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de Groot-Hedlin, C, Constable S.  2004.  Inversion of magnetotelluric data for 2D structure with sharp resistivity contrasts. Geophysics. 69:78-86.   10.1190/1.1649377   AbstractWebsite

We have developed a linearized algorithm to invert noisy 2-D magnetotelluric data for subsurface conductivity structures represented by smooth boundaries defining sharp resistivity contrasts. We solve for both a fixed number of subsurface resistivities and for the boundary locations between adjacent units. The boundary depths are forced to be discrete values defined by the mesh used in the forward modeling code. The algorithm employs a Lagrange multiplier approach in a manner similar to the widely used Occam method. The main difference is that we penalize variations in the boundary depths, rather than in resistivity contrasts between a large number of adjacent blocks. To reduce instabilities resulting from the breakdown of the linear approximation, we allow an option to penalize contrasts in the resistivities of adjacent units. We compare this boundary inversion method to the smooth Occam inversion for two synthetic models, one that includes a conductive wedge between two resistors and another that includes a resistive wedge between two conductors. The two methods give good agreement for the conductive wedge, but the solutions differ for the more poorly resolved resistive wedge, with the boundary inversion method giving a more geologically realistic result. Application of the boundary inversion method to the resistive Gemini subsalt petroleum prospect in the Gulf of Mexico indicates that the shape of this salt feature is accurately imaged by this method, and that the method remains stable when applied to real data.

Constable, S, Constable C.  2004.  Observing geomagnetic induction in magnetic satellite measurements and associated implications for mantle conductivity. Geochemistry Geophysics Geosystems. 5   10.1029/2003gc000634   AbstractWebsite

Currents induced in Earth by temporal variations in the external magnetic field have long been used to probe mantle electrical conductivity, but almost exclusively from sparsely distributed land observatories. Satellite-borne magnetometers, such as flown on Magsat, Orsted, and Champ, offer the prospect of improved spatial coverage. The approach we have taken is to isolate induction by harmonic Dst ("disturbance storm time'') excitation of the magnetospheric ring current in satellite magnetic measurements: this is done by removing the magnetic contributions of the main (core) magnetic field, the crustal magnetic field, and ionospheric fields (cause of the daily variation) using Sabaka et al.' s [2000, 2002] CMP3 comprehensive model. The Dst signal is then clearly evident in the midlatitude satellite passes lower than 50 degrees geomagnetic latitude. At higher latitudes, auroral and field aligned currents contaminate the data. We fit the internal and external components of the Dst signal for each equatorial pass, exploiting the fact that the geometry for the internal and external components is different for the azimuthal and radial vector components. The resulting timeseries of internal and external field variations shows that the Dst signals for the dawn passes are half those of the dusk passes. The sum of equatorial external and internal components of the field averaged over dawn and dusk passes provides an excellent estimate for the Dst index, and may in fact be superior when used as a proxy for the purposes of removing induced and magnetospheric fields from satellite magnetic data. We call this estimate satellite Dst. Cross spectral analysis of the internal and external timeseries shows both greater power and higher coherence in the dusk data. We processed the transfer function between internal and external dusk timeseries to provide globally-averaged, frequency dependent impedances that agree well with independently derived estimates. We estimate Earth's radial electrical conductivity structure from these impedances using standard regularized inversion techniques. A near-surface conductor is required, of thickness less than 10 km with a conductivity-thickness product almost exactly that of an average Earth ocean. Inversions suggest that an increase in conductivity at 440 km depth, predicted by recent laboratory measurements on high pressure phases of olivine, is not favored by the data, although, as in previous studies, the 670 km discontinuity between the upper and lower mantle is associated with a two orders of magnitude jump in conductivity. A new feature in our inversions is a further increase in lower mantle conductivity at a depth of 1300 km. A global map of the internal (induced) component of the magnetic field provides a qualitative estimate of three-dimensional (3-D) variations in Earth electrical conductivity, demonstrating graphically that the satellite data are responsive to lateral variations in electrical conductivity caused by the continents and oceans.

