Acoustic imaging of objects buried in soil

A Comprehensive Review of Acoustic Methods for Locating Underground Pipelines

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.

On the detection of objects buried at a shallow depth using seismic wave reflections

This paper concerns the detection of shallow (of the order 1 m) buried objects using seismic excitation. Time-extended signals are used to generate a compressional wave using a shaker attached to the ground. The wave propagates through the ground, reflects off a buried object and is captured by an array of geophones on the surface. The envelopes of the cross-correlation functions between the measured ground velocities and the excitation signal are calculated and summed to generate a cross-sectional image of the ground. The wide cross-correlation peaks caused by high ground attenuation are partially compensated for by using the generalized cross-correlation function called the phase transform. Simple simulations are conducted to demonstrate the method, and some field experiments have been carried out aimed at the detection of a buried concrete pipe. In the experiments the pipe could be detected using the method proposed, with experimental and simulated data producing good agreement.

Surface response of a fractional order viscoelastic halfspace to surface and subsurface sources

Previous studies by the second author published in this journal focused on low audible frequency (40-400 Hz) shear and surface wave motion in and on a viscoelastic material representative of biological tissue. Specific cases considered were that of surface wave motion on a halfspace caused by a finite rigid circular disk located on the surface and oscillating normal to it [Royston et al., J. Acoust. Soc. Am. 106, 3678-3686 (1999)] and compression, shear, and surface wave motion in a halfspace generated by a subsurface finite dipole [Royston et al., J. Acoust. Soc. Am. 113, 1109-1121 (2003)]. In both studies, a Voigt model of viscoelasticity was assumed in the theoretical treatment, which resulted in agreement between theoretical predictions and experimental measurements over a limited frequency range. In the present article, the linear viscoelastic assumption in these two prior works is revisited to consider a (still linear) fractional order Voigt model, where the rate-dependent damping component that is dependent on the first derivative of time is replaced with a component that is dependent on a fractional derivative of time. It is shown that in both excitation source configurations, the fractional order Voigt model assumption improves the match of theory to experiment over a wider frequency range (in some cases up to the measured range of 700 Hz).

An implementation of synthetic aperture focusing technique in frequency domain

A new implementation of a synthetic aperture focusing technique (SAFT) based on concepts used in synthetic aperture radar and sonar is presented in the paper. The algorithm, based on the convolution model of the imaging system developed in frequency domain, accounts for the beam pattern of the finite-sized transducer used in the synthetic aperture. The 2D fast Fourier transform (FFT) is used for the calculation of a 2D spectrum of the ultrasonic data. The spectrum is then interpolated to convert the polar coordinate system used for the acquisition of ultrasonic signals to the rectangular coordinates used for the presentation of imaging results. After compensating the transducer lobe amplitude profile using a Wiener filter, