Chave, A.D.

Filloux, J.H.

García, X.

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INSTRUMENTATION VIEWPOINTSessions29THE PERMANENT SEAFLOOR GEOMAGNETIC OBSERVATORYThe Permanent Seafloor Geomagnetic ObservatoryX. Garcia (1), A.D. Chave (2), J.H. Filloux (3)(1) Dublin Institute for Advanced Studies,5 Merrion Sq., Dublin 2, IrelandTel: +353-1-6621333 email: xavi@cp.dias.ie(2) Woods Hole Oceanographic Institution, Woods Hole, MA, USA.(3) SCRIPPS Institute of Oceanography, La Jolla, CA, USA1. IntroductionEarth’s magnetic fi eld fl uctuates over a wide range of spatial and temporal scales. These variations contain fundamental information about both the solid earth and the environment outside the planet. Detailed study of the geomagnetic fi eld addresses a range of fi rst-or-der problems in the deep mantle and core, lithosphere, oceans, and the solar-terrestrial environment. In addition to these basic research themes, geomagnetic measurements have practical applications in navigation, resource assessment and exploration, hazard mitigation, and control and protection of the telecommunications and power grids.The mechanism for generating the geomagnetic fi eld remains a fundamental problem in physics and geophysics [1]. Studies of the geodynamo require the comparison of theory and numerical mod-els with long term observations of the geomagnetic fi eld on a global scale. In order to improve our models we need to extend the existing observatory network into the ocean basins [2].We have built a Seafl oor Geomagnetic Observatory (SGO) that can provide some of this functionality.In addition to geomagnetic fi eld data, geoelectric fi eld data fi nd ap-plication in electromagnetic induction and studies of the core. In par-ticular, geoelectric data can be used to provide a constraint on the size and variability of the toroidal magnetic fi eld at the core-mantle boundary. The geoelectric fi eld also contains information about the large-scale oceanic velocity fi eld C-2 through the motional induction eff ect [3; 4].In this paper we present our Seafl oor Geomagnetic Observatory that will be part of the geomagnetic network soon.2. The components of a Seafl oor geomagnetic ObservatoryThe components of a SGO consist of fi ve separate systems: sensors, packaging, data recording/transmission, observatory timing and control, and power. The sensor component is divided in a geoelectric sensor and a geomagnetic sensor system.The SGO that we have built was going to be installed on the Hawaii-2 Observatory site (H2O)[5], which provided power (48 V at 100 W), timing (1 msec accuracy) and two-way communications (80 kbits/s using RS-422 protocol). Thus only the sensor and packaging issues were considered in the design.The geoelectric system consists of fully redundant, three-component (north, east and vertical) sensors contained in a non-metallic frame. The electric potential is measured at the Ag-AgCl electrodes at the end of mutually-orthogonal, 6 m long salt bridge pipes which con-nect to a set of water switches or choppers [6]. In order to minimize corrosion-induced electric fi elds, the entire geoelectric assembly  is fabricated from fi berglass and plastic, while the electronics are con-tained in a glass instrumentation housing.The geomagnetic sensor system consists of a fl uxgate magnetom-eter, a set of suspended magnet variometer sensors, an Overhauser scalar fi eld magnetometer, a gyroscope, tilt sensors, and data control electronics. These components are mounted on a triangular titanium sled which is placed on a heavy concrete anchor base at the seafl oor. The concrete base serves to provide a maximally stable monument which yields rough azimuthal and vertical orientation of the geo-magnetic sled through a set of locating pins.The commercially-produced ringcore vector fl uxgate sensor is cur-rently used in land observatories and is manufactured by B. Narod. This system requires about 1W for three component running contin-uously. We also include a suspended magnet sensor that measures magnetic fi eld fl uctuations relative to an arbitrary reference [6]. The absolute measurements on the ocean bottom are done using a scalar magnetometer . All these systems follow the INTERMAGNET stand-ards [7].The key to obtain the absolute declination