Constable, CG, Constable SC.  2004.  Satellite magnetic field measurements: applications in studying the deep earth. The state of the planet : frontiers and challenges in geophysics. ( Sparks RSJ, Hawkesworth CJ, Eds.).:147-160., Washington, DCS.l.: American Geophysical Union ;International Union of Geodesy and Geophysics   10.1029/150GM13   Abstract
Everett, ME, Constable S, Constable CG.  2003.  Effects of near-surface conductance on global satellite induction responses. Geophysical Journal International. 153:277-286.   10.1046/j.1365-246X.2003.01906.x   AbstractWebsite

A 3-D finite-element simulation of global electromagnetic induction is used to evaluate satellite responses in geomagnetic dipole coordinates for harmonic ring-current excitation of a three-layer mantle overlain by a realistic near-surface conductance distribution. Induced currents are modelled for lithospheric and asthenospheric upper-mantle conductivities in the range sigma= 10(-4) -0.1 S m(-1) . The magnetic scalar intensity B is calculated at a typical satellite altitude of 300 km. At short periods, T = 2 and 12 h, the induction signal owing to the near-surface conductance is large when a resistive upper mantle is present, but drops off with increasing mantle conductivity. At longer periods, T = 2 d, the near-surface induction signal is generally much smaller and nearly independent of upper-mantle conductivity. The near-surface induction signal is very sensitive to the electrical conductivity of the lithospheric mantle, but only moderately sensitive to that of the asthenospheric mantle. Induced currents are confined to the heterogeneous surface shell at periods of less than 2 h, and flow predominantly in the mantle at periods of longer than 2 d. In the intervening period range, induced currents are partitioned between the near-surface and the upper mantle. These results indicate the importance of carrying out a full 3-D analysis in the interpretation of satellite induction observations in the period range from hours to days.

Korte, M, Constable S, Constable C.  2003.  Separation of external magnetic signal for induction studies. First CHAMP mission results for gravity, magnetic and atmospheric studies. ( Reigber C, Luehr H, Schwintzer P, Eds.).:315-320., Berlin: Springer Abstract
Key, K, Constable S.  2002.  Broadband marine MT exploration of the East Pacific Rise at 9 degrees 50 ' N. Geophysical Research Letters. 29   10.1029/2002gl016035   AbstractWebsite

[1] We present the first use at a mid-ocean ridge of a recently developed broadband marine magnetotelluric (MT) instrument. The extended high frequency performance of the instrument allows resolution of electrical resistivity structure at shallower depths than traditional marine MT sensors. Our two-dimensional inversion model from data collected at four MT sites on the East Pacific Rise (EPR) at 9degrees50'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 well with seismic tomography and seafloor compliance results. Resistivities beneath the ridge imply a crustal partial melt fraction of 1-20%. A total melt volume of about 0.75 km(3) per kilometer of ridge implies an average melt residence time of about 1000 years.

Constable, S, Duba A.  2002.  Diffusion and mobility of electrically conducting defects in olivine. Physics and Chemistry of Minerals. 29:446-454.   10.1007/s00269-002-0260-8   AbstractWebsite

Electrical conductivity of lherzolite (65% olivine), measured as a function of time after changes in the oxygen fugacity (f(O2)) of the surrounding CO(2)/CO atmosphere, is used to infer the diffusivity of the point defects responsible for conduction in olivine. A total of 63 equilibration runs at temperatures of 900, 1000, 1100, and 1200 degreesC were fit using nonlinear parameter estimation to recover time constants (directly related to diffusivity) and conductivity steps. An observed f(O2) dependence in the time constants associated with re-equilibration implies two defect species of fixed diffusivity but with f(O2)-dependent concentrations. Although the rate-limiting step may not necessarily be associated with a conducting defect, when time constants are converted to diffusivities, the magnitudes and activation energies agree extremely well with the model for magnesium vacancies (the slower species) and small polarons (holes localized on Fe(3+)) derived by Constable and Roberts (1997). This earlier study used an independent method of simultaneous modeling of thermopower and electrical conductivity as a function of f(O2) and temperature, on data from a different type of sample (a dunite). We observe that at high f(O2) where polarons dominate over magnesium vacancies in the defect population, re-equilibration is dominated by magnesium vacancy diffusion, and vice versa (at low f(O2) magnesium vacancies dominate and re-equilibration proceeds at the faster rate associated with polaron mobility). We interpret this to suggest association between the cation vacancies and polarons, as has been suggested by Tsai and Dieckmann (1997), making the concentration of the minority defect the rate-limiting step in the oxidation/reduction reactions.

