IMI30 - 30kHz Deep Towed Mapping System
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The Hawaii Mapping Research Group (HMRG) has been funded by the U.S. National Science Foundation to build and operate IMI30, a deep-towed 30kHz phase-difference bathymetric sidescan sonar (OCE-0097822 ). The system simultaneously acquires true sidescan sonar imagery, phase difference bathymetry, and multibeam subbottom data, with the ability to host other user-supplied instruments as well (such as towed magnetometers).
IMI30 is functionally similar to the HAWAII MR1 and IMI120 towed sonars, sharing fundamental electonics characteristics, data acquisition computers, and sonar processing software. These shared characteristics resultin lower costs for each system, and a larger pool of engineers, technicians and scientists familiar with the maintenance, operations, and data processing requirements of this family of sonars. IMI30 is designed to be fully portable for use on any class I or II UNOLS vessel, and on other suitable vessels of opportunity. Once operational, IMI30 is offered to all NSF researchers as part of the University of Hawai`i shared-use equipment pool.
- Specifications
- Overview
- Technical Description
- Image Gallery
IMI120 : Specifications
IMI30 A deep-towed, full-ocean depth (6000 m), 30-kHz multibeam phase-difference bathymetric sidescan sonar.
Swath width Swath width is limited only by signal-to-noise ratio, providing bathymetry swaths of 2-3 km and sidescan swaths of 4-5 km.
Speed Towing speeds up to 4 knots, limited by the ability of the system to maintain appropriate altitude above the seabed and towfish motion induced by towing speed. IMI30 uses a depressor weight, but drag on the cable determines how fast the system can be towed in different water depths . Typical towing speeds are 2 knots in 4000 m depths and 4 knots in 500 m depths.
Bathymetry IMI30 phase-difference bathymetry supports contour intervals of < 1% of towfish altitude.
Sidescan Cross-track pixel size < 0.75 m, 24-bit
Subbottom A sub-bottom profiler is available as a separate option and is deployed on the clump weight that is towed ~40-m forward of the IMI towfish; this arrangement minimizes weight and increases space for other sensors on the towfish.
Configurable transmit A variety of transmit settings are available and user-selectable, including swept frequency and coded pulse transmissions to increase signal-to-noise. levels . This level of control allows transmission characteristics to be optimized for the objectives of each survey.
Raw digital acoustic data Raw acoustic data are logged in real time, allowing quantitative studies of seafloor backscatter to be performed. Digital data are 24 bit, which provides high dynamic range and eliminates the need for time varying gain (TVG) data compression.
Staffing All shipboard operations are conducted by a crew of four people: a survey chief, a system engineer, and two watchstanders.
Portability Can be operated on all UNOLS class-1 and -2 vessels, some UNOLS class-3 vessels, and commercial ships of opportunity.
Rapid deploymentsThe IMI30 is transportable via air freight, along with the minimum required handling equipment for its deployment and operation. This capability allows rapid-response deployments.
Add-on sensorsThe subsurface electronics network of digital signal processors are designed to handle inputs from other sensors, allowing researchers to deploy mission-specific instruments either on or near the towfish and/or the depressor weight.
Towfish stability IMI30 is towed at a relatively high altitude (500-700 meters) compared to higher frequency, lower range sonars such as the IMI120. By towing at a higher altitude, the towfish can fly over seafloor topography without changing wire out, resulting in less towfish motion, which introduces changes in look angle from ping to ping that are relatively easy to account for when processing bathymetry but result in degradation of the sidescan sonar imagery. Towing at a higher altitude means a more stable towfish, which results in a cleaner sidescan image.
Nested surveys IMI30 hardware and software are compatible with the IMI120 and MR1 sonars, allowing nested surveys during the same research cruise using the same deck handling gear, computer acquisition systems, processing software and shipboard staff.
Reliability High reliability because engineering, operational and data processing skill sets are common across multiple sonars (IMI120, IMI30 and HAWAII MR1) -- engineers and scientists familiar with one system will be familiar with all.
Good for NSF Cost savings realized by NSF because HMRG already operates and maintains the HAWAII MR1 sonar. The IMI30 shares the MR1's deck and lab equipment, therefore there is little added cost to maintain the IMI30 and its associated handling and computing equipment. These cost savings translate to lower day rates charged to NSF.
Overview: IMI30 - Advantages
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The IMI30's 30-kHz operational characteristics (30-kHz frequncy, ~500-m tow height) provide sufficient resolution to identify small fault scarps, lava flows, or submarine channels as well as a much faster rate of coverage than higher frequency sonars . For example, the 120-kHz IMI120 has a maximum sidescan swath width of 1 km and is towed at ~1 knot; thus a 40 km x 100 km survey of a midocean ridge spreading segment requires at least 90 days of IMI120 survey time. Although the meter-scale resolution of IMI120 sidescan is ideal for detailed studies of outcrop-scale geological features such as eruptive vents and hydrothermal mounds on midocean ridges (Scheirer, 2000), it is an inefficient tool for covering large areas.. Figure 1 shows the amount of seafloor that can be surveyed per hour by the IMI120, the HAWAII MR1, and the IMI30. The IMI30 has a swath width of ***3*** km and a tow speed 2-2.5 knots and is thus able to survey at least ***5*** times the area of IMI120 over a given period.
