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McFadden, PL, Constable SC.  1983.  The Estimation and Removal of a Linear Drift from Stacked Data. Journal of Geophysics-Zeitschrift Fur Geophysik. 53:52-58. 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.

Webb, SC, Constable SC, Cox CS, Deaton TK.  1985.  A Sea-Floor Electric Field Instrument. Journal of Geomagnetism and Geoelectricity. 37:1115-1129. AbstractWebsite
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.

Cox, CS, Constable SC, Chave AD, Webb SC.  1986.  Controlled-Source Electromagnetic Sounding of the Oceanic Lithosphere. Nature. 320:52-54.   10.1038/320052a0   AbstractWebsite

The attenuation of ionospheric signals in the frequency range 0.06–24 Hz by sea water effectively precludes using the magnetotel-luric method to study the electrical structure of the upper oceanic lithosphere. We have carried out a dipole–dipole electromagnetic sounding in the North Pacific by injecting electromagnetic signals into the ocean and sea bed. The crust at the site is 25 Myr old and has a thin sediment cover. The technique, similar to that used in earlier work1,2, involves dragging a horizontal dipole antenna along the sea floor. The electric fields that propagated through the resistive basement were detected by seafloor receivers at ranges of 10–65 km. As the ambient electric field is very small (varying from 10−18 V2 m−2 Hz−1 at 0.1 Hz to 10−24 V2 m2 Hz−1 above 1 Hz; ref. 3), the controlled-source signals could be easily monitored. Our data are consistent with a simple one-dimensional Earth model consisting of a 3–7-km-thick crustal layer of moderate conductivity (~0.001 S m−1) underlain by a thicker region of very low conductivity (<2 × 10−5 S m−1). The results suggest an upper mantle water content of at most 0.1% by volume.

Webb, SC, Constable SC.  1986.  Microseism Propagation Between 2 Sites on the Deep Sea-Floor. Bulletin of the Seismological Society of America. 76:1433-1445. AbstractWebsite
Constable, SC, Parker RL, Constable CG.  1987.  OCCAMS Inversion - A Practical Algorithm for Generating Smooth Models From Electromagnetic Sounding Data. Geophysics. 52:289-300.   10.1190/1.1442303   AbstractWebsite

The inversion of electromagnetic sounding data does not yield a unique solution, but inevitably a single model to interpret the observations is sought. We recommend that this model be as simple, or smooth, as possible, in order to reduce the temptation to overinterpret the data and to eliminate arbitrary discontinuities in simple layered models. To obtain smooth models, the nonlinear forward problem is linearized about a starting model in the usual way, but it is then solved explicitly for the desired model rather than for a model correction. By parameterizing the model in terms of its first or second derivative with depth, the minimum norm solution yields the smoothest possible model. Rather than fitting the experimental data as well as possible (which maximizes the roughness of the model), the smoothest model which fits the data to within an expected tolerance is sought. A practical scheme is developed which optimizes the step size at each iteration and retains the computational efficiency of layered models, resulting in a stable and rapidly convergent algorithm. The inversion of both magnetotelluric and Schlumberger sounding field data, and a joint magnetotelluric‐resistivity inversion, demonstrate the method and show it to have practical application.

de Groot-Hedlin, C, Constable S.  1990.  Occam Inversion to Generate Smooth, 2-Dimensional Models from Magnetotelluric Data. Geophysics. 55:1613-1624.   10.1190/1.1442813   AbstractWebsite

Magnetotelluric (MT) data are inverted for smooth 2-D models using an extension of the existing 1-D algorithm, Occam’s inversion. Since an MT data set consists of a finite number of imprecise data, an infinity of solutions to the inverse problem exists. Fitting field or synthetic electromagnetic data as closely as possible results in theoretical models with a maximum amount of roughness, or structure. However, by relaxing the misfit criterion only a small amount, models which are maximally smooth may be generated. Smooth models are less likely to result in overinterpretation of the data and reflect the true resolving power of the MT method. The models are composed of a large number of rectangular prisms, each having a constant conductivity. Apriori information, in the form of boundary locations only or both boundary locations and conductivity, may be included, providing a powerful tool for improving the resolving power of the data. Joint inversion of TE and TM synthetic data generated from known models allows comparison of smooth models with the true structure. In most cases, smoothed versions of the true structure may be recovered in 12–16 iterations. However, resistive features with a size comparable to depth of burial are poorly resolved. Real MT data present problems of non‐Gaussian data errors, the breakdown of the two‐dimensionality assumption and the large number of data in broadband soundings; nevertheless, real data can be inverted using the algorithm.

