TOPOGRAPHIC CORRECTION OF SEAFLOOR MAGNETOTELLURIC DATA USING FLATTENING SURFACE MODELING

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1 Session EM7: Ocean Studies EM7-1 TOPOGRAPHIC CORRECTION OF SEAFLOOR MAGNETOTELLURIC DATA USING FLATTENING SURFACE MODELING Kiyoshi Baba (Woods Hole Oceanographic Institution, Seafloor magnetotelluric data are strongly affected by topographic variation because of the high contrast in electrical conductivity between seawater and lithosphere. Accounting for seafloor topography is very important in estimating reliable conductivity structures beneath the seafloor. There is difficulty, however, in incorporating topographic variation into numerical modeling, because the vertical extent of topographic variation is much smaller than the scale of target mantle structures and because fine meshed numerical modeling and inversion capable of resolving topography is numerically challenging. One realistic approach to estimating conductivity structure from seafloor MT data is inverting data corrected for topographic effects in a model space without topographic variation using conventional inversion methods. Nolasco et al. (1998) showed that the synthetic response to an assumed mantle structure with topography and the response to the same mantle structure without topography are required to calculate the topographic effect and remove it from observed data. I use a flattening surface three-dimensional modeling (FS3D) method (Baba & Seama, 2002) to calculate the synthetic response instead of the thin sheet modeling method used by Nolasco et al. (1998). FS3D is 3-D forward modeling method based on the finite difference method using staggered grids and incorporating topography as variation in the conductivity and permeability of two layers just above and below the flattened surface. The benefits of using FS3D are, 1) it doesn t have short period limitations, as the thin sheet approximation does, 2) it can treat complex (even 3-D) structures beneath the seafloor, and 3) it can reduce the number of mesh points substantially. FS3D is extended in two ways in this study. One is to extend the algorithm to handle a wide range of topographic variation and the sea-land boundary. The other is to incorporate along axis anisotropy in conductivity to address recent issues in oceanic mantle studies. I present the principles of extended FS3D in detail and show some results of tests of the topographic correction for synthetic data.

2 EM7-2 ELECTRICAL CONDUCTIVITY STRUCTURE OF UPPER MANTLE BENEATH THE PHILIPPINE SEA PLATE Kiyoshi Baba (Woods Hole Oceanographic Institution, Hisashi Utada (Earthquake Research Institute, University of Tokyo) Nobukazu Seama (Research Center for Inland Seas, Kobe University) Hiroaki Toh (Faculty of Science, Toyama University) Masahiro Ichiki (Japan Marine Science and Technology Center) Seafloor magnetotelluric (MT) data were collected using 6 ocean bottom electromagnetometers (OBEMs) in the Philippine Sea during 8 months from October 1999 June 2000 to study the electrical conductivity structure beneath the Philippine Sea Plate. The observation array covered the three major basins constructing the Philippine Sea Plate: West Philippine Basin, Shikoku-Parece Vela Basin, and Mariana Trough. They are back arc basins associated with the subduction of the Pacific Plate, and are considered to have been formed by successive episodes in this order. The goal is to constrain the tectonic evolution of the Philippine Sea Plate with related to the age difference in the lithosphere from the electrical conductivity structure. In this study, the MT responses are inverted to smooth one-dimensional (1-D) models for each observation site, and then difference in models is examined. Higher dimensional analysis is not yet made because the sites distribute sparsely and independently on the surface geological feature. We use the apparent resistivity and phase calculated from the square root of the determinant of the MT impedance tensor for 1-D Occam s inversion (Constable et al., 1987). Topographic effect, which is very important for seafloor MT data to account for, is calculated using Flattening Surface 3-D modeling (Baba & Seama, 2002). Final models are obtained by the iteration of the inversion and the topographic correction process (Koyama 2002). Resulting models show clear age dependence; the high conductivity zones appear about 30km depth beneath the Mariana Trough (0Ma), about 80km depth beneath the Parece Vela Basin (~22Ma), and about 100km beneath the West Philippine Basin (>57Ma), suggesting that the cooling process of the lithosphere for the back arc basins are same as for the normal oceanic plate. We can also see relatively resistive asthenosphere beneath the Mariana Trough in comparison with those beneath the other two basins. This feature might be associated with the present active back arc spreading.

