Seafloor mapping AUV
MBARI has developed autonomous underwater vehicles (AUVs) with capabilities to map the seafloor with higher resolution than is possible with hull-mounted or towed sonar systems.
There are now two AUVs with these capabilities in the MBARI fleet. The vehicles are equipped with four mapping sonars that operate simultaneously during a mission: a swath multibeam sonar, two sidescan sonars, and a sub-bottom profiler. The multibeam sonar produces high-resolution bathymetry (analogous to topography on land), the sidescan sonars produce imagery based on the intensity of the sound energy’s reflections, and the subbottom profiler penetrates sediments on the seafloor, allowing the detection of layers within the sediments, faults, and depth to the basement rock. All components are rated to 6,000 meters depth. The vehicles are launched on programmed missions and run on their own battery power until they return to the ship, as programmed, for recovery.
In honor of MBARI’s long-time Board member Dr. D. Allan Bromley of Yale University, who passed away in 2004, the first Mapping AUV was christened the D. Allan B.
Purpose and motivation
A fundamental activity in marine science is to use mapping technology to image the structure and character of the seafloor. Sonars that are hull-mounted or towed can provide high quality seafloor maps in shallow water, but cannot show details in seafloor features such as lava flows or slumps in typical ocean depths.
Using platforms mounted with high-frequency sonars that can operate in deep waters is necessary to map the seafloor at high resolution. Since high-frequency sound is required to obtain high-fidelity maps of the seafloor, and high frequencies are attenuated by sea water, the sonar must be brought close to the seafloor to produce the highest quality maps. Platforms in the past have consisted of submersibles, which are expensive, noisy and erratic, or towed systems, which in deep water and especially near rough seafloor, can be dangerous and slow and produce data that is not precisely located and contaminated by ship motion. AUVs provide a faster, more nimble platform to produce very high-quality data sets especially in the deep ocean, and can accomplish this task efficiently and reliably.
New high-resolution maps of the seafloor are expected to:
- Drive new science (such as sediment transport from shelf to deep sea)
- Enable deep-sea resource management (such as habitat surveys)
- Help in planning & installing seafloor observatories (such as MARS)
- Dorado- class autonomous underwater vehicle (AUV)
- Named the D. Allan B.
- Size: 0.53 meters (1.7 feet) in diameter; 5.3 meters (17.3 feet) long
- Three modular sections
- Hull: ABS plastic (acoustically transparent at the relevant frequencies and provides structural strength)
- Syntactic foam between housings provides buoyancy
- Weight: 680 kilograms in air
- Endurance: 17.5 hours
- Speed: 1.5 meters per second (5.4 kilometers per hour, or 3 knots)
- Depth rating: 6,000 meters
- The AUV is shaped similar to a torpedo
- Altitude: typically flown 50 to 100 meters above the seafloor
- Inertial Navigation System (INS) and Doppler Velocity Log (DVL) navigation: rated to 6000 meters 1
- Range: 55-85 kilometers depending on sonar load
- Turning diameter: less than 20 meters
- Maximum climb/dive rate: more than 30 meters/minute
- Operable: From MBARI R/V Rachel Carson and R/V Western Flyer, and from blue water UNOLS vessels.
- Nose Section:
- Conductivity, temperature, and depth (CTD) sensor (SeaBird SBE-49 fastCat)
- Lithium-ion batteries
- Mid-body module:
- Reson 400 kHz Multibeam Sonar
- Flat receive array (model 7125)
- 0.94 degree by 0.94 degree beams
- 256 beams across a 150 degree swath
- Edgetech FS-AU Sonar Package
- 110 kHz chirp sidescan
- 410 kHz chirp sidescan
- 2-16 kHz chirp subbottom profiler
- More about sonars
- Reson 400 kHz Multibeam Sonar
- Tail section:
- Kearfott inertial navigation system with Doppler velocity log
- Paroscientific pressure sensors
- Main vehicle computer
- Ultra-short baseline, and acoustic modem for communications
- Articulated propeller inside a circular duct for propulsion
- 5 kilowatt-hour Eagle-Pitcher secondary cells in a 1 atmosphere glass housing
- 3 x 2 kilowatt-hour lithium-polymer pressure-tolerant batteries
- MBARI-patented propulsion system
- Brushless DC motor and gear box
- Double-gimballed ring-wing duct moves vertically for elevator, and horizontally for rudder
- Propeller moves with the duct
- 52 Newtons (12 lbf) of thrust at 300 rpm
- Freewave RF modem, 57.6 kilobits per second.
