Cable-based transport observations in the Southeastern Caribbean
Agusta Flosadottir
Abstract:
Flow through the island passages that separate the southern Caribbean Sea from the Atlantic Ocean feeds the Florida Current and Gulf Stream, forming an important link in both the wind-driven and thermohaline circulations of the North Atlantic. A recorded history of the variability of this flow will contribute to our understanding of ocean circulation and its role in the Earth's climate. With the generous permission of the owners of the Eastern Caribbean Fibre System, a commercial telecommunication system linking the southern islands of the Antilles chain, cable-voltage observations have been initiated across the southernmost island passage between the Atlantic and the Eastern Caribbean, the Grenada Passage between the islands of Grenada and Trinidad. The work is being done by a group of scientists at the Universities of Washington and Miami, and at the U.S. National Oceanic and Atmospheric Administration (NOAA). The installation will be discussed and initial data presented.
Deep MT sounding in Fennoscandia using sumbarine cable voltages measurements
Nick A. Palshin and Peter Sigray
Abstract:
One of the most useful ways for scientific use of submarine cables is to measure natural electric potential variations over large spacing. Submarines cables, used as long horizontal receiving antennas grounded at their end, comprises low-cost efficient systems for on-land measurements of electric field. The advantages of such systems are evident, the longer is the receiving line the larger is the signal level, while the noises caused by the electrodes and grounding conditions do not increase. We will present the first results of deep MT sounding using a submarine cable obtained in Fennoscandian Shield.
Beginning from 1999 a long-term cable voltages have been measured between Gotland Island and Swedish Mainland in the Baltic Sea. The primary aim of this study to monitor the temporal variability of water transport using motionally induced electric field. The observational system uses the submarine cable, with its cable ends near to Visby on Gotland and Vastervik on the Swedish Mainland. Both stations were grounded locally with copper grids buried in the immediate neighbourhood of the sea. The mantle of the fibre cable was used for connecting the local ground at Gotland with the acquisition system placed on the Swedish mainland. The magnetic field variations are measured with a 3-axial fluxgate magnetometer (Bartington Mag-03) placed inside the Västervik station building. The mantle and the local ground as well as the signals of the fluxgate magnetometer are connected to low-pass filters, all with a designed cutoff frequency of 20 mHz. The filters were deliberately made out of passive electrical components so as to avoid unnecessary grounding between the signal sources and the final AD-conversion. After filtering, the signals were fed into a high-resolution digital multimeter (Keithley Instruments, model K2000) with the capacity to record 10 independent channels. The scanning rate was set to 0.1 Hz, well above the cut-off frequency of the filter, and with a 0.2-second time delay between the channels. Finally, an ordinary portable computer, with the prime task of storing data, was used to communicate with the millimetre through a GPIB-interface. The built-in modem of the computer was used for data transfer to Institute of Meteorological Institute in Stockholm, and moreover served as an invaluable tool for checking the status of the system.
Using potential variations over a large spacing, studies could be also made on electrical conductivity distribution deep in the Earth. Magnetotelluric (MT) responses derived from the submarine cable measurements are essentially better quality in comparison with traditional technique utilizing short receiving antennas of about 100-200 meters length. Gotland cable voltage time series are used for accurate estimation of apparent resistivity and impedance phase in a period range up to one day. 1D inversion of the observed transfer function is discussed. The conductivity structure obtained is compared with the results of the Baltic Electromagnetic Array Research (BEAR) project aimed for deep electromagnetic sounding using a shield-wide magnetotelluric and magnetometer array of simultaneous long period recordings and focused on determining the electrical conductivity of the upper mantle beneath the ancient Fennoscandian Shield.
Micro-tsunami detected by cabled ocean-bottom tsunami meters off Sanriku
Tatsuo Ohmachi and Kiyoshi Suyehiro
Abstract:
Some of real-time cabled tsunami meters observe water pressure at the ocean-bottom to estimate wave height of the sea surface disturbance. However, the water pressure at the ocean-bottom is usually contaminated by several factors to the extent that the estimation is hardly possible. Among the
factors, earthquake ground motion causes large pressure vibration. The present study aims to obtain quantitative relations between the seawater pressure and the wave height of tsunami following an oceanic earthquake by means of numerical computation. On this basis, dynamic ocean-bottom displacement due to seismic faulting is considered in the numerical computation. According to the result, the relation between water pressure at the ocean-bottom and wave height at the sea surface varies with the period of water pressure. A simple procedure is presented to estimate tsunami phase from the water pressure following an earthquake. The procedure has been successfully applied to the actual water pressure recorded by the offshore tsunami meters during a moderate-to-strong inter-plate earthquake (M6.1) off Sanriku, Japan. The 3-dimensional tsunami numerical computation demonstrates the validity of the procedure. From the numerical computation, the amplitude of the water pressure due to earthquake
motion is found to be sometimes much larger than that of the water pressure due to tsunami.