3 EM7-3 ELECTROMAGNETIC SOUNDING AT THE LUCKY STRIKE AXIAL VOLCANO, MID-ATLANTIC RIDGE Neville Barker (School of Ocean and Earth Science, Southampton Oceanography Centre, Empress Dock, Southampton, SO14 3ZH, UK. Martin Sinha, Lucy MacGregor (Southampton Oceanography Centre) The ISO-3D Team 1 In September 1999, a marine controlled source electromagnetic (CSEM) sounding experiment was carried out over the Lucky Strike hydrothermal field, on the mid-atlantic ridge. Both the measurement errors and data scatter obtained were small compared to previous mid-ocean ridge (MOR) data. Analysis of the data is ongoing, and thorough 1- dimensional modelling and inversion of the data is now complete. By combining the final resistivity models obtained with previous geochemical, physical and geophysical results from the site, it will be possible to better constrain the sub-surface properties of the hydrothermal system. Results from 1-D Occam inversions of the data have shown that the bulk resistivity structure at Lucky Strike is broadly similar to other MOR sites, with resistivity values most similar to the Reykjanes Ridge. Layer over halfspace 1-D models of the data from each instrument, and normalised electric field data now demonstrate the high level of lateral variation in the crustal resistivity structure across the area, along with an apparent increase in crustal conductivity over the centre of the hydrothermal site. Clear systematic trends are visible in the normalised data, with amplitude anomalies visible over horizontal lengthscales of hundreds of metres to several kilometres. These anomalies appear to be related to 3-dimensional resistivity structure, and make the dataset amenable to future 3- D modelling. 1 Other members of the ISO-3D Team are: Miranda, J.M.; Junge, A.; Flosad ttir, A.H.; Santos, F.M.; Luis, J. Luis; Louren o, N.; Soares, A.; Dean, S.; Cheng, Z.; Riches, S.

4 EM7-4 ELECTRICAL ANISOTROPY IN THE SEAFLOOR: CSEM RESULTS FROM APPLE James Behrens (Institute of Geophysics and Planetary Physics, Scripps Institution of Oceanography, University of California San Diego) Lucy MacGregor (School of Ocean and Earth Science, Southampton Oceanography Centre) Steven Constable (Institute of Geophysics and Planetary Physics, Scripps Institution of Oceanography, University of California San Diego) Mark Everett (Department of Geology and Geophysics, Texas A&M University) Certain events in the life cycle of oceanic lithosphere are apparently two-dimensional, such as formation of crust at axial spreading centers, and deformation at the lithosphere - asthnosphere boundary. These processes may result in an electrically anisotropic oceanic lithosphere by creating conductive pathways in preferred orientations. Controlled source electromagnetic (CSEM) soundings and magnetotelluric (MT) soundings were made during the Anisotropy and Physics of the Pacific Lithosphere Experiment (APPLE), carried out in February/March 2001 approximately 1000 km west of San Diego, California. Twenty seafloor electromagnetic field sensors were deployed (and recovered with data) during this experiment. The deep-towed EM transmitter (DASI) was a 100 m horizontal electric dipole, was towed in a 30 km radius circle around a central core and perimeter array of recorders, a radial tow of 70 km total range, and other unplanned tows forced upon us by weather conditions. DASI transmitted a 4 Hz square wave throughout the CSEM phase of the experiment. Processing of the CSEM data reveals evidence of electrical anisotropy. In particular, at a range of 30 km and a frequency of 4 Hz, transmitted azimuthal-mode electromagnetic fields are attenuated more by about a factor of two when measured from west to east than from north to south. This supports earlier results from the PEGASUS experiment, which proposed that oceanic lower crust and upper mantle with east-west trending lineaments of increased conductivity will exhibit greater attenuation of electric fields in the east-west direction. These lineaments may be deposits of conductive minerals (e.g. oxides, graphite) stretched out along the direction of plate motion. There are also data from shorter source-receiver offsets (2-20 km). While anisotropic effects are not as easily identifiable, these tows represent both radial and azimuthal CSEM modes. Analysis of the difference between these two modes is used to identify any heterogeneities in the survey area, further constraining our estimate of the electrical anisotropy. A useful by-product of this experiment has been to refine the 1D conductivity profile of the oceanic lithosphere first established by the PEGASUS experiment and its predecessors.

