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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.

Barak, O, Key K, Constable S, Ronen S.  2018.  Recording active-seismic ground rotations using induction-coil magnetometers. Geophysics. 83:P19-P42.   10.1190/geo2017-0281.1   AbstractWebsite

Most of the current rotational sensing technology is not geared toward the recording of seismic rotations' amplitudes and frequencies. There are few instruments that are designed for rotational seismology, and the technology for building them is currently being developed. There are no mass industrial producers of seismic rotation sensors as there are for geophones, and only one current sensor model can be deployed on the ocean bottom. We reviewed some current rotational-seismic acquisition technologies, and developed a new method of recording rotations using an existing, robust and field-deployable technology that had seen extensive use in large exploration surveys: induction-coil magnetometers. We conducted an active seismic experiment, in which we found that magnetometers could be used to record seismic rotations. We converted the magnetometer data to rotation-rate data, and validated them by comparing the waveforms and amplitudes with rotation rates recorded by electrokinetic rotation sensors.

Carvajal, M, Araya-Cornejo C, Sepulveda I, Melnick D, Haase JS.  2019.  Nearly instantaneous tsunamis following the Mw 7.5 2018 Palu earthquake. Geophysical Research Letters. 46:5117-5126.   10.1029/2019gl082578   AbstractWebsite

The tsunami observations produced by the 2018 magnitude 7.5 Palu strike-slip earthquake challenged the traditional basis underlying tsunami hazard assessments and early warning systems. We analyzed an extraordinary collection of 38 amateur and closed circuit television videos to show that the Palu tsunamis devastated widely separated coastal areas around Palu Bay within a few minutes after the mainshock and included wave periods shorter than 100 s missed by the local tide station. Although rupture models based on teleseismic and geodetic data predict up to 5-m tsunami runups, they cannot explain the higher surveyed runups nor the tsunami waveforms reconstructed from video footage, suggesting either these underestimate actual seafloor deformation and/or that non-tectonic sources were involved. Post-tsunami coastline surveys combined with video evidence and modeled tsunami travel times suggest that submarine landslides contributed to tsunami generation. The video-based observations have broad implications for tsunami hazard assessments, early warning systems, and risk-reduction planning. Plain Laguage Summary Tsunami hazard assessment is routinely based on assessing the impacts of long-period waves generated by vertical seafloor motions reaching the coast tens of minutes after the earthquake in typical subduction-zone environments. This view is inadequate for assessing hazard associated with strike-slip earthquakes such as the magnitude 7.5 2018 Palu earthquake, which resulted in tsunami effects much larger than would normally be associated with horizontal fault motion. From an extraordinary collection of 38 amateur and closed circuit television videos we estimated tsunami arrival times, amplitudes, and wave periods at different locations around Palu Bay, where the most damaging waves were reported. We found that the Palu tsunamis devastated widely separated coastal areas within a few minutes after the mainshock and included unusually short wave periods, which cannot be explained by the earthquake fault slip alone. Post-tsunami surveys show changes in the coastline, and this combined with video footage provides potential locations of submarine landslides as tsunami sources that would match the arrival times of the waves. Our results emphasize the importance of estimating tsunami hazards along coastlines bordering strike-slip fault systems and have broad implications for considering shorter-period nearly instantaneous tsunamis in hazard mitigation and tsunami early warning systems.

Chave, AD, Constable S, Edwards RN.  1991.  Electrical exploration methods for the seafloor. Electromagnetic methods ub appied geophysics. 2( Nabhigian M, Ed.).:931-966., Tulsa: Society of Exploration Geophysics Abstract
Constable, S, Duba A.  1990.  Electrical-Conductivity of Olivine, a Dunite, and the Mantle. Journal of Geophysical Research-Solid Earth and Planets. 95:6967-6978.   10.1029/JB095iB05p06967   AbstractWebsite