Monitoring tne Kuroshio in the Tokara Strait and Izu Islands region by using submarine cables
K. Rikiishi , Y. Hashimoto, H. Matsuda and M. Michigami
Abstract:
The Faraday's law of electro-magnetic induction predicts that an ocean current induces a cross-stream voltage. To monitor the temporal variation of volume transport of the Kuroshio, we are measuring the voltage across the Tokara Strait since 1999 and Izu Islands since 1997 by using submarine cables. Voltages are measured at intervals of about a second, and their 10 minute averages are automatically sent via the telephone line to remote laboratories once a day.
The voltages are compared with the cross-stream sea level differences, and generally good agreements are seen between the two time series. When the Kuroshio hits the Miyake Island at which the sea level and voltage are observed, however, coastal sea level there becomes higher than the prediction by about 20 cm. In the Tokara Strait, the temporal variation in sea level difference leads that in voltage by about three days, suggesting that the Kuroshio current begins to change first in the subsurface layer and then in the surface layer.
Conversion factors from voltage to volume transport have been estimated by comparing the tidal component of voltage with the tidal current from a numerical model by Matsumoto et al. (2000). It is found that a volt of the voltage corresponds to a volume transport of 60 Sv for the Izu Island region, and 25 Sv for the Tokara Strait. By using these factors, the mean volume transport of the Kuroshio is estimated to be about 50 Sv at the Izu Island region, and the variation range is about 6 -7 Sv at the Tokara Strait.
Ocean Transport Monitoring in the Beginning Region of the Kuroshio using OKITAI&OLU Submarine Cables
Momoki Koga, Cho-Teng Liu , Sesar Villanoy and Keisuke Taira
Abstract:
Both the OKITAI(Okinawa-Taiwan) and the OLU(Okinawa-Luzon) cable voltages are measured to monitor the Kuroshio transport near the entrance into the East China Sea. The 10-days low passed OKITAI cable voltage (data period Jun.1998-Dec.2001) is well correlated with the 10-days low passed sea level difference between Suao (northeast coast of Taiwan) and Ishigaki Island at the entrance into the East China Sea (Corr. Coeff.=0.49). The formula to estimate the Kuroshio transport based on the OKITAI cable voltage is derived, assuming the standard deviation of the cable estimated transport is equal to that of the sea level estimated transport (Yang et al.2001). [Cable estimated transport](Sv)= A0 + A1*[Cable voltage](Volt), where A0= 20.87 (Sv) and A1= 53.69(Sv/Volt). The formula is consistent with our 4 days' ship-boarded ADCP current observation crossing the Kuroshio along and near the OKITAI cable (R/V Hakuho-maru, Sep. 2000; KH-00-4 cruise). The relationship among two submarine cable voltages and the sea level difference will be examined.
Three-Dimensional Electrical Conductivity Structure Beneath North Pacific by Using a Submarine Cable Network
Takao Koyama, Hisayoshi Shimizu and Hisashi Utada
Abstract:
In this paper, we present a model of three-dimensional electrical conductivity structure in the mid-mantle beneath the North Pacific by a semi-global three-dimensional inversion of EM data. We used data of
voltage variations obtained by eight submarine cables as well as magnetic field variations at eight geomagnetic observatories/stations and C responses published Fujii and Schultz [2002].
We assumed that 3-D heterogeneous conductivity distribution can be separated into radially symmetric (1-D) structure and 3-D perturbation, in which the 1-D reference model beneath the North Pacific by Utada et
al. [2003] is used. 3-D perturbation was inversely estimated so that the observation-calculation misfit is minimized with an additional mathematical constraint such that perturbation is smooth in space. For this analysis, a new three-dimensional inversion algorithm was developed by combining the steepest descent and the quasi-Newton methods. An integral equation solver using the modified IDM (e.g. Singer, 1995) was
used as a forward solver.
