This paper reviews the nature of substrate vibration within aquatic environments where seismic interface waves may travel along the surface of the substrate, generating high levels of particle motion. There are, however, few data on the ambient levels of particle motion close to the seabed and within the substrates of lakes and rivers. Nor is there information on the levels and the characteristics of the particle motion generated by anthropogenic sources in and on the substrate, which may have major effects upon fishes and invertebrates, all of which primarily detect particle motion. We therefore consider how to monitor substrate vibration and describe the information gained from modeling it. Unlike most acoustic modeling, we treat the substrate as a solid. Furthermore, we use a model where the substrate stiffness increases with depth but makes use of a wave that propagates with little or no dispersion. This shows the presence of higher levels of particle motion than those predicted from the acoustic pressures, and we consider the possible effects of substrate vibration upon fishes and invertebrates. We suggest that research is needed to examine the actual nature of substrate vibration and its effects upon aquatic animals.
Seismic interface waves generated by seabed impacts are believed to have biological importance. Various wave types are of interest to seismologists, who can minimize the unwanted, but often dominant, ground roll waves with suitable instrumentation. Waves made by dredging and piling have been measured using geophones and found to be of this interface type, which propagate much more slowly than the pressure waves in the water column above. Short interface wavelets of a few cycles were modeled using transient finite element analysis (FEA). Wavelets with low losses have been modeled using graded sediment data from the literature. They do not radiate energy away from the interface because the evanescent acoustic pressures they generate decay rapidly with distance from the seabed. Associated water particle velocities are much greater than would be expected from similar acoustic pressure measurements in a free field. This motion is significant to aquatic life which is dependent on inertial sensors (otoliths, etc.) to respond to the environment. Additional amplification of the horizontal seabed motion of the adjacent water is predicted for a short seismic wavelet modeled in a graded solid seabed. Further recent analysis studied the distribution of the energy flux within the sediment layers.
Acoustic noise sources are usually characterised by source levels measured in specific directions. However for some applications the average over all directions is useful, and this can also be specified as the total acoustic output power output.Power measurements are closely related to the measurement of source levels. Whilst the source level concept is based on free field spherical spreading, some benefits of the power output rating are best seen by considering the extreme reverberant environment of a test tank. Test tanks have been shown by a variety of authors to be useful in this respect over the past 40 years.One example of the benefits is the possibly of providing a good estimate of a reverberant noise field by the summation of the noise powers of different source contributions. This technique is used in airborne acoustic problems such as the design of factory environments where it is desirable to keep the reverberant field levels below the safety limit such as 85 dBA required by statute to minimise the risks of damage to the workers hearing. There is a need to deliberately avoid the detailed directivity information in order to predict the long term overall spatial average, and the consideration of noise power and reverberation makes this simple.Also, where the average over all directions of source outputs or hydrophone sensitivities is required, reverberant test tanks can provide such data directly, in a way analogous to the use of reverberant air chambers. This technique will be discussed in detail using results taken from work over the past decade. Whereas the theory due to Sabine characterises the reverberant volume by a reverberation time the analysis described here uses an alternative technique better able to explore the limits of performance.This analysis can be applied to the prediction of the total noise power of complex machinery including ROVs and even ships. Whilst not appropriate for highly detailed work, such an approach can provide a very economic way to assist in the estimation of the effects of noise on the environment and also on the performance of some sonar systems.As has been discussed elsewhere the prediction of possible environmental impact by machinery required for a specific task (perhaps a pipe-laying operation) cannot use the detailed directional data on the possible systems to be employed. However, knowledge of the output acoustic power does allow the nature of the various sources to be compared easily, and can also be used to reassure those determining the risk, especially if it becomes clear that the risks are low.
Ground roll waves traveling across the seabed provide extra information, their direction of rotation, compared with plane waves in fluids or solids. Idealized Rayleigh waves are "retrograde" in that their horizontal particle motion opposes the direction of travel of the wave when the interface is raised. A single point measurement near the seabed can determine this rotation. In water, there are associated evanescent pressure waves that are largely confined to the bottom, likely to affect fish and other creatures near the seabed during pile driving. The directional information may prove key to the lifestyle of such creatures.
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