Laboratory studies of the electrical conductivity of rocks and minerals are vital to the interpretation of electromagnetic soundings of the Earth's mantle. To date, the most reliable data have been collected from single crystals. We have extended these studies with electrical conductivity measurements on a dunite from North Carolina, in the temperature range of 600°–1200°C and under controlled oxygen fugacity. Observations of conductivity as a function of oxygen fugacity and temperature demonstrate that conduction in the dunite is indistinguishable from conduction in single olivine crystals. Thus the common practice of exaggerating the single-crystal conductivities to account for conduction by grain boundary phases in the mantle is unnecessary. Because the dunite conductivity is consistent with that published for single crystals under similar conditions, we have made a combined analysis of these data. Conductivity as a function of temperature between 600° and 1450°C displays three conduction mechanisms whose activation energies may be recovered by nonlinear least squares fitting, yielding activation energies of 0.21±2.56×10−19 J (0.13±1.60 eV) below 720°C, 2.56±0.02×10−19 J (1.60±0.01 eV) between 720°C and 1500°C and 11.46±0.90×10−19 J (7.16±0.56 eV) above 1500°C. The behavior of conductivity as a function of oxygen fugacity is well explained by a model in which an ƒo2-independent population of charge carriers is supplemented at high oxygen fugacities with a population that is proportional to ƒo20.3. This parametrization produces a clear correlation of the ƒo2 dependent term with iron content, which is otherwise obscured by variations in conductivity among olivines.

Constable, S.  2011.  EM Instrumentation. Encyclopedia of solid earth geophysics. ( Gupta HK, Ed.).:604-608., Dordrecht: Springer Abstract
Constable, SC.  2007.  Geomagnetism. Treatise on Geophysics. 5( Schubert G, Kono M, Eds.).:237-276.: Elsevier Abstract
Constable, S.  2006.  Marine electromagnetic methods-a new tool for offshore exploration. Leading Edge. 25:435-437.   10.1190/1.2193225   AbstractWebsite

Electromagnetic survey methods, used for decades mainly for mining exploration on land, have burst upon the offshore hydrocarbon exploration scene over the past five years. This move, driven by the costs and challenges associated with the deepwater exploration environment, has left many companies scrambling to understand new and unfamiliar technologies, and has resulted in a sudden expansion of the portfolios of the few "nonseismic" geophysicists left in the oil industry. Of the two main methods, magnetotelluric (MT) surveys are at least somewhat familiar from their use on land as a reconnaissance tool and for problem-solving in areas of poor seismic performance, and to a large extent can be applied to the marine environment in a similar way to their terrestrial counterparts. The other dominant technology, controlled-source electromagnetic (CSEM) sounding, has featured much less in land exploration, but more importantly behaves so very differently in the deepwater marine environment that it might as well be considered a completely separate method. Indeed, this has resulted in the early advocates of its use in offshore exploration creating new (and more marketable) names, such as seabed logging by Statoil and remote reservoir resistivity mapping by ExxonMobil

Constable, S, Shankland TJ, Duba A.  1992.  The Electrical-Conductivity of an Isotropic Olivine Mantle. Journal of Geophysical Research-Solid Earth. 97:3397-3404.   10.1029/91jb02453   AbstractWebsite

In order to extend the useful temperature range of interpretation of olivine electrical conductivity-sigma we have used the nonlinear iterative Marquardt technique to fit experimental data over the range 720-degrees-1500-degrees-C to the parametric form sigma = sigma-1e(-A1/kT) + sigma-2e(-A2/kT), where k is Boltzmann's constant and T is absolute temperature. The model describes conduction by migration of two different thermally activated defect populations with activation energies A1 and A2, and preexponential terms sigma-1 and sigma-2 that depend on number of charge carriers and their mobility and that may be different for each crystallographic direction. A combined interpretation of recent high (San Carlos olivine) and low (Jackson County dunite) temperature measurements has been made that demonstrates that a single activation energy A1 for all three crystallographic directions adequately fits the data. The parametric fits show that the high-temperature conduction mechanism has far greater anisotropy than the low-temperature mechanism, consistent with previous assignments to ionic and electronic conduction, respectively. The geometric mean of the conductivity in the three directions is approximately sigma-BAR 10(2.402)e-1.60eV/kT + 10(9.17)e-4.25eV/kT S/m and is presented as a model for isotropic olivine, SO2, appropriate from 720-degrees-C to above 1500-degrees-C, at oxygen fugacities near the center of the olivine stability field. It is observed that the magnitudes of sigma-1 for the three crystal directions are similar to the ratios of the inter-ionic distances between the M1 magnesium sites in olivine, to within 5%, consistent with Fe3+ preferring the M1 site below 1200-degrees-C.

