From May 3 – June 13, 2025, 8 CSSF team members supported the Korean Institute of Ocean Science and Technology (KIOST) aboard the R/V Isabu for their 2025 Indian Ocean Expedition. The science team’s objective was to estimate the depth distribution of hydrothermal ore bodies on eight previously identified massive sulfide deposits along the Central Indian Ridge. To do this, our teams were to conduct a type of specially designed ROV-based Controlled-Source Electromagnetic Method (CSEM) survey that was tailored to the study area and tested for the very first time using ROPOS. This initiative was a collaboration between the Canadian Scientific Submersible Facility, Scripps Institute of Oceanography and Ocean Floor Geophysics (based out of Vancouver, Canada).

Leg 1: The focus of Leg 1 was to conduct our highly anticipated CSEM surveys. Using ROPOS, we placed devices called Deployed Undersea Electromagnetic Source Instruments (DUESIs; developed by Scripps Institute of Oceanography; photos on the left) on the seafloor, which were free, battery-powered, and had acoustic releases. They acted as dipole generators, creating an electromagnetic field, and allowed the science team to detect self-potential anomalies below the seafloor, which are characteristic of seafloor massive sulfides. The DUESIs were launched on their own; then, we used ROPOS to move them to the most effective spots. Our team put the science team’s electronic source with the DUESIs and pulled them out to 50 m as opposed to 1 m, as instructed in the package. This increased the dipole moment and gave us more power to detect the strength of the electromagnetic field using ROPOS. We placed 2–3 down per site, which allowed for the detection range to match the morphology of the seabed. ROPOS had been fitted with triple-axis electromagnetic receivers that could detect the anomalies down to ~8–9 m below the seafloor. The receivers extended out 3 m (white; main photo above) to prevent the sub’s electrical noise from interfering with the signal and logging system for the electromagnetic data. When conducting the survey, we flew ROPOS at a constant altitude of 3 m above the seabed. This close-proximity operation enhanced signal strength, while using multiple frequency bands minimized interference. As a result, the survey achieved superior results compared to the regular method.

Leg 2: The focus of Leg 2 was biology and geology sampling at 6 sulfide deposits. We had ROPOS mostly in normal science configuration, which included sample collection tools such as the suction sampler and Niskin bottles, apart from the addition of a series of autonomous dissolved oxygen sensors. During this leg, we had to recover two dives because of marginal sea conditions of swells at ±2.25 m.

A highlight for the science team was the discovery of 2 new active vent sites at the ONNARE and DAUM vent fields, while for Keith Tamburri, our Operations Manager, it was being able to see the depth of the electrical field. A highlight for both the science and ROPOS teams was conducting the survey with the CSEM setup for the first time on an ROV and ground truthing the existing seafloor data, that had been taken with a multibeam echosounder.

Our team faced two challenges on this expedition. The first was to understand how the DUESIs and sensors worked to collect the best data (e.g. altitude and speed) for the science team. Our team overcame this by getting tips from the Ocean Floor Geophysics staff on what has worked best for them in the past. The second challenge was the inclement weather and making the call of whether to go, or not to go forth with operations. Keith Tamburri, our Operations Manager, monitored the sea conditions at the stern of ship for ~10–20 min at a time to ensure that the triple-axis sensors stayed out of harm’s reach. Our team had to perform two recoveries because there were potentially harmful swells at ±6 m and wind speeds of up to 40 knots, and, at one point, because the rough weather spiked the tension in the umbilical cord to a 16 000-lb snap load. Keith T. noted, “They do appreciate our efforts during these challenging situations” and that “everything works out in the end”.