MR1

HAWAII MR1 LaunchThe HAWAII MR1 is a shallow towed sidescan sonar system that collects digital bathymetry and acoustic imagery data in all ocean depths. MR1 is portable, and can be deployed by the Hawaii Mapping Research Group from research or survey vessels anywhere in the world. With acoustic imagery swaths up to 25 km wide and a survey speed of 9 knots, MR1 can image up to 415 square kilometers per hour. This high survey rate, along with MR1’s high resolution, make it an ideal wide-area seafloor survey tool.

 


Specs


MR1 : Specifications

Towfish
 Dimensions (Length x Width x Height)  195″ (4.95m ) x 50″ (1.27m) x 38″ (0.97m)
 Tow body Weight (in air)  approx. 3500 lbs (1,600Kg)
 Depressor Weight (in air)  approx. 2200 lbs (1,000Kg)
 Typical Operating Depth  80 -120 metres
 Operating Temperature  0 – 40 degrees Celcius
Deck Equipment
 Electrical Voltage  440-460 VAC, 3 phase
 Electrical Frequency  57-63 Hz
 Eletrical Current – Maximum  72 A Full Load
 Electrical Current – Typical  approx. 30-50 A Normal Load
 Operating Temperature  -10 to40 degrees Celcius
Hydraulic Power Unit
Dimensions (Length x Width x Height)  52″ (1.32m) x 44″ (1.12m) x 60″ (1.52m)
Weight  approx. 1500 lbs (700Kg)
Towing Winch
 Dimensions (Length x Width x Height)  100″ (2.54m) x 52″ (1.32m) x 84″ (2.13m)
 Weight (with cable)  approx. 9000 lbs (4,100Kg)
Launch and Recovery System
 Dimensions (Length x Width x Height)  240″ (6.10m) x 96″ (2.44m) x 102″ (2.60m)
 Weight  approx. 25,000 lbs (11,500Kg)
Laboratory Equipment
 Electrical Voltage  115 VAC, +/- 10% (with UPS)
 Electrical Frequency  47-63 Hz
 Electrical Current – Maximum  38 A
 Electrical Current – Typical  approx. 15 A
 Operating Temperature  0 – 40 degrees Celcius
 Operating Relative Humidity  5 – 80%, non-condensing


Overview


MR1 is a portable side-scanning seafloor imaging system that simultaneously acquires digital bathymetry (swath width ~ 3.4 times water depth) and sidescan sonar imagery (swath width ~ 7.5 times water depth). The system’s sonar transducers are housed in a 4.5-m-long vehicle that is towed beneath the surface mixed layer (60 to 100 m) at ship speeds of 8 to 10 knots.  A 1600 kg depressor weight is towed about 50 m in front of the towfish, thus mostly decoupling it from ship motion.   The MR1 towfish is extremely stable due to its multi-body towing configuration and its large righting moment. As a result, MR1 has successfully operated in rough sea conditions (up to sea state 6) that typically cause performance degradation in hull-mounted systems due to bubble masking and violent ship motion.

A detailed technical description of the system design.

Electronic Configuration

The MR1 towfish uses separate transducer arrays on its port and starboard sides, which operate at different frequencies (11 and 12 kHz, respectively) to minimize crosstalk. Each array contains two rows of elements spaced one-half wavelength apart, and both rows are driven by 10 kW amplifiers to transmit acoustic pulses 1-10 msec long. This high power capability results in a high signal-to-noise ratio, which improves the quality of bathymetric measurements and widens the swath of seafloor over which sidescan data are collected. MR1 uses an advanced network of digital signal processors on the towfish and in the shipboard data acquisition electronics to control the operation of the sonars, and to process acoustic data.

Bathymetric Capabilities

MR1 determines bathymetry by using the phase difference of the signals from dual arrays to measure the angle of returning echoes on either side of the towfish. Flexible data processing software allows power, ping rate and beam spacing to vary in order to maximize the swath width and data quality with changing seafloor depth and acoustic properties. MR1 accounts for ray bending due to acoustic velocity gradients in the water by applying an empirical transfer function based on data collected during a calibration test at the beginning of each survey, and the system can be re-calibrated at any time during the survey to account for changing water column properties. The bathymetric precision for MR1 is within 1.5% of Sea Beam Classic, based on a quantitative comparison between Sea Beam and the MR1 sister-system SEAMAP, which was developed for the U.S. Navy.  In addition to the 6 sidescan transducers, the MR1 towfish houses heading, attitude and depth sensors.