5 EM7-5 Correction of Motional Electric Field Measurements for Galvanic Distortion Alan D. Chave (Deep Submergence Laboratory, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA; Douglas S. Luther and Chrisopher S. Meinen (Department of Oceanography, University of Hawaii, Honolulu, Hawaii 96822, USA) The theory of galvanic distortion of the motional electric field (i.e., the field induced by ocean current flow through Earth s magnetic field) is developed from first principles, suggesting that seafloor conductivity heterogeneity at a range of scales can produce changes in the electric field direction and amplitude that are specific to a single location. A procedure to correct for the distortion is then derived from the theory. This is based on estimation of inter-site transfer tensors for the electric fields in the high frequency limit where external (ionospheric and magnetospheric) induction is dominant. A decomposition of the measured tensor is derived which expresses it as the product of a set of distortion tensors and the underlying, undistorted transfer tensor. The algorithm may be applied simultaneously to a set of sites and assessed statistically, yielding the undistorted electric field uniquely at each site except for a single site-dependent multiplicative scalar which must be obtained from other data. Because the distortion is frequency-independent, the same tensor may be used to undistort the low frequency, motional induction component that is of interest in oceanography. This procedure is illustrated using an electric field data set collected in the Southern Ocean in as the SubAntarctic Front Dynamics Experiment which is heavily distorted by galvanic processes.

6 EM7-6 Crustal Resistivity Structure at 9 o 50 N on the East Pacific Rise: Preliminary Results of an Electromagnetic Survey Rob. L. Evans 1, Spahr C. Webb 2 and the RIFT-UMC Team 3 1. Department of Geology and Geophysics, Woods Hole Oceanographic Institution 2. Lamont Doherty Earth Observatory 3. RIFT-UMC Team: W. Crawford, IPGP, Paris; C. Golden, MPL, Scripps Institution of Oceanography; K. Key, IGPP, Scripps Institution of Oceanography; L. Lewis, Arnold Orange Associates, Houston, TX; H. Miyano, Chiba University, Japan; E. Roosen, Dept of Geology and Geophysics, Woods Hole Oceanographic Inst; D. Doherty, Scripps Institution of Oceanography. We report the first results of an extensive electromagnetic survey of the East Pacific Rise (EPR) at 9 o 50 N, which used the magnetometric resistivity (MMR) technique to measure the electrical resistivity structure of the seafloor in the vicinity of the spreading center. The survey featured 10 seafloor magnetometers and over 200 transmission stations. Magnetometers were placed on the ridge crest in areas of known hydrothermal activity, in axial sites devoid of venting, and further off-axis to a distance of approximately 4 km. Data collected at off-axis sites show higher seafloor resistivities than at axial sites. This response is opposite to that expected from porosity controlled resistivity structure, with a thicker high-porosity extrusive layer off-axis, as required by seismic data. An explanation for the reduced axial resistivities is that the uppermost few hundred meters of crust are much hotter beneath the ridge crest than a few kilometers off-axis, lowering the pore-fluid resistivity. Within the sheeted dike complex, the cumulative effects of hydrothermal alteration and infilling by late-stage eruptions are expected to reduce porosities significantly as the crust ages. Thus, we explain the differences in resistivity on and off-axis through a combination of lower porosity off-axis and higher temperatures on-axis throughout the upper-crust. At greater depth, (> 1 km), on and off-axis resistivities are similar, suggesting that temperatures at 1 km depth must be similar across the ridge to distances of 4-5 km. The resistivity gradient with depth, both on and off-axis, is lower than that seen at 13 o N in a previous experiment. The low resistivity at depths around 1 km is consistent with the known higher levels of magmatic and hydrothermal activity in the 9 o N section of the EPR. Resistivities are higher at 9 o N than on the Cleft segment of the Juan de Fuca ridge, which has a much thicker extrusive layer, and probably similar thermal conditions, as the segment erupted at nearly the same time as the 9 o 50 N segment of the EPR.