The 1-D reference model consists of 17 layers with 50 km thickness. Perturbation is assumed to exist in the depth range from 350 km to 850 km in a quarter of the Earth centered by the North Pacific, that is, in
the horizontal range from 90 to 270 degrees of longitude and from 0 to 90 degrees of co-latitude. This region of perturbation is divided into 12 x 6 x 5 grids, i.e., total 360 grids of 15 degrees laterally and 100 km radially with unknown conductivity parameters, while the grid size for calculation is 3 degrees horizontally and 50 km in the depth. In the oceanic region, the ocean-land contrast makes a significant effect on EM responses, so we took into consideration the oceanic effect by including a priori ocean distribution with 3 S/m conductivity in the inversion. Total number of data parameters which are complex MT and GDS responses are 330.
We have made a checkerboard test and found the spatial resolution depends on the distribution of observation sites (cables and magnetic stations). Considering this, following features can be pointed out that can be comparable to results of seismic tomography (e.g. Fukao et al., 2001; Megnin and Romanowicz, 2000):
1) There is a conducting zone in the transition zone beneath Hawaii with a conductivity perturbation of factor of three.
2) The uppermost lower mantle beneath Philippines is less conductive by a half.
3) The transition zone beneath Mariana is more conductive by three times.
Assuming the heterogeneities purely of thermal origin, we found that they correspond to temperature anomalies of 200 - 300 degrees.
Acoustic thermometry time series in the North Pacific (P)
Bruce M. Howe, Brian. D. Dushaw, James A. Mercer, Peter F. Worcester, and the NPAL Group (John. A. Colosi, Bruce C. Cornuelle, Brian. D. Dushaw, Matthew. A. Dzieciuch, Bruce. M. Howe, James. A. Mercer, Robert. C. Spindel, and Peter. F. Worcester)
Abstract:
Acoustic measurements of large-scale, depth-averaged temperatures are continuing in the North Pacific as a follow on to the Acoustic Thermometry of Ocean Climate (ATOC) project. An acoustic source is located just north of Kauai. It transmits to six receivers to the east at 1-4-Mm ranges and one receiver to the northwest at about 4-Mm range. The transmission schedule is six times per day at four day intervals. The timeseries were obtained from 1998 through 1999 and, after a two year interruption because of permitting issues, began again in January 2002 to continue for at least another five years. The intense mesoscale thermal variability around Hawaii is evident in all timeseries; this variability is much greater than that observed near the California coast. The paths to the east, particularly those paths to the California coast, show cooling this year relative to the earlier data. The path to the northwest shows a modest warming. The acoustic rays sample depths below the mixed layer near Hawaii and to the surface as they near the California coast or extend north of the subarctic front. The temperatures measured acoustically are compared with those inferred from TOPEX altimetry, ARGO float data, and with ECCO (Estimating the Circulation and Climate of the Ocean) model output. This on-going data collection effort, to be augmented over the next years with a more complete observing array, is a contribution to the CLIVAR effort and can be used, for example, for separating whole-basin climate change from low-mode spatial variability such as the Pacific Decadal Oscillation (PDO). [Work supported by the Office of Naval Research.]
Detectability of decadal variations of the surface electric potential generated by oscillating zonal flows in earth's core (P)
Hisayoshi Shimizu, Hisashi Utada
Abstract:
Electric field observations using planetary scale submarine cables have been collected since the early 1990's. One of the aims is obtaining the strength of the toroidal magnetic field and its variations at the core mantle boundary (CMB). Several constraints on the toroidal magnetic field at the CMB have previously been obtained. For example, Shimizu et al. (1998) analyzed submarine cable electric potential data for the decadal toroidal field variation amplitude at the CMB, showing that it is between the same order as to 10 times larger than the amplitude of the poloidal field variation. However, whether a toroidal field of this strength can be generated by the dynamo has not been determined so far. In this paper, the plausibility is tested using a simple mean-field kinematic dynamo, and the detectability of the electric potential difference at Earth's surface generated by the geodynamo is re-examined.
The kinematic dynamo has the core, the D'' layer, and the rest of the mantle. The D'' layer is assumed to be 10-100 times more conductive than the rest of the mantle. Electromagnetic field variations are generated by a prescribed oscillating zonal flow having a period of 30 years. It is confirmed that the strength of the toroidal field variation obtained by Shimizu et al. (1998) can be generated by such a dynamo if the conductance of the mantle is of order 10^7-10^8 S.
