Best Practice Guide for Underwater Particle Motion Measurement for Biological Applications.

Abstract

The problem: All sound comprises fluctuations in pressure and particle motion (PM) and all fishes and many aquatic invertebrates detect PM. Noise is unwanted or harmful sound, and underwater anthropogenic noise is a global pollutant. Therefore, a large proportion of marine life is potentially threatened by PM created by anthropogenic activity. There is building evidence that anthropogenic noise is detrimental to the health and survival of fishes and aquatic invertebrates, but the importance of PM to these effects remains unclear because until recently very few PM measurements have been taken, with studies mainly relying on sound pressure measurements to estimate PM exposure levels. In theory, PM cannot be predicted effectively from sound pressure in certain physical conditions. These physical conditions tend to be near the surface and the bottom, or in shallow water such as near shore, in lakes and rivers etc., where most aquatic life is found. Thus, there is a need to measure PM to establish the levels at which aquatic life can detect sound, levels at which adverse effects from anthropogenic activity occur, and to establish the boundaries for the physical conditions where sound pressure can or cannot be used to predict PM. PM sensors are becoming commercially available, while some scientists are also making their own, leading to an increase in the number of scientists taking PM measurements around the world. There is a need for guidance that helps scientists to understand what PM is and when it needs to be measured, how to select and calibrate instruments for such measurements, how to properly take measurements and then how to process and report the data for consistency and comparability between studies. ----- Our solution: Our solution is this ‘Best Practice Guide’, with the following providing an outline of our key findings on best practice for PM measurement for biological applications. A frequently posed question is: ‘Do I need to measure PM?’ In order to fully answer this, the expertise of a biologist is required to determine whether PM is biologically relevant, and the expertise of an acoustician is required to determine whether PM could be calculated from pressure measurements. This best practice guide introduces the biological applications of PM measurements in Chapter 1, the scope of the guide in Chapter 2 and some of the basic physical principles of PM in relation to sound pressure in Chapter 3. It is the recommendation of the authors of this guide that PM is of biological relevance if it can be heard (e.g., by the accelerometer-like ears of fishes), if it risks causing injury, or if sound source direction is of interest to a biological study. It is also our recommendation that the magnitude (though not necessarily the direction) of PM can be calculated in situations where a plane wave or spherical spreading from a monopole source are reasonable approximations, but in other situations PM should be measured. Factors that could affect plane wave conditions depend on source type, frequency and distance from reflective boundaries. Essentially, in shallow water conditions in open water and in tanks, both sound pressure and PM need to be measured to describe the complete sound field. The physics justifying ‘how shallow is shallow?’ is explained in Chapter 4 of this guide. The maths describing these relationships has been programmed into a simple calculator where the user can enter physical properties of the acoustic environment and the sound of interest and receive a recommendation about whether PM should be measured or not. This calculator is attached to this guide as supporting material. Reporting only the pressure component risks underestimating impacts due to PM, particularly close to the source where PM is higher. However, we always recommend reporting sound pressure alongside PM to contextualise PM measurements. Measuring sound pressure gradients can also be an effective way to measure PM. In Chapter 5, basic guidance is provided on how to measure sound pressure, including the specifications of system components that can be used and how to deploy them. Measurement systems consist of hydrophone(s), conditioning preamplifier(s), analogue-to-digital converter(s) and a computer. The measuring system may consist of individual components or as an integrated system such as an autonomous recorder. Making effective measurements requires selecting hydrophones with a sensitivity that is well matched to the signals of interest and that the entire system is calibrated over the frequency range of interest. Sound propagation in and on the seabed can influence sound propagation in the water column. Therefore, a brief overview of behaviour of sound in the seabed is provided in Chapter 6. Interface waves and shear waves can affect the soundscape and these can be measured using geophones coupled to the seabed. Once the need for measuring PM is established, selecting equipment with which to make measurements is critical. We cover instrumentation in Chapter 7. PM sensors are most commonly based on accelerometers but can also be based on geophones or hydrophone arrays. The entire PM sensor must be calibrated for measurements to be meaningful, and this is covered in Chapter 8. Water salinity, PM sensor waterproofing and suspension methods all influence an instrument’s calibration. For best practice, report the calibration in SI units and include the phase response and percentage uncertainty. Instructions for how to deploy PM sensors to make measurements in open water and in tanks are given in Chapter 9, with checklists to guide the reader. Due to the diversity of instruments and possible suspension types, this guidance is relatively general. PM recordings are acoustic data that are often stored as ‘wav’ files, with the main difference from pressure recordings being that PM data involve multiple channels; one for each axis of motion. Therefore, once calibrated, much of the data processing steps are the same for PM data as for sound pressure data. Chapter 10 includes a schematic representation of common analysis pathways and signposts the reader to detailed guidelines on signal processing that are already published. Intensity and integrating data from the three axes to calculate the PM magnitude are covered as well. Reporting PM is a key area to standardise at this early stage of the field, to ensure comparability between studies, which is the topic of Chapter 11. It is considered essential to report acceleration, with the option to report other quantities such as velocity or displacement if they are relevant to the study. We recommend reporting both linear and logarithmic quantities (i.e., linear and levels) and comparisons between PM and pressure should be like-for-like in terms linear or logarithmic units. For reporting levels in decibels, we provide a table of recommended reference values and recommend reporting the reference value with the quantity rather than the unit to avoid confusion. Finally, in Chapter 12 the key requirements and recommendations of this guide are summarised for making successful measurements. The science of this field is at an early stage and for this reason it is necessary for someone in the measurement team to understand the technicalities well, thus the summary cannot be taken as a short cut. Why now? ----- There is an increased public and industrial awareness of PM and a concern that PM from anthropogenic activities can adversely affect aquatic life. These concerns create a need for scientific research involving PM measurements, and increase the likelihood that Environmental Impact Assessments will be required to address PM in the future. There is currently no agreed standard for measuring PM, but scientists need to be able to produce universally comparable data. Our best practice guide is well timed as the field of PM bioacoustics is set to blossom. The current number of scientists making PM measurements is relatively small, thus a significant proportion of the community could attend a workshop that would allow for constructive discussions that will guide the field as it develops. The inclusive process of writing this best practice guide Following an in-depth literature review, an interim best practice guide was initially written by the authors. The authors of this guide comprise a multidisciplinary team that have a range of expertise spanning fish biology, the physics of underwater sound, practical experience of making underwater PM recordings, and analysing and reporting the resulting data. The interim best practice guide was rigorously reviewed by a panel of scientists active in this research field, the funder, and several global leaders in this field; and selected independent expert scientists active in the research area of measuring PM. The draft guidance was discussed by this group of selected scientists, the scientific panel, and all the authors at a webinar held in September 2020. At the webinar, directed discussions occurred on topics for which the authors could not agree a best practice. Polls by the webinar contingency followed each topic, which provided guidance for the authors to arrive at consensus on all topics. These agreements are incorporated into this guide. Our intention is to test this best practice guide in a field trial before producing a final version that will serve as a standard for research and industry. The development of this guidance document was sponsored by the E&P Sound and Marine Life Joint Industry Programme (JIP). Other documents the reader may wish to consult include a self-consistent set of procedures developed for use in JIP-sponsored projects (Ainslie et al., 2017; de Jong et al., 2020), and other self-consistent sets developed by the soundscape monitoring projects Atlantic Deepwater Ecosystem Observatory Network (ADEON; e.g., Ainslie et al., 2020a; Heaney et al., 2020) and Joint Monitoring Programme for Ambient Noise North Sea (JOMOPANS, e.g., Wang and Robinson, 2020).