7 EM7-7 Using EM to Understand Groundwater Processes on the Continental Shelf Rob. L. Evans Department of Geology and Geophysics, Woods Hole Oceanographic Inst Dan Lizarralde Earth and Atmospheric Science, Georgia Institute of Technology Ann Mulligan Marine Policy Center, Woods Hole Oceanographic Inst The flux of nutrient-rich waters from land to the oceans is a fundamental component of biogeochemical cycles. This flux occurs through a variety of mechanisms: river input; groundwater discharge into estuarine marshes; and submarine groundwater discharge (SGD) directly through the seafloor. While riverine flux into the oceans is easily quantified, offshore groundwater transport and submarine discharge remain poorly understood. We present data from two survey locations off North Carolina, one very close to shore and one approximately 20 km from shore. In both locations we have used EM profiling along with high resolution Chirp seismic reflection and see evidence for groundwater discharge to the seafloor. The first survey was in Long Bay, N.C., where recently observed temperature and geochemical signals from well sites have been interpreted by Moore and co-workers as evidence for relatively high flux submarine groundwater discharge (SGD) a substantial distance offshore. Our data show evidence of submarine karst formation associated with this SGD zone. We suggest that groundwater transport to this location is most likely through a deep regional aquifer, and that this water percolates upward through a limestone unit within which we observe significant lateral contrasts in apparent porosity. These contrasts may have focused groundwater flow, creating a positive feedback between mixing, dissolution and discharge. Conditions favorable to the formation of similar locally punctuated sites of high flux SGD are likely to exist along the mid- to inner shelf of the southeastern U.S., where carbonate aquifers are prevalent. We have also surveyed a coastal environment, around Wrightsville Beach, North Carolina. Here, a number of channels incise an Oligocene sequence, which we interpret as a hydrologic capping unit, and in places appear to intersect the top surface of a shallow limestone aquifer. Based on the complex geophysical signature of these channels, we suggest that they act as high permeability conduits, allowing groundwater to flow from the limestone up to the seafloor. This assertion is consistent with hydrologic models of the region which predict that pore-fluid with salinity fresher than seawater can be found in the channels to distances of several kilometres from the shoreline.

8 EM7-8 MAGNETOTELLURIC MEASUREMENTS ON NORTH-POLAR ICE OVER THE GAKKEL RIDGE Hartmut Joedicke, Gerhard Kapinos, Birger Lahrmannn (Univ. Muenster, Germany; In a feasibility study as part of the joined US/German project AMORE 2001, the conductivity structure of the oceanic crust and uppermost mantle was studied on northpolar ice over the Gakkel ridge by means of magnetotelluric measurements. Forming the northward continuation of the Midatlantic Ridge, the Gakkel ridge extends from NE Greenland to the Laptev Sea, and is characterised by an extremely low spreading rate of about 1 cm/year. The main unknown factors of MT measurements in that area are the little known properties of the magnetic source field at high magnetic latitudes north of the polar light oval, the drift of the ice floes, and the water currents which may distort the electric field and, thus, the stability of the magnetotelluric transfer functions. Due to intense bathymetric measurements carried out during the cruise of the two icebreakers RV Healey and RV Polarstern the topography of the ocean bottom was well known. 5 sites were deployed along the central valley of the Gakkel ridge with recording times between 1 and 9 days. The time series exhibit high coherency between magnetic and electrical fields realized on the main within the used period range of pulsations (30 s 600 s) and variations (600 s 3 h). However, also time sections exist whith remarkably low coherencies during phases of sufficient source field strength which reduces the amount of usable data. The drift, documented by GPS records, proved to be extremly irregular from time to time with velocities exceeding 700 m/h, which was larger than expected. In contrast, the rotation of ice floes was of minor importance. The influence of electric fields associated with ocean currents was not yet analysed, it may be restricted to periods longer than about 3 hours. First 1D models from selected data sets show good agreement between calculated and bathymetric water depths. Little resolution was achieved in the depth range of oceanic crust and upper mantle up to now. Nevertheless, in comparison with the more sophisticated conductivity model presented for the Reykjanes Ridge (south of Iceland) by M. Sinha et al. generally good agreement between the model curves derived from his model and our data is found, too. Therefore, for a comparably detailed insight into the electrical properties of the Gakkel ridge a-priori informations, in particular, structural data need to be included.