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.

Constable, S, Kowalczyk P, Bloomer S.  2018.  Measuring marine self-potential using an autonomous underwater vehicle. Geophysical Journal International. 215:49-60.   10.1093/gji/ggy263   AbstractWebsite

The marine self-potential (SP) method is used to explore for hydrothermal venting and associated seafloor mineralization. Measurements are commonly made in deep water using instruments towed close to the seafloor, which requires dedicated ship time, is limited to slow speeds, and is subject to navigation errors. Instead, we mounted a three-axis electric field receiver on an autonomous underwater vehicle (AUV), and tested the method with data collected in the Iheya area of the Okinawa Trough, off Japan. Parts of this prospect have documented hydrothermal venting and seafloor massive sulfide (SMS) deposits. An International Submarine Engineering Limited explorer-class AUV was fitted with a controlled-source electromagnetic (CSEM) amplifier and logging system, modified to collect DC SP data using silver chloride electrodes on approximately 1 m dipoles. A 1 km x 1 km area was surveyed with a flight pattern of six lines, collected three times to assess repeatability and noise levels. The entire data set was collected in a single day on station with a 10 hr AUV deployment. Flying height was 70 m, navigation errors were less than 3 m, collection speed was 1.1 m s(-1) and electric field noise levels were less than 5 mu V m(-1). Localized anomalies of 0.3 mV m(-1) were observed, from which potentials were estimated using regularized inversion, yielding negative SP anomalies of 15-25 mV. Modelling electric field data as dipoles shows that the negative poles causing the anomalies are localized near the seafloor with a diffuse return current deeper than 1000 m below seafloor. Apparent conductivities as high as 30 S m(-1) were derived from CSEM data collected during the same deployment, which strongly suggests that SMS mineralization is associated with one of the SP anomalies, although the localization near the seafloor and the lack of a dipolar signal suggest that the causative mechanism for the SP anomalies is due to hydrothermal venting. In either case, we have demonstrated that AUV-mounted instrument systems are an efficient, effective and low noise means of collecting marine SP data.

Constable, S.  2007.  Conductivity, Ocean floor measurements. Encyclopedia of Geomagnetism and Paleomagnetism. ( Gubbins D, Herrero-Bervera E, Eds.).:71-73.: Springer Abstract
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, S.  1991.  Magnetic Appraisal Using Simulated Annealing - Comment. Geophysical Journal International. 106:387-388.   10.1111/j.1365-246X.1991.tb03900.x   AbstractWebsite
Constable, SC, McElhinny MW, McFadden PL.  1984.  Deep Schlumberger Sounding and the Crustal Resistivity Structure of Central Australia. Geophysical Journal of the Royal Astronomical Society. 79:893-910.   10.1111/j.1365-246X.1984.tb02875.x   AbstractWebsite

Three 200 km Schlumberger resistivity soundings have been conducted over the central Australian shield, using telephone lines to obtain the large electrode spacings. These represent the first crustal scale controlled source electrical study to be carried out in this continent. A computer controlled data acquisition system was used which allowed precise measurements to be made with only modest emission currents (0.1–0.5 A).The three soundings, centred on the towns of Renner Springs, Wauchope and Aileron, showed the southern part of the study area (the Arunta Block) to be an order of magnitude more resistive than the more northerly section (the Tennant Creek Block). This difference correlates with the higher heat flow of the Tennant Creek Block. A lowering of apparent resistivity at large electrode spacings for one sounding (Wauchope) is taken to indicate the presence of a low resistivity layer in the middle crust, at a depth less than 20 km. However, the effect of the highly conductive overburden characteristic of inland Australia, combined with the large transverse resistance of the crust, prevented the other two soundings from detecting such a layer. Without support from these two soundings, it is impossible to be sure that the lowered resistivity at Wauchope is not caused merely by lateral variations in near-surface resistivity.The data also show that crustal resistivities are much lower than the expected values for dry rock, whether or not a low resistivity layer is included in the model. This implies a widespread occurrence of free water in the crust, with greater amounts occurring at depth if the low resistivity zone exists.

