A porous elastic model for acoustic scatter from manganese nodules

Including porosity introduces absorption into the previously developed scattering analysis for elastic spheres. Acoustic propagation in porous media has two distinct compressional modes, one in which the fluid and the structure move in phase with each other and the other in which they move in opposite phase. Expressions for the complex wavenumbers of both waves are derived, using the parameters for manganese nodules. A modal analysis with a pressure boundary condition indicates that the first kind of compressional wave is lightly attenuated and carries most of the energy of the coupled system. The second compressional wave is highly attenuated and effectively loses all of its energy in the first centimeter of the nodule. This second wave is very important for consideration of nodule-scale dimensions since it represents the dominant loss mechanism. A method was developed to form a single effective compressional wave attenuation for both compressional waves by employing a pressure and velocity boundary condition at the surface of the nodule and using a spatially integrated intensity match. An effective wave attenuation is generated for the compressional wave in the non-porous elastic model which accounts for porous attenuation and which is based on a parameter defined as travel distance L. If L is assumed to be proportional to nodule radius, the attenuation is approximated as constant for the frequency range of interest and is independent of nodule radius. An effective attenuation is predicted. The shear attenuation is found to exhibit a frequency squared form dependence for manganese nodules. However, shear attenuation is very small for ideal nodules and is negligible compared to the compressional loss. The elastic model of the nodule was numerically altered to accommodate complex wavenumbers in shear and compression. Four distinct regions of attenuation are observed in individual reflectivity, R_θ, versus ka plots and discussed. The importance of including attenuation is readily apparent from observing the transformation of the elastic non-porous results. Scattering experiments were performed on manganese nodules and experimental individual reflectivity versus ka plots were generated. Good agreement is found for the spherical Atlantic nodules using values in the third region of attenuation. From these experiments, an estimation of attenuation is made and compared with the theoretical prediction. The influence of attenuation on individual scattering and multiple scattering is investigated. Attenuation tends to shift the major bottom reflectivity, C_R, peak toward lower ka values. This affects the prediction of the acoustic response of manganese nodule deposits.

Name thesis:degree_name

Master of Science

Level thesis:degree_level

masters

Discipline thesis:degree_discipline

Aerospace and Ocean Engineering

Department dc:contributor.department

Aerospace and Ocean Engineering

Grantor dc:publisher

Virginia Polytechnic Institute and State University

Year dc:date.issued

1982

Author dc:creator

• Riggins, David

Statement dc:rights

• In Copyright

Licence dc:rights.uri

• http://rightsstatements.org/vocab/InC/1.0/

Language dc:language.iso

en_US

Handle dc:identifier.uri

http://hdl.handle.net/10919/80236

OAI identifier oai:identifier

oai:vtechworks.lib.vt.edu:10919/80236