Launch and Recovery System

The MR1 system is designed for portability and ease of use on different ships. The launch and recovery of the MR1 towfish is accommodated by a hydraulic Launch and Recovery System (LRS) that can be mounted on the fantail of any suitable ship. The LRS features a hydraulic tilt-bed assembly that supports the towfish, and can be extended aft and tilted up to deploy or capture the tow vehicle. For data acquisition, the system is lowered into the water and attached to a 50 m-long umbilical cable that in turn is attached to a one-ton depressor weight. The weight is required because the towfish is slightly positively buoyant (so that the towfish will return to the surface if the tow cable breaks), and because the resulting two-body towing configuration decouples the towfish from the heave of the ship. The depressor weight is attached to the ship by a cable that passes through a tow point on the LRS to a winch mounted forward of the LRS. In addition to mechanically supporting the towfish, the tow cable carries power down to the towfish and data up to the ship.

MR1 Deck Gear Drawing

The HAWAII MR1 Launch and Recovery System (LRS) has a footprint equivalent to a standard 20-foot shipping container, and is mounted on the aft end of the work deck. The towfish cradle tilts and slides to deploy and recover the tofish and depressor weight. To accommodate the movement of the cradle, the LRS must be mounted flush with the aft end of the work deck, and have 15 feet of overhead. Controls for the system’s hydraulics are located on the starboard aft corner of the LRS, and clear access is required to both sides of the LRS during deployment and recovery.

Installation Step 6
Installation Step 7

The LRS contains ISO corner fittings that may be used to secure the LRS to the ship using twistlocks that can be welded directly to the workdeck. The LRS also features bolt holes spaced at 2 foot intervals that allow the LRS to be attached to angle brackets that can be bolted or welded to the deck.

Installation Step 9
Installation Step 10

The MR1 tow winch needs to be mounted at least 20 feet forward of the LRS to allow the tow cable to level wind on the drum as its hauled in. The winch can be bolted or welded to the deck. The hydraulic power pack should be mounted in a dry location within 40 feet of the LRS and winch. See MR1 Physical Specifications for power requirements and other exciting facts.

Installation Step 11
Installation Step 12

The MR1 travels with a 20-foot shipping container outfitted to serve as an engineering or data processing laboratory. On ships where the work deck is wet, the lab and other deck gear need to be lifted high and dry, with elevated catwalks to allow dry access and safe operation.

Sidescan Sonar Systems

History

Harold Edgerton, a professor of electrical engineering at the Massachusetts Institute of Technology, developed sidescan sonar technology for use in the civilian community in the early 1960’s. Prior to developing sidescan, Edgerton was known for his work with photographic systems and “stopping time” via pictures. He applied a similar approach to mapping the seafloor using “flashes” of sound to create a narrow “image” of the underlying terrain and then stacking these images in a continuous, long “picture” as the sonar was towed along by a ship.  Many of the names that we now associate with underwater exploration and mapping are people who trained with Edgerton during the early days of sidescan development; Marty Klein and Ed Curley were students of Edgerton who would later go on to found Klein Associates Inc. and EPC where you can purchase sidescan sonars and other ocean instrumentation.  Another Edgerton student was Sam Raymond, founder of Benthos, which markets transponders and deep-ocean camera systems.

How Sidescan Sonars Work

Sidescan Sonar System in 1965Sidescan sonars are among the simplest seafloor mapping tools that you can use. In their towed configuration they consist of a subsurface unit called a towfish, a cable for towing and transmitting information, and a topside system that is used to process data and transmit commands. Sidescan sonars that work on autonomous underwater vehicles (AUV’s) don’t require the cable; instead commands are pre-programmed into the AUV’s onboard computer guidance system.  In the AUV case, a topside computer is still useful for downloading data and evaluating the performance of the sonar while at sea.