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.

Constable, S.  1993.  Conduction by mantle hydrogen. Nature. 362:704-704.   10.1038/362704a0   AbstractWebsite
Constable, S.  2013.  Review paper: Instrumentation for marine magnetotelluric and controlled source electromagnetic sounding. Geophysical Prospecting. 61:505-532.   10.1111/j.1365-2478.2012.01117.x   AbstractWebsite

We review and describe the electromagnetic transmitters and receivers used to carry out magnetotelluric and controlled source soundings in the marine environment. Academic studies using marine electromagnetic methods started in the 1970s but during the last decade these methods have been used extensively by the offshore hydrocarbon exploration industry. The principal sensors (magnetometers and non-polarizing electrodes) are similar to those used on land but magnetotelluric field strengths are not only much smaller on the deep sea-floor but also fall off more rapidly with increasing frequency. As a result, magnetotelluric signals approach the noise floor of electric field and induction coil sensors (0.1 nV/m and 0.1 pT) at around 1 Hz in typical continental shelf environments. Fluxgate magnetometers have higher noise than induction coils at periods shorter than 500 s but can still be used to collect sea-floor magnetotelluric data down to 40-100 s. Controlled source transmitters using electric dipoles can be towed continuously through the seawater or on the sea-bed, achieving output currents of 1000 A or more, limited by the conductivity of seawater and the power that can be transmitted down the cables used to tow the devices behind a ship. The maximum source-receiver separation achieved in controlled source soundings depends on both the transmitter dipole moment and on the receiver noise floor and is typically around 10 km in continental shelf exploration environments. The position of both receivers and transmitters needs to be navigated using either long baseline or short baseline acoustic ranging, while sea-floor receivers need additional measurements of orientations from compasses and tiltmeters. All equipment has to be packaged to accommodate the high pressure (up to 40 MPa) and corrosive properties of seawater. Usually receiver instruments are self-contained, battery powered and have highly accurate clocks for timekeeping, even when towed on the sea-floor or in the water column behind a transmitter.

Constable, SC.  1990.  Marine Electromagnetic Induction Studies. Surveys in Geophysics. 11:303-327.   10.1007/bf01901663   AbstractWebsite

In reviewing seafloor induction studies conducted over the last seven years, we observe a decline in single-station magnetotelluric (MT) experiments in favour of large, multinational, array experiments with a strong oceanographic component. However, better instrumentation, processing techniques and interpretational tools are improving the quality of MT experiments in spite of the physical limitations of the band limited seafloor environment, and oceanographic array deployments are allowing geomagnetic depth sounding studies to be conducted. Oceanographic objectives are met by the sensitivity of the horizontal electric field to vertically averaged motional currents, providing the same information, at much greater reliability and much lower cost, as an array of continuously operating current meter moorings.The seafloor controlled source method has now become, if not routine, at least viable. Prior to 1982, only one seafloor controlled source experiment has been conducted; now at least three groups are involved in the experimental aspects of this field. The horizontal dipole-dipole configuration is favoured, although a variant of the magnetometric resistivity method utilising a vertical electric transmitter has been developed and deployed. By exploiting the characteristics of the seafloor environment, source receiver spacings unimaginable on land can be achieved; on a recent deployment dipole spacings of 90 km were used with a clear 24 Hz signal transmitted through the seafloor. This, and prior experiments, show that the oceanic upper mantle is characteristically very resistive, 105 Ω m at least. This resistive zone is becoming apparent from other experiments as well, such as studies of the MT response in coastal areas on land.Mid-ocean ridge environments are likely to be the target of many future electromagnetic studies. By taking available laboratory data on mineral, melt and water conductivity we predict to first order the kinds of structures the EM method will help us explore.