measurement is to achieve an absolute azimuth reference (usually to true north) at the seafl oor. For this we use a gyroscope and the magnetic fi eld sensors, previ-ously calibrated against a standard observatory.Tilt sensors provide local vertical reference for the magnetic fi eld sen-sors, and measure high frequency motion of the sensors caused by either seafl oor movement or vortex shedding in near-bottom cur-rents.The physical design and packaging of the SGO required careful thought to avoid corrosion problems and allow for ease of instal-lation and maintenance. Corrosion is easily dealt with by using ti-tanium, glass, and plastic for all components of the observatory in contact with sea water. The major issue includes the construction of a monument base to hold the observatory in order for it to remain as stable as possible on the seafl oor, and it has to allow for easy de-ployment and recovery of sensors without appreciably altering  their orientation or location.The system also contains a PC104 that is programmed to switch on and off  instruments, gather data from diff erent sensors, time stamp them and deliver them through the cable, so they can be monitored in real time.This system was completed in summer 2003. After, we ran a series of tests in two geomagnetic observatories: Chambon-La-Foret (France) and Victoria (Canada). These led to measurements of the angles be-tween the two vectorial sensors (fl uxgate and suspended magnet) with respect to the gyro. Initial deployment on the H2O cable was scheduled for October that same year, although because of techni-cal problems the H2O cable is down and the deployment had to be cancelled.Figure 1. SGO being calibrated against land observatory in Victoria (Canada).M2INSTRUMENTATION VIEWPOINTSessions303. ConclusionsWe have successfully created the fi rst ocean bottom geomagnetic observatory. One of the most important and diffi  cult tasks (orienta-tion of measurements) is done using a gyroscope. The system is de-signed to deliver geomagnetic and geoelectric data in real-time.4. References[1] B.A. Buff et, Earth’s core and the geodynamo, Science, 288, 2007-2012, 2000.[2] National Research Council, “The National Geomagnetic Initiative”, Washing-ton D.C.: National Academy Press, 246 pp., 1993.[3] J.C. Larsen, Transport and heat fl ux of the Florida Current at 27 N derived from cross-stream voltages and profi ling data: Theory and observations, Phil. Trans. R. Soc. Lond. A338, 169-236, 1992.[4] A.H. Flosadottir, J.C. Larsen & J.T. Smith, Motional induction in North Atlan-tic circulation models, J. Geophys. Res., 102, 10353-10372, 1997.[5] A.D. Chave, F.K. Duennebier, R. Butler, R.A. Petitt, Jr., F.B. Wooding, D. Harris, J.W. Bailey, E. Hobart, J. Jolly, A.D. Bowen, and D.R. Yoerger, H2O: The Hawaii-2 Observatory,in L. Beranzoli, P. Favali, and G. Smriglio (eds.), Science-Technology Synergy for Research in the Marine Environment: Challenges for the XXI Century, Develop-ments in Marine Technology Series 12, Amsterdam: Elsevier, pp. 83-92, 2002.[6] J.H. Filloux, “Instrumentation and experimental methods for oceanic stud-ies”, in J.A. Jacobs (ed.), Geomagnetism, Vol. 1, New York: Academic Press, 143-248, 1987.[7] INTERMAGNET, “INTERMAGNET Technical Reference Manual”, 73 pp., 1994.OBSERVATORIES AND LANDERS TO STUDY OXYGEN DYNAM-ICS IN THE MARINE ENVIRONMENTAnders Tengberg, Elin Almroth, Sara Andersson, Anna Apler, Per Hall and Madeleine NilssonDept. of Chemistry, Göteborg University, SwedenIntroduction:Most chemical and biological processes result in changes in disolved oxygen concentrations. Oxygen is therefore a prime parameter to measure in marine environmental studies. Because of technological limitations of existing sampling methods and sensors long term mon-itoring of oxygen was not possible until the introduction of accurate and stable optical oxygen sensors. This technology has revolution-ized the possibilities to measure oxygen and has also open the door to obtain two dimensional pictures of the oxygen distribution at the sediment-water interface. In this presentation we will demonstrate new insights in the dynamics of oxygen in the marine environment by giving numerous examples ranging from shallow coastal areas to the deep Arctic Ocean. The presented investigations