IMI30

IMI30 HMRG developed the 30-kHz, deep-towed bathymetric sidescan, the IMI30, with funding from the National Science Foundation (NSF). This 30-kHz system fills an operational gap between broad coverage, but low-resolution 12-kHz systems and high resolution, but limited coverage 120-kHz systems.   IMI30 is designed to be fully portable for use on any class I or II UNOLS vessel, and on other suitable vessels of opportunity.  When deployed from UNOLS vessels, winch and wire are coordinated through the National Science Foundation’s winch and wire pool; on other deployments acquisition of the winch and wire are coordinated and negotiated with the customer.  For deployments at 6000 m, up to 10 km of 0.681″ electro-optical wire is required.  Twelve IMI30 cruises have been conducted since 2000 and these data are archived on RAID arrays at HMRG.


Specs


IMI30 : Specifications

Towing Depth To 6000 m; 200-800 m above seafloor
Range 50-3000 m
Towfish Dimensions (LxWXH) 2.5 x 1.3 x 1.3 m
Towfish Weight 730 kg in air (~1600 lb)
Depressor Weight 730  kg (~1600 lbs)
Towfish Operating Temperature 0-40 deg. C.
Wire Specification 0.681 electro-fiberoptic wire; 10,000 m

Electrical Specifications

Computers Linux
Lab Equipment: Voltage 115 VAC, +/- 10% with UPS
Lab Equipment: Electrical Frequency 47-63 Hz.
Lab Equipment: Current ~15 A normal load; 38 A full load
Operating Temperature & Humidity 0-40 deg. C; 5-80% humidity

Bathymetric Sidescan Sonar

Transducer Frequencies 24-30 kHz
# of Transducers 6 arrays
Transducer Source Level +216 dB re 1µPa @ 1 m
Horizontal/Vertical Beamwidths 1.5 x 60-90 degrees
Pulse Length Programmable:  6-96 cycles
Receiver Gain Not programmable

Standard and Optional Sensors

Heading PNI Corp. TCM2.5
Attitude PNI Corp. TCM2.5
Depth Applied Microsystems SV&P
CTD (optional) SeaBird 37SI
Magnetometer (optional) Honeywell HMR2300 Digital
Sub-bottom Profiler (optional) HMRG-designed subbottom profiler
Attitude and heading for Sub-bottom PNI Corp. TCM2
Sound velocity (optional) Optional


Overview


IMI30 is a portable side-scanning seafloor imaging system that simultaneously acquires digital bathymetry and sidescan sonar imagery (swath width up to 5000 m). The system’s sonar transducers are housed in a 2.5-m-long vehicle that is towed 200-800 m above the seafloor at ship speeds of 1-3 knots.  The tow vehicle is towed about 40 m behind a depressor weight to decouple the vehicle from ship motion.  Several depressors are available ranging from 700 to 1000 kg in weight.  Due to this multi-body towing configuration, the IMI30 tow vehicle is extremely stable.   As a result, IMI30 has successfully operated in rough sea conditions (up to sea state 5) that typically cause performance degradation in hull-mounted systems due to bubble masking and violent ship motion. In addition to the six sonar transducer arrays and sonar acquisition electronics, the IMI30 system can be configured to house a compass, a CTD, a sound velocity and depth sensor, an attitude sensor and a magnetometer.  A depressor weight can optionally house instrumentation including a subbottom profiler with sonar acquisition electronics, a compass, an attitude sensor, and a sound velocity and depth sensor.

Electronic Configuration

The IMI30 tow vehicle uses 6 separate transducer arrays on its port and starboard sides (1 transmit and 2 receive arrays on each side), which operate at  frequencies between 24 and 30 kHz to minimize crosstalk. Each receive array contains two rows of elements spaced nominally one-half wavelength apart.  Operating frequencies, pulse lengths and transmit power can be programmed while the system is in operation. The 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.  IMI30 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.  Other significant features include digitization at the receiver, multiple channels of quadrature-detected data, and a high-bandwidth optical fiber link to the surface.

Bathymetric Capabilities

IMI30 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 vehicle. Flexible data processing software allows power, ping rate and frequency to vary in order to maximize the swath width and data quality with changing seafloor depth and acoustic properties. IMI30 accounts for ray bending due to acoustic velocity in the water by applying an empirical transfer function based on data collected during a calibration test at the beginning of each survey.

Auxiliary Sensors

Additional sensors can be mounted on either the tow vehicle or the depressor weight, depending upon project needs.  A Honeywell HNR2300 Digital magnetometer is often deployed on the towfish body itself.  A subbottom profiler, designed and built by HMRG engineers, can be deployed on the depressor weight.  This subbottom profiler is designed to use similar electronics as the IMI30 towfish and is programmable for frequency and pulse lengths; a frequency of 4 kHz is the usual configuration for this sensor.  This subbottom profiler also uses a dedicated heading and attitude sensor that is also located on the depressor weight.