Data Acquistion and Processing
Topside data acquisition and processing hardware will be the same for IMI30 as HAWAII MR1, and IMI120 (the forthcoming replacement for MR1), so engineering and data processing personnel will maintain familiarity across all platforms. IMI30 will use HMRG's suite of sonar acquisition and processing software, which is currently used for both the MR1 and IMI120 sonars. Because the HMRG software is adaptable to any phase-difference bathymetric sonar, ongoing efforts to improve HMRG software will immediately benefit the IMI30, the IMI120, and the HAWAII MR1.
Shipboard Operations
IMI30 will use existing HMRG deck hardware for launch, recovery, and towing operations. Deck hardware includes a towing winch, a Launch & Recovery System (LRS), a hydraulic power pack, and a 20-foot container that serves as a seagoing engineering lab. The system will be designed so it can be launched/recovered using A-frames or cranes in addition to the LRS. IMI30 will use fiber optic telemetry, and will be towable from either the armored deep-sea fiber optic cable available on some UNOLS vessels, or from the portable UH fiber-optic cable and winch.
Data processing will be conducted using HMRG's network of Sun Solaris and PC linux workstations. HMRG provides a crew of four people to conduct all sonar operations: a survey chief, a system engineer, and two sonar operators. These four conduct all sonar operations, including all deck operations and system maintenance, real-time operation of the sonar, and shipboard data processing. No other personnel are required.
The following section lists personnel required to deploy and operate the IMI30 towed system. Scientific party watchstanders will be identified from the shipboard party to assist in vehicle control, data logging, and watch command.
The Hawaii Mapping Research Group (HMRG) supplies seven individuals for the navigation and flying stations, including a data processor to assist with reduction of the data. In cases where the chosen vessel does not have a winch capable of supporting either a fiber optic or coaxial cable, one will be supplied with an operator.
Position |
Science |
HMRG |
Navigator |
0 |
3* |
Flyers |
0 |
3 |
Watch Leader |
3 |
0 |
HMRG Party Chief |
0 |
1 |
Data Loggers |
3 |
0 |
Non-watch Data Processor |
0 |
2 |
Total |
8 |
6* |
Operation requires a minunum of four people per watch section to maintain round-the clock operation. Watch sections are generally four hours on, eight hours off. A watch section is divided as follows:
Watch Leader
Carries out mission objectives during the watch and reports directly to the Chief Scientist. Coordinates activities from one watch to another. Responsible for navigational plan and analyzing real-time data.
Flyer
“Flies” the IMI120A at a predetermined altitude (usually 100 meters) to provide optimum acoustic coverage of the seafloor.
Navigator
Operates surface and subsea navigation. Interacts with the ship’s crew to adjust heading and/or speed. A navigator is only required if LBL navigation is collected.
Sonar Data Logger
Monitors and is responsible for logging IMI120 sonar data.
Non-Watch Data Processor
Primarily concerned with the production of processed data products off-line.
HMRG Party Chief
Final processing and data integration of sonar. Works with the science party to assess the quality of the collected data.
Technical Description
How a 30kHz system fills a resolution gap in the NSF supported mapping systems pool.
Seafloor mapping (bathymetry and sidescan sonar) is a recognized necessity for marine geophysical research. NSF supports a variety of multibeam and sidescan sonars for ocean mapping, which are available on a shared-use basis. However, until the development of the IMI30 system, a gap existed between higher-resolution/slow-survey-rate towed sonars (IMI120) and lower-resolution/fast-survey-rate surface-towed (HAWAII MR1) or hull-mounted multibeam sonars. The IMI30 intermediate-frequency deep-towed sonar is a critical tool for mapping missions that require the identification of features having length scales of tens of meters (small faults, lava flow boundaries, seafloor channels) over survey areas of several thousand square kilometers. IMI30 fulfills this role and is operated as a shared-use NSF facility. The IMI30 system includes a 30-kHz bathymetric sidescan sonar, a subbottom profiler, motion sensors, a three-axis magnetometer, CTD, sound velicometer, and supports other sensors that can be mounted on or towed behind the sonar towfish and/or the depressor weight.
Rapid Delivery of Final Charts and Processed Data
HMRG has developed data processing techniques to allow fully-processed HAWAII MR1 data sets to be delivered at sea. For every survey since 1997, all data were fully processed into bathymetry and sidescan charts in near-real time, and delivered as final products while still at sea. In addition to satisfying PIs needs for rapid data throughput, this approach has saved money because shore-based data processing (which always takes longer to perform than the 24/7 processing that occurs at sea) has rarely been needed.
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 delivers all digital data from each survey to the involved PIs. However, HMRG also maintains an on-site data archive to insure the secure, redundant storage of all raw and processed data collected by HMRG sonars. Currently supported media include 8mm data cartridges, digital linear tape, and CD-ROM, as well as on-line data repositories. HMRG migrates digital data to faster, higher density storage media as new technologies become available, and future plans include porting all HMRG data to DVD.
Archive Status??? Are we still providing these services?
HMRG has standardized the archival and dissemination of HAWAII MR1 data, and will follow the same model for data from the IMI30. 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.
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/MR1/Archives/MR1_ArchiveFrame.htm
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.
Image Gallery:IMI30 Configuration
Click image to enlarge.
Click image to enlarge.