Author Identifiers

Ying Liu
ORCID: 0000-0003-2101-3401

Date of Award

2021

Degree Type

Thesis

Degree Name

Doctor of Philosophy

School

School of Engineering

First Advisor

Professor Daryoush Habibi

Second Advisor

Associate Professor Douglas Chai

Abstract

Radially layered cylindrical acoustic waveguide is one of the most common waveguide structures, as well as a physical model prototype for many practical applications, such as boreholes and buried pipelines. Study on the wave propagation in radial-layered cylindrical waveguides can provide a theoretical basis for acoustic methods in reservoir exploration and fluid resource transportation, including acoustic well logging, underground pipeline detection and location. This PhD study is conducted from two aspects: one is the monopole acoustic well logging in determining velocities of heterogenous formation based on the borehole acoustics; and the other is research on acoustic wave propagation within buried pipeline systems based on the thin shell theory.

A theoretical model is established firstly to investigate the characteristics of wavefield within a borehole surrounded by heterogeneous formation, where an additional layer with different velocities from original homogenous formation is included to simulate the radial velocity distribution in practice. The arrival time difference and P-wave amplitude variation in time domain are observed and compared against the waveforms from homogeneous formation model. In order to study the contributing factors of wave amplitude variation, the current head wave theory is employed and its applicability in heterogeneous formation model is also investigated. It is found that the velocity difference of two formation layers, Poisson's ratio difference, the thickness of the additional layer and the source frequency contribute together to determine the magnitude of the disturbance to the original homogenous formation, which is the main applicable condition of the current head wave theory.

Based on our established forward mode, the simulated waveforms in time domain are found to carry the valuable information of heterogeneous formation velocities. Therefore, a stepwise inversion method is proposed to image the radial profiles of formation P- and S-wave velocities. Instead of determining the formation velocity variation and its corresponding radial position simultaneously as traditional methods do, the inversion procedure is divided into two steps: 1) the velocity array is determined by semblance processing of contiguous receiver pairs of acoustic array data; and 2) the thickness of the layer (radial position) is obtained based on ray theory. The modelling-based inversion results and the application to field data validates its efficiency and accuracy in profiling both P- and S -wave velocities of formation.

Buried pipeline systems share the same category of physical model prototype with borehole, i.e. a radial-layered cylindrical waveguide. However, the thickness of pipe wall is usually very smaller or much smaller than the radius. To address this special case, various thin shell theories have been developed. In this research, an established analytical model of buried fluid-filled pipes is deployed to study the axisymmetric wave motion. The behaviour of gas dominated wave is investigated and compared against water-dominated wave. It is observed that the gas-dominated wave in gas pipes cannot radiate into surrounded soil as effectively as water-dominated wave in water pipes because of the weak coupling between gas and pipe-soil. For buried gas pipes, the soil displacements due to radiation of shell-dominated wave are stronger than gas-dominated wave, which differs from buried water pipe. Hence, exciting shell dominated wave is beneficial to generating stronger vibration signals and obtaining the location information, which would optimise the current vibro-acoustic method in locating buried gas pipes.

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Available for download on Wednesday, January 10, 2024

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