Jean H. Filloux

Magnetotelluric imaging of the Society Islands hotspot

In April-June 1989, seafloor magnetotelluric data across and along the leading edge of the Tahiti hotspot were obtained. The magnetotelluric response functions were found to be strongly influenced by bathymetric and island effects, and a new procedure for modeling and removing this distortion using a thin sheet approach combined with the measured water depths is introduced. The corrected response functions are consistent with a two-dimensional structure. Inversion of the data shows a slightly higher conductivity (relative to a reference site located away from the hotspot) down to 130 km depth beneath the active area southeast of Tahiti underlain by a more resistive structure. There is a suggestion for a change in conductivity in the 400–450 km depth range, which is consistent with elevated temperatures. This result is consistent with a mantle plume of limited extent (less than 150 km radius) located near the leading edge of the Tahiti hotspot. The magnetotelluric data provide no evidence for lithospheric thinning or for a strong thermal influence over a large area, as would be required by a superswell model.

Techniques and instrumentation for study of natural electromagnetic induction at sea

Abstract Electromagnetic fluctuations in the ocean have external sources above (ionospheric) and below (secular variation of the earths magnetic field), and internal, purely oceanic sources associated with interaction between water velocity fields and the earths field. Energy diagrams indicative of the electromagnetic activity in the sea are presented. From the latter, estimates of the resolution required in electromagnetic research at sea can be made. Absolute minima of 1 γ and 0.05 μV/m are necessary for magnetic and electric fields, respectively. Because the ocean shields overhead sources at frequencies above a few hundred c/h and because motional fields have weak signatures, a resolution at least 10 times higher would considerably enhance the scope of such research. The response of electric field instruments to motionally induced fields depends upon whether they are fixed or drifting, but both types respond similarly to fields of external origin. The most stringent limitation to electric field sampling in the sea is the difficulty in achieving low-noise electrical continuity between measuring circuits and sea water. Even the best matched silver—silver chloride electrodes introduce variable electrochemical signals hard to maintain below a millivolt. These mask very low frequency signals unless sophisticated techniques such as electrode switching are used.

Low-frequency, motionally induced electromagnetic fields in the ocean: 2. Electric field and Eulerian current comparison

The theoretical relationship between the motional horizontal electric field (HEF) and the seawater conductivity-weighted vertical average of horizontal water velocity is validated at subinertial frequencies with seafloor point measurements of HEF and moored measurements of horizontal water velocity collected in 1986–1987 in the central North Pacific during the Barotropic, Electromagnetic and Pressure Experiment (BEMPEX). The comparison is limited principally by inaccurate estimation of vertically averaged water velocity due to weak vertical coherences among the current meters and excessive rotor stalls at the deepest instruments. In the BEMPEX area, conductivity weighting results in only a trivial baroclinic contribution to HEF, so that HEF is an accurate measure of the vertically averaged water velocity (or, transport divided by depth) at periods greater than approximately 5 days. Furthermore, the actual transport divided by the depth is nearly identical to the barotropic (approximately depth-independent) component of motion. The magnetic field at the seafloor is found to have no detectable relationship to horizontal water currents at subinertial frequencies.

Magnetotelluric exploration of the North Pacific: progress report and preliminary soundings near a spreading ridge

Abstract Intensive sea floor magnetotelluric field work has been carried out over the North Pacific for the past four years including a total of six experiments, each involving the deployment of up to eighteen instruments, in clusters or arrays. Because sea floor magnetotelluric exploration today is still subject to a broad range of often stringent limitations, an important objective of this program is to assess the potential contribution of this technique to submarine geophysics by checking the consistency of its predictions with more generally accepted knowledge. A progress report is presented on this subject. In one of the recent experiments which took place on the Cocos plate in the immediate vicinity of the Pacific Rise crest at 12°N, the electrical conductivity is shown to rise rapidly at a depth as shallow as 20 km and to remain relatively high for at least another 150 km. This finding is in harmony with earlier electrical conductivity models of the upper mantle proposed for two widely different provinces of the North Pacific. The experiment reported here includes two stations located 35 and 120 km to the east of the axis of the spreading center, and with crustal ages of 0.8 and 2.7 My respectively. The conductivity profiles are remarkably similar at both stations, with only a slight decrease in the total conductance in the upper 200 km for the eastern station, the one most distant from the ridge crest. This lateral conductivity evolution is consistent with the progressive cooling of the new lithosphere and with the increased distance from ascending magma. Another feature common to both soundings is the presence of a significant, though small, anisotropy of the sea floor impedance that does not seem explainable in terms of the coastal effect. A similarity of origin with seismic velocity anisotropy observed on the Cocos plate is not excluded. The conductivity structure proposed for this Pacific Rise area appears remarkably similar to that of the Atlantic Ridge crest beneath northeastern Iceland.

