IMI30: CHIRP Subbottom Profiler
Subbottom data are an important component of many seafloor studies. The long fore/aft aperture of the IMI-30 towfish presents an exciting possibility: broadband hydrophones can be used to receive both 30 kHz sidescan signals as well as low-frequency signals appropriate for subbottom profiling. By including a 1-meter wide athwartships array of low frequency (2 to 6 kHz) transmitters, the IMI-30 can simultaneously serve as a multibeam subbottom profiler as well as a bathymetric sidescan sonar.
Figure 1 illustrates how the system will transmit a low-frequency subbottom beam that is wide in the fore/aft direction and narrow (~20°) across track. The system would form a receive beam that is narrow (~10°) in the fore/aft direction, resulting in a 10°x20° footprint. Although wide by multibeam bathymetry standards, these beam widths are narrow compared to other subbottom profilers. The ability to tow this system deeply will result in a small physical footprint. The system electronics will be able to transmit arbitrary waveforms, so swept-frequency ("chirp") pulses (or any other kind of coded pulse) can be transmitted. Chirp sonars are useful because the low frequency energy results in deep penetration, while the high frequency energy can be used to resolve more detail in the shallow subbottom. Similar shallow water subbottom profilers have recently yielded detailed profiles with penetration to 45 meters (e.g. Driscoll, 1999).
Figure 1. Multibeam Subbottom Profiler The subbottom profiler will transmit chirp or coded pulses using frequencies from ~2 to ~6 kHz. The signal will be transmitted from an 1 meter wide array of transducers mounted athwartships on the towbody, resulting in a transmit beam that is wide in the fore-aft direction and narrow (~20°) across track (A). Echoes will be received on a 2-meter-long array of hydrophones mounted along the sides of the towfish, so that an even narrower (~10°) receive beam can be formed (B). The intersection of the transmit and receive beams forms the 10° x 20° acoustic footprint of the subbottom profiler (C). When combined, data from each transmission create a subbottom profile that has a small footprint relative to profilers that do not form narrow receive beams (D). The proposed sonar design uses the sidescan hydrophones to receive the subbottom signal, taking advantage of the long aperture to form the narrow receive beam. This design is made possible by new broadband hydrophones, and by using digital signal processors (DSPs) in the towfish to differentiate and process the different frequencies of the sidescan and subbottom systems.