Ellingsrud, S, Eidesmo T, Johansen S, Sinha MC, MacGregor LM, Constable S.  2002.  Remote sensing of hydrocarbon layers by sebed logging (SBL): results from a cruise offshore Angola. The Leading Edge. 21:972-982. Abstract
Eidesmo, T, Ellingsrud S, MacGregor LM, Constable S, Sinha MC, Johanson S, Kong FN, Westerdahl H.  2002.  Sea Bed Logging (SBL), a new method for remote and direct identification of hydrocarbon filled layers in deepwater areas. First Break. 20:144-152. Abstract
MacGregor, L, Sinha M, Constable S.  2001.  Electrical resistivity structure of the Valu Fa Ridge, Lau Basin, from marine controlled-source electromagnetic sounding. Geophysical Journal International. 146:217-236.   10.1046/j.1365-246X.2001.00440.x   AbstractWebsite

In December 1995 we carried out a comprehensive controlled-source electromagnetic survey of the Valu Fa Ridge at 22 degrees 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 which included wide-angle and reflection seismics, bathymetry and potential field measurements. Electromagnetic signals at frequencies between 0.25 and 40 Hz were transmitted from a horizontal electric dipole towed close to the seafloor and were recorded by an array of 11 sea-bottom receivers fit ranges of up to 20 km from the source. Over 80 hr of data, consisting of the magnitude of the horizontal electric field at the seafloor, were collected. These data have extremely low scatter compared to similar data from previous surveys. The data were interpreted using a combination of 1- and 2-D forward modelling and inversion. The vertical resistivity gradient in the upper crust at the Vain Fa Ridge is abnormally low, with resistivities of less than 10 Ohm m observed throughout layer 2 of the crust to a depth of 3 km. This is significantly more conductive at depth than the axis of the slow-spreading Reykjanes Ridge at 57 degrees 45'N, and the fast-spreading East Pacific Rise at 13 degreesN, where similar data sets have been collected in the past. Although the structure of layer 2 is well constrained by the electromagnetic data, its extremely low resistivity causes rapid attennuation of electromagnetic signals diffusing through it, and hence the data are not sensitive to the structure in layer 3, in particular the structure of the melt lens or surrounding low-velocity zone. The seismic velocity structure of the Valu Fa Ridge, determined from the coincident wide-angle seismic study, is similar to that observed at other mid-ocean ridges, with a steep seismic velocity gradient through layer 2 (although overall velocities are slightly lower). The seismic velocity anomaly calculated relative to an average off-axis structure is also small. This suggests that the very low resistivities observed at the axis are not caused by an upper crust of abnormally high porosity. However, hot and/or saline fluids permeating the crust can explain the low resistivities without affecting the seismic velocity. Since the conductive region extends unbroken from 3 km depth to the seafloor, it is probable that these fluids circulate to (or close to) the magma chamber itself.

Hoversten, GM, Constable SC, Morrison HF.  2000.  Marine magnetotellurics for base-of-salt mapping: Gulf of Mexico field test at the Gemini structure. Geophysics. 65:1476-1488.   10.1190/1.1444836   AbstractWebsite

A sea-floor magnetotelluric (MT) survey was conducted over the Gemini subsalt petroleum prospect in the Gulf of Mexico (GOM) to demonstrate that the base of salt can be mapped using marine magnetotelluric (MMT) methods. The high contrast in electrical resistivity between the salt and the surrounding sediments provides an excellent target for MMT. The Gemini salt body, located at 28 degrees 46' N 88 degrees 36' W, is a relatively complex shape buried 2-5 km below the sea floor in I-km-deep water. Its geometry has been previously determined using 3-D seismic prestack depth migration with well log control. In order not to confuse limitations in interpretation technique with limitations in data acquisition, numerical forward and inverse modeling guided the survey design to locate a profile that would be amenable to 2-D inversion, even though the body was clearly 3-D. The seismic imaging of the base of salt along the chosen profile is considered good, thus providing a good control for testing the MT method. In many other areas of the GOM, and indeed other portions of the Gemini structure itself, seismic imaging of the base of salt is problematic. Data were collected using autonomous sea-floor data loggers equipped with induction coil magnetic sensors and electric field sensors consisting of silver-silver chloride electrodes connected to an ac-coupled amplifier originally designed for sea-floor controlled-source studies. Nine sites of excellent quality MT responses were obtained. Smooth 2-D inversion of the data produce a confined resistive anomaly at the comet location and depth, and recently developed sharp-boundary 2-D inversion recovers base of salt in excellent agreement with the seismic models. Simple perturbation analysis shows that base of salt has been resolved to within 5-10% of burial depth.

