The underwater sound field from vibratory pile driving

Abstract

Underwater noise from vibratory pile driving was observed using a vertical line array placed at range 16 m from the pile source (water depth 7.5 m), and using single hydrophones at range 417 m on one transect, and range 207 and 436 m on another transect running approximately parallel to a sloping shoreline. The dominant spectral features of the underwater noise are related to the frequency of the vibratory pile driving hammer (typically 15-35 Hz), producing spectral lines at intervals of this frequency. The mean-square pressure versus depth is subsequently studied in third-octave bands. Depth and frequency variations of this quantity observed at the vertical line array are well modeled by a field consisting of an incoherent sum of sources distributed over the water column. Adiabatic mode theory is used to propagate this field to greater ranges and model the observations made along the two depth-varying transects. The effect of shear in the seabed, although small, is also included. Bathymetric refraction on the transect parallel to the shoreline reduced mean-square pressure levels at the 436-m measurement site.

Structure-Borne Wave Radiation by Impact and Vibratory Piling in Offshore Installations: From Sound Prediction to Auditory Damage

“…On the contrary, the secondary noise path, which consists of the Scholte waves, seems to govern the noise field in the seawater at low frequencies in the vicinity of the seabed surface. This is again in line with the measurements presented in [29], albeit for a different installation setup and pile dimensions, in which it was observed that the Scholte mode contributes to more than 30 dB in the 63-Hz octave band. • The pressures in the water column are significantly lower when compared to the ones generated by the impact hammer.…”

“…This is in line with the observations and the model developed by Dahl et al [29] in which the vibratory pile source is modeled as an incoherent line source (and not as a coherent one with a predefined time delay, as was the case when impact piling was examined). As mentioned in [29], any sense of line-source spatial coherency is lost or at the very least rendered vastly more complex with effects not observable in the ensuing pressure field. In other words, the Mach wave radiation from a coherent source traveling down the pile surface is not seen in the case of vibratory piling.…”

“…In this section, the noise field is investigated for the case of a foundation pile installed with a vibratory hammer. Although the studies on this subject are limited [28,29], the current trend of the wind industry towards the use of vibratory hammers of larger dimensions may lead to increased environmental concern regarding the generated underwater noise. The pile and soil properties are already given in Section 3.…”

“…At this low frequency range, only a few modes propagate in the seawater, i.e., the bulk of the energy irradiates into the soil domain. The contribution of the few propagating modes into the fluid region and its experimental identification are discussed extensively in [29].…”

“…Noise radiation from vibratory piling has attracted much less attention due to the fact that the impact on the marine ecosystem is considered to be limited. Nevertheless, a few studies do exist in which acoustic radiation by vibratory piling is discussed [28,29]. Given the current trend in the offshore wind industry towards piles of larger dimensions, the use of vibratory hammers of larger sizes is inevitable.…”