| An active source electromagnetic sounding technique was used to study the electrical conductivity structure of the basement rocks in the area of the East Pacific Rise at 21(DEGREES)N latitude as part of the RISE project in 1979. The source was a length of cable, insulated except for bared ends, deployed on the sea floor and powered from shipboard; this antenna, once straightened and oriented by dragging on the bottom, formed an end grounded horizontal dipole source. Through this antenna were fed a number of different tailored waveforms, which between them gave transmissions at the frequencies 0.25 Hz, 0.50 Hz, 0.75 Hz, 1.50 Hz, and 2.25 Hz with roughly equal dipole strengths on the order of 2.5 x 10('4) ampere-meters. The transmitter was deployed approximately 23 kilometers from the crest of the spreading ridge, and a recording receiver instrument package was deployed about 5 kilometers from the crest, on the same side as the transmitter, in such a way that the transmitter and receiver were separated by a distance of close to 19 kilometers; the positions of both the transmitter and the receiver were monitored by means of a pre-established bottom transponder net. The receiver was designed to measure the horizontal components of the bottom electric field in its vicinity at the transmitted frequencies, and by means of a low noise amplifier system and a signal stacking scheme was able to measure field strengths on the order of 10('-10) volts per meter with a signal-to-noise ratio of five or better at all of the frequencies in question. The data collected by the receiver was normalized against the transmitted signal intensities, with data amplitudes in the range from 2 x 10-15 volts per ampere-meter('2) to 1 x 10('-14) volts per ampere-meter('2) resulting. Preliminary modeling based on data amplitudes (ignoring the associated phase data) indicated the presence of a zone with a conductivity of 0.004 S/m or less having a thickness of several kilometers and located at a depth beneath the sea floor no greater than 3 kilometers and probably less than 2 kilometers; also, the existence of any magma intrusion into this zone or above it with a mean thickness of more than 200 meters was ruled out, as magma is about as conductive as seawater, and therefore would have contrasted noticeably with the surrounding rock if present in any quantity. The complete data set was processed by means of a data inversion scheme developed from the Frieman-Kroll method for calculation of induced electromagnetic field intensities; the results of this work suffer from considerable uncertainty, but are roughly consistent with the above picture. They suggest a conductivity roughly on the order of that of seawater immediately beneath the seafloor, and a fairly thick region with a conductivity less than 4 x 10('-3) S/m no more than 1.5 kilometers under the seafloor and falling below 10('-3) S/m at depths on the order of 5 to 8 kilometers; for greater depths a reasonable interpretation of the inversion results on the basis of geophysical arguments gives a continued rise with increasing depth. These data therefore argue against the lateral extension of a molten magma chamber more than a kilometer or so into the region above the Mohorovicic discontinuity (between 5 and 6 kilometers beneath the seafloor in the neighborhood of the ridge) and between the transmitter and receiver sites. Conductance as low as 10('-4) S/m suggests that magma, originally molten beneath the ridge crest, has cooled and solidified well before it is 400,000 years old. This is consistent with the large degree of geothermal vent activity observed in the ridge crest area, which implies a considerable extent of convective hydrothermal cooling of the crustal rock in the vicinity, although it is possible to construct reasonable crustal models that cool the magma to this extent without resort to deep hydrothermal circulation. |