Fullekrug, M, Constable S.  2000.  Global triangulation of intense lightning discharges. Geophysical Research Letters. 27:333-336.   10.1029/1999gl003684   AbstractWebsite

A global network of three electromagnetic measurement instruments is used to simultaneously record time series of globally observable Extremely-Low-Frequency (ELF) magnetic field disturbances which propagate with little attenuation around the globe within the Earth-ionosphere cavity. The triangulation of individual lightning flashes results in a picture of the temporal evolution of intense lightning discharge occurrences on the planetary scale during April 1998. The lightning flash charge moments are calculated with the short pulse approximation of the normal mode expansion. The majority of the triangulated lightning discharges exhibit charge moments with a potential to excite mesospheric sprites and similar to 5-20% may account for air breakdown at sprite altitudes in similar to 50-70 km height.

Heinson, G, Constable S, White A.  2000.  Episodic melt transport at mid-ocean ridges inferred from magnetotelluric sounding. Geophysical Research Letters. 27:2317-2320.   10.1029/2000gl011473   AbstractWebsite

Oceanic crust is generated at mid-ocean ridges by decompression melting of upwelling mantle at depths of between 50 and 120 km. Geodynamic and geochemical models of upwelling, melt extraction, and melt emplacement into crustal magma reservoirs present a variety of possible migration geometries, most of which assume steady-state or near steady-state processes. Here we present results from marine magnetotelluric (MT) measurements, carried out as part of the RAMESSES experiment on the slow spreading Reykjanes Ridge, which support a model of melt extraction and migration that is episodic, rather than steady-state.

Everett, ME, Constable S.  1999.  Electric dipole fields over an anisotropic seafloor: theory and application to the structure of 40 Ma Pacific Ocean lithosphere. Geophysical Journal International. 136:41-56.   10.1046/j.1365-246X.1999.00725.x   AbstractWebsite

Seismic anisotropy has been detected in the oceanic crust and upper mantle, and likewise it is geologically reasonable to expect that a certain amount of lateral anisotropy exists in seafloor electrical properties. Anisotropy in Earth properties can often lead to surprising effects on geophysical responses that are not anticipated from simple isotropic theories. Here, we investigate the effects of lateral anisotropy on the frequency-domain, controlled-source electromagnetic (CSEM) response of a uniaxially conducting, non-magnetic seafloor excited by a horizontal electric dipole whose moment is oriented obliquely with respect to the electrical strike direction. A 'paradox of anisotropy' is observed, in which the seafloor electric field strength is enhanced in the most conductive direction of the seafloor. This enhancement is opposite to what one would expect based on naive isotropic theory. We also show that it is possible in certain circumstances to extract the along-strike electrical conductivity from marine controlled-source electromagnetic data using only isotropic modelling. The extraction of across-strike conductivity, however, requires full anisotropic modelling. The physical insight into electromagnetic induction in uniaxial media that is presented here should greatly assist the geological interpretation of marine CSEM experimental data. Applying our algorithm to the PEGASUS data set (CSEM data collected over 40 Ma Pacific Ocean lithosphere) produces a model with conductivity in the fossil spreading direction that is seven times greater than the conductivity perpendicular to spreading. Strain-aligned mineralogical fabric, as predicted by tectonic modelling, would explain our result, with enhanced conductivities caused by hydrogen conduction along the olivine a-axis or connected accumulations of trace conductors such as graphite or magnetite.

Heinson, G, White A, Constable S, Key K.  1999.  Marine self potential exploration. Exploration Geophysics. 30:1-4.   10.1071/EG999001   AbstractWebsite

Recent marine self potential (SP) measurements south of Eyre Peninsula, South Australia and Rose Canyon, San Diego, California have been made. In both cases, a series of horizontal electrode dipoles were towed close to the seafloor in water depths of up to 100 m to measure the electric potential gradients generated by mineralisation beneath the seafloor. A proton-precession magnetometer was also towed at the surface in the Eyre Peninsula experiment. Marine SP measurements show significantly lower noise levels than land measurements, due to the uniform marine environment and low contact resistance of the electrodes, so that anomalies of less than a few tens of microvolts per metre can be detected. The major source of noise is from ocean swell and waves, which may be minimised by coherent stacking of signals, and by bandpass filtering. South of Eyre Peninsula SP electric field anomalies of 100 µV/m and width 2 km were observed in a number of traverses perpendicular to the trend of an onshore mylonite zone. Little correlation exists between the SP and magnetic data, suggesting that the SP sources are probably due to non-ferrous minerals such as graphite, and/or from the electrokinetic effect of groundwater flow through the fracture zone.