Constable, S.  2006.  SEO3: A new model of olivine electrical conductivity. Geophysical Journal International. 166:435-437.   10.1111/j.1365-246X.2006.03041.x   AbstractWebsite

Standard Electrical Olivine 3 (SEO3) is a new model of electrical conductivity for olivine as a function of temperature and oxygen fugacity, and is based on a previous study of point defect behaviour in a dunite. That study produced quantitative estimates of mobility and concentration for small polarons and magnesium vacancies as a function of temperature and oxygen fucacity. Unlike earlier SO1 and SO2 models of olivine conductivity, the dunite sample was buffered by pyroxene, and avoided alteration at high temperature. The low-temperature extrapolation produced by SEO3 is in good agreement with SO2, while the high-temperature extrapolation is significantly more conductive, which is in general agreement with many field studies that show the Earth's upper mantle to be more conductive than predicted by SO2 for reasonable temperatures. Plausible variations in mantle oxygen fugacity correspond to about half an order of magnitude variation in electrical conductivity, equivalent to about 100 degrees in temperature.

Constable, S.  2007.  Induction from satelite data. Encyclopedia of Geomagnetism and Paleomagnetism. ( Gubbins D, Herrero-Bervera E, Eds.).:413-416.: Springer Abstract
Constable, S, Cox CS.  1996.  Marine controlled-source electromagnetic sounding .2. The PEGASUS experiment. Journal of Geophysical Research-Solid Earth. 101:5519-5530.   10.1029/95jb03738   AbstractWebsite

The marine controlled-source electromagnetic sounding method developed over the past 15 years at Scripps Institution of Oceanography employs a towed seafloor electric dipole transmitter of moment 4 x 10(4) Am and multiple free-vehicle seafloor electric field recorders. A survey of 40 Ma normal oceanic lithosphere in the northeast Pacific using frequencies of 0.25 to 24 Hz and synchronous stacking of 0.25- to 12-hour-duration detected signals at transmitter-receiver ranges between 5 and 95 km. One-dimensional electrical conductivity structure is recovered from the data using the Occam process of nonlinear regularized inversion. Repeated inversion of a model terminated with an essentially infinite conductor or resistor demonstrates that the maximum depth of inference for this experiment is about 30 km, well into the upper mantle, with bounds placed on conductivity to depths of 60 km. Structure shallower than about 1 km is comparable to that obtained by a similar experiment on the East Pacific Rise and by borehole logging, with a sharp increase in resistivity at depths of 600-800 m, although strictly our experiment is sensitive only to integrated square root of conductivity, or total attenuation, in the surface layers. The lower crust and upper mantle has a resistivity between 2 and 7 x 10(4) Omega m and a transverse resistance of at least 10(9) Omega m(2), suggesting at most 0.3% volume fraction of free water in the lower crust and some form of conductivity enhancement over mineral conductivity in the uppermost mantle. Although resolution is weak, below 30 km our data are compatible with a dry olvine model of mantle conductivity-temperature.

Constable, S, Heinson G.  2004.  Hawaiian hot-spot swell structure from seafloor MT sounding. Tectonophysics. 389:111-124.   10.1016/j.tecto.2004.07.060   AbstractWebsite

