours at water depths up to 6000 m, making
tens of measurements during a single lowering makes this system adaptable to a wide variety of geothermal objectives. This system can be operated by a scientific team of 4 to 6 people, with modest help from shipboard technicians and the ship officers and crew for launch and recovery. Details of the historic, theoretical and constructional background of multi-penetration probes are given in Hyndman et al. [1979], Villinger and Davis [1987] and Davis [1988].
The design of the violin bow and strength lance provides both the mechanical robustness to withstand repeated insertions and withdrawals from the sediment, and sensitivity needed to make highly accurate measurements. The small diameter sensor tube and thermistor string is held in tension by the lance, and is located far enough from the lance to be thermally decoupled from it over the period of the measurement. Temperature times series used for both the determination of the thermal gradient and thermal conductivity are logged into solid-state memory in a data logger located in the probe weight stand. Other parameters logged by the system include time, pressure (depth), water temperature, tilt, and a stable reference resistance. Acoustic telemetry during an active survey relays enough data to the surface so that instrument performance can be monitored (see discussion below of shipboard equipment needed to receive data by acoustic telemetry).
A multipenetration heat flow station starts with the lowering of the instrument to the bottom. A 12-kHz pinger is attached to the wire about 50 m above the instrument to monitor the distance between the probe and bottom. The probe is driven into sediments by gravity and temperatures within the sediment are measured with equally spaced thermistors. Following collection of data for equilibrium temperatures and thermal gradient, in situ thermal conductivity is measured. A heat pulse is generated with a calibrated current applied to a heater wire that extends along the length of the sensor tube. The temperature decay of the heat pulse gives a measure of thermal conductivity [Lister, 1979]. With the completion of a measurement, the instrument is hoisted 100–500 m above the sediment (depending on intended measurement spacing), the ship is maneuvered to a new position, and the process is repeated. In this manner a navigated transect of heat flow measurement can be obtained relatively quickly. Heat flow measurements can generally be made at a rate of 1-2 hours per measurement, approximately 15 minutes for the actual measurement and 45 to 90 minutes to reposition the ship and probe.
Autonomous outrigger instrumentation
An autonomous outrigger probe system was developed at UCSC in collaboration with Antares Datensysteme GmbH, Stuhr, Germany, based on an initial development by H. Villinger and colleagues at University of Bremen (Pfender and Villinger, 2002). The original probes were about the size of a felt-tip pen, contained a data logger and lithium battery, and had a thermistor sensor installed at the end of a 2.5-cm long probe tip (Fig. 2). The thin wall of the tip allowed for rapid probe response, and the tools had an operating range of –5 to 50°C, nominal resolution of a few mK, and a maximum working depth of 6 km. Non volatile memory stores up to 65k readings collected at a time interval of 1 s to 24 hrs. One of the most innovative aspects of the Antares probes was use of the pressure housing and probe tip for logger communication and programming without opening the tool, avoiding exposure of the electronics to marine conditions.
The Bremen probes were placed inside outrigger mounts that were screwed on to modified core barrels. This system worked well during initial testing, but modifications were made to make the probes more robust and permit use on conventional core barrels (of essentially any diameter). The UCSC outrigger system uses two kinds of modified Antares probes (Fig. 2). A shorter sensor tip on the standard (“short”) probe reduces the potential for bending, but limits the accuracy of extrapolations to in situ temperature and remains subject to damage if the probe encounters hard material. A second probe design uses the same pressure housing and electronics, but has the thermistor 10 cm from the end of a much longer probe tip. The long probe tip is run up and over a Delrin (low conductivity) fin, which is fixed to a core barrel mount, so that the sensor is positioned in the center of a cut-out in the fin (Fig. 2). The pressure housing is secured in a clam-shell holder at the back end of the mount. This design generates accurate data during short deployments (desirable to assure recovery of the coring system) and is extremely robust. Except for the Delrin fin on the long probe, all other major outrigger components are built from 17-4 precipitation-hardened steel alloy. Fins on both short and long mounts detach and can be interchanged at sea in case of damage or for use with core barrels having different diameters. When secured with multiple pieces of stainless steel banding, compressing rubber mat material below the mount, these assemblies remain fixed on the core barrel through repeated deployments. In fact, during >200 outrigger probe deployments on one expedition in 2002, there were no cases of data loss to electronic or mechanical failure.