Constable, SC, Orange AS, Hoversten GM, Morrison HF.  1998.  Marine magnetotellurics for petroleum exploration Part I: A sea-floor equipment system. Geophysics. 63:816-825.   10.1190/1.1444393   AbstractWebsite

Induction in electrically conductive seawater attenuates the magnetotelluric (MT) fields and, coupled with a minimum around 1 Hz in the natural magnetic field spectrum, leads to a dramatic loss of electric and magnetic field power on the sea floor at periods shorter than 1000 s, For this reason the marine MT method traditionally has been used only at periods of 10(3) to 10(5) s to probe deep mantle structure; rarely does a sea-floor MT response extend to a 100-s period. To be useful for mapping continental shelf structure at depths relevant to petroleum exploration, however, MT measurements need to be made at periods between 1 and 1000 s. This can be accomplished using ac-coupled sensors, induction coils for the magnetic field, and an electric field amplifier developed for marine controlled-source applications. The electrically quiet sea floor allows the attenuated electric field to be amplified greatly before recording; in deep (l-km) water, motional noise in magnetic field sensors appears not to be a problem. In shallower water, motional noise does degrade the magnetic measurement, but sea-floor magnetic records can be replaced by land recordings, producing an effective sea-surface MT response. Field trials of such equipment in l-km-deep water produced good-quality MT responses at periods of 3 to 1000 s: in shallower water, responses to a few hertz can be obtained. Using an autonomous sea-floor data logger developed at Scripps Institution of Oceanography, marine surveys of 50 to 100 sites are feasible.

Hoversten, GM, Morrison HF, Constable SC.  1998.  Marine magnetotellurics for petroleum exploration, Part II: Numerical analysis of subsalt resolution. Geophysics. 63:826-840.   10.1190/1.1444394   AbstractWebsite

In areas where seismic imaging of the base of salt structures is difficult, seaborne electromagnetic techniques offer complementary as well as independent structural information. Numerical models of 2-D and 3-D salt structures demonstrate the capability of the marine magnetotelluric (MT) technique to map the base of the salt structures with an average depth accuracy of better than 10%. The mapping of the base of the salt with marine MT is virtually unaffected by internal variation within the salt. Three-dimensional anticlinal structures with a horizontal aspect ratio greater than two can be interpreted adequately via two-dimensional inversions. Marine MT can distinguish between salt structures which possess deep vertical roots and those which do not. One measure of the relative accuracy of MT acid seismic methods can be made by considering the vertical and lateral position errors in the locations of interfaces caused by neglecting velocity anisotropy in migration. For the shallow part of the section where two-way travel times are on the order of 1 s, the vertical and lateral position errors in the locations of salt-sediment interfaces from 2-D MT inversion is more than twice the expected migration error in reflectors in transversely isotropic sediments, such as those in the Gulf of Mexico. Deeper in the section where two-way times are on the order of 4 s, lateral position errors in migration become comparable to those of the MT inverse, whereas seismic vertical position errors remain more than a factor of two smaller than MT errors. This analysis shows that structural mapping accuracy would be improved using MT and seismic together.

Tolstoy, M, Constable S, Orcutt J, Staudigel H, Wyatt FK, Anderson G.  1998.  Short and long baseline tiltmeter measurements on axial seamount, Juan de Fuca Ridge. Physics of the Earth and Planetary Interiors. 108:129-141.   10.1016/s0031-9201(98)00091-0   AbstractWebsite

Long-term observations of seismic activity and ground deformation at mid-ocean ridges and submarine volcanoes are required for an understanding of the spatial and temporal characteristics of magma transport and intrusion. To make precise records of tilt on the seafloor we have installed short baseline tiltmeters in six ocean bottom seismometers (TILT-OBS) and developed a long baseline (100-500 m) two-fluid tiltmeter (LBT). In the TILT-OBS, the seismometer platform is levelled to better than 1 degrees after deployment. The tiltmeter consists of a pair of electrolytic bubble sensors mounted on a secondary levelling stage on the seismometer platform. The levelling stage uses two motor-driven micrometers on a triangular mounting plate to bring the sensors to null. The sensitivity of these tiltmeters is 0.05 mu rad, at a dynamic range of 0.2 mrad. A long baseline instrument was developed to achieve a better spatial average of deformation. Most approaches used on land to measure stable long baseline tilt cannot be applied to a submarine instrument, but tiltmeters in which the pressure of a fluid in tubes is measured are amenable to installation on the seafloor. The development resulted in a device that is essentially a center-pressure instrument folded back on itself, with fluids of different densities in the two tubes. During July to September 1994, these instruments were deployed on Axial Seamount, on the Juan de Fuca Ridge off Washington state, for a test of their relative performance on volcanic terrain, yielding 9 weeks of continuous data (seismic, tilt, and temperature) from five TILT-OBS and one long baseline instrument. Drift on all instruments was of the order of 1 mu rad/day, with higher frequency variations of order 5-10 mu rad. Initial drift on the TILT-OBS is shown to be associated with platform settling rather than with the sensor or its mounting. High frequency noise is coherent across instruments and tidal in character, and we conclude that tidal currents moving the sensors are responsible. (C) 1998 Elsevier Science B.V. All rights reserved.

