With the global awareness of the need to make energy cleaner, marine construction, typically wind farms, has multiplied in recent years. The presence of marine mammals during offshore infrastructure work (pile driving, drilling, dredging) is now a major environmental concern because it has been shown that they could be harmed by exceeding noises.
To safeguard species and their natural habitats, more and more local legislation imposes a cap on sound levels from offshore activities. Wind farms developers, therefore, are required to measure, monitor, and mitigate noise caused by building work, and underwater noise monitoring regulations are enforced as a means of protecting aquatic life.
Underwater acoustics firm RTSYS has developed the processing and communication capabilities of its recorders and buoys for this type of monitoring. If autonomous recorders are perfectly suitable for the background-noise surveys before the installation phases, surveillance during the actual construction requires real-time information in order to adapt as fast as possible. For that reason, floating systems, allowing transmission of data through telemetry, are preferred.
A new buoy from the company uses a motherboard with synchronous data acquisition, one output, and four inputs (SDA14).
The buoy has been designed for the acquisition of acoustic signals from passive or preamplified hydrophones. It integrates four synchronized analog receivers capable of recording four different sound sources simultaneously. Its wide-band analog input has a frequency of 1.25 MHz with a dynamic range greater than 100 dB, which guarantees an efficient signal-to-noise ratio. The system is designed to operate in standalone mode and can be remotely operated and monitored using Wi-Fi, Iridium, or the Global System for Mobile Communications. The data is recorded and stored in an uncompressed .wav format.
The analog inputs are preamplified with a fixed gain and can be either single ended or differential. The inputs can process wide-dynamic-range signals from the hydrophone sensor. The cutoff frequency of the high-pass filter and the amplifier gain can be customized for each channel.
In addition to the four acoustics channels, the buoy includes a serial port input, which can be used to connect any kind of single- or multiparameter probe to record various oceanographic and water quality parameters (e.g., temperature, turbidity, salinity, conductivity, dissolved oxygen, pH), which can be very practical in some occasion. As an example, turbidity is often monitored to assess effects on seabed sediments.
The SDA14 motherboard is also capable of embedded signal processing and can calculate various sound levels such as zero to peak sound pressure level, weighted and unweighted sound exposure level, weighted and unweighted sound pressure level, and third octave band pressure levels from 20 Hz to 25 kHz. All of these calculations follow the applicable standards, including the ANSI S1 11-2004 standard.
Additionally, the raw data can be streamed from the motherboard to a coprocessing board, which can embed different kinds of processing algorithms, typically vessel or mammal detection.