The measurements of airborne and underwater noise radiated by a Griffon BHT130 hovercraft were conducted in the Ural-Caspian Channel and in the North Caspian Sea. This type of hovercraft is being used for all-season cargo and crew transportation to oil and gas platforms within the environmentally sensitive area of the Ural River estuary known for its abundant bird and fish fauna. Several field campaigns were organized from 2017 to 2022 to measure and analyze acoustic noise levels simultaneously in the air and underwater at various sites and hovercraft speeds. Airborne noise levels were estimated according to ISO 2922:2020, 2021. Underwater noise study included not only acoustic pressure recordings but also particle velocity measurements with a self-designed pressure gradient sensor (PGS), which is important since the hearing of the majority of fish perceives the sound in terms of particle motion. This study is the first to report the particle velocity levels formed underwater during hovercraft passages. The minimum levels of underwater noise, 100 dB re 1 µPa (pressure), 45 dB re 1 nm/s (particle velocity), and airborne noise, 93 dBA re 20 µPa (pressure), normalized to a distance of 25 m were observed for the hovercraft passages at a cruising speed of 7–15 m/s. Thus, this speed interval can be recommended as an optimum to minimize an acoustic impact on ornitho- and fish fauna. The directivity of the hovercraft noise was estimated for the first time and utilized for noise mapping of the Ural-Caspian Channel. The possible hydrodynamic effect of a passing hovercraft is discussed.

The experimental results of measurement of the scalar and vector characteristics of an acoustic field in shallow water with a carbon bottom are analyzed. Simultaneous recording of the sound pressure...

Cross-correlation functions of ambient and shipping noise were measured by near-bottom hydrophones in 100 meter-deep water in the Straits of Florida. This paper is based on cross-correlations in the 20-100 Hz frequency band between two hydrophones 5 km apart. The propagation conditions between the hydrophones were nearly range-independent. The noise cross-correlation functions provides an approximation to the deterministic Green's functions, which describes sound propagation in opposite directions between the hydrophones. Using a background sound-speed profile, 44 eigenrays were found and relevant perturbation matrices were calculated for propagation with and against the flow. Modelled Green's function has 10 peaks, which have been matched to the peaks of the cross-correlation function.Sound speed and flow velocity profiles in water have been retrieved by minimizing the mismatch between positions of the modelled and measured peaks. Acoustic frequencies being low, errors in ray-theoretical modeling limit the accuracy of the sound speed profile reconstruction. However, due to the reciprocity principle, the current velocity profile inversion remains accurate and agrees with independent measurements. The feasibility of retrieving geoacoustic parameters of the seafloor in addition to the water column parameters will be discussed.

Using the data obtained in the 2012 noise-interferometry experiment in the Straits of Florida, this paper investigates the feasibility of retrieving dispersion curves of acoustic normal modes from two-point noise cross-correlation functions. Simultaneous, continuous records of ambient and shipping noise by two near-bottom hydrophones separated by 5 km in 100 meter-deep water have been cross-correlated to obtain an approximation to the deterministic acoustic Green's function.Dispersion curves of the four lowest-order normal modes have been retrieved from spectrograms of the noise cross-correlation function. The techniquedeveloped in the paper allows one to measuredispersion of individual normal modes to a good accuracy without employing any controlled sound sources or hydrophone arrays. Normal modedispersion curves provide ample information for solving inverse problems. Our results suggest the possibility of monitoring complex dynamics of the coastal ocean with acoustic daylight by using simple and inexpensive passive acoustic systems.

In an ecologically sensitive region—the Ural River Delta Nature Reserve near the Caspian Sea shelf—simultaneous measurements of underwater and airborne noise from a Griffon BHT130 hovercraft were conducted. The measurements were designed to assess the potential negative impact of hovercraft noise on the fish and birds of the reserve. In addition to determining sound pressure levels, a gradient-type vector receiver, formed by four spatially spaced hydrophones, was used to estimate the absolute particle velocity levels of underwater noise. Such measurements are an important part of underwater noise research, as a significant proportion of fish perceive sound as particle vibrations. Simultaneous recording of underwater and airborne noise allowed us to evaluate their mutual correlation. The resulting correlation between the underwater and airborne noise levels from the hovercraft can be used to simplify the estimation of the upper limit of underwater noise based on the measured airborne noise levels. The measured and estimated maximum underwater noise levels of the hovercraft were significantly lower than those of conventional submersible motor-driven vessels, making the hovercraft an attractive alternative for operation in areas with particularly high requirements for permissible levels of anthropogenic noise impact in the aquatic environment.