Seafloor mapping AUV
MBARI has developed an autonomous underwater vehicle (AUV) with capabilities to map the seafloor with higher resolution than is possible with hull-mounted or towed sonar systems.
The MBARI mapping AUV is a torpedo-shaped vehicle equipped with four mapping sonars that operate simultaneously during a mission. The sonars are 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 vehicle is launched on programmed missions and runs on its own battery power until it returns 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 mapping AUV was christened the D. Allan B.
Purpose and motivation
A fundamental activity in oceanography is to use mapping technology to image the structure and character of the seafloor. Sonars that are hull-mounted or towed do provide high quality seafloor maps in shallow water, but cannot show elusive seafloor features such as lava flows or slumps at depths of more than 100 meters. 1
Using platforms mounted with high-frequency sonars that can operate in deep waters is required in order to achieve one meter resolution of the seafloor. These platforms consists of expensive, noisy, and erratic submersibles.
MBARI has created an autonomous underwater vehicle (AUV) to efficiently map high-resolution images on the ocean floor.
Autonomous underwater vehicles provide important advantages for seafloor mapping, especially in the deep ocean. Since high-frequency sound is required to obtain high-fidelity maps of the seafloor, and high frequencies are attenuated by sea water, it is necessary to get close to the seafloor to produce the highest quality maps. Typically this is done by towing sonars close to the bottom. However, in deep water, especially near rough seafloor, this can be dangerous and slow and produce data contaminated by ship motion. The AUV provides a faster, more nimble platform to produce very high-quality data sets.
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
- 200 kHz forward looking sonar
- Mid-body module:
- Reson 200 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 200 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 or stern of the ship using the launch and recovery system (LARS). MBARI AUV technicians download the mission script onto the AUV while it floats at the ocean surface. They then check to make sure that all of the systems and instruments are fully functional.
The AUV receives a command to dive below the ocean floor. The vehicle will sometimes be launched in waters deeper than 130 meters. When this occurs, the global positioning system (GPS) and the doppler velocity log (DVL) do not simultaneously start working.
So the inertial navigation system for the AUV has to be aided using an ultra-short baseline tracking instrument, instead using GPS and DVL. Since the AUV is already programmed with its mission objective, it is usually not tracked, which means the research vessel can conduct other tasks or go back to dock 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 200 kHz Reson multibeam sonar is the primary mapping sensor. The Flight Systems Development Working (FSDW) system has dual sidescans and a subbottom profiler that takes images of the seafloor’s structure.
The multibeam sonar can operate up to 50 meters from the seafloor with a resolution of one meter since towards the edge of the swath, the beam footprints are larger. The sonar is capable of mapping 12 kilometers of the seafloor in a 17.5 hour mission at 50 meters altitude.
Three mapping sonar systems aboard the mapping AUV
The primary mapping sensor is a Reson 7125 200 kilohertz multibeam sonar. It produces backscatter intensity and swath bathymetry. It generates bathymetry data at one meter lateral resolution in autonomous surveys flying at 60-90 meters altitude. It can generate data at 0.5 meter resolution when mounted to an remotely operated vehicle flying at 20 meters altitude. The vertical precision is 0.30 meters (limited by pressure sensor).
The bathymetry beam footprints are as small as 0.5 meters across . The bathymetry grid at right has a lateral resolution of one meter.
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 correcting roll and pitch biases, editing the sonar data, and adjusting the navigation data, post-survey.
After the AUV mission, the multibeam processing package MB-system processes the various sonar instruments’ data. Processing the data involves correcting for pitch and roll errors, navigation adjustment, and editing erroneous soundings from bathymetry data.
It is important to adjust for navigation since the AUV accumulates many errors when using the inertial navigation system data. 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. The MBnavadjust tool can correct navigation errors by correlating overlapping seafloor features.
The sidescan data is processed by using the multibeam bathymetry instead of using a flat surface. The subbottom profiler data can be processed using MB-System. Various data products from the sensors can be used to produce grids, mosaics, and maps.
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 roll and pitch are logged and can be accounted for in post-processing.
Editing sonar data
Sonar data are edited with MB-System’s package “mbedit”. 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.
AUV multibeam data maps
Closeups of Axial’s AUV-generated bathymetry, gridded at 1 m resolution
Click maps to view larger versions
Maps and images courtesy of National Oceanic and Atmospheric Administration (NOAA), 2006