- Iridium phone
- Radio Direction finder (RDF)
- Sonardyne Fusion Ultra-short baseline (USBL) MF, 19 kilohertz (kHz) down, 27 kilohertz (kHz) up
- Slight positive buoyancy (~8 pounds buoyant)
- Emergency 10 kilogram drop weight with internal and remote acoustic trigger
- Homerpro acoustic beacon, Radio Direction Finder, strobe light
- When on the surface, Iridium calls home to give a position
The AUV is usually deployed over the side of a ship using a crane. MBARI AUV technicians communicate by radio to the AUV while it floats at the ocean surface to download the mission script to it and check that all of the systems and instruments are fully functional. Then the AUV receives a command to dive.
Once submerged it is no longer in contact with the global positioning system (GPS). The doppler velocity log (DVL) can lock onto the bottom when the AUV is within 130 m of the seafloor. In waters deeper than 130 m, the navigation of the AUV is updated on descent using ultra-short baseline (USBL) acoustic tracking from the ship until the DVL locks on to the sea floor. Then the inertial navigation system (INS) takes over, aided by the DVL. Since the AUV is programmed with its mission script, it is usually not tracked during the survey, which means the research vessel can conduct other tasks and recover the AUV at a later time period.
The mapping AUV maps the seafloor by emitting sound at various frequencies that reflect off the bottom and return to receivers on the vehicle. The amount of time the sound takes to return and the energy with which it is returned are processed to make “images” of the shape and hardness of the seafloor. The vehicle is programmed to “mow the lawn” (moving back and forth across a segment of the seafloor) to fully cover a region of interest.
The sonar instruments are held within a titanium frame. The 400 kHz Reson multibeam sonar is the primary mapping sensor. The Flight Systems Development Working (FSDW) system has dual sidescan sonars and a subbottom profiler that takes images of the seafloor’s structure.
When flown at 50 meters above the seafloor, the resolution of the multibeam sonar is one meter. The vehicle is usually programmed to fly at 5.56 km/hr (3 kt) speed and with 150 m line spacing. Battery life is about 18 hours, and when descent and ascent time is considered, at 1500 m depth the vehicle is capable of mapping 90 kilometers of seafloor in a mission.
Three mapping sonar systems aboard the mapping AUV
The primary mapping sensor is a Reson 7125 400 kilohertz multibeam sonar. It produces swath bathymetry and backscatter intensity. The bathymetry data is one meter lateral resolution in surveys flown at 50 meters altitude, and lower resolution if flown at higher altitudes. The vertical precision is 0.30 meters (limited by pressure sensor).
Edgetech 110 and 410 kilohertz chirp sidescan sonars image the seafloor character and fine-scale features at ~10 centimeters resolution.
Edgetech 2-15 kilohertz chirp subbottom profiler images subsurface sediment structure. It achieves up to 50 meters penetration with 10-centimeter vertical resolution.
Excellent navigation is critical to mapping
The current navigation system used on the mapping AUV is the Kearfott SeaDevil inertial navigation system (INS). It also includes the doppler velocity log (DVL) as well as a ring laser gyro. If the DVL continuously tracks the seafloor, the real-time navigation deviation is 0.05% of the total distance traveled. The Inertial Navigation System also provides data on the vehicle attitude (pitch, heading, and roll). A Paroscientific Digiquartz pressure sensor can precisely measure vehicle depth at a standard deviation of 0.3 meters from depths of 3000 to 6000 meters. MBARI AUV technicians plan missions by using the interactive application MBgrdviz, which is part of the MB-System.
For the vehicle to fly at a safe and uniform altitude over the seafloor, missions are planned over the most reliable maps available of the area. To ensure that the vehicle executes the mission, and for the high-resolution maps to be accurate, the position and orientation of the AUV must be precisely known and logged during the mission, and this operational data is used during post-mission data processing.
Navigation during the dive
The navigation equipment includes an inertial navigation system (INS) that is integrated with a doppler velocity log (DVL) and laser ring gyros to measure the vehicle’s position and altitude (see Vehicle specifications). Control algorithms use this data to maintain a stable platform and to record the vehicle’s track.
The missions start on the surface where the vehicle achieves a valid global positioning system fix and begins a spiral descent. Since reliable bottom tracking is not possible during descent, the AUV relies on inertial navigation and position updates sent from the support ship: ultra short baseline (USBL) tracking data of the vehicle is packaged on the ship and transmitted in messages over an acoustic modem link to the AUV. The vehicle responds to these messages with vehicle status messages.
After operational depth is achieved, the AUV starts the mission designed using the multibeam procesing package MB-System “Mbgrdviz“. Missions are typically composed of a sequence of straight lines that connect at waypoints. The control algorithm uses the navigated position to compute the distance of the vehicle from the line joining the previous waypoint to the next. This position “error” is the input to a control loop that computes a heading command and positions the rudder.
Successful navigation during the dive and all post-processing corrections require precision timing between sonar pings and periods of listening to prevent acoustic interference.
