IMI30: Characteristics
IMI-30 A deep-towed, full-ocean depth (6000 m), 30 kHz multibeam phase-difference bathymetric sidescan sonar plus multibeam subbottom profiler.
Swath width Swath width will be limited only by signal-to-noise, allowing bathymetry and sidescan swaths of at least 6 km wide, perhaps greater.
Speed Towing speeds up to 8 knots, limited by the ability of the system to maintain appropriate altitude above the seabed. IMI-30 will use a depressor weight, but drag on the cable will determine how fast the system can be towed, and we anticipate that towing speed will vary with water depth (for example, 2 knots in 4000 meter water, 8 knots in 500 meters water).
Bathymetry IMI-30 phase-difference bathymetry that will support contour intervals of < 1% of towfish altitude (perhaps better).
Sidescan Cross-track pixel size < 0.75 m, 24-bit
Subbottom Integrated chirp (2 to 6 kHz) multibeam subbottom profiler
Configurable transmit A variety of transmit settings will be available and user-selectable, including swept frequency and coded pulse transmissions to increase signal-to-noise. This level of control allows transmission characteristics to be optimized for the objectives of each survey.
Raw digital acoustic data Raw acoustic data will be logged in real time, allowing quantitative studies of seafloor backscatter to be performed. Digital data will be 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 deployments The IMI-30 will be transportable via air freight, along with the minimum required handling equipment for its deployment and operation. This capability will allow rapid-response deployments.
Add-on sensors The subsurface electronics network of digital signal processors will be designed to handle inputs from other sensors, allowing researchers to deploy mission-specific instruments either on or near the towfish (for instance, a magnetometer can be towed behind the fish).
Towfish stability IMI-30 will be towed at a relatively high altitude (500-700 meters) compared to DSL-120 (50-100 meters). By towing at a higher altitude, the towfish can fly over seafloor topography without changing wire out, resulting in less towfish motion. Towfish motion introduces changes in look angle from ping to ping. These changes are relatively easy to account for when processing bathymetry, but the changes in imaging geometry 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 IMI-30 hardware and software will be compatible with the DSL-120 and MR-1 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 (DSL-120, IMI-30 and HAWAII-MR1) -- engineers and scientists familiar with one system will be familiar with all.
Good for NSF Cost savings will be realized by NSF because HMRG already operates and maintains the HAWAII MR-1 sonar. The IMI-30 will share the MR-1's deck and lab equipment, therefore there will be little added cost to maintain the IMI-30 and its associated handling and computing equipment. These cost savings translate to lower day rates charged to NSF.