Sinha, MC, Constable SC, Peirce C, White A, Heinson G, MacGregor LM, Navin DA.  1998.  Magmatic processes at slow spreading ridges: implications of the RAMESSES experiment at 57 degrees 45 ' N on the Mid-Atlantic Ridge. Geophysical Journal International. 135:731-745.   10.1046/j.1365-246X.1998.00704.x   AbstractWebsite

This paper is the first in a series of three (this issue) which present the results of the RAMESSES study (Reykjanes Axial Melt Experiment: Structural Synthesis from Electromagnetics and Seismics). RAMESSES was an integrated geophysical study which was carefully targeted on a magmatically active, axial volcanic ridge (AVR) segment of the Reykjanes Ridge, centred on 57 degrees 45'N. It consisted of three major components: wide-angle seismic profiles along and across the AVR, using ocean-bottom seismometers, together with coincident seismic reflection profiles; controlled-source electromagnetic sounding (CSEM); and magnetotelluric sounding (MT). Supplementary data sets included swath bathymetry, gravity and magnetics. Analyses of the major components of the experiment show clearly that the sub-axial geophysical structure is dominated by the presence and distribution of aqueous and magmatic fluids. The AVR is underlain by a significant crustal magma body, at a depth of 2.5 km below the sea surface. The magma body is characterized by low seismic velocities constrained by the wide-angle seismic data; a seismic reflection from its upper surface; and a region of anomalously low electrical resistivity constrained by the CSEM data. It includes a thin, ribbon-like melt lens at the top of the body and a much larger region containing at least 20 per cent melt in a largely crystalline mush zone, which flanks and underlies the melt lens. RAMESSES is the first experiment to provide convincing evidence of a significant magma body beneath a slow spreading ridge. The result provides strong support for a model of crustal accretion at slow spreading rates in which magma chambers similar to those at intermediate and fast spreading ridges play a key role in crustal accretion, but are short-lived rather than steady-state features. The magma body can exist or only a small proportion of a tectono-magmatic cycle, which controls crustal accretion, and has a period of at least 20 000 years. These findings have major implications for the temporal patterns of generation and migration of basaltic melt in the mantle, and of its delivery into the crust, beneath slow-spreading mid-ocean ridges.

MacGregor, LM, Constable S, Sinha MC.  1998.  The RAMESSES experiment - III. Controlled-source electromagnetic sounding of the Reykjanes Ridge at 57 degrees 45 ' N. Geophysical Journal International. 135:773-789.   10.1046/j.1365-246X.1998.00705.x   AbstractWebsite

A controlled-source electromagnetic sounding survey centred on an axial volcanic ridge (AVR) segment of the Reykjanes Ridge at 57 degrees 45'N was performed as part of the RAMESSES experiment. Low-frequency (0.35-11 Hz) electromagnetic signals were transmitted through the crust to an array of horizontal electric field recorders at the seafloor to ranges of 15 km from the source, which was a 100 m long horizontal electric dipole towed at heights of 50-80 m from the seafloor. Coincident seismic and magnetotelluric studies were conducted during the rest of the RAMESSES experiment. Data were interpreted using a combination of 1-D forward modelling and inversion, and iterative forward modelling in two dimensions. On the axis of the AVR, the resistivity at the seafloor is 1 Ohm m. There is a steep resistivity gradient in the upper few hundred metres of the crust, with the resistivity reaching approximately 10 Ohm m at a depth of 500 m. In order to explain the low resistivities, the upper layer of the crust must be heavily fractured and saturated with sea water. The resistivity increases with distance from the axis as the porosity decreases with increasing crustal age. The most intriguing feature in the data is the large difference in amplitude between fields transmitted along and across the AVR axis. A significant zone of low-resistivity material is required at approximately 2 km depth beneath the ridge crest in order to explain this difference. It is coincident with the low-velocity zone required by the seismic data, and has a total electrical conductance in excellent agreement with the results of the magnetotelluric study. The low-resistivity zone can be explained by the presence of a body of partially molten basalt in the crust. Taken together, these results provide the first clear evidence for a crustal magma chamber at a slow spreading mid-ocean ridge. The data constrain the melt fraction within the body to be at least 20 per cent, with a melt volume sufficient to feed crustal accretion at this segment of the ridge for of the order of 20 000 years. Since this body would freeze in the order of 1500 years, this finding lends support to the hypothesis that, at slow spreading rates, crustal accretion is a cyclic process, accompanying periodic influxes of melt from the mantle to a crustal melt reservoir.