The amplitude of the electric voltage variation at Earth's surface generated by the kinematic dynamo is controlled by the strength of the alpha-effect and the shear flow. A dynamo which generates the expected amplitude of the dipole field variation produces an electric potential of order 50-100mV/1000km at the surface of the Earth. To detect a 10mV or larger signal is not difficult if there is no other source of electric potential. However, the dynamo signal originated is mixed with electric fields generated near the surface of Earth from induction due to the external field variations and motional induction due to the ocean flow. If we consider the decadal time scale, induction due to external fields is very small (about 1mV/1000km for 10year period), so that its influence may be neglected. Decadal variations of ocean currents are poorly characterized at present. If we suppose that the pattern of the ocean current does not change but its intensity oscillates by 20%, the amplitude variations may be as large as several tens of mV/1000km. Although the amplitude is large, the variation of electric potential due to the secular change of the ocean flow can be corrected in principle if the ocean flux is monitored for decadal time scale. It is essential to make the correction to extract information on the geodynamo properly.
Double Seismic Zone in the Hokkaido Island, Southern Kurile Arc, Derived from off-Kushiro permanent OBS and Land-based Observations (P)
Tomoki WATANABE, Riyo OTSUKA, Hitoshi MIKADA, Kenji HIRATA, Hiroaki TAKAHASHI, Minoru KASAHARA, and Kiyoshi SUYEHIRO
Abstract:
Double seismic zone beneath the Tohoku region in the northeastern Japan arc is a well-known evidence in seismology. Compared with the case in Tohoku region, in the Hokkaido region, southern Kuril arc, double seismic zone has been obscured due to low seismicity in the upper seismic belt. A permanent cabled system southeast off Hokkaido Island with three ocean bottom seismographs (OBSs), developed by Japan Marine Science and Technology Center (JAMSTEC), began its operation in July 1999. Hypocenters are determined at JAMSTEC, by using OBS and land-based observations in Hokkaido and Tohoku region. In the present study, we suggest that hypocenter distribution between June 2001 and December 2002, characterizes a double seismic zone with a vertical interval of 45km, which begins 40km off Nemuro Peninsula, eastern tip of the Hokkaido Island bordering on the Pacific. The upper belt emerges at 40km depth. It may not be seemed that the double seismic zone is a uniform distribution along the southern Kuril arc. Off-Nemuro double seismic zone is only found in east area from cabled OBSs. There exists an area where double seismic zone disappears, which corresponds with the area just under OBSs, nevertheless the earthquake detection is thought to be well-functioned. Morphologically, this disappeared zone corresponds with the area where eastern and western end mar
ks Kushiro Submarine Canyon and Erimo seamount, respectively. 1952 Tokachi-oki earthquake (M8.2) occurred in this region. In northwestern region from the area with this disappeared seismic zone, just beneath Hokkaido Island, double seismic zone appears with almost same vertical interval as that found off-Nemuro area.The depth upper seismic belt in this zone emerges is 60km. Along southern Kuril arc in the whole Hokkaido region, these two seismic zones seem to be connected continuously, with depth of 40km to 120km in the upper seismic belt, and 80km to 150km in the lower belt, with a dip angle of about 35degree.
Long-term electromagnetic field observational network in Russian Primorye (P)
V.M. Nikiforov, N.A. Palshin, S.S. Starzhinskii and V.A. Kouznetsov
Abstract:
An electromagnetic (EM) field observational network is being constructed in Russian Primorye. The studies are aimed to determine deep and large-scale 3-D electrical conductivity distributions in the Earth. The method employs a commercial telephone network to measure voltage differences with long dipole lengths ranging from 10 to several tens of kilometres. At present the network consists of several telephone lines constituting two nodes near the cities of Vladivostok and Ussuriysk. Due to general permission granted by ROSTELOCOM and Local Administration the telephone lines network will be increased in the near future and new longer lines will be used for electric field long-term measurements. The large-scale voltage difference across the Japan Sea using the JASC submarine cable crossing the Japan Sea have been carried out since 1996. Additionally to existing geomagnetic observatories, geomagnetic field variations are permanently recorded in two sites at Vladivostok magnetic observatory (VLA) and at Popov Island (PPI).
