The DMON is a passive acoustic instrument that is capable of recording and processing audio in realtime. We have implemented a low-frequency detection and classification system (LFDCS; Baumgartner and Mussoline 2011) on the DMON to identify marine mammals calls from a variety of autonomous platforms, including gliders, profiling floats, and moorings (Baumgartner et al. 2013). The LFDCS produces pitch tracks of sounds with relatively high signal-to-noise ratios (> 10 dB above background). Pitch tracks describe changes in the fundamental frequency (pitch) of a call over time. Scientists typically examine spectrograms to identify marine mammal calls, such as the one below, and pitch tracks are derived from these spectrograms.
Play the audio depicted in the spectrogram above (sped up by a factor of 12 to make these low-frequency calls audible)
Spectrograms are a method to visualize sound, and they describe how the frequency (pitch) of sounds change over time. They also describe the loudness or amplitude of a sound (in the spectrogram above, calls that are louder appear brighter). Very simply, the LFDCS seeks to "draw a line" through each call in the spectrogram. These lines, or pitch tracks, can be used to describe the call. The pitch tracks depicted below were generated from the spectrogram above (the colors indicate the loudness of the call; cooler colors are quieter, warmer colors are louder).
Note that the calls shown in the spectrogram are easily recognizable by their corresponding pitch tracks. The LFDCS extracts simple statistics of each pitch track (e.g., duration, average frequency, rate of change of frequency with time) and uses quadratic discriminant function analysis to classify each call. Classification is based on a known set of calls contained in a call library that is defined prior to deployment of the DMON/LFDCS. Through communications with the glider, and ultimately with a shore-based computer via Iridium satellite transmission, the DMON/LFDCS remotely reports tallies of calls for each call type in its call library, as well as a subset of pitch tracks. Because of limitations in the amount of data that can be transmitted via satellite, the DMON/LFDCS relays a maximum of 8 KB of pitch track data every hour.
The DMON/LFDCS has been or is currently being implemented on several Autonomous platforms:
Slocum glider: Slocum gliders (Teledyne Webb Research) operate by moving up and down in the water column using a buoyancy pump. The pump only operates at the beginning and end of a dive; therefore the vehicle is silent during descent and ascent, making it ideal for passive acoustic monitoring. Short wings provide lift and passively generating forward momentum during descent and ascent. Slocum gliders are capable of moving horizontally at slow speeds (< 1 knot), and their endurance is limited to 3-5 weeks when powered using alkaline batteries. They are equipped with oceanographic sensors, a global positioning system (GPS) receiver, and an Iridium satellite modem that allows two-way communication with a shore-side computer. The vehicle is navigated simply by providing new waypoints to the vehicle during Iridium communication sessions; the vehicle is capable of self-navigating to these waypoints by adjusting its roll and yaw to change direction. The DMON hydrophone is mounted externally in faired acoustically transparent polyurethane, so flow noise is negligible.
Profiling float: APEX profiling floats (Teledyne Webb Research) also move up and down in the water column using a buoyancy pump, but they lack wings to provide lift and forward momentum. They therefore passively drift with the ocean currents and cannot be accurately navigated. To retard the movement of a profiling float, the vehicle can be "parked" at or near the bottom where the sub-tidal currents are typically quite slow, and the frequency with which they return to the surface to transmit data to a shore-side computer is kept quite low. Like the Slocum glider, profiling floats are equipped with oceanographic sensors, a GPS receiver, and an Iridium satellite modem, and the DMON hydrophone is mounted externally in acoustically transparent polyurethane.
Wave glider: Wave gliders (Liquid Robotics, Inc.) consist of a surface float (resembling a surfboard) and a tethered "sub" that use the energy of surface waves to provide propulsion and solar-charged batteries to provide power for the navigation system, instrumentation, and Iridium modem. Like the Slocum glider, the wave glider can be navigated by supplying new waypoints during Iridium communication sessions. However, its endurance is theoretically unlimited, making it ideal for persistent use in remote and harsh environments over time scales of months to years. The DMON hydrophone is suspended several meters below the "sub" in a hydrodynamic towfish, and the hydrophone cable features mid-length floatation to decouple the motion of the vehicle from that of the hydrophone.
Moored buoy: The WHOI Mooring Group has developed a successful moored buoy design for a variety of acoustic applications. The DMON instrument and hydrophone are suspended at depth on the mooring line between an anchor and a subsurface float. This float, in turn, is attached to a surface buoy via a "stretch hose" with helical electromagnetic conductors embedded in the walls of the hose. The hose can stretch to nearly twice its relaxed length and is thus capable of decoupling the motions of the surface buoy from the mooring line below the subsurface float. This ensures a quiet acoustic environment for passive acoustic monitoring. The conductors in the stretch hose and electromagnetic cable allow (1) power to be delivered to the DMON from the alkaline battery pack in the surface buoy well, and (2) detection data to be delivered from the DMON/LFDCS to a data logger in the surface buoy. The data logger continuously collects and stores these data and eventually transmits them to a shore-side computer via an Iridium satellite modem on a pre-determined schedule (e.g., once every hour).
Click on one of the studies above, and then click on one of the platforms to view time series or spatial summaries of reported marine mammal detections. You can also review a subset of detections for a specified time by viewing pitch tracks in the large table below the plots. This table is comprised of 15-minute tallies of detected and classified calls, and if pitch tracks were reported during the 15-minute tally period, these will be viewable by clicking on the "PT" link. Viewing the pitch tracks can help evaluate the context and patterning of calls. For more information, see
Baumgartner, M.F. and S.E. Mussoline. 2011. A generalized baleen whale call detection and classification system. Journal of the Acoustical Society of America 129:2889-2902. PDF
Baumgartner, M.F., D.M. Fratantoni, T.P. Hurst, M.W. Brown, T.V.N. Cole, S.M. Van Parijs, and M. Johnson. 2013. Real-time reporting of baleen whale passive acoustic detections from ocean gliders. Journal of the Acoustical Society of America 134:1814-1823. PDF