Self-potential mapping using an autonomous underwater vehicle for the Sunrise deposit, Izu-Ogasawara arc, southern Japan

Abstract

We performed a simultaneous survey of self-potential and plume turbidity using an autonomous underwater vehicle (AUV) above the Sunrise deposit in the Myojin Knoll caldera of the Izu-Ogasawara arc. A 10-m-long electrode rod, on which five electrodes referenced with a common electrode were mounted, was connected at the tail of an AUV. The survey was conducted at a typical speed of 2 knots, covering the 1500 m × 1500 m area with a typical spacing of survey lines of 100 m. With AUV altitude of 100 m above the seafloor, a negative self-potential anomaly of a few millivolts was observed. The self-potential anomaly was found to spread 300 m × 300 m. The self-potential is probably attributable to the geo-battery mechanism: electric current is generated by redox reactions occurring around an ore body crossing a redox contrast. Assuming that the source of the self-potential is an electric current dipole, we can image a southward-dipping dipole with the moment of approximately 10 3 A m, approx. 30 m below the southern part of the ore deposit. Anomalies of turbidity, which are correlated to ambient temperature and which are signatures of discharged hydrothermal fluids, were distributed more broadly than the self-potential. Some turbidity anomalies were found without self-potential anomalies. They were probably transported by the ocean current. Spatial decoupling between the self-potential and turbidity anomalies suggests that the direct contribution of hydrothermal fluids to the self-potential anomalies is probably a secondary effect. The survey altitude of 100 m and the survey speed of 2 knots in the present study represent practical limitations for the self-potential survey when active hydrothermal fields are targeted. We have observed that the self-potential method responds exclusively to the presence of hydrothermal ore deposits. This behavior differs from other methods for exploring seafloor hydrothermal ore deposits: The geomagnetic method responds not only to ore deposits but also to volcanic bodies. The plume method can detect remote hydrothermal activities, but the source locations are not necessarily specified. The self-potential method is useful as an excellent exploration tool, particularly for initial surveys.

Marine DC resistivity and self-potential survey in the hydrothermal deposit areas using multiple AUVs and ASV

“…Autonomous underwater vehicles (AUVs), particularly because of their stable posture control without a towing cable, are powerful platforms for scientific investigations and industrial work including the exploration of hydrothermal ore deposits below the seafloor. Kawada and Kasaya (2018) conducted a dense SP survey using a middle-class AUV "JINBEI" as well as mapping turbidity and side-scan sonar data at the same time. They detected a negative SP anomaly related to known hydrothermal deposits that are localized compared to a widely spreading turbidity anomaly distribution.…”

Gradient Self-Potential Logging in the Rio Grande to Identify Gaining and Losing Reaches across the Mesilla Valley

“…The waterborne gradient SP logging approach differs from most land-based SP surveys in that both SP electrodes are mobile (no fixed reference electrode) along a profile or grid in a river or lake, and the electrical circuit is completed at each measurement location by the contact between the SP electrodes and the surface water instead of the aquifer [37]. Immersion in surface water reduces contact resistance between the electrodes and the surface water, which enhances the signal-to-noise ratio, and meaningful anomalies of less than a few tens of microvolts have been measured and published [38][39][40][41][42][43]. Recently, waterborne gradient SP logging enabled the characterization of reach-scale heterogeneous hyporheic-driven groundwater and surface-water exchange between the lower Guadalupe River and the Carrizo-Wilcox aquifer in central Texas [37] (in their Figure 1), identified meter-scale groundwater discharge locations in the Quashnet River of Cape Cod, Massachusetts [44], and identified gaining and losing reaches of the Colorado River where it crosses the Bee Creek Fault, and is incised into the surficial exposures of the rocks that comprise the lower and middle zones of the Trinity Aquifer in central Texas [43,45].…”

Deep-Sea DC Resistivity and Self-Potential Monitoring System for Environmental Evaluation With Hydrothermal Deposit Mining

“…In the marine environment, SP survey, as well as electrical and electromagnetic explorations, has been attracting attention for hydrothermal deposit exploration (Kawada and Kasaya, 2017;Safipour et al, 2017). Constable et al (2018) and Kawada and Kasaya (2018) carried out SP surveys using autonomous undersea vehicles (AUVs) and successfully detected negative anomalies associated with hydrothermal deposits. Kasaya et al (2009) detected precursory electric potential changes associated with seismically generated turbidity flows by connecting electro-magnetometers and pressure gauges to cables laid off Hatsushima.…”