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, 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.

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.  1991.  Magnetic Appraisal Using Simulated Annealing - Comment. Geophysical Journal International. 106:387-388.   10.1111/j.1365-246X.1991.tb03900.x   AbstractWebsite
Evans, RL, Constable SC, Sinha MC, Cox CS, Unsworth MJ.  1991.  Upper Crustal Resistivity Structure of the East Pacific Rise Near 13-Degrees-N. Geophysical Research Letters. 18:1917-1920.   10.1029/91gl02305   AbstractWebsite

An active source electromagnetic (EM) sounding has been conducted on the axis of the East Pacific Rise (EPR) at 13-degrees 10' N. 1D inversion and modelling techniques, seeking resistivity as a function of depth, have been applied to 8 Hz amplitude data collected along the ridge crest. Resistivity is seen to increase monotonically between 50 m and 1 km below the seafloor, increasing from approximately 1-OMEGA-m. We observe no intrinsic difference in upper crustal resistivity structure between the rise axis and 100 000 year old crust. Inferred surface porosities of 20% are larger than those recorded in 5.9 my old crust in DSDP hole 504B. Our data do not require, and lack sufficient information for, the reliable inclusion of a conductive termination to the model below 1.2 km.

Constable, SC.  1992.  Electrical studies of the Australian lithosphere. Geological Society of Australia, Special Publication. 17:121-140. Abstract
Vanyan, LL, Kaoldayev NA, Palshin NA, Constable SC.  1992.  On anisotropy of electrical conductivity in the oceanic lithosphere. Fizika Zemli. 5:79-85. Abstract
Heinson, G, Constable S.  1992.  The Electrical-Conductivity of the Oceanic Upper Mantle. Geophysical Journal International. 110:159-179.   10.1111/j.1365-246X.1992.tb00719.x   AbstractWebsite

Previous inversions of sea-floor magnetotelluric (MT) sounding data have predicted upper mantle electrical conductivities which are more than an order of magnitude higher than laboratory measurements of the conductivity of olivine would suggest, and controlled source-field electromagnetic (CSEM) soundings require a lithospheric mantle conductivity of 3 x 10(-5) Sm-1, which is so low that the electromagnetic (EM) coast effect would produce more anisotropy in MT soundings than is observed. We address these issues by constructing an olivine mantle model for conductivity and examining the inversion of MT data from three sea-floor sites, and show that the incompatibilities can be largely resolved if the effects of oceans and coastlines are considered. Our mantle model is based on recent measurements of olivine conductivity, the conductivity of tholeiite melt, a thermal model for the upper mantle based on lithospheric cooling and the temperature of alpha --> alpha + beta olivine transition, the pyrolite model of mantle petrology, and conductivities derived from CSEM sounding. We propose Archie's Law with exponent 2 and interconnected tubes as realistic lower and upper bounds for the effect of partial melt on rock conductivity, and Archie's Law with exponent 1.5 as the preferred estimate. The 1-D forward response of this model is not compatible with observed sea-floor MT data. Three data sets presented by Oldenburg (1981) pass a test for one-dimensionality based on the size of the residuals when fit with Parker's D+ algorithm, but two of the three soundings fail a test for independence of residuals. We also find that the presence of the upper mantle 'high-conductivity zone' previously inferred from these data is highly dependent on data misfit and not required when the misfit criterion is relaxed a reasonable amount. Re-inversions of the MT data produce models which are incompatible with our petrological model of mantle conductivity. However, by adding an ocean with various coastlines of simple geometry to our petrological model and solving the forward 3-D problem using thin-sheet analysis we predict MT responses which are distorted in a manner that is remarkably similar to the observed data.

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.  1993.  Conduction by mantle hydrogen. Nature. 362:704-704.   10.1038/362704a0   AbstractWebsite
Constable, S.  1993.  Constraints on Mantle Electrical-Conductivity from Field and Laboratory Measurements. Journal of Geomagnetism and Geoelectricity. 45:707-728.   10.5636/jgg.45.707   AbstractWebsite