have been done combining a wide range of instrumentation mounted on both fi xed bottom platforms and autonomous landers. The rationale of using oxygen in environmental investigations will be described as well as the techniques. We will demonstrate that such combined measure-ments will make it possible not only to know more about the oxygen dynamics per se but also to use oxygen as a proxy for the ongoing biological, chemical and physical processes in the water column and at the bottom.Rationale of using oxygen to monitor the overall water quality: In near shore waters1. Biological production/consumption: During primary produc-tion in oceans and lakes CO2 and nutrients are incorporated by phytoplankton to create organic material (chemically written as (CH2O)106(NH3)16H3PO4). As a result of this process oxygen is pro-duced. Chemically the reaction can be expressed as: 106 CO2 + 122 H2 + 16 NHO3 + H3PO4   (CH2O)106(NH3)16H3PO4 + 138 O2. As written in the equation the approximate atomic ratios between oxy-gen, carbon, phosphorus and nitrogen are O138C106:N16:P1 which is known as the Redfi eld/Richards ratio. The phytoplankton can then be decomposed or consumed by zooplankton or fi sh which will in turn generate organic matter in the form of fecal pellets and dead organisms. The particulate organic matter sinks towards the bottom and on the way it can be further decomposed (by e.g. bacteria) and dissolved before it reaches the seafl oor. At the bottom organic matter is subjected to decomposition (mineralization), dissolution, and buri-al. Through mineralization oxygen is consumed and nutrients can be recycled to the water. This process can approximately be expressed as the reverse of the reaction described above.2. Water mixing: Oxygen conditions in the aquatic environment are aff ected by water circulation. Horizontal currents can bring in water which has been more exposed to bottoms with high oxygen demand or to discharge of industrial or urban waste water with chemical (COD) or biological oxygen demand (BOD).3. Air-water exchange: The dissolution of oxygen in surface water is aff ected by variations in air pressure according to the common gas law. The role of atmospheric air pressure changes on the total stock of dissolved oxygen is normally minor compared to oxygen varia-tions induced by biological production/consumption. Nevertheless it should be taken into account especially during the less productive parts of the year.4. Pollution induced oxygen consumption: Pollution from industry (mainly COD) and urban population (BOD) can play a signifi cant role in boosting the oxygen demand. This might lead to serious biological damages and will also signifi cantly infl uence on chemical equilibra-tions.Since oxygen concentrations are coupled to the four above listed fac-tors measuring oxygen in diff erent levels will mirror the sum of these changes. To be able to separate the reasons for oxygen concentration changes from each other it is crucial to measure water circulation, salinity and temperature in parallel. Useful information is also gained by adding sensors for chlorophyll, particles and light (if the water is shallow).Technology to study oxygen dynamics: In combination with other measurements, briefl y described above, we have used three diff erent techniques to study the oxygen dy-namics.1. Incubations: We have measured oxygen consumption as well as the total carbonate (TCO2) and nutrient production in-situ by mak-ing parallel chamber incubations of the sediment and the overlying water. This work has been done with autonomous landers which sink freely to the sea fl oor. There chambers are gently pushed into the sediment leaving about 20 cm of overlying water. Incubations start when lids are closed and stirrers start to mix the chamber water. Dur-ing incubations, which generally lasts for 36-48 h the oxygen concen-tration is continuously monitored in the enclosed waterusing oxygen optodes and samples (ten from each chamber) are au-tomatically collected into syringes. The water samples are analysed on-board once the lander has been recovered. The evolution in sol-ute concentrations with time gives information about the degrada-tion and burial of organic matter.M2

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