Underground pipelines are vital means of transporting fluid resources like water, oil and gas. The process of locating buried pipelines of interest is an essential prerequisite for pipeline maintenance and repair. Acoustic pipe localization methods, as effective trenchless detection techniques, have been implemented in locating underground utilities and shown to be very promising in plastic pipeline localization. This paper presents a comprehensive review of current acoustic methods and recent advances in the localization of buried pipelines. Investigations are conducted from multiple perspectives including the wave propagation mechanism in buried pipe systems, the principles behind each method along with advantages and limitations, representative acoustic locators in commercial markets, the condition of buried pipes, as well as selection of preferred methods for locating pipelines based on the applicability of existing localization techniques. In addition, the key features of each method are summarized and suggestions for future work are proposed. Acoustic methods for locating underground pipelines have proven to be useful and effective supplements to existing localization techniques. It has been highlighted that the ability of acoustic methods to locate non-metallic objects should be of particular practical value. While this paper focuses on a specific application associated with pipeline localization, many acoustic methods are feasible across a wide range of underground infrastructures.

Waveform-based seismic location methods can reliably and automatically image weak seismic sources, such as microseismic events and microtremors. Besides the classical diffraction stacking operator which is based on the one-way traveltime, correlation-based imaging methods are another subcategory of waveform-based methods using differential traveltime. In this work, we systematically analyse the existing correlation-based methods and propose a novel hybrid correlation stacking method, which belongs to waveform-based relative location methods. The double differential traveltime from an event pair (i.e. a master event and a target event) to a receiver pair is used to stack the corresponding double correlation waveforms in this new approach. We generalize the correlation-based methods using a unified formula by describing cross-correlation stacking with beamforming. A thorough analysis of these imaging operators using synthetic and field data examples reveals their different characteristics of imaging resolution and level of redundancy, and a moderate level of redundancy can ensure both the accuracy and stability of correlation-based imaging methods, while an extremely high or low level of redundancy will hinder their performance in locating weak seismic events. The examples also demonstrate the potential disadvantage of using multiple phases with inaccurate velocity models for waveform-based location methods.

The acoustic problem of an eccentric drill collar in a fluid-filled borehole has been of interest in the field of acoustic logging while drilling (ALWD) in recent years. To reduce the effects of tool eccentricity on ALWD measurements, studies on acoustic responses under such conditions are essential. This study therefore has developed an analytical method to investigate borehole wavefields with an off-center monopole ALWD tool in fast and slow formations. By evaluating the contributions of compressional and shear branch points, the effects of the tool eccentricity on individual formation primary and shear head waves are investigated. Results illustrate that tool eccentricity only affects the excitation properties and has almost no effect on the extracted velocities. The joint analysis of synthetic full waveforms and dispersion diagrams with varying eccentricity degrees indicates that multipole modes are excited when the tool is off center, and their excitation amplitudes gradually increase with the increasing eccentricity, especially in the direction of tool movement. Moreover, the dispersion analysis reveals that the two modes with intersections in the centered case are coupled when the tool becomes eccentric. In particular, the coupling performance between the Stoneley and flexural modes is the most prominent. Furthermore, the effects of tool eccentricity on the monopole acquisition method, that is, the sum of waveforms received at four orthogonal azimuths, are evaluated. Results show that the above summation method can effectively reduce the effects of slight or moderate eccentricity. However, for large or extreme eccentricity, reducing or eliminating the effects of eccentricity on the Stoneley wave is a challenge for this method. Based on the above analyses, measurements may not be reliable for formation evaluation in the case of extreme eccentricity, especially for Stoneley wave applications.

The borehole dipole shear-wave reflection imaging method has great potential in heterogeneous reservoir explorations because of its deep investigation depth and relatively large reflection amplitude. However, the generally used shear horizontal (SH) reflection approach can only indicate the reflector strike and has an inherent defect in azimuth ambiguity. We have developed a multicomponent cross-dipole array acoustic measurement with four azimuthally distributed receiver arrays and a method using reflected dipole P-waves to eliminate the azimuth ambiguity caused by the SH reflection. The recorded data include cross-dipole waves with four components and two combined dipole-monopole waves that stack the data of the four azimuthally distributed receivers induced by each dipole source. A theoretical analysis indicates that the dipole compressional reflection is sensitive to the reflector azimuth. Therefore, the cross-dipole waves are first used to determine the reflective interface strike with the SH reflection. The compressional reflections obtained from the cross-dipole data and the combined dipole-monopole data are then processed to identify the correct azimuth. The effectiveness and accuracy of the method are validated via synthetic and field data examples in a soft formation. Our method may potentially solve the azimuth ambiguity problem in borehole acoustic reflection imaging and fully use cross-dipole acoustic measurements.