Abstract
We conducted a self-potential survey at an active hydrothermal field, the Izena hole in the mid-Okinawa Trough, southern Japan. This field is known to contain Kuroko-type massive sulphide deposits. This survey measured the self-potential continuously in ambient seawater using a deep-tow array, which comprises an electrode array with a 30-m-long elastic rod and a stand-alone data acquisition unit. We observed negative self-potential signals not only above active hydrothermal vents and visible sulphide mounds but also above the flat seafloor without such structures. Some signals were detectable >50 m above the seafloor. Analysis of the acquired data revealed these signals’ source as below the seafloor, which suggests that the self-potential method can detect hydrothermal ore deposits effectively. The self-potential survey, an easily performed method for initial surveys, can identify individual sulphide deposits from a vast hydrothermal area.
Highlights
IntroductionSuch methods can reveal the two-dimensional or three-dimensional distribution of subsurface electrical conductivity, which is sensitive to the occurrence of metal-containing materials such as sulphide minerals
Exploring methods that are applicable to visible and to buried ore deposits should be chosen out of line-survey or mapping methods from the viewpoint of efficiency. Such exploration methods are established in on-land environments, efficient methods for marine environments are not necessarily the same as those on land
In on-land environments, the standard technique of exploring subsurface ore deposits is a remote sensing geophysical survey combined with geological information of the target area
Summary
Such methods can reveal the two-dimensional or three-dimensional distribution of subsurface electrical conductivity, which is sensitive to the occurrence of metal-containing materials such as sulphide minerals These methods, which are being developed for use in marine environments[5,6,7,8], might be used effectively in the ocean, but they are difficult to handle as tools for initial surveys without detailed information of the target area. Reliable dataset[10]; the method responds to various sources of electric current, such as fluid flow and temperature gradient[11]; and noise near the ground surface is occasionally high[12] For these reasons, as a method of exploration in land environments, the self-potential method is minor compared to electromagnetic methods with active sources. Because few means other than the self-potential method can detect subsurface fluid flow, the self-potential method in the imaging subsurface fluid flow overcomes the shortcomings described above[10,13]
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