With both outrigger designs, the fin holds the logger away from the core barrel so that a time series of in situ temperatures can be measured before temperatures are disturbed by the frictional heating resulting from the core barrel penetrating the sediments. Tools are placed on the core barrel in a slight spiral pattern (radial offset of 10-15 deg), so that penetration of one probe does not disturb the sediments where the next probe will penetrate. After the core barrel penetrates the seafloor, it is left in place for ~5-6 minutes to achieve partial thermal equilibration. Experience shows that leaving the core in the bottom for a short time has a negligible influence on pullout tension in a variety of sediments, even when 3–4 outriggers are run.
Thermal conductivity determination using needle probe
Sediment thermal conductivity can vary significantly over short distances laterally and vertically, and heat flow data are prone to errors and misinterpretation if thermal conductivity is not determined. Sediment thermal conductivity measurements are especially important when quantifying the influence of recent changes in bottom water temperature, appraising unusual lithologies like sulfide or gas hydrate, estimating fluid seepage rates from curved gradients, or evaluating subtle spatial variations in heat transport. Lab testing of sedimentis required for accurate interpretation of outrigger probe data, both to extrapolate in situ temperatures to equilibrium conditions, and to calculate heat flow from the resulting thermal gradient. In addition, even when in situ data are collected with a multipenetration heat flow system, it is often desirable to recover core material and test it at fine resolution.
The needle probe method is standard for determining the thermal conductivity of marine sediments [Von Herzen and Maxwell, 1959]. Marine sediments are recovered in a plastic core liner. After the recovered material equilibrates to room temperature, a small hole is drilled in the side of the liner, and a needle probe is inserted. The probe contains a wire loop with known heating characteristics, and a thermistor is positioned near the middle of the probe. The temperature response of the probe during continuous or pulse heating through the wire allows thermal conductivity to be determined by fitting observational data to an analytical solution to a radial heat diffusion equation.
UCSC researchers have supplied to the Marine Heat Flow Capability a needle probe system designed and constructed by R. Von Herzen and colleagues at WHOI in 1996 (Fig. 3A). Although this system works, hardware components are aging and replacements are difficult to acquire, and the customized software used for operating the system (on a Win98 computer) is no longer supported. This system will be made available to users, and a new system will be purchased for longer-term use and increased reliability.
Necessary Shipboard Equipment
Heat flow and coring surveys can be run from many UNOLS vessels, but they are much easier on ships that have a dynamic positioning (DP) capability. This allows the ship to be positioned over a specific target, and to be moved a short distance between adjacent measurements or cores. Successful surveys can be run on non-DP ships, but this requires particular care and skill from the ship’s officers and others who drive the vessel during survey activities.
Both the multipenetration heat flow system and piston- and gravity-coring systems used for deployment of outrigger probes generally require use of a 9/16 trawl wire and winch system. Most large UNOLS ships have such a system available, but it is worth verifying that the system is in good working condition and that necessary spare components are available to make repairs at sea, if needed, including sufficient spare cable. On some ships, scientists are allowed to operate the winch during heat flow and coring operations. In this case, it will be especially important to have a working depth and weight indicator for real time assessment of wire out and penetration.
The UNOLS ship operators are generally prepared to supply 12-kHz pingers for use on the wire above the heat flow and coring gear, which is used to determine the height of the instrument above bottom and the potential need to pay out more cable once the instrument penetrates the seafloor. Because this instrument is so important to running heat flow surveys, it is a good idea to have two available, along with spare batteries and other parts.
A separate 12-kHz pinger in the weight stand of the multipenetration heat flow probe is used for data telemetry to assess real-time instrument status and make rough estimates of heat flow during measurement. Receiving data from this pinger, and from the 12-kHz pinger on the wire above the tool, requires that the ship have a 12-kHz transducer (generally hull-mounted) and a display system (digital or analog) for viewing 12-kHz data in real time.
Launching and recovering heat flow and coring equiment generally requires additional deck equipment, including one or more cranes and an A-frame, as well as ropes and assistance from shipboard officers, technicians, and/or deck crew. We can provide guidance on what launch and recovery systems have been used in the past on UNOLS vessels, and project PIs are encouraged to work directly with ship operators (once a project has been funded and a ship has been assigned for the project) to make sure that appropriate equipment and supplies are available for individual cruises.
We will also supply Marine Heat Flow Capability users with a notebook computer that is preloaded with all necessary software for acquisition and initial processing of heat flow and thermal conductivity data.