A global geomagnetic response function, sensitive to the average radial electrical conductivity structure of Earth's mantle to depths of at least 1800 km, is obtained by averaging published, single-site response functions estimated at periods between 10(5)-10(7) seconds from magnetic observatory records. Although the error bars on the global response function are mostly smaller than 5%, Parker's D+ algorithm demonstrates compatibility with a one-dimensional model, both in terms of magnitude and distribution of data residuals. Smooth models in the sense of minimum first and second derivatives of log(conductivity) with log(depth) show conductivities increasing from 0.01 S/m 200 km deep to 2 S/m at a depth of 2000 km. Geotherms inferred from these conductivities using a laboratory model for the temperature dependence of dry subsolidus olivine yield temperatures of 1750-degrees-C at a depth of 410 km; hotter than the 1400-degrees-C for this depth inferred from published values for the equilibrium boundary of the olivine alpha --> alpha + beta transition. Inclusion of a sharp jump in conductivity at the 660 km seismic discontinuity lowers the electrogeotherm to 1600-degrees-C at 410 km, while an explicit penalty on the conductivity at this depth demonstrates that a temperature of 1400-degrees is compatible with the global response function if 1000 S of additional conductance is included above 200 km. The electrical conductivity below the jump at 660 km is 1 S/m increasing to 2 S/m at 2000 km, in excellent agreement with recent diamond anvil measurements of lower mantle materials. Extension of the global response to higher frequencies is possible using data from magnetic satellites. One such study is shown to be in general agreement with the averaged response.

Heinson, G, Constable S, White A.  1993.  The electrical conductivity of the lithosphere and asthenosphere beneath the coastline of southern california. Bulletin of Australian society of Exploration in Geophysics. 24:194-200. Abstract
de Groot-Hedlin, C, Constable S.  1993.  Occam's Inversion and the North American Central Plains Electrical Anomaly. Journal of geomagnetism and geoelectricity. 45:985-999.   10.5636/jgg.45.985   Abstract

The COPROD2 magnetotelluric (MT) data set obtained over the North American central plains conductivity anomaly in the Canadian shield features a predominantly one-dimensional response at periods shorter than 14 s and a predominantly two-dimensional response at longer periods (to 910 s). A subset of the COPROD2 data consisting of both strike-parallel and strike-perpendicular components of the MT apparent resistivity and phase data at periods between 14-910 s was subjected to the Occam's inversion process, which attempts to find maximally smooth models which fit a data set to a specified misfit. After 9 iterations a model was found fitting these data to 10% in resistivity and 2.9° in phase, and after a few more iterations excess structure was removed to reveal three discrete conductive zones of resistivity less than 1 Ωm at depths of 8-22 km in a relatively resistive background layer of 100-1000 Ωm. The inversion algorithm was modified to take advantage of the 1D structure of the shorter periods, which were inverted to obtain a model representative of surface sediments which extend to a depth of about 2 km. This surface structure was included in the long-period inversion by imposing a second penalty term in the regularized inversion, and the resulting model featured a broader, more complex conductive anomaly and a pronounced, westward-dipping fabric in the mid- to lower-crustal rocks. Graphitic rocks can account for the highly conductive parts of the models.

Duba, A, Constable S.  1993.  The Electrical-Conductivity of Lherzolite. Journal of Geophysical Research-Solid Earth. 98:11885-11899.   10.1029/93jb00995   AbstractWebsite

Electrical conductivity as a function of oxygen fugacity (f(O2)) between 10(-5) Pa and 1 Pa, temperature between 700-degrees-C and 1200-degrees-C, and time over a period of 1700 hours are reported for a lherzolite nodule from Mount Porndon, Australia. Analysis of complex impedance collected at 100 Hz, 1 kHz, and 10 kHz indicates that there is significant frequency dispersion in this rock at temperatures below 900-degrees-C - 1000-degrees-C. By choosing the resistance of an equivalent parallel RC network at the frequency having minimum phase, the effect of dispersion is largely avoided. Conductivity as a function of time, collected following changes in f(O2) at 1010 and 1200-degrees-C, has been analyzed to determine the diffusivity of defects relating f(O2) to the electrical conduction mechanism in olivine. The diffusivities so obtained are in remarkably good agreement with those determined from strain measurements during creep tests, which implies that magnesium vacancies are the rate-limiting step for conductivity reequilibration after f(O2) changes. A longer-term process is observed in this rock in which the conductivity drifts upward or downward with a time constant of hundreds of hours after initial reequilibration to gas mix (f(O2)) changes. We speculate that this long-term drift could be related to equilibration of the iron distribution between coexisting olivine and pyroxene as a function of f(O2). The sense is an increase in conductivity (olivine gains iron) at low f(O2) and vice versa. The lherzolite conductivity data are not significantly different from measurements made on olivine single crystals and polycrystals, even though the rock contains about 34 modal % pyroxene. This consistency of laboratory measurements of electrical conductivity of olivines from many localities and geological settings supports the use of recent models relating mantle temperature with electrical conductivity in the interpretation of mantle geo/electromagnetic soundings.

