Joint Interpretation of Marine Self‐Potential and Transient Electromagnetic Survey for Seafloor Massive Sulfide (SMS) Deposits: Application at TAG Hydrothermal Mound, Mid‐Atlantic Ridge

Abstract

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.

Plain Language Summary

Seafloor massive sulfide (SMS) deposits contain abundant amounts of Cu, Zn, Au, and Ag, which are of growing economic interest for our civilization. In recent years, the exploration of SMS deposits has entered into a new era with the goal of better assess and characterize these resources. In this perspective, geophysical methods are in high demand to localize SMS deposits and obtain their three‐dimensional geometry. The self‐potential (SP) method provides an efficient non‐intrusive and cost‐effective approach to image SMS deposits. Previous surveys have documented the existence of recordable negative SP anomalies above the seafloor in association with SMS deposits. SP tomography can be used to assess the 3D geometry of SMS deposits. That said, the conductivity pattern below the seafloor is unknown in previous SP surveys while it is an important ingredient in the inversion of the SP data. Active source measurements like the transient electromagnetic (TEM) method can be used to obtain the electrical conductivity distribution below the seafloor. So, we propose to combine SP and TEM data to improve our ability to image SMS deposits. We apply the new methodology to a famous submarine hydrothermal field located in the Atlantic mid‐oceanic ridge Trans‐Atlantic Geotraverse and investigated in the past by various scientific teams. Our study demonstrates that the combination of the two methods has the ability to better image SMS deposits by eliminating source current ghosts associated with conductivity contrasts.

Key Points

• A joint self‐potential (SP) and transient electromagnetic survey has been performed at Trans‐Atlantic Geotraverse (TAG) hydrothermal field (Mid‐Atlantic Ridge)

• The transient electromagnetic method provides the electrical conductivity needed for SP tomography

• The inversion of the SP data reveals the distribution of the massive sulfide deposits at TAG