The determination of resistivity and porosity of the sediment and fractured basalt layers near the Juan de Fuca Ridge

Nobes, David C.; Law, L. K.; Edwards, R. N.

doi:10.1111/j.1365-246x.1986.tb03830.x

Cited by 39 publications

(34 citation statements)

References 28 publications

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“…The technique is based on the magnetometric resistivity principle and has been deployed successfully on several occasions and in a variety of locales. For example, soundings have been obtained in the coastal waters of British Columbia and in Middle Valley on the Juan de Fuca Ridge (Edwards et aI., 1985;Nobes et al, 1986). A miniaturized version of the equipment has also been constructed and deployed from the submersible Alvin for small-scale studies in a polymetallic sulfide region (Wolfgram et aI., 1986).…”

Section: Frequency-domain Controlled Source Electromagneticsmentioning

confidence: 98%

Seafloor Electromagnetic Exploration Methods

Chave¹

1990

Gorda Ridge

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Electrical exploration of the marine environment is just beginning, but interest in the application of electromagnetic approaches is rising rapidly. A brief review of seafloor electromagnetic exploration principles and techniques is given, with an emphasis on the midocean ridge environment. The physical processes governing the electrical conductivity of rocks are discussed. Two types of seafloor electromagnetic exploration systems are then described. The first uses a frequency-domain controlled source to inject energy into the seafloor and has been used for both shallow and deep sounding of the oceanic lithosphere. The second uses a time-domain controlled source and is under development as a tool for rapid, near-bottom surveying.

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Section: Frequency-domain Controlled Source Electromagneticsmentioning

confidence: 98%

Seafloor Electromagnetic Exploration Methods

Chave¹

1990

Gorda Ridge

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“…This relation indicates that the amplitude of the induced horizontal magnetic field is inversely proportional to the resistivity of the seafloor and to the square of the source-receiver separation; the larger magnetic field amplitude represents a smaller resistivity for a given separation. Nobes et al (1986Nobes et al ( , 1992 applications of the technique. Evans et al (1998Evans et al ( , 2002 determined resistivity models at depths up to 1 km beneath the seafloor, both at the ridge axis and further away.…”

Section: Introductionmentioning

confidence: 99%

“…Tada et al (2005) presented a one-dimensional electrical resistivity structure over a region of hydrothermal circulation in the spreading axis of the central Mariana Trough using an MMR experiment. All the previous studies (Nobes et al, 1986(Nobes et al, , 1992Evans et al, 1998Evans et al, , 2002Tada et al, 2005) used a "vertical" bipole electric current as a source, which was transmitted from several stationary transmission stations. We call this the "stationary source method" hereafter.…”

Section: Introductionmentioning

confidence: 99%

A continuously towed vertical bipole source for marine magnetometric resistivity surveying

et al. 2013

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We propose to use an approximately vertical bipole electric current towed by a ship as a source for a Magnetometric Resistivity (MMR) method. This proposal requires the precise positioning of the bottom electrode for the bipole source, and our newly developed MMR system achieved this. We conducted an MMR experiment in the central Mariana Trough, and we obtained data using two different methods along a survey line: one method towed a bipole source transmitting continuously along the survey line, and the other used a conventional vertical bipole source transmitting at several stationary transmission stations along the survey line. We found that the towed bipole source tilted from the vertical by an angle of 8 degrees at the maximum during the MMR experiment. We compared the results from the two methods to evaluate the towed bipole source method. Our results indicate that the tilted bipole source approximates well with the vertical bipole source at the mid-point between the surface and the bottom electrodes. Since the towed bipole source method requires much less survey time and the results show a higher spatial resolution, it is a powerful tool for MMR experiments to image a shallow oceanic crustal resistivity structure efficiently.

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“…The magnetometric resistivity (MMR) technique is useful for revealing the electrical resistivity structure of the oceanic crust (e.g., Edwards et al 1981). The first application of the MMR technique for exploring an electrical resistivity structure of an active hydrothermal system was implemented off the Juan de Fuca Ridge by Nobes et al (1986), Nobes et al (1992). Evans et al (1998) conducted a MMR experiment at the Juan de Fuca Ridge.…”

Section: Introductionmentioning

confidence: 99%

Electrical Resistivity Structure of the Snail Site at the Southern Mariana Trough Spreading Center

Matsuno

Kimura

Seama

2014

Subseafloor Biosphere Linked to Hydrothermal Systems

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The electrical resistivity of the oceanic crust is sensitive to the porosity of the crust and the fluid temperature within crustal fractures and pores. The spatial variation of the crustal porosity and the fluid temperature that is related to a hydrothermal circulation can be deduced by revealing an electrical resistivity structure of the oceanic crust involving a hydrothermal site. We carried out a magnetometric resistivity experiment using an active source to reveal an electrical resistivity structure of the oceanic crust at the Snail site on the ridge crest of the Southern Mariana Trough. Active source electric currents were transmitted along and across the ridge axis in a 4,000 m 2 area including the Snail site. Five ocean bottom magnetometers were deployed around the Snail site as receivers to measure the magnetic field induced by the transmission of the active source electric currents. The amplitude of the induced magnetic field was calculated by maximizing data density and the signal to error ratio in the data, and locations of the transmissions were determined using several types of calibration data. An optimal 1-D resistivity structure of the oceanic crust, averaged over the experimental area, was deduced by least squares from the data of the amplitude of the magnetic field and the location of the transmission. After calculating magnetic field anomalies, which are deviations of the observed amplitude from the prediction of the optimal 1-D resistivity model, an optimal 3-D resistivity structure was deduced from the magnetic field anomalies through trial and error 3-D forward modeling. The optimal 1-D resistivity structure is a two-layer model, which consists of a 5.6 Ω-m upper layer having a 1,500 m thickness and a 0.1 Ω-m underlying half-space. Using Archie's law and porosity profiles of the oceanic crust, the resistivity of 5.6 Ω-m at depths ranging from 800 to 1,500 m suggests the presence of hightemperature fluid related to the hydrothermal circulation. The resistivity of 0.1 Ω-m below 1,500 m depth may represent a magma mush that is a heat source for the hydrothermal circulation. The optimal 3-D resistivity structure includes a conductive anomaly (0.56 Ω-m in approximately 300 m 2 area down to 400 m depth) immediately below the Snail site, two resistive anomalies (56 Ω-m with slightly larger volumes than the conductive anomaly) adjacent to the conductive anomaly on the across-ridge side, and three conductive anomalies away from the Snail site. The conductive anomaly immediately below the Snail site suggests hydrothermal fluid, and the adjacent resistive anomalies suggest areas of low porosity. The size and distribution of the conductive and resistive anomalies near the Snail site constrains the size and style of the hydrothermal circulation.

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The determination of resistivity and porosity of the sediment and fractured basalt layers near the Juan de Fuca Ridge

Cited by 39 publications

References 28 publications

Seafloor Electromagnetic Exploration Methods

Seafloor Electromagnetic Exploration Methods

A continuously towed vertical bipole source for marine magnetometric resistivity surveying

Electrical Resistivity Structure of the Snail Site at the Southern Mariana Trough Spreading Center

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