The long receiving lines and long-term measurements enabled to obtain the electric field with a high signal-to-noise ratio and with observed responses relatively free from the effects of small-scale near-surface heterogeneity with a scale length shorter than the typical electrode spacing. The complete set of response functions are estimated between the respective voltage differences, horizontal magnetic and vertical magnetic fields. The set includes impedances and tippers as well as horizontal electric and magnetic tensors. Transfer functions are accurately estimated in a wide period range. Long electrode spacing also enables physically meaningful direct comparison between the observations and model responses is feasible even if the finescale features of near-surface heterogeneity are ignored.
EM field and transfer functions observed in Primorye is characterized by coast effect screening deep conductivity structure. For better understanding of the spatial and frequency behavior of transfer functions prognostic model of 3D conductivity structure has been constructed and the numerical modeling has been carried out. Prognostic conductivity model is based on ETOPO5 bathometry and a reference profile concept. We found that integral resistivity value about 108 Ohm-m2 allows as to fit experimental data partly. Thus, we managed to fit induction arrows behavior at Pacific coast of the Japan (e.g. Kakioka, Onagawa and Hatizio) as well as impedance phase obtained using the JASC cable. Hypothetical deep conducting fault zones associated primarily with the plate boundaries will be included in our conductivity model. Such subvertical zones are acting as shortening current paths allow electric currents to penetrate from the nearsurface conducting water layer to the deeper mantle conductors. The aim is to check the hypothesis that the lithospheric mantle is much more resistive that is was estimated using homogeneous model of the lithosphere and the low values of integral resistivity were obtained due to the influence of the sub-vertical conducting zones (deep faults).
Measurements of motionally induced voltages in the Ria de Aveiro lagoon(Portugal) (P)
Rita Nolasco, Fernando A. Monteiro Santos, Antonio Soares, Nick A. Palshin, Patricia Represas and Joao M. Dias
Abstract:
Techniques based on motional induced electric field, which is induced by the water flow across earth's magnetic field, has motivated an increase of interest of oceanographers regarding studies of large scale ocean flows. The major part of these experiences has been applied to large scale ocean studies. In this paper we present the preliminary results of the implementation of these techniques to smaller scales systems like the one of Ria de Aveiro lagoon (Portugal). In fact, Ria de Aveiro is an estuary connected with the Atlantic Ocean through a single narrow artificial channel (300 m width and depths varying from 3 to 25 m). The current velocity varies from 0 to 3 m/s with typical values of about 1-1.5 m/s. The measurements were carried out simultaneously at three horizontal receiving antennas. A submarine cable crossing the channel at the entrance of the lagoon (Barra channel) allows the estimation of the water flow at this place. Complementing this observational system, a fluxgate magnetometer and two electrical dipoles were installed in land. One of these dipoles was installed in the N-S direction and the other one in the E-W direction, parallel and perpendicularly to the West coast, respectively. The differences of potential are measured using a 16-bit ADC-Campbell datalogger at 60 s intervals. Ag/AgCl non-polarizable electrodes, specially designed for the sea application were used in the lagoon antenna. In land non-polarizable electrodes of Pb/PbCl2 were used. Non-continuous measurements of water conductivity and temperature are also available. From the first records (obtained between July 23 and November 13, 2002) it was observed that the spectrum of W-E component of the electric field (Ey) is clearly dominated by the frequencies P1/K1. The spectrum of the N-S component (Ex) is dominated by frequencies S2/K2 and P1/K1 but also by the M2. It must be noted that at Barra the lagoon is a channel oriented in the East-West direction. An oceanic current in that direction due to tides may generate an electric field in the North-South direction. This justify the presence of the tide components in the Ex component. The spectrum of the voltages measured at the ends of the cable is dominated by semidiurnal M2, S2/K2 and P1/K1 frequencies. The amplitude of the M2 is higher than S2/K2, and higher than any other frequencies in the spectrum. This means that the tidal induced electric field is stronger than the other components of the electric fields. The cable time-series also provides the visualization of the spring-neap tidal cycle and the diurnal-inequality characteristic of mixed tidal regime. The preliminary analysis of the collected data, shows that there is a good potential to obtain the water transport (by tidal and residual flows) through the canal by measuring the differences of electrical potential. The residual flows, originated by non-tidal driven currents, must have a relevant role in the study of the interchange mechanisms between the lagoon and the ocean.