Yesterday afternoon we deployed the MBARI Mapping AUV (autonomous underwater vehicle) for its first survey of the voyage (below).
The MBARI AUV aboard the R/V Kilo Moana.
Unlike ROV Jason, the AUV is not attached to the ship when it makes a dive (that’s the “autonomous” part). This AUV is a ~6 m (20 ft) long torpedo-shaped robot that flies a pre-programmed path to map the bathymetry of the sea floor (topography, if on land) using sonar. We are using it on this trip to repeat a path it has run in past years to detect vertical movements of the volcano.
Jenny Paduan and Dave Caress evaluating the AUV data.
An interesting behavior shown by both Axial Seamount and Kilauea Volcano in Hawaii (arguably two of the best-instrumented volcanoes on Earth), is vertical deformation through the eruption cycle: during eruptions, the summit rapidly drops several meters (during the 2015 eruption at Axial, the caldera floor dropped 2.5 m, or 8.2 ft,), and then as the magma chamber refills toward the next eruption, the summit slowly rises again. Because the AUV can travel nearly 100 km in a single survey, it can map well outside the caldera. That will give us a picture of how the rest of the volcano is responding, and augment the precision measurements being made using the ROV Jason on this trip and the long time-series of measurements by instruments deployed inside the caldera.
Deploying the MBARI AUV.
The AUV was lifted by a crane from the aft deck and set into the water behind the ship (right). It was then released to execute the planned mission, which included spiraling down to a depth ~ 50 m above the sea floor then driving the pattern we programmed for it. We listened for its acoustic signal for a while to ensure that it knew where it was and that its systems were functioning properly. When it left acoustic range of the ship, we turned our attention back to the ROV Jason, which had been suspending off bottom so the ship could maneuver during the AUV launch and now could get back to work; its dive will continue for another day.
The multibeam sonar on the AUV pings an array of sound downward then listens for the echo of the pings after they have bounced off the sea floor back to the vehicle. From the amount of time it takes for the sound to make that return trip, the depths along a 250 m (820 ft) wide swath of seafloor below the vehicle are calculated. When the vehicle flies at 50 m (~165 ft) altitude above the sea floor the resulting map has a lateral resolution of 1 meter and vertical resolution of ~10 cm, meaning that things 1 m (3 ft) square by 10 cm (4 in) tall can reliably be seen. When the same patch of sea floor is mapped repeatedly, height changes that exceed 20 cm (8 in) are apparent. In this way, we have been able to map changes due to faulting, landslides, and lava flows. An example of mapping new lava flows is shown from the 2011 eruption here at Axial, where we made a depth difference map by subtracting the pre-eruption surface from the post-eruption surface (map below).
AUV depth difference map showing the eruption.
The map reveals flows that were up to 15 m thick, their fissure systems (black is negative depth change), intricate lava channels (as thin veneer), and ponded flow margins (thick accumulations). From such maps, calculations can be made of the area and volume of flows, and together with data acquired during the eruption from other instruments on the sea floor, we can also calculate the average eruption rate. With the repeat AUV surveys we are conducting on this expedition, we are attempting to measure even more subtle depth changes between surveys that are due to the volcano inflating between eruptions, as part of a wider effort to monitor the volcano. For example, we know that Axial Seamount has already re-inflated more than 1.5 m (~ 5 feet) since the 2015 eruption, which is easily measureable by repeat mapping with the AUV. These revolutionary measurements help us understand processes inside the volcano that are hidden from view, such as the depth and geometry of the magma reservoir beneath the caldera and at what rate magma is being supplied between eruptions.
