Research Projects

Seismic-Invertebrate Impact Review

Preliminary findings from a modeling study carried out
at Cape Breton University by Dr. Geoffrey Lee-Dadswell.

Experimental attempts to establish whether seismic testing has any impact on crabs and the crab fishery have been hampered by a lack of theoretical work on the subject. As a result, experimenters have not known what effects to look for or what threshold sound levels might be problematic for crabs. The study has aimed at beginning the necessary theoretical work to guide future experiments examining the physical effects of seismic testing on invertebrates.

The approach has been to consider a simple model of a snow crab and, using basic acoustical theory, calculate the amplitude of vibrations inside the crab. In the first stage of the study the crab was modeled as a spherical, elastic shell, with a fluid interior. Material properties of crab tissues were estimated and combined with the calculated vibrational amplitudes to predict what minimum sound levels might cause physical damage.

The overall preliminary conclusions of the first stage of the project are:

• No direct physical damage to snow crabs is expected to be caused by seismic pulses. The assessment has shown that dangerous heating of the crab’s internal tissues is not a likely outcome of any realistic seismic test pulse.

• The exception is that the separation of the outer membrane of certain organs might be directly caused by seismic pulses. This is because the total amplitude of vibration in the crab's tissues could be as large as a millimeter, which may be sufficient to cause membrane separation. Further study would be needed to determine whether this is the case.

• Crabs can likely feel their internal organs vibrate as seismic pulses pass through them. This makes it likely that crabs will respond and possibly experience stress as a result of seismic testing. More study would be needed to determine this.

• The present study has been hampered by a lack of data on the physical characteristics of crab tissues. It is recommended that experimental work be done to determine certain physical quantities of crab tissues. The physical characteristics most needed are: speed of sound in the crab cuticle (and seasonal variation of this value); shear and bulk viscosity of various representative crab tissues; speed of transverse waves through various representative crab tissues; elastic limits (especially yield point) of various representative crab tissues. This will make it possible to better predict the effects of seismic pulses on crabs with greater confidence.

In an initial study such as this one, it is only possible to make very rough estimates. However, it is felt that the estimate arrived at should be good to within an order of magnitude and will be useful as a starting point for future studies. Additionally, it will help to establish what areas require further work.

The second stage of the study is now in progress. The model has been refined by considering the crab's tissues as an elastic solid with a very low speed of transverse waves rather than as a fluid, as was done in the first stage. Considerable effort has been devoted to finding ways to identify resonances of the crab's tissues. The plot below shows the maximum amplitude of oscillation in the tissues as a function of frequency. The peaks in the plot indicate resonances. The animation below the plot shows the particle displacement in a cross section through the middle of the crab for a typical resonance frequency.