Seafloor mapping AUV

Interior of the mapping AUV, with components called out. © MBARI 2006

Computer-Aided-Design (CAD) drawing of the batteries, sensors, and computers assembled in the interior (top), surrounded with syntactic foam for flotation (middle), and encased in the plastic outer fairings (bottom).
© MBARI 2006

General Specifications

• 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.

Multibeam receive ring, viewed from aft. © MBARI 2005

• Nose Section:
  • Conductivity, temperature, and depth (CTD) sensor (SeaBird SBE-49 fastCat)
  • Lithium-ion batteries
  • Fluorometer
• 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
• 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

Power Options

• 5 kilowatt-hour Eagle-Pitcher secondary cells in a 1 atmosphere glass housing
• 3 x 2 kilowatt-hour lithium-polymer pressure-tolerant batteries

Propulsion

Tailcone of the AUV © MBARI 2005

• 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

Surface Communications

• Freewave RF modem, 57.6 kilobits per second.
• Iridium phone
• Radio Direction finder (RDF)

Submerged communications

• Sonardyne Fusion Ultra-short baseline (USBL) MF, 19 kilohertz (kHz) down, 27 kilohertz (kHz) up

Safety

• 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.

 Cartoon of AUV mapping the seafloor with a swath of sound. © MBARI 2009

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

Multibeam sonar

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).

Sidescan sonar

This map was generated from sidescan data collected in Monterey Canyon. Dark patches are areas of low reflectivity. © MBARI 2006

Edgetech 110 and 410 kilohertz chirp sidescan sonars image the seafloor character and fine-scale features at ~10 centimeters resolution.

Subbottom profiler

This image was created with the AUV’s subbottom profiling sonar. It shows layers of sediments draping the walls of the inner Monterey Canyon. David Caress © 2005 MBARI

Edgetech 2-15 kilohertz chirp subbottom profiler images subsurface sediment structure. It achieves up to 50 meters penetration with 10-centimeter vertical resolution.

Kearfott INS/DVL/GPS SeaDevil. © MBARI 2005

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.

Navigation performance

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.

The bathymetry map above shows the original real-time INS navigation in red overlain with the final adjusted navigation in black. The navigation adjustments for this survey are modest: the largest relative adjustment required to match overlapping features is 30 meters.
© MBARI 2005

• 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

Plan to “mow the lawn” (survey grid) in Mbgrdviz.
© MBARI 2005

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

Screen grab of mbedit in use. © MBARI 2005

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.

Navigation adjustment

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

Monterey Canyon

This three-dimensional view of the Monterey Canyon channel was made from multibeam bathymetry data collected with the mapping AUV. Ripples and erosion channels are visible in the canyon axis. David Caress © 2005 MBARI

The Canyon Processes project maps the axis of Monterey Canyon regularly to detect bathymetric changes due to sediment transport events. The MARS project used the AUV to map the cable route.

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.

 Side-by-side comparison of multibeam bathymetry showing the improvement that AUV-collected data offers over ship-collected data. The area includes part of the 1998 flow and the “International District” chimneys (see map below for close-up). (Left) ship-collected bathymetry at 20 m resolution. (Right) AUV-collected bathymetry at 1 m resolution. (After Figure 3 of Paduan et al., 2009).

 (Left) Map of the “International District” hydrothermal vent field on the east side of Axial caldera, where several large sulfide chimneys were discovered on ROV ROPOS dive R1014, guided by MBARI AUV bathymetry collected just days before. (Figure 10 of Paduan et al., 2009). (Right) El Guapo is one of the new chimneys found on R1014. It is venting water at 338^oC (640^oF) and stands 13m (43ft) tall, the largest active vent yet found at Axial Seamount. (Image courtesy of NOAA, 2006.)

AUV map of a lava pond on the floor of Axial caldera that breached a levee and continued to flow northward. Color ramp is -1544m (blue) to -1539m (orange). (Figure 14 of Clague et al., 2013).

AUV before and after difference map of the 2011 flows on the caldera floor. Color ramp shows 0 to +15 m depth change. (After Figure 2 of Caress et al., 2012).