The sidescan sonar towfish typically includes the sonar transducers, electronics and their pressure housings and flotation. Over the decades as the technology for creating flotation and electronics has improved and become more compact, sidescan towfish have evolved from cumbersome systems (Figure 1) into lightweight vehicles that can be launched from small platforms with limited handling capabilities (Figure 2).

Moden Sidescan Sonar Instrument

Figure 1 left | Figure 2 right

Sidescan sonars work by projecting a narrow beam of sound to either side of a towfish and recording the strength and time of the returned echo. The strength of the echo depends on a number of parameters including the operating frequency of the sonar and the distance to the seafloor, the angle of incidence of the sound, the slope of the seafloor being ensonified (or insonified), and the type of substrate.

Schematic of sidescan sonar data collection and interpretationFigure 3 (left)- Schematic diagram of a sidescan sonar towed instrument ensonifying the seafloor (top) and the sidescan data record created (bottom). The intensity of sound reflected back from rocks, sediment and other features provides information on the distribution and characteristics of the seafloor morphology. Strong reflections (high backscatter) from boulders, gravel and positive topographic features facing the instrument are white and weak reflections (low backscatter) from finer sediments and shadows are in black.

To produce the fan-shaped beam of sound shown in yellow in Figure 3, sidescan arrays are long in the along-track direction and narrow in the across-track direction (see Figure 2).  In general, the longer the array, the narrower the along-track angular width of the sonar fan-shaped beam. Array length also varies as a function of operational frequency; lower frequency systems need to have longer arrays than higher frequency systems.  Here’s an example of a row of transducer elements from the University of Hawaii’s (UH’s) IMI-30 sonar:
IMI30 Transducers

Figure 4 – A row of six transducer elements from UH’s IMI-30 (30 kHz) sonar system.  For scale, the elements are 3.3cm long and 1.8cm wide.  The transducers  are mounted in a brass housing.

The sound waves projected by sidescan sonars and the echoes that the sonar receives are waves with a wavelength is directly related to the operational frequency of the system through the equation:

f * λ =  c

where:

f is the operating frequency (cycles/second),
λ is the wavelength (typically centimeters),
and c is the speed of sound in water (~1500 m/sec)

For some typical systems:

Operating frequency
Example Systems
Wavelength
12 kHz
Seabeam, EM-120, MR1
12.5 cm
30 kHz
EM-300, IMI-30
5.0 cm
100 kHz
Klein, EM-1002 (95 kHz)
1.5 cm

 Data

Each time a pulse of sound or “ping” is transmitted, a sidescan sonar measures the time it takes the sound wave to travel to the seafloor and be echoed back to the sonar as well as the amplitude (or strength) of the echo.  Figure 5 shows an end-on schematic of the transducers in Figure 4 with an incoming echo.  Schematics of the type in Figure 5, showing a uniform incoming waveform, are fairly common.   In reality the echo amplitude is highly variable; the parameter that doesn’t change is the wavelength of the sound wave:

Sideview of a transducer array

Figure 5(left) – View of the transducer array in Figure 4 from the end with an idealized incoming echo.

Waveforms

Figure 6 – – Variable amplitude waveform (black) superimposed on the idealized waveform (gray) being received at the transducer in Figure 5.  Sidescan sonars measure variations in wave amplitude as a function of time. 

An example of one ping of data from a sidescan sonar is shown in Figure 7.

One ping of MR1 Data

Figure 7 – One ping of data collected by a sidescan sonar system.  The strength of the echoed sound wave is plotted along the y-axis and time in seconds is plotted along the x-axis.  The blue waveform is a component of the actual values (voltages) that are measured by the transducers arrays.  The components are combined to produce the discrete magnitudes (red crosses).  These data were collected by a shallow-towed 12 kHz sonar system located almost 5000 meters above the seafloor.  Because it takes sound more than six seconds to travel from the sonar and back again at a speed of ~1500 m/sec, there is no amplitude recorded in the early part (left-hand side) of the figure.  After ~6.5 seconds, the first returned echo, called the bottom detect, is finally received by the system.  The strength of the measured echo generally decreases for an additional eight seconds until it is time to stop recording data and transmit another ping.