9 EM7-9 PRELIMINARY MARINE MT RESULTS FROM THE ANISOTROPY AND PHYSICS OF THE PACIFIC LITHOSPHERE EXPERIMENT (APPLE) Kerry Key (Scripps Institution of Oceanography, UCSD, La Jolla, CA, USA; Graham Heinson (Dept. of Geology and Geophysics, Adelaide University, Adelaide, Australia) Steven Constable (Scripps Institution of Oceanography, UCSD, La Jolla, CA, USA) Antony White (SoCPES, Flinders University, Bedford Park, Australia) We present preliminary magnetotelluric (MT) and geomagnetic depth sounding (GDS) results from the Anisotropy and Physics of the Pacific Lithosphere Experiment (APPLE). APPLE included both controlled source EM and MT components in order to provide constraints on the depth and alignment of anisotropic conductivity structure in both the crust and upper mantle. A key goal of the MT component is to provide insights into electrical conduction mechanisms in the mantle, particularly the proposal that hydrogen dissolved in olivine enhances the conduction in the a axis direction. The main survey was located on 30 Ma old lithosphere, about 1000 km west of San Diego, USA. The core location consisted of two long period MT instruments ( s: ELFMT), two broadband MT instruments ( s: BBMT) along with four long wire electric field receivers (LEM). Around the core four ELFMT and four BBMT instruments were deployed at 45 azimuths in a 30 km radius to provide constraints on lateral heterogeneities in conductivity structure that may masquerade as mantle anisotropy. In order to study mineral scale anisotropy, macro scale anisotropy due to the coast effect must be deconvolved from the data. This was facilitated by deployments of four ELFMT instruments along a transect from the core to the base of the continental slope during APPLE, augmented with four BBMT sites in 1500 m water on the continental shelf offshore San Diego and six BBMT sites in m water offshore Torrey Pines Beach, California. Initial processing of MT data yielded impedances that are two dimensional (2D), with the largest split between the two principle MT modes and significant GDS response nearest the continental margin. Impedance strike angles are relatively constant in the period band seconds, but exhibit a frequency dependent strike rotation which amounts to about 20 degrees at the longest period of data (5x10 4 s). We augment the examination of strike angle with 2D modeling of bathymetry to help differentiate coast effect and mantle anisotropy.

10 EM7-10 MEASURING THE ELECTRICAL RESISTIVITY OF THE CRUST AT THE EAST PACIFIC RISE 9 50 N Kerry Key (Scripps Institution of Oceanography, La Jolla, California, USA, kkey@ucsd.edu) Steven Constable (Scripps Institution of Oceanography, La Jolla, California, USA) We present the first use of a recently developed broadband marine magnetotelluric (MT) instrument at a mid ocean ridge. The attenuation of source fields by sea water, the effect of bathymetry, and the existence of vertical electric fields all serve to make seafloor MT very different than land MT. By using AC coupled sensors, we extend the high frequency performance of the new instrument, allowing resolution of electrical resistivity structure at much shallower depths than the traditional marine MT method. Our two dimensional inversion model from data collected at five seafloor MT sites on the East Pacific Rise at 9 50 N demonstrates the viability of the method to image electrical resistivity structure in both the crust and shallow mantle. While our pilot experiment falls far short of the coverage needed to provide rigorous constraints on structure, a low resistivity zone in the crustal portion of the inversion model agrees exceptionally well with seismic tomography results from a nearby section of the ridge. Resistivities beneath the ridge imply a crustal partial melt fraction of 3 9%.

