Controlled electromagnetic sources for measuring electrical conductivity beneath the oceans: 1. Forward problem and model study

Exact closed-form expressions for the electromagnetic induction fields produced by vertical and horizontal current sources in the conducting ocean overlying a one-dimensional earth are derived from the Maxwell equations. Numerical methods for the evaluation of the solutions are given, including correction for the finite size of real sources. Simple models of the electrical conductivity structure of the ocean crust and lithosphere are deduced from geologic, petrologic, and laboratory data, and their electromagnetic response is modeled. Horizontal electric dipole sources produce much larger field amplitudes than their vertical counterparts for a given frequency and range, and the horizontal electric field offers superior received signal performance. Reflections of electromagnetic waves from the sea surface and thermocline must be considered for low enough frequencies or long ranges. Estimates of the ambient noise level from natural electromagnetic sources in the frequency range 0.01–10 Hz are presented. The ability of controlled sources to determine features of the conductivity of the ocean crust and upper mantle, especially low conductivity zones, is demonstrated. If the mantle conductivity is low enough, horizontal ranges of 50 km and conductivity estimates to over 20 km depth can be achieved.

[1]  A. Chave.  On the theory of electromagnetic induction in the Earth by ocean currents , 1983 .

[2]  P. Pistek,et al.  An Active Source EM Method for the Seafloor , 1981 .

[3]  O. Phillips.  The dynamics of the upper ocean , 1966 .

[4]  R. N. Edwards,et al.  On measuring the electrical conductivity of the oceanic crust by a modified magnetometric resistivity method , 1981 .

[5]  S. Webb,et al.  Electromagnetic fields induced at the seafloor by Rayleigh‐Stoneley waves , 1982 .

[6]  P. Morse,et al.  Methods of theoretical physics , 1955 .

[7]  A. Chave,et al.  Electromagnetic induction fields in the deep ocean north-east of Hawaii: implications for mantle conductivity and source fields , 1981 .

[8]  Thomas N. L. Patterson,et al.  Algorithm 468: algorithm for automatic numerical integration over a finite interval [D1] , 1973, CACM.

[9]  G. Watson.  Bessel Functions. (Scientific Books: A Treatise on the Theory of Bessel Functions) , 1923 .

[10]  C. Cox,et al.  Electromagnetic active source sounding near the East Pacific Rise , 1981 .

[11]  J. Filloux.  Magnetotelluric soundings over the northeast Pacific may reveal spatial dependence of depth and conductance of the asthenosphere , 1980 .

[12]  James R. Wait,et al.  THE ELECTROMAGNETIC FIELDS OF A HORIZONTAL DIPOLE IN THE PRESENCE OF A CONDUCTING HALF-SPACE , 1961 .

[13]  Walter L. Anderson,et al.  Computer program; numerical integration of related Hankel transforms of orders O and 1 by adaptive digital filtering , 1979 .

[14]  J. Filloux.  Magnetotelluric exploration of the North Pacific: progress report and preliminary soundings near a spreading ridge , 1981 .

[15]  T. Shankland,et al.  Conductivity in fluid‐bearing rocks , 1974 .

[16]  M. Manghnani,et al.  Electrical conductivity of ultramafic rocks to 1820 kelvin , 1978 .

[17]  W. Campbell,et al.  Integrated power spectra of geomagnetic field variations with periods of 0.3–300 s , 1976 .

[18]  T. Shankland.  Electrical conduction in rocks and minerals: Parameters for interpretation , 1975 .

[19]  R. Coleman.  Tectonic setting for ophiolite obduction in Oman. , 1981 .

[20]  P. Pistek,et al.  Electromagnetic fluctuations induced by wind waves on the deep‐sea floor , 1978 .

[21]  T. Madden,et al.  Generalized thin sheet analysis in magnetotellurics: an extension of Price's analysis , 1980 .

[22]  N. Christensen,et al.  The seismic velocity structure of a traverse through the Bay of Islands Ophiolite Complex, Newfoundland, An exposure of oceanic crust and upper mantle , 1978 .

[23]  C. Cox.  Electromagnetic induction in the oceans and inferences on the constitution of the earth , 1980 .

[24]  W. Brace,et al.  Electrical conductivity of serpentinized rocks to 6 kilobars , 1973 .

[25]  A. Souriau.  The upper mantle beneath Ninetyeast Ridge and Broken Ridge, Indian Ocean, from surface waves , 1981 .

[26]  A. Chave.  Lithospheric structure of the Walvis Ridge from Rayleigh wave dispersion , 1979 .

[27]  J. Pallister,et al.  Samail Ophiolite plutonic suite: Field relations, phase variation, cryptic variation and layering, and a model of a spreading ridge magma chamber , 1981 .

[28]  C. J.,et al.  The Electromagnetic Field of Long and Intermediate Water Waves ' , 2022 .

[29]  R. Horne,et al.  The effect of pressure on the electrical conductivity of sea water , 1963 .

[30]  J. P. Greenhouse,et al.  Geomagnetic variation sounding of the asthenosphere beneath the Juan de Fuca Ridge , 1981 .

[31]  H. Taylor,et al.  An oxygen isotope profile in a section of Cretaceous oceanic crust, Samail Ophiolite, Oman: Evidence for δ18O buffering of the oceans by deep (>5 km) seawater-hydrothermal circulation at mid-ocean ridges , 1981 .