Browsing by Subject "Low-frequency"
Now showing 1 - 2 of 2
- Results Per Page
- Sort Options
Item Development of a standing-wave apparatus for calibrating acoustic vector sensors(2014-08) Lenhart, Richard David; Wilson, Preston S.; Sagers, Jason D.Underwater acoustic pressure transducers measure pressure fluctuations, a scalar parameter of the acoustic field. Acoustic vector sensors contain an omnidirectional pressure transducer (omni) and also bi- or tri-axial sensing elements that respond to either the particle velocity or pressure gradient of the acoustic field; which are vector quantities. The amplitude of the signal output of each directional channel of a vector sensor is proportional to the orientation relative to the direction of acoustic pressure propagation. The ratio of the signal amplitudes between two directional channels and the cross-spectra between the vector sensor omni and directional channels enable one to estimate the bearing to the source from a single point measurement. In order to accurately estimate the bearing across the usable frequency band of the vector sensor, the complex sensitivities of the omni and directional channels must be known. Since there is no standard directional reference transducer for a comparative calibration, the calibration must be performed in an acoustic field with a known relationship between the acoustic pressure and the acoustic particle velocity. Free-field calibrations are advantageous because this relationship is known for both planar and spherical wave fronts. However, reflections from waveguide boundaries present a practical limitation for free-field calibrations, especially at low frequencies. An alternative approach is to perform calibration measurements in a standing-wave field, where the relationship between pressure and particle velocity is also known. The calibration facility described in this thesis is composed of a laboratory-based, vertically-oriented, water-filled, elastic-walled waveguide with a piston velocity source at the bottom end and a pressure release boundary condition at the air/water interface at the top end. Some of the challenges of calibrating vector sensors in such an apparatus are discussed, including designing the waveguide to mitigate dispersion, mechanically isolating the apparatus from floor vibrations, understanding the impact of waveguide structural resonances on the acoustic field, and developing the calibration algorithms. Data from waveguide characterization experiments and calibration measurements are presented along with engineering drawings and calibration software.Item Low-frequency attenuation measurements of fluids(2019-06-19) McCann, Michael Ryan; Spikes, Kyle; Tisato, NicolaPore fluids significantly affect the elastic responses of rocks. Rock-physics models can be used to predict how pore fluids affect the elastic responses of fully or partially saturated rocks. Thus, to identify fluids in the subsurface, knowing the anelastic properties of such fluids, such as attenuation, is useful. In addition, new technologies to assess and monitor hydrocarbon exploration rely on the precise determination of the anelastic properties of hydraulic fracturing fluids. Moreover, the anelastic properties of fluids depend on the frequency of the wave propagating through the fluid. Methodologies to measure high-frequency anelastic properties of fluids have been widely reported. What have not been established are methodologies to measure the low-frequency anelastic properties and attenuation of fluids in a laboratory setting, aside from shear viscosity. Using the low-frequency properties and attenuation, rather than the already known high-frequency properties of pore fluids, will provide more accurate estimates for rock physics models and seismic data. To address this situation, a laboratory machine has been designed, built, and calibrated to measure the low-frequency attenuation and bulk modulus of fluids at frequencies from 0.1 to 10 Hz. Deionized water and aqueous guar gum solutions have been tested. Results for measurements of attenuation and bulk modulus of water agree with the literature. Measurements of guar gum solutions show that attenuation is greater than 0.01 with higher concentration samples having higher attenuation. This might be explained by energy being lost during the breakup of weak and strong bonds in the guar gum polymer chains. A higher concentration provides more bonds to break up which leads to more energy being lost which increases attenuation. The present study will help improve seismic methods and rock physics models by incorporating low-frequency attenuation values of fluids.