Anderson, G, Constable S, Staudigel H, Wyatt FK.  1997.  A seafloor long-baseline tiltmeter. Journal of Geophysical Research-Solid Earth. 102:20269-20285.   10.1029/97jb01586   AbstractWebsite

Long-term monitoring of seismicity and deformation has provided constraints on the eruptive behavior and internal structure and dynamics of subaerial volcanoes, but until recently, such monitoring of submarine volcanoes has not been feasible. Little is known about the formation of oceanic crust or seamounts, and we have therefore developed a stand-alone long-baseline tiltmeter to record deformation on active seafloor volcanoes. The instrument is a differential pressure, two-fluid sensor adapted for use on the seafloor, combined with an autonomous data logger and acoustic navigation/release system. The tiltmeter can be Installed without use of remotely operated vehicles or manned submersibles and, to first order, is insensitive to noise driven by temperature or pressure gradients. We recorded 65 days of continuous data from one of these tiltmeters on Axial Seamount on the Juan de Fuca Ridge during a multidisciplinary experiment that included ocean bottom seismographs, magnetotelluric instruments, and short-baseline tiltmeters. After instrument equilibration the 100-m-long tiltmeter provided a record with long-term drift rates of 0.5-5 mu rad day(-1) and higher frequency variations of the order of 5-10 mu rad. Comparison with records of subaerial volcanic tilt shows that this instrument can discriminate volcanic deflation events, though none occurred during our deployment, a conclusion supported by nearby short-baseline tilt and bottom pressure recordings. The short-and long-baseline data constrain volcanic inflation of Axial Seamount to be below 0.5-1 mu rad day(-1) during mid-1994. Analysis of the long-baseline tilt data in conjunction with electric field, temperature, and short-baseline tiltmeter data shows that high-frequency signals are largely driven by ocean currents. Improved coupling between the tiltmeter and seafloor should reduce this noise, improve stability and drift, and further enhance our ability to record tilt related to active submarine volcanism.

Constable, S, Roberts JJ.  1997.  Simultaneous modeling of thermopower and electrical conduction in olivine. Physics and Chemistry of Minerals. 24:319-325.   10.1007/s002690050044   AbstractWebsite

Measurements of conductivity and thermopower as a function of oxygen fugacity (f(O2)) are used to derive a model for conduction in olivine. Thermopower at 1000-1200 degrees C is between 50 and 400 mu V/K and has a positive f(O2) dependence, and electrical conductivity exhibits approximately a 1/11 power dependence on f(O2). However, small polarons, considered to be the conducting defect in olivine at these temperatures, would produce a larger thermopower than observed, with a negative f(O2) dependence, as well as 1/6 power dependence of conductivity on f(O2) At least one other conducting defect species must be invoked to explain the observed magnitude and f(O2) dependence of thermopower. An electron/polaron model cannot be made to fit the conductivity and thermopower data well, but a polaron/magnesium vacancy model fits the data if a constant polaron or magnesium vacancy term is included. Concentrations from our fits are consistent with predictions from theoretical models, and our analysis predicts a transition from polaron dominance in conduction to magnesium vacancy dominance at around 1300 degrees C, as has been previously inferred from other data.

Sinha, MC, Navin DA, MacGregor LM, Constable S, Peirce C, White A, Heinson G, Inglis MA.  1997.  Evidence for accumulated melt beneath the slow-spreading Mid-Atlantic ridge. Philosophical Transactions of the Royal Society of London Series a-Mathematical Physical and Engineering Sciences. 355:233-253.   10.1098/rsta.1997.0008   AbstractWebsite