The barotropic electromagnetic and pressure experiment: 1. Barotropic current response to atmospheric forcing

During 1986–1987, an array of horizontal electrometers (which measure the subinertial barotropic velocity) was deployed for 11 months in the central North Pacific to study atmospherically forced motions in a low eddy kinetic energy region. Significant coherence is found to be ubiquitous between the barotropic velocity and one or more surface atmospheric variables over the North Pacific. The maximum squared coherence between currents and wind stress curl at all periods greater than 3 days is typically 0.6, rarely below 0.4, and occasionally above 0.8, and is dominantly nonlocal, although local coherence is seen at some sites, especially at the shorter periods. Both the coherence and the intersite pattern similarity are strongest at the periods of relative maxima in the barotropic current autospectra (e.g., at 6–8, 10–14, and 20–50 days). The locations at which the wind stress curl forces the motions at each site and for each period have been determined. While there is substantial intersite variability, some of the observed patterns of squared coherence over the North Pacific are very similar to those predicted by simple, linear, smooth-bottomed, quasi-geostrophic models of atmospherically forced barotropic currents. The forcing location is occasionally found to be the same spot for all oceanic instruments, especially at the autospectral maxima in barotropic current. More commonly, distinct forcing locations are found for each instrument even when closely spaced, suggesting topographic influence. In other cases, multiple forcing locations are implicated for the motions at a single station. All of this site- and period-dependent variability of the forcing location is incompatible with the simple models and is due either to the known spatial inhomogeneity of the curl forcing field or to topographic effects. The importance of the former is emphasized using simple arguments based on analytic transfer functions between the atmospheric and oceanic variables and on observed wavenumber spectra for wind stress curl. Quantitative assessment of topographic effects must await suitable numerical models. In addition, maximum likelihood wavenumber spectra of the barotropic currents have been estimated in those bands with an adequate number of coherent sensors. The wavenumbers at the spectral peaks are compatible with the dispersion relation for Rossby waves modified by the observed, large-scale, linearly sloping topography which shoals to the northeast. The observed group velocities are in general quite consistent with the inferred forcing locations based on the ocean-atmosphere coherences.

Abyssal currents during the formation and passage of a warm-core ring in the East Australian Current

Abstract Measurements of currents and temperatures at abyssal depths in the Tasman Sea are compared with near-surface observations of the East Australian Current (EAC) System to ascertain the extent to which the deep and near-surface flows are related. The deep measurements come from three instruments which were moored at different locations on the Tasman Abyssal Plain from early December 1983 to late March 1984. They included an Anderaa current meter, recording velocity and temperature, a second temperature sensor, and an instrument which recorded fluctuations in temperature and the vertical component of the ambient electric field (from which zonal velocity is inferred). During this period a meander of the East Australian Current moved southwards and pinched off to form a warm-core ring. Both the current meter and the vertical electric field instrument recorded current surges to the east when the surface front first arrived at their positions. The maximum speed recorded by the current meter was 35 cm s−1 (when the flow was southward). Strong abyssal currents (over 10 cm s−1) were usually in the same approximate direction as the surface current at the EAC front and associated with the movement of the front before the ring pinched off. The abyssal temperature fluctuations increased in magnitude with the increase in water velocity as the front moved past, but the temperature records do not indicate that any thermal effect of the eddy extended to the sea floor.

