Surface buoys are frequently chosen for data collection programs that require multiple measurement parameters or real-time current profiling. Surface buoy-mounted ADCPs can be deployed in two primary configurations: self-contained mode or as an integrated component of the buoy system. The integrated setup is generally preferred, as it allows for power supply from the buoy, facilitates real-time data transmission, and provides remote access through satellite or other communication networks. For details on instrument integration, please see the Integrator's Manual specific to your sensor.
Tilt is a critical factor that must be considered when working with surface buoys. If the instrument is likely to experience significant inclination, bin mapping adjustments are necessary to correct for potential distortions in the data. Additionally, excessive tilt can interfere with the ADCPs compass readings. To address this, an attitude and heading reference system (AHRS) can be integrated with instruments from the Signature series, either within the ADCP itself or as a separate component on the buoy. Next to a magnetometer and an accelerometer, the AHRS is also equipped with a gyroscope. The combination of all three sensors allows for a compensation of motion on a moving system which is particularly valuable to increase single ping precision. For further details on the AHRS, please see this FAQ.
Hazards and challenges
Before deploying an ADCP on a surface buoy, several factors should be carefully evaluated. Since the buoy is floating on the sea surface it is endangered by vessel traffic and fishing operations, which could strike the buoy and thereby disrupt or damage the instrument. Additionally, surface buoys are susceptible to unintended interactions, such as vessels tying up to them or acts of vandalism. These incidents can impact the buoy’s stability, compromise data quality, and even result in loss of power or communication capabilities.
Another persistent challenge in buoy-mounted deployments is the presence of air bubbles near the ADCP transducer. These bubbles originate from wave action, surface turbulence, or trapped air pockets released by the buoy itself. In high sea states, bubble formation intensifies, and the instrument’s profiling range can be substantially reduced. Compounding this issue, waves sloshing against the buoy or support frame can generate broadband structural noise, further degrading the signal-to-noise ratio (SNR).
To address this, several strategies are used:
- Suspending the ADCP below the buoy on a rigid pole, frame, or cage structure to place it beneath the bubble-rich surface layer
- Designing the buoy to minimize turbulence, using hull shapes or baffling to reduce bubble generation
Despite these measures, some performance degradation during rough conditions is inevitable and instruments deployed at the sea surface will always have a reduced range compared to a upward looking orientation!
Integrated deployment
Deploying an acoustic instrument as an integrated component of a buoy system requires careful planning and rigorous assessment of potential electromagnetic interference (EMI). Interference from electrical noise can significantly degrade both recorded and transmitted data, particularly in environments where multiple electronic systems are operating concurrently. It can manifest in the data as an overall elevated noise floor or periodically raised return signal strength. We recommend testing the assembled system before deployment to investigate potential interference sources.
Electrical noise may originate from:
- the buoy’s on-board power supply
- DC/DC converters
- other operational sensors or electronic equipment installed on the buoy
These sources can introduce noise that overlaps with the acoustic instrument’s operational frequency range, especially when power supplies use switching regulators or are poorly filtered. Different instrument models operate at different frequencies, and therefore be susceptible to different types of noise. For instance, a buoy system optimized for a Signature 55 may experience different interference characteristics when swapped with a Signature 1000.
To minimize interference and ensure high-quality data collection:
- Power Supply Compatibility: Always verify that any external power supply (other than the one delivered with the instrument) has a switching frequency that lies outside the acoustic sensor’s operational bandwidth.
- Measurement Timing: If multiple acoustic instruments or sensors are deployed on the same buoy, their acoustic signals can interfere with one another. Staggering their measurement intervals—so they do not ping simultaneously—can reduce cross-talk and improve overall data quality.(See also: Acoustic interference between multiple instruments)
- Shielding and Grounding: Proper cable shielding and grounding practices can help reduce EMI transmission paths and protect signal integrity. Ensure that the grounding plate is in contact with seawater.
Note: When working with Gen2 instruments or the Signature series, a spectrum analyzer test can be carried out, to identify individual noise sources and systematically remove them.
Electrical noise sources will impact instrument operating at different frequencies to a variable extent. Therefore a renewed noise test will be necessary, if switching the instrument.
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