By Rachel Teasdale
Overcast skies with blustery winds (up to 20 knots) and moderate (swells of 6-8 ft).
What’s happening today?
ROV Jason launched this morning on the last incubator experiment dive; AUV Sentry was launched this afternoon and a bottom pressure recorder (BPR) was acoustically released from the seafloor and recovered this evening.
|Bathymetric map of Axial white box shows area discussed in high resolution maps discussed below.|
Making Lava Maps
To make a map of a volcano or other feature above sea level can be done using direct observations and GPS to record geologic features directly on a base map. Unfortunately, it’s not so easy at sea, where neither light nor satellite communications penetrate the deep ocean. Even the strong lights of the ROV Jason only penetrate 10-20 m (32-64 ft). Instead, sound waves, which are transmitted through water, are used for mapping the bathymetry of the ocean floor.
Bathymetric maps show the depth to the seafloor and at Axial Seamount they reveal the caldera, Axial’s rift zones and other topographic features (see map). Bathymetry is represented using color coding to distinguish high areas (shallow depths in warm colors) from low areas (deeper depths in cool colors). This “underwater topography” is similar to a map used to find ones way along a hiking trail, but shows the topography of the seafloor instead.
However, the resolution of bathymetric maps depends on how close to the seafloor the sonar data are collected. Sonars on ships can map about at 30-m resolution at this depth (1500 m), whereas sonars on underwater vehicles, which can “fly” much closer to the seafloor, can map at 1-m resolution. A high-resolution map is like focusing a blurry picture – it allows you to see much more detail.
As noted in the previous AUV Sentry blog , Sentry uses its multibeam sonar to map the seafloor using a pre-programmed set of tracks that have it systematically fly over the floor of the caldera in an overlapping zig-zag pattern, like “mowing the lawn,” with each strip overlapping the next for thorough coverage. Once AUV Sentry returns to the ship, data from the mapping survey is processed to build bathymetric maps. To convert data into the maps, Jenny Paduan of the Monterey Bay Aquarium Research Institute (MBARI) processes the data to remove tides and other navigational effects to precisely represent seafloor depths on a bathymetric map.
On this cruise, the multibeam sonar mapping is helping to map the location of new lava flows, and the resolution is good enough to describe the morphology of those flows, which can be used to extract the volume of lava erupted, the style and the rate of emplacement. For instance, recent mapping in the caldera has allowed a better view of the lava flows erupted there in April 2015. While lights on ROV Jason can show geologists a view of approximately 10 m (32 ft) in front of the vehicle, Jenny’s maps with AUV data reveal high resolution views of much larger areas.
An example of this resolution is in a series of maps below. On the left is a map of an area in the NE part of the caldera (see location box in map at top of this page), mapped before the April 2015 eruption. Jenny generated the middle map from AUV Sentry data from August 2015. Areas in the white boxes (A and B) are notably different from the pre 2015 eruption map, as seen in the Difference map on the right, which is developed by subtracting the “Before Eruption” data from the “After Eruption” data.
The area enclosed by Box A contains a new flow lobe emplaced during the 2015 eruption and is shown at a larger scale in the image below (left), which also reveals a collapse area at the center of the large flow. An example of a much much smaller but similar collapse is shown below right.
Similarly, the area enclosed in Box B was filled with narrow flow lobes and channels (“Before Eruption”) but those channels are now filled in and flattened out by the 2015 lavas. This can also be seen in the “Difference” map, which depicts more shallow areas (higher sea floor topography shown with warmer colors) after the eruption.
Box C in the “After Eruption” map above is shown at larger scale in the map below (left), which reveals a top-view of vertical pillars that were once channels for water to flow vertically though the lava flow. Those vertical channels were preserved when the molten interior of the flows drained, leaving the pillars left standing in the flow field (see image below right).
The high resolution multibeam sonar data collected by AUV Sentry allows fine details of new lavas to be resolved in the resulting maps. While ROV Jason can photograph similar features, it cannot access the extensive areas that AUVs can map, so often the AUV maps are used to identify features that merit detailed exploration with ROV Jason. The combination of widespread, high resolution mapping is a critical contribution to the characterization of the distribution of the 2015 lava flows, yet another important step towards understanding this active submarine volcano.
In addition to creating high-resolution bathymetric maps, another goal of the sonar mapping by AUV Sentry is to look for changes in seafloor depth due to deformation related to inflation and deflation since previous surveys were collected. This data will be used to complement the pressure measurements and expand the spatial coverage of the deformation measurements in the future.