The navigation requirement for MBARI seafloor mapping operations is that the real-time navigation error at the end of the survey be no worse than half a swath width. This allows the navigation post-processing software,Mbnavadjust, to locate overlapping and crossing swaths. It then matches bathymetric features and adjusts the navigation so that the precision is equivalent to the lateral resolution of the bathymetry data.
- Real-time navigation: 0.05% of distance traveled, CEPR with continuous DVL bottom lock. After traveling 10 kilometers there is a 50-50 chance that the accrued navigation error is more than 5 meters. There is a one in 100 chance that the error is more than 13 meters.
- Post-processed navigation: Approaches the lateral resolution of the multibeam bathymetry. In a 50 meter altitude survey, the relative navigation error is less than 3 meters.
The raw sonar and navigation data must be processed before maps can be made
The multibeam processing package MB-System is used extensively for planning the surveys, and for post-survey correction of roll and pitch biases, editing erroneous soundings from the bathymetry data, piecing together multiple surveys, and adjusting the navigation data. Data products include bathymetry grids, sidescan mosaics, subbottom profiles, and GIS-compatible files.
Planning the survey
Surveys are planned with MB-System’s package “Mbgrdviz”. Altitude off the bottom, line spacing, and crossings can all be specified. Missions are typically composed of a sequence of straight lines that connect at waypoints. Missions are downloaded to the vehicle over a radio link before the dive. Waypoints can also be sent to the ship’s bridge.
Roll and pitch bias correction
The vehicle’s attitude (roll, pitch, and heading) are logged and accounted for in post-processing. Biases introduced by slight differences in alignment between the sensors can be analyzed and corrected.
Editing sonar data
Sonar data are edited with MB-System’s package “mbedit” and 3-D ping viewer “mbeditviz”. All the beams within each ping of the swath are displayed for editing. Bad beams can be flagged so that they will not be considered during further processing.
Programmed into the deployment’s track are several crossings so that drift during the dive can be corrected later. MB-System’s utility “Mbnavadjust” is used to match features in overlapping swaths and adjust the navigation. It is important to adjust for navigation since errors accumulate during each survey. For a 17.5 hour mission, the upper limit on navigation error is 44 meters with a standard deviation of 10 to 20 meter errors.
AUV multibeam data maps
Axial Seamount, Juan de Fuca Ridge
The Submarine Volcanism project has been mapping the summit and rift zones of Axial Seamount since documenting the flows of the 1998 eruption with ship-board multibeam sonar a few months afterward. Subsequent expeditions have coupled AUV bathymetry with ROV observations, allowing precise determinations of the extent of new lava flows after the 2011 and 2015 eruptions.
Paduan, J., Clague, D., Caress, D., & Thomas, H. (2016). High-resolution AUV mapping and ROV sampling of mid-ocean ridges. Presented at the Marine Technology Society / Institute of Electrical and Electronics Engineers Oceans Conference.
Clague, D. et al. (2014), Eruptive and tectonic history of the Endeavour Segment, Juan de Fuca Ridge, based on AUV mapping data and lava flow ages, Geochem Geophys Geosystems, 15(8), 3364–3391, doi:10.1002/2014GC005415.
Clague, D. et al. (2013), Geologic history of the summit of Axial Seamount, Juan de Fuca Ridge, Geochem Geophys Geosystems, 14(10), 4403–4443, doi:10.1002/ggge.20240.
Paull, C., D. Caress, W. Ussler, E. Lundsten, and M. Meiner-Johnson (2011), High-resolution bathymetry of the axial channels within Monterey and Soquel submarine canyons, offshore central California, Geosphere, 7(5), 1077–1101, doi:10.1130/GES00636.1.
Paull, C., W. III, D. Caress, E. Lundsten, J. Covault, K. Maier, J. Xu, and S. Augenstein (2010), Origins of large crescent-shaped bedforms within the axial channel of Monterey Canyon, offshore California, Geosphere, 6(6), 755–774, doi:10.1130/GES00527.1.
Paduan, J., Caress, D. W., Clague, D. A., Paull, C. K., & Thomas, H. (2009). High-resolution mapping of erosional, tectonic, and volcanic hazards using the MBARI mapping AUV. Rendiconti Online Società Geologica Italiana, 7, 181–186.
Caress, D. W., H. Thomas, W. J. Kirkwood, R. McEwen, R. Henthorn, D. A. Clague, C. K. Paull, J. Paduan, and K. L. Maier (2008), High-resolution multibeam, sidescan, and subbottom surveys using the MBARI AUV D. Allan B, Marine habitat mapping technology for Alaska, 47–69. [online] Available from: http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.464.1712&rep=rep1&type=pdf