Terminology

The terms “sidescan,” “backscatter” and “reflectivity” are often used interchangeably, and incorrectly, when describing data collected by sidescan sonar. For the record, these are the correct definitions of these terms:

Sidescan is the correct term for the instrument that is used to collect the data.
Backscatter is the measure of the amplitude of a reflected acoustic signal bouncing off the seafloor (or riverbed, etc.).  Backscatter results from the small (centimeter scale) reflective surfaces on the seafloor.
Reflectivity refers to the broader scale reflective properties of the seabed (i.e., the side of a hill, the flat region directly under a sonar system).

Examples of reflection

The strength of any given “target” depends on both its reflective properties and the extent to which it contributes to the backscattered signal. It is assumed that the surface reflects incoming sound waves equally in all directions:

Figure 8(left) – Ideal diffuse reflection at a surface (left) versus diffuse reflection of a surface with a directional component (right).

The first echoed return is usually assumed to come from directly under the sonar vehicle, and this assumption is probably correct 80-90% of the time.  However, in regions having significant topographic relief (mid-ocean ridges, continental slopes, fracture zones) this can be inaccurate.  In Figure 7 the strength of the echo diminishes rapidly at first, which results from the decreasing strength of the reflectivity (the specular or mirror-like return) from underneath the vehicle).  This trend continues until the time reaches ~13 seconds, when there is a second peak in the data.  This corresponds to the first multiple return; that is, the point when sound waves making two roundtrips from the vehicle to the seafloor interferes with echoes completing one round trip from vehicle to seafloor but at a high angle to the vertical.
By juxtaposing successive sidescan pings, it is possible to make an image of seafloor textures in response to sound waves emanating from a particular sonar system (Figure 11). It is important to remember that the measured amplitude will vary as a function of wavelength; towed at the same altitude above the seafloor a 30 kHz sidescan sonar and a 12 kHz sidescan sonar will record different amplitudes. One of the most common mistakes made by novices to sidescan sonar data interpretation is to assume that the magnitude of the return echo has a geological meaning; that is, that magnitudes in one range correspond to lava flows and magnitudes in another range correspond to coral reefs. There has been significant effort to quantify the data collected by sidescan sonars and systematically characterize the seafloor surface, but to date these experiments have not been successful. Nevertheless, the resulting imagery allows scientists to investigate the processes that contribute to seafloor creation and modification.

MR1 Sidescan Sonar Data

Figure 11 -Swath of MR1 sidescan data showing round volcanic cones and tectonic lineations. The labels on the left side of the figure hard and easy parts of the sidescan data swath to interpret.


Data



Cruise List


Secure Data Archive

The perpetual integrity of data sets is a priority for the marine geoscience community. As expressed by the FUMAGES Solid Earth Working Group, “the rapidly growing acquisition rate of data and samples requires more effective and standardized data management and publication. We risk losing vast amounts of data in the files or hard drives of individual investigators. Data bases, sample archives and standardized data management are necessary complements to publication of research papers, and are likely to be of even longer-lasting value.”

HMRG has standardized the archival and dissemination of HAWAII MR1 data. All metadata associated with HMRG sonar surveys are currently accessible via the world wide web, as are cruise reports, data processing documentation, and chart products. Proprietary data sets are password protected, allowing researchers immediate access to all available information regarding their surveys, while maintaining the confidentiality of their data. Once data sets enter the public domain, password protection is turned off so that the data become universally available. We encourage you to visit our data archive at:

http://www.soest.hawaii.edu/HMRG/cms/mr1-archive/

Use the following instructions to view MR1 data and documentation for Dr Patricia Fryer’s 1997 Marianas survey:

1. From the left-hand panel, select “1997 Cruises”

2. From the 1997 survey menu, select “MW9719”

3. Use the menu options to view general cruise information, sidescan and bathymetry charts, and data processing documentation.

HMRG delivers all digital data to the involved PIs.  All MR1, IMI30, and IMI120 data are stored at SOEST on mirrored disk arrays and on tape, CD-ROM, and/or DVD.

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