Launch and Recovery System

The IMI30 system is designed for portability and ease of use on different ships. For deployment, the towfish is lifted by a crane or A-frame and lowered into the water.  It is attached to a 40 m-long umbilical cable that in turn is attached to a 700 to 1000-kg depressor weight. The weight is required because the towfish is slightly positively buoyant (so that the towfish will return to the surface if the umbilical 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 is spooled from a winch mounted on the deck of the ship.  In addition to mechanically supporting the towfish, the tow cable carries power down to the towfish and data up to the ship.

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

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


2011/2012 Mariana Forearc and Trough

During cruise TN273 HMRG’s near-real-time display of IMI30 backscatter data superimposed on existing multibeam bathymetric data was used to make preliminary selections of potential dredge and core sites.

The IMI30 was used to map areas in the Southeast Marina Forearc Rifts and the southernmost Mariana Trough Spreading Center during cruise TN273 aboard the University of Washington’s research vessel Thomas G. Thompson, which left Guam on Dec. 12, 2011, and returned on Jan. 22, 2012.  This collaborative effort under co-Principal Investigators (PIs) Drs. Fernando Martinez, Katherine Kelley, and Robert Stern was funded by the National Science Foundation (NSF) for field work and subsequent lab study of the tectonic and magmatic evolution of an intraoceanic arc as characterized by the SE Marina Arc rifts (SEMFR) and southernmost Marina Trough spreading center.

Launch of the IMI30 towfish from the back deck of
the R/V Thompson.

IMI30 towfish and depressor weight on the back deck
of the R/V Thompson

The recent discovery of volcanism in the SEMFR suggests that mantle wedge asthenosphere has been drawn into the rifts.  The combined deep-tow sidescan sonar (IMI30) and dredging/wax-coring cruise was designed to determine if the volcanic products recovered by dredging and coring directly reflect variations in slab fluxes and mantle melting associated with varying P-T conditions and breakdown of hydrous minerals in the underlying slab. The regional bathymetric and geophysical data synthesis allows the PIs to kinematically model rift evolution.

Left: Standard HMRG bathymetric and sidescan plots from data collected during TN273.  Note that IMI30 sidescan swath widths are greater than the corresponding bathymetric swath widths.

 

 

2006 Hawaiian Islands Survey

In 2006 the IMI30 field program had a short cruise between the islands of Maui and Hawaii to test the engineering improvements implemented earlier that year. The IMI30 performed very well in 2000-2500 mwd, collecting high resolution bathymetry and sidescan on both sides of the sonar. The 3-D perspective belowt shows a comparison of IMI30 bathymetry and the bathymetric compilation dataset under development at SOEST.

IMI30 3D Bathymetry

Two-dimensional charts of the IMI30 sidescan and bathymetry collected during June 2006 are shown below.

IMI30 Sidescan

IMI30 Bathymetry

3-D perspective views of the same data are depicted in the below figures. In the 3-D image of bathymetry, contours indicate ten meter depth intervals; the sidescan data are overlain on the same terrain model produced by the bathymetry to allow direct comparison. The subbottom system was operating during this survey, but the area had virtually no sediment cover to image.

IMI30 Bathymetr 3D
Click On The Image Above To See A Larger View

IMI30 Sidescan 3D
Click On The Image Above To See A Larger View

2004 Lau Basin

IMI30 Survey - Fernando Martinez

During its first scientific program in 2004, the IMI30 collected sidescan and bathymetry data for the Lau Basin, but only on the port side of the instrument. Sidescan data agreed well with analogous data collected by the IMI120, and showed far more detail than the data produced by the Kilo Moana’s hull-mounted EM120 (below). Problems resulted mainly from the potted transducers that flooded.

Nested Surveys Example
Click On The Image For A Larger View

DSL120 and IMI30 on the deck
The IMI30 (left) and the IMI120 (right) were both used on the Lau Basin Survey.

2004 Beaufort Sea Survey

Healy Frozen In

In 2005, IMI30 was mobilized on the icebreaker Healy to map the surface and shallow subsurface of the Beaufort Margin, north of Alaska. Unfortunately, ice conditions were too severe to launch the IMI30 except for one fixed station and a four hour survey just north of Barrow, Alaska (the figure above shows the Healy frozen in ice, which lasted for four days of the survey). The first deployment on station (below) was sufficient to demonstrate that the new IMI30 subbottom system was working as expected.

IMI Arctic Deployment

The subbottom data below shows reflectors consistent with those imaged by the Knudsen system that is hull-mounted on Healy.

IMI30 Subbottom
Click On Image For A Larger Version


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