Underwater Infra-Sound Resonator for Long Range Acoustics and Seismic Survey

There is a growing interest for a very low frequency sound source in the frequency range below 20 Hz for such applications as Arctic under-ice acoustic, far-range navigation, communications and Thermometry, sub-bottom seismic profiling, et cetera. The ultra-low frequency sound propagates without attenuation and loss of coherency at a very far distance covering the water column from the surface to the ocean floor. Another aspect of the same problem, which has been in an increasing focus of oil and gas producers, is the reducing the impact of noise from traditional air-guns on marine mammals. A coherent sound source can be a quieter and more benign to marine mammals. The major oil companies, Shell, Exxon and Total, are sponsoring are sponsoring the Marine Vibrator Joint Industry Project MVJIP. Marine Vibrators are a coherent type of seismic source, which less harmful for marine inhabitants and gives a clearer, more precise and higher resolution imaging of the bottom formations, structures, and deposits. Teledyne Webb Research is one of the participants in the MVJIP. Teledyne Webb Research has many years of experience in a deep water sound source development showing that to build a sound source with a frequency below 20 Hz is a hard task due to a very large emitted volume velocity or product of aperture area to its linear displacement. For sound pressure level (SPL) larger than 200 dB re 1 uPa at 1 meter the volume displacement at 5 Hz cycle can be tens of liters. Systems with rigid or flexural vibrating diaphragm with a large aperture area are difficult to build, and usually not efficient or have a very narrow bandwidth. Highly efficient frequency sweeping sound sources on the base of tunable organ pipes show very good performance for 150 - 2000 Hz frequency bandwidth. This technology can potentially reach a frequency bandwidth 70-100 Hz, while keeping a high efficiency performance. However, a further decrease of the frequency will be hard to achieve because of the organ pipe growing dimension. The expected complication from such giant design demands us to look for other more simple approaches for underwater sound emitting. As one of the participants in the Marine Vibrator JIP, Teledyne Webb Research is developing a coherent seismic marine sound source technology based on the application of an underwater, gas filled bubble resonator as a very low frequency seismic source. This innovative system is a promising candidate for a high power, highly efficient, and coherent seismic source. The gas-filled bubble offers the large radiating area and was shown to be a good impedance transformer with very high radiation efficiency. The bubble sound source has a simple design using a standard commercial off-the-shelf driver. The elastic membrane supports high volume displacement with a large radiation aperture and prevents cavitation damage. Large volume displacement and velocity support the large radiation power. The sound sources have very small coupling effects in water and can work together in a large phased array. An infra-sound transducer with a resonator in the form of an underwater bubble or balloon made from an elastic material is different from the known engineering solution in the way of seismic wave generating. However, the physics of the dynamics is similar to the physics of air released from an air-gun. The equation of the dynamics of spherical bubbles was first derived and used by Rayleigh (1917) and then Plesset (1949). The most general form of the equation of the dynamics with additional terms due to surface tension and viscous effects in the bubble surface condition is widely known as the Rayleigh-Plesset equation. The equation derived from the general Navier-Stokes equation and is non-linear and includes components, which are important for infra-sound oscillations with high amplitude. The practical bubble has a shape different from spherical. Its internal pressure oscillations are comparable with the difference of static gravity forces and acoustic-gravity oscillations and are part of its dynamics. The real Q-factor of a practical bubble is smaller than theoretical. The real problems of a practical giant bubble resonator are considered in the present project. This research includes the simplified linear acoustic model of the bubble and complete 2D and 3D finite-element analysis of an engineering structure and membrane material. The theoretical research and computer simulation predict the experimental research in the Teledyne acoustical pool and Woods Hole Oceanographic Institution's dock. Different variety of drivers were tested. The membrane dynamics study included bubble shape deformation due to gravitational effects. The acoustic-gravity oscillations from the air bubble close to the sea surface were analyzed. It was demonstrated that a cylindrical bubble resonator can be towed with maximum speed up to 8 knots. The research gave practical numbers for Q-factor, resonance frequencies and sound pressure levels. The experimental bubble resonator has shown good performance with a maximum SPL close to 200 dB and frequency in a range of 5-20 Hz. The parameters of the underwater bubble resonator were reasonably close to the COMSOL simulations. Application of COMSOL finite element analysis allowed source parameters estimation and avoided a long series of water tests with parameter adjustment. The theoretical and experimental research of an underwater gas-filled bubble proved that it is a promising practical approach for a very low frequency sound source, which can find applications for long-range acoustic systems, and as a coherent source for a marine seismic survey.