Deep‐sea polymetallic sulfides associated with hydrothermal systems are considered a potential viable resource for base and precious metals. The Southwest Indian Ridge (SWIR, Indian Ocean) hosts active and inactive hydrothermal systems. Inactive hydrothermal fields are more abundant than active fields but are difficult to remotely characterize. We report a deep‐sea self‐potential investigation to locate inactive ore deposits at the Yuhuang hydrothermal field on the ultraslow‐spreading SWIR. A horizontal array of six nonpolarizing AgCl electrodes are attached to a deep‐towed transient electromagnetic system to record the electrical field at a height of 40 m above the seafloor. The observed electrical field strength near a previously recognized sulfide deposits (verified by drilling) reaches an amplitude of 0.5 mV/m. Negative self‐potential anomalies (~ −27 mV) were observed by integrating the measured electrical field. The high electrical conductivities (up to 12 S/m) of sulfide samples measured in the laboratory and the oxidized sulfides recovered from the surface of the deposit suggest that the self‐potential anomalies are due to sulfide mineralization and corrosion of the polymetallic sulfides. Inversion of the self‐potential data reveals a localized body with a thickness of ~65 m, which is interpreted as the Yuhuang ore deposit. Our field data demonstrate that the self‐potential method is a useful exploration method to identify and image seafloor massive sulfide deposits hosted in inactive hydrothermal fields at mid‐ocean ridges.

The self‐potential (SP) method is expected to play an important role in the exploration of seafloor massive sulfide (SMS) resources. Since the redox potential changes with depth below the seafloor, SMS deposits are the source of an electrical (source) current density, generating in turn a recordable electrical field. This electrical field can be remotely measured at and above the seafloor. By integrating this electrical field, we obtained a so‐called SP profile or map. The electrical conductivity distribution of SMS deposits is an important ingredient in the inversion of these SP data in order to distinguish between causative primary sources (i.e., associated with the SMS deposits themselves) and secondary current sources associated with conductivity contrasts below the seafloor. Such ingredient is therefore necessary to properly localize SMS deposits and eliminating ghosts associated with high conductivity contrasts. The conductivity structure of the seafloor is however unknown in most SP surveys. This study presents the first joint interpretation of transient electromagnetic and SP surveys. The field data were acquired at the Trans‐Atlantic Geotraverse (TAG) hydrothermal mound. The conductivity structure of TAG mound was obtained by inverting transient electromagnetic data. This electrical conductivity distribution is then used to invert the SP data. The obtained (primary) source current distribution delineates the geometry of the SMS deposits. The deposits are located in the center of the mound as well as at its edges, in a way that is consistent with water column data. Combining the two geophysical methods improves our ability to better decipher the fine geometry of SMS deposits below the seafloor.

Seafloor massive sulfide (SMS) deposits contain abundant metals, such as Cu, Zn, Au, and Ag, which are of growing economic interest. Currently, the exploration of SMS deposits has entered into a new stage of resource assessment, so we need a geophysical method to rapidly obtain the 3D structure of these SMS deposits. We report here a self-potential (SP) survey performed with an autonomous underwater vehicle (AUV) at the Yuhuang hydrothermal field on the ultraslow-spreading Southwest Indian Ridge with a water depth ranging from 1300 m to 2200 m. The electric-field sensors are attached to the tail of the AUV to record the electric field approximately 50 m above the seafloor. The electric field strength is observed to reach an amplitude of 0.6 mV/m at this known SMS deposit. SP tomography is used to obtain the 3D structure of the ore body, and benchmark numerical tests are first performed on synthetic cases to optimize the inversion algorithm parameters to estimate the ore location. The 3D SP tomography reveals a south-sloping ore body, which is 100 m long in the north–south direction and nearly 200 m long in the east–west direction. The vertical extent of the ore body is approximately 100 m. These results suggest that an AUV-based SP survey and tomography are potentially useful exploration methods to characterize the geometry of SMS deposits. This method may play an important role in resource assessment in concert with other geophysical methods, such as magnetic and induced-polarization surveys.

The marine self-potential (SP) method is currently playing an increasing role in the exploration and resource evaluation of seafloor massive sulfide (SMS) deposits. SP surveys are conducted using autonomous underwater vehicles (AUV), which yield multicomponent electric field datasets. By comparing with the single-component electrical field data used to date, the inversion of these multicomponent data is expected to provide a more accurate description of the 3D structure of SMS deposits beneath the seafloor (like gradiometry in gravity surveys). We introduce an inversion algorithm specifically adapted to multicomponent SP data. A synthetic model demonstrates that the inversion of multicomponent datasets allows us to better recover the amplitude of the current density and the morphology of the ore bodies compared to using a single component of the electrical field. Next, we apply our approach to a multicomponent SP dataset collected during the DY58 oceanic cruise at the Yuhuang hydrothermal field on the Southwest Indian Ridge. Subsequently, we reconstruct the three-dimensional (3D) geometry of the SMS deposits beneath the seafloor. The AUV-based SP system with the collection of multicomponent SP data inversion appears to be a powerful tool in the exploration and evaluation of seafloor sulfide resource and, in the future, could be used in concert with induced polarization data.

Ore bodies generate natural electrical fields that are measurable at the ground surface. The ground surface signature of this electrical field is called a self-potential anomaly. We developed a sandbox experiment to monitor the evolution of a self-potential anomaly associated with redox processes mediated by bacterial activity at the surface of a buried metallic object crossing the water table. A Bio-Electrochemical Cell (BEC) is formed by a metal bar connecting the upper, oxygen-rich, part of the tank and an aquifer containing an electron donor in the form of acetate. The self-potential response was observed during a period of 327 days. The tomography of the self-potential signature confirms that self-potential tomography is able to locate the metallic target acting as a BEC. In addition, we performed redox potential, pH, and electrical potential measurements over a vertical cross-section of the tank at several time steps to obtain an idea of where the redox front is located. The distributions of the redox potential and pH further demonstrated the development of the oxidation-reduction chemical processes facilitated by the BEC as bacterial communities developed around the metallic bar. The electrical potential anomaly shows that the bacterial communities followed a short period of exponential growth, then a longer period of a sustained population. These results demonstrate the usefulness of the self-potential method in monitoring redox processes at the surface of a buried ore body. Further works will need to combine such self-potential anomalies with induced polarization anomalies through joint inversion.