Variability of the wind stress curl over the North Pacific: Implications for the oceanic response

The subinertial frequency-wavenumber structure and spatial coherence of wind stress curl, and their spatial and temporal variability, are described with the emphasis on characteristics which affect the oceans response to the wind stress curl forcing function. Wind stress curl over the North Pacific between 25 o and 60oN was computed for 3 years (1985-1988) from the Fleet Numerical Oceanography Center wind product. The range from winter peak to summer trough for the mean and variance are typically a factor of 3-4 and = 10 respectively, with superimposed interannual changes of up to a factor of 2. Power spectra vary seasonally and interannually in an essentially frequency-independent manner that is consistent with the curl variance. However, the spectrum over the period band 5-100 days often fails a statistical test for whiteness at the 95% level. Zonal and meridional wavenumber spectra were estimated for 13 sites distributed around the central-eastern North Pacific using the maximum likelihood method. Directional trends with frequency are comparable to those from earlier studies if 3-year-long data segments are analyzed, with approximate wavenumber symmetry except for the zonal term at mid-latitudes for periods shorter than 10 days, where eastward propagation is dominant. However, spectra for shorter data sections sometimes display eastward and westward excursions which are only weakly similar over distances of =1000 km and essentially dissimilar over longer separations. The spatial correlation structure of wind stress curl is shown typically to have a main lobe of =1000 km and multiple intercorrelation lobes separated by > 2000 km. The strength and location of the intercorrelation peaks vary slowly with time. These results suggest that curl behavior is more complex than was previously believed, that the use of long-term averages of or simple parameterizations for the frequency-wavenumber spectrum of wind stress curl in model studies may be unrealistic, and that more attention to actual curl characteristics at the time oceanic measurements are collected will be required to reconcile models with observations.

An ocean bottom, D component magnetometer

The distribution of electric conductivity in the crustal and upper mantle materials beneath the ocean may be estimated from measurements of the relationship between the magnetic fluctuations and the induced electric field at the ocean bottom. Techniques for the measurement of the electric field have been available for a few years. The horizontal magnetic fluctuations to the magnetic east, usually called D, can be recorded with a simple instrument placed on the sea floor at any depth. This instrument uses a magnet pair which orients itself among the main horizontal field H. The coupling of the magnets to the mirror of a sensitive optical lever is delayed until the instrument has reached the bottom. There is no need to perform any orientation in situ. The instrument resolves 1 γ or less and has a dynamic range of at least 2500 γ. It is capable of recording for approximately 40 days at the rate of 30 readings per hour on self‐contained dry cells. It is lowered to the sea floor and recovered by means of a moo...

The Tasman Project of Seafloor Magnetotelluric Exploration: experiment and observations

Abstract A major experiment in natural geomagnetic induction has been carried out by making seafloor magnetotelluric observations on a line of sites between Australia and New Zealand; simultaneous observations were made by land magnetometers on the Australian continent. The large volume of data recorded shows a great variety of phenomena of electromagnetic induction in the earth and sea. Parkinson arrows and magnetotelluric impedances have been computed for the seafloor sites. The Parkinson arrows show a strong coast effect near the Australian continent and a weaker coast effect over the submerged Lord Howe Rise. The occurrence of smoothly varying arrows across the Tasman Sea suggests a simple, large-scale pattern for electromagnetic induction in the Tasman Sea. The interpretation of Parkinson arrows at seafloor sites is shown to require special care, due to the attenuation and phase-shift effects caused by the ocean on seafloor horizontal magnetic field fluctuations. The magnetotelluric impedances exhibit a strong anisotropy at all sites in the Tasman Sea. The orientation of the impedance axes is constant across the Sea and the anisotropy may be attributed to the two-dimensional shape of the Tasman Sea. Calculations based on the distance scale of the magnetotelluric anisotropy, interpreted as an ocean-continent boundary effect, give a lower bound for the integrated resistivity of the Tasman Sea oceanic lithosphere of order 107 Ω m2, corresponding to an estimate of 2 × 102 Ω m for its average resistivity.

Observations of the Boundary Current System at 26.5°N in the Subtropical North Atlantic Ocean*

Abstract Five current meter moorings and four horizontal electric field records spanning June 1990–February 1992 are used to describe the mean structure and variability of the vertically averaged velocity field and volume transport extending 425 km east of Abaco, The Bahamas, at 26.5°N. Examination of zonal and meridional velocity sections shows that, while meandering may explain part of the variability, there is substantial evidence for pulsation of the core DWBC velocity in the record. Intermittently strong northward flow is observed 225 km east of Abaco that is significantly coherent and out of phase with the currents closer to the boundary at periods of 50–100 d and longer, suggesting recirculation. This is never observed 315 km offshore. At 380 km offshore and extending east at least 60 km, a strong, dominantly southward flow is observed that is coherent with both that near Abaco (in phase) and in the intervening recirculation zone (out of phase) at long periods. The net mean transport (over 17 month...