Evans, RL, Sinha MC, Constable SC, Unsworth MJ.  1994.  On the Electrical Nature of the Axial Melt Zone at 13-Degrees-N on the East Pacific Rise. Journal of Geophysical Research-Solid Earth. 99:577-588.   10.1029/93jb02577   AbstractWebsite

The first controlled source electromagnetic experiment directly on a ridge, with the potential to identify the presence of an axial melt body beneath a fast-spreading center, was conducted at 13-degrees-N on the East Pacific Rise (EPR) in 1989. Transmission for 36 hours was achieved by a deep towed horizontal electric dipole source, of moment 6000 Am, operating at frequencies between 1/4 and 8 Hz. Signals from the source were recorded by seven seafloor electric field receivers positioned both along the ridge crest and 5 km to the east on 100,000-year-old crust. Data above ambient noise levels were obtained at ranges of up to 10 km. The results of modeling observed electric field amplitudes reveal that resistivities in the uppermost crust are very low (approximately 1 OMEGAm), indicating a heavily fractured, high-porosity surficial layer. Below this topmost layer, the upper 2 km of crust is found to be moderately resistive (approximately 100 OMEGAm). We find no evidence for a large conductive axial melt body with dimensions on the order of kilometers in the middle or upper crust. If a partial melt body is present, which is continuous along strike and which comprises a connected, and therefore conductive, melt texture, it must be of very limited volumetric extent. This picture is consistent with recently proposed models of a thin sill-like melt lens with across strike dimensions of no more than 1 km and probably with smaller vertical extent. The larger region below the sill, characterized by low seismic velocities, must contain at best a very small melt fraction distributed in isolated pockets, providing further evidence that the EPR at 13-degrees-N is currently in a state of relative magmatic quiescence.

Heinson, G, Constable S, White A.  1996.  Seafloor magnetotelluric sounding above axial seamount. Geophysical Research Letters. 23:2275-2278.   10.1029/96gl01673   AbstractWebsite

Axial Seamount is a large, active, ridge axis volcano located on the central segment of the Juan de Fuca Ridge in the northeast Pacific Ocean. Magnetotelluric (MT) data have been collected at three sites, approximately 4 km apart around the eastern rim of the volcano, during a 65-day deployment. MT responses, in the bandwidth of 10(2) - 10(5) s, are almost isotropic, with a weakly-defined principal direction of strike parallel to the main topographic trends of Axial Seamount, and are relatively flat over the whole bandwidth. Apparent resistivities are of the order of 7 - 20 Omega m, and phases are as low as 30 degrees at the short periods. Diagonal terms of the MT tensor are an order of magnitude smaller than the off-diagonal terms, suggesting that three-dimensional effects on the data are minimal. Two-dimensional inversions suggest that seafloor bathymetry and the distant coastlines have a surprisingly small effect on the MT response, and one-dimensional inversions fit the MT data to within the errors with no serial correlation in the residuals. A low crustal resistivity is the most robust part of the model, probably due to seawater in fractures and possibly due to a magma chamber. An electrical asthenosphere, although less well constrained, exists over a depth range 30 - 60 km, and the resistivity of this region is compatible with about 8% fraction of melt.

Petersons, HF, Constable S.  1996.  Global mapping of the electrically conductive lower mantle. Geophysical Research Letters. 23:1461-1464.   10.1029/96gl01412   AbstractWebsite

It is known that the electrical conductivity of the Earth's mantle increases to about 1 S/m at depths between 500 and 1000 km. This increase could equally well be ascribed to thermally activated conduction or to enhanced conductivity in high pressure mineral phases. Thermal activation would produce a smooth conductivity profile while conductivity associated with abrupt phase changes would also vary abruptly. Unfortunately, geomagnetic data alone cannot distinguish between these two models. However, under the assumption that the variation is indeed abrupt, we seek the best estimate for the depth to a conductivity jump. A peak in the geomagnetic spectrum around a period of 27 days produces an electromagnetic response at this period with least uncertainty and least bias associated with breakdown of an assumed P-1(0) source-field geometry. Fitting such data using a simple two-parameter model of a buried conducting sphere, we estimate a conductivity of 1.18+/-0.10 S/m at a depth of 650+/-20 km. The coincidence of this result with estimated depths to the 660 km seismic discontinuity provides independent support for the hypothesis that the observed abrupt change in the elasticity of the mantle is also accompanied by an equally abrupt change in the electrical conductivity. Both physical properties are presumably associated with a mineral transition from an olivine-dominated upper mantle composition to perovskite/wustite assemblage.