Seafloor magnetotelluric (MT) data were collected at seven sites across the Hawaiian hot spot swell, spread approximately evenly between 120 and 800 km southwest of the Hawaiian-Emperor island chain. All data are consistent with an electrical strike direction of 300degrees, aligned along the seamount chain, and are well fit using two-dimensional (2D) inversion. The major features of the 2D electrical model are a resistive lithosphere underlain by a conductive lower mantle, and a narrow, conductive, 'plume' connecting the surface of the islands to the lower mantle. This plume is required; without it the swell bathymetry produces a large divergence of the along-strike and across-strike components of the NIT fields, which is not seen in the data. The plume radius appears to be less than 100 km, and its resistivity of around 10 Omegam, extending to a depth of 150 km, is consistent with a bulk melt fraction of 5-10%. A seismic low velocity region (LVR) observed by Laske et al. [Laske, G., Phipp Morgan, J., Orcutt, J.A., 1999. First results from the Hawaiian SWELL experiment, Geophys. Res. Lett. 26, 3397-3400] at depths centered around 60 kin and extending 300 kin from the islands is not reflected in our inverse model, which extends high lithospheric resistivities to the edge of the conductive plume. Forward modeling shows that resistivities in the seismic LVR can be lowered at most to 30 Omegam, suggesting a maximum of 1 % connected melt and probably less. However, a model of hot subsolidus lithosphere of 10(2) Omegam (1450-1500 degreesC) within the seismic LVR increasing to an off-swell resistivity of >10(3) Omegam (<1300 degreesC) fits the MT data adequately and is also consistent with the 5% drop in seismic velocities within the LVR. This suggests a 'hot, dry lithosphere' model of thermal rejuvination, or possibly underplated lithosphere depleted in volatiles due to melt extraction, either of which is derived from a relatively narrow mantle plume source of about 100 km radius. A simple thermal buoyancy calculation shows that the temperature structure implied by the electrical and seismic measurements is in quantitative agreement with the swell bathymetry. (C) 2004 Elsevier B.V. All rights reserved.

Constable, SC.  1985.  Resistivity Studies Over the Flinders Conductivity Anomaly, South-Australia. Geophysical Journal of the Royal Astronomical Society. 83:775-786.   10.1111/j.1365-246X.1985.tb04337.x   AbstractWebsite

Seven Schlumberger resistivity soundings with maximum current electrode spacings of 20 km have been conducted south of Lake Frome in South Australia. These experiments were done partly to test new electrical sounding equipment and partly to investigate a large conductivity anomaly previously delineated by other workers using magnetometer array and MT methods (the ‘Flinders’anomaly). These previous studies left some doubt as to the depth to the conductive region responsible for the anomaly.The electrical soundings did not detect a buried conductive zone, which constrains it to lie deeper than 5–7 km. However, the study did show the surface sediments of the region to be very conductive; resistivities of 2–9 μm were measured over thicknesses of 50–400 m, with sediment thickness inferred to be up to 2 km to the north of the studied area. This raises the question of whether current channelling in the surface sediments could have been responsible for the earlier results. Simple modelling and application of the criteria given by Jones suggest this may be so.The equipment used for this study is a low power (200 W), computer controlled system which employs synchronous stacking and other signal processing to achieve signal to noise improvement ratios of up to 1000.

Constable, S.  2010.  Ten years of marine CSEM for hydrocarbon exploration. Geophysics. 75:A67-A81.   10.1190/1.3483451   AbstractWebsite

Marine controlled-source electromagnetic (CSEM) surveying has been in commercial use for predrill reservoir appraisal and hydrocarbon exploration for 10 years. Although a recent decrease has occurred in the number of surveys and publications associated with this technique, the method has become firmly established as an important geophysical tool in the offshore environment. This is a consequence of two important aspects associated with the physics of the method: First, it is sensitive to high electrical resistivity, which, although not an unambiguous indicator of hydrocarbons, is an important property of economically viable reservoirs. Second, although the method lacks the resolution of seismic wave propagation, it has a much better intrinsic resolution than potential-field methods such as gravity and magnetic surveying, which until now have been the primary nonseismic data sets used in offshore exploration. Although by many measures marine CSEM is still in its infancy, the reliability and noise floors of the instrument systems have improved significantly over the last decade, and interpretation methodology has progressed from simple anomaly detection to 3D anisotropic inversion of multicomponent data using some of the world's fastest supercomputers. Research directions presently include tackling the airwave problem in shallow water by applying time-domain methodology, continuous profiling tools, and the use of CSEM for reservoir monitoring during production.