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.

Development of the Seafloor Mapping AUV and software for processing the sonar data has been a major focus of the Seafloor Mapping Lab. 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)
• Provide spatial context for ROV dives and enables targeted sampling

General Specifications

• Two Dorado- class autonomous underwater vehicle’s (AUV) designed and built by MBARI
• Size: 0.53 meters (1.7 feet) in diameter; 5.3 meters (17.3 feet) long
• Three modular sections
• Hull: ABS plastic farings held together by circular joining rings (acoustically transparent at the relevant frequencies and provides structural strength)
• Syntactic foam between housings provides buoyancy
• Weight: 680 kilograms in air
• Endurance: 19 hours
• Speed: 1.5 meters per second (5.4 kilometers per hour, or 3 knots)
• Depth rating: 6,000 meters
• Altitude: typically flown 50 to 100 meters above the seafloor
• Inertial Navigation System (INS) and Doppler Velocity Log (DVL) navigation
• Maximum range: 95 kilometers
• Turning diameter: less than 20 meters
• Dive and climb rates: typically 25 m/min descending and 50 m/min ascending
• Ship operations most often from MBARI’s R/V Rachel Carson but has been operated from other vessels ranging from a 24 m (80 ft) pilot boat to a 110 m (360 ft) icebreaker.

AUV Nose:

• Conductivity, temperature, and depth (CTD) sensor (SeaBird SBE-49 fastCat)
• Water sound speed sensor
• 10 kWhr of Lithium-ion batteries

AUV Mid-body Payload:

• Teledyne SeaBat T50-S 400 kHz Multibeam Sonar
  
  • 1.0 degree by 0.5 degree beams
  • 1024 beams across a 170 degree swath
• Edgetech 2205 Sonar Package
  • 110kHz chirp sidescan
  • 1-6 kHz chirp subbottom profiler

AUV Tail:

• Kearfott SeaDevil Inertial Navigation System (INS) with integrated 300 kHz Doppler velocity log (DVL)
• Paroscientific Digiquartz pressure sensor
• Main vehicle computer
• Ultra-short baseline (USBL) tracking sonar beacon
• Benthos acoustic modem for communications
• Articulated propeller inside a circular duct for propulsion and control

Power Options

• Dual 5 kWhr lithium ion battery packs in spherical glass housings

Propulsion

• 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.
• Two Iridium satellite modems
• Radio Direction finder (RDF) beacon

Submerged communications

• Sonardyne Ranger 2 USBL, 19 kHz down, 27 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, emails position via Iridium satellite network

The AUV is usually deployed and recovered 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.

When an articulating crane with a capture head is available, as on MBARI’s AUV mother vessel, R/V Rachel Carson, AUV recovery is accomplished by maneuvering the ship alongside the AUV and hooking into the lifting bale. Otherwise, a small boat may be used to bring the AUV to the ship, and manually hook the bale to the ship’s crane.

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. There are three mapping sonar systems aboard the mapping AUV described below

Multibeam sonar

The primary mapping sensor is a Reson 7125 400 kHz multibeam sonar. It produces swath bathymetry and backscatter intensity. The bathymetry data is 1 meter lateral resolution in surveys flown at 50 meters altitude, and lower resolution if flown at higher altitudes. The vertical precision is 0.10 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 kHz chirp sidescan sonar images the seafloor character such as surficial texture, near surface density variations and other fine-scale features at ~0.1 m resolution.

Subbottom profiler

Edgetech 1-6 kHz chirp subbottom profiler images subsurface sediment structure. It achieves up to 50 m penetration with 0.1 m vertical resolution.

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 INS also provides data on the vehicle attitude (pitch, heading, and roll). The pressure sensor can precisely measure vehicle depth at a standard deviation of 0.1 meters up to 6000 m depth. Control algorithms use this data to maintain a stable platform and to record the vehicle’s track.

Mission Planning and Execution

For the vehicle to fly at a safe and uniform altitude over the seafloor, missions are planned over the most reliable maps available, typically using bathymetry collected using ship-mounted sonars. The mission route is planned interactively in a visualization tool that is part of the MB-System software package.

Deep water missions start on the surface where the vehicle obtains a valid global positioning system (GPS) fix and begins a spiral descent. Without DVL bottom lock, the INS drifts rapidly. In order to stabilize the INS position during decent, USBL tracking of the AUV by the ship is relayed to the AUV using the acoustic modem. Once the AUV is close enough to the seafloor that the DVL can measure velocity relative to the bottom, USBL aiding is no longer needed and is stopped. Once operational depth is achieved, the AUV starts the designed mission. Survey missions are typically composed of a sequence of straight lines that connect at waypoints.

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 tool in MB-System,Mbnavadjust, to locate overlapping and crossing swaths. This tool matches bathymetric features in overlapping swaths and solves for a navigation model with relative precision equivalent to the lateral resolution of the bathymetry data.