11 EM7-11 MARINE MAGNETOTELLURIC MAPPING OF SUB-VOLCANIC STRUCTURE IN THE FAROE-SHETLAND BASIN Lisl Lewis (AOA Geophysics Inc., and San Diego State University) Steven Constable (Scripps Institution of Oceanography), Arnold Orange (AOA Geophysics Inc.), Kerry Key (Scripps Institution of Oceanography). In the summer of 2001, AOA Geophysics Inc. and Scripps Institution of Oceanography collected 116 sites of marine magnetotelluric (MMT) data in the Faroe- Shetland Basin using a fleet of 12 seafloor broadband MT instruments. This was a proprietary survey over several recently acquired blocks, but 27 calibration sites were occupied to link the frontier prospects to existing wells and known geology in the area West of Shetlands and in the Faroe Islands. The eastern Faroe-Shetland basin in the North Atlantic Ocean is rich with hydrocarbon-bearing sediments, but to the west the sedimentary section is overlain by the massive Tertiary volcanics of the Faroe Shelf. The boundary and geologic structure between the viable hydrocarbon prospects in the east and ultra-thick basalt section in the west is naturally a question of great interest to the petroleum industry. Seismic data yield ambiguous results in this region, and the need for another geophysical method is clear. The large conductivity contrast between the resistive basalts and the conductive sediments make electrical methods a viable tool for resolving the geologic structure of the sub-basalt section. The region is notorious for strong ocean currents, and indeed we see instrument noise modulated at tidal frequencies. However, we are able to process through the noise to obtain good results at frequencies that range from to 0.2 Hz. Results from the MMT calibration sites close to the wells agree with the major boundaries encountered in the well logs. The presence of low resistivity sub-volcanic sediments, along with their thickness and structural arrangement, can be confirmed by the MMT data. Forward modeling and 2D inversions further clarify the geologic structure of this region and provide an excellent illustration of how EM methods are practical and necessary tools for solving the sub-volcanic problems of the Faroe-Shetland Basin and similar provinces.

12 EM7-12 PROPERTIES OF CRUSTAL FLUIDS AT THE VALU FA RIDGE, LAU BASIN, AND THEIR RELATIONSHIP TO ACTIVE HYDROTHERMAL CIRCULATION, FROM JOINT ANALYSIS OF CONTROLLED SOURCE ELECTROMAGNETIC AND SEISMIC DATA. L.MacGregor, A. Greer, M.C.Sinha, School of Ocean and Earth Science, Southampton Oceanography Centre, U.K. S.Constable, I.G.P.P., Scripps Institution of Oceanography, California, U.S.A. The complementary nature of electromagnetic and seismic measurements for the study of active spreading centres has been demonstrated in a variety of previous experiments. Whilst the seismic method responds well to structural contrasts, electromagnetic methods are more sensitive to the bulk properties of a medium. Seismic velocity depends strongly on, among other things, porosity and crack geometry. In contrast electrical resistivity is much more sensitive to the distribution of cracks, their interconnectedness, and the properties of the pore fluids. By collecting both data types, therefore, better constraints on the lithology and pore fluid properties can be obtained than from either technique alone. This in turn gives valuable information on the presence and properties of seawater or hydrothermal fluids in the upper crust, and bodies of partial melt at deeper levels in the crust or upper mantle. In December 1995 we carried out a comprehensive controlled source electromagnetic (CSEM) survey of the Valu Fa Ridge at 22 o 25'S in the Lau Basin. The Valu Fa Ridge is a back-arc spreading centre of intermediate spreading rate, and is a site of extensive hydrothermal activity. Seismic studies have imaged a melt lens at an average depth of 3.2 km below the seafloor, surrounded by a zone of lowered seismic velocity, interpreted as a region of partial melt in the crust. The electromagnetic experiment was part of a multidisciplinary study that included wide angle and reflection seismology, bathymetry and potential field measurements. Electromagnetic signals at frequencies between 0.25 Hz and 40 Hz were transmitted from a horizontal electric dipole towed close to the seafloor, and were recorded by an array of eleven sea bottom receivers at ranges of up to 20 km from the source. The data collected were analysed using a combination of 2.5-dimensional forward modelling and inversion to determine the resistivity structure of the crust to a depth of around 3 km below the seafloor. In this paper we use a joint effective medium theory approach to combine results from the electromagnetic and seismic experiments, in order to study the bulk properties of the upper crust and the fluids therein.