Page 40 - Summer2022

WEIRD DATA IN UNDERWATER ACOUSTICS on the right track and that the faulty bearing(s) ulti- mately played an important role in the degraded data quality on a commercially important acoustics survey” (personal email, 2022, used with permission). As an example of co-interference of active acoustic sys- tems, an acoustic modem, acoustic Doppler current profiler (ADCP), echosounder, and passive acoustic array may all be used at the same time on a single platform. In Figure 3, bottom left, interference from a 20-kHz WHOI micromodem shows up in a 333-kHz echosounder; the ADCP signal is also seen in the echosounder data as single-time specks with varying range. In Figure 3, bottom right, interference from a 20-kHz WHOI micro- model also affects bearing estimates in a target-tracking problem, showing up as strong cross-bearing features in beamformed passive acoustic data. Electrical Noise One of the biggest challenges to designing and build- ing any effective acoustics system is electrical noise. In underwater acoustics, electrical noise problems are exac- erbated by the restricted space in the required waterproof enclosures so that noisy components are generally physi- cally nearby noise-sensitive elements. Furthermore, the limited pins available on underwater connectors often result in combined ground lines and limited sensor isola- tion. Those same underwater connectors are notorious for degrading signal quality. As a result, underwater electrical systems need to be designed extremely carefully to ensure isolation between noisy and sensitive components. One common issue is the sensitivity of underwater acoustic systems to the resonances of platform power sources and loading on a power system; a system with low noise plugged into a ship laboratory when the ship is running on battery can experience a sudden high-noise floor with severe narrow- band frequency interference when the ship’s generator kicks in. Another challenge arises from other sensors or systems in use on the same platform. It is extremely common to locate multiple acoustic systems on the same platform with the same power system. Active acoustic systems can interfere with recordings on a different acoustic sensor acoustically through sound put into the water, through the power system, through electromag- netic interference, or through signal ground lines. Other systems, such as actuators, radio communication systems, and spinning drives, can also create a combination of acoustic, vibration, power, and signal noise that interferes with most acoustic sensors. Any active voltage convert- ers can also create interference that impacts acoustic data recordings (Figure 3, top) where a significant part of the spectrogram is obscured by power system noise). These issues should be taken into consideration when designing, building, and deploying systems and when analyzing data. Unexpected Reflections, Scattering, and Clutter Sound is used to explore the ocean because it provides so much information, but sometimes that informa- tion is difficult to understand. Unexpected reflections and scattered sound are another source of surprises in underwater acoustic data. The most frequent issues caused by unexpected reflections are signal saturation, false positive detection, or confounding between the “signal” targets you want in your experiment and other targets (“clutter”). For example, in an experiment to count fish, an echosounder might show returns from fish but also returns from copepods, or changes in water density, shear instabilities, bubbles, and kelp (Stanton et al., 2021). Man-made objects and structures also can cause unex- pected reflections and scattering, even in the most controlled of experiments, as related by Aubrey Espana of the APL-UW when her team was running a seabed target-scattering experiment and found that a surprise ship reflection limited their working area: “During BAYEX14, a side lobe from our source reflected from the bottom of the boat. The timing of the path was such that it overlapped the 15m target line. So essentially, we couldn’t put any objects out at that ground range. The boat was in a 4-pt moor, so moving it was not an option” (personal email, 2022, used with permission). Reflections off boats, vessels, and underwater structures can have significant effects on sensing, navigation, and communication systems by presenting echoes that can confound the desired signal. Because acoustic systems are often mounted on some kind of platform, the plat- form itself can be a pernicious and difficult to identify source of reflection, as Nicholas Rypkema of WHOI observed firsthand: 40 Acoustics Today • Summer 2022