The analysis of data from a multi-component geophysical experiment conducted on a segment of the slow-spreading (20 mm yr(-1)) Mid-Atlantic Ridge shows compelling evidence for a significant crustal magma body beneath the ridge axis. The role played by a crustal magma chamber beneath the axis in determining both the chemical and physical architecture of the newly formed crust is fundamental to our understanding of the accretion of oceanic lithosphere at spreading ridges, and over the last decade subsurface geophysical techniques have successfully imaged such magma chambers beneath a number of intermediate and fast spreading (60-140 mm yr(-1) full rate) ridges. However, many similar geophysical studies of slow-spreading ridges have, to date, found little or no evidence for such a magma chamber beneath them. The experiment described here was carefully targeted on a magmatically active, axial volcanic ridge (AVR) segment of the Reykjanes Ridge, centred on 57 degrees 43' N. It consisted of four major components: wide-angle seismic profiles using ocean bottom seismometers; seismic reflection profiles; controlled source electromagnetic sounding; and magneto-telluric sounding. Interpretation and modelling of the first three of these datasets shows that an anomalous body lies at a depth of between 2 and 3 km below the seafloor beneath the axis of the AVR. This body is characterized by anomalously low seismic P-wave velocity and electrical resistivity, and is associated with a seismic reflector. The geometry and extent of this melt body shows a number of similarities with the axial magma chambers observed beneath ridges spreading at much higher spreading rates. Magneto-telluric soundings confirm the existence of very low electrical resistivities in the crust beneath the AVR and also indicate a deeper zone of low resistivity within the upper mantle beneath the ridge.

Constable, SC, Heinson GS, Anderson G, White A.  1997.  Seafloor electromagnetic measurements above Axial Seamount, Juan de Fuca Ridge. Journal of Geomagnetism and Geoelectricity. 49:1327-1342. AbstractWebsite

Magnetotelluric (MT) data were collected at three sites around the eastern rim of the caldera of Axial Seamount, on the Juan de Fuca Ridge. The seamount has been observed to be volcanically and hydrothermally active over the last ten years, and is therefore an excellent target for electromagnetic induction studies on the seafloor. This paper follows an initial interpretation by Heinson et al. (1996) with a more complete analysis of the MT data, to investigate both oceanographic induction effects and the resistivity structure beneath the seamount. From time series analysis of electric field data using multitaper methods, coherences between electric field data from different sites are significant at the 95% confidence level at periods less than 1 day, and generally greater than 0.8 at solar and ocean tidal periods. Spectral peaks at 16.7 hours and 4 days are observed; the former is due to inertial currents in the area, and the latter is probably a ridge-trapped Rossby wave. Robust MT impedance tensors are derived using a remote-reference, and tensor decomposition shows that there is no galvanic distortion and almost isotropic responses at each site. The MT data are inverted for 1D structure, and more complex 3D forward models used to assess the lateral extent of the resistivity structure. 1D inversions show that the data are consistent with a crust with a very high electrical conductance of 1200 +/- 200 S and an asthenosphere of 5-50 Omega.m at a depth of 40 km, connected by a low resistivity lithosphere of 50-100 Omega.m. The low resistivity lithosphere acts as a leakage path to the mantle for induced currents in the ocean. 3D forward modelling suggests that this region may be present only beneath Axial Seamount, surrounded by a resistive lithosphere of 500-50,000 Omega.m. The tectonic implications from these models are that a small fraction of melt is presently migrating from the melt source in the mantle to a crustal magma chamber beneath Axial Seamount. Bulk estimates of melt fractions are 1-10% for the asthenosphere, and 1% between the asthenosphere and the crustal magma chamber, although melt may be concentrated in fractures or pipes.

Flosadottir, AH, Constable S.  1996.  Marine controlled-source electromagnetic sounding .1. Modeling and experimental design. Journal of Geophysical Research-Solid Earth. 101:5507-5517.   10.1029/95jb03739   AbstractWebsite

Numerical forward modeling, predicting an observable response given a mathematical representation of the Earth, is an important component of practical exploration work. In addition, derivatives which relate changes in response to changes in the Earth model are useful for experimental design and are a crucial element of linearized inversion techniques. Differentiation of kernels followed by numerical integration using a fast Hankel transform provides an efficient combination of forward and sensitivity modeling for frequency-domain horizontal electric dipole-dipole sounding over a layered seafloor. Our code is validated against an independent forward modeling technique using a mode analysis and against central difference derivatives. Efficiency is important in the application of regularized inversion to large data sets; we give an example from the East Pacific Rise, requiring 2000 elements in the Jacobian matrix. We illustrate the use of forward modeling and discrete analogs of the Frechet kernels to provide aid to physical intuition and experimental design in the context of the electrical conductivity of the oceanic lithosphere. By using the most favorable parts of range-frequency space, experiments using current technology should be capable of distinguishing a thicker, less resistive, from a thinner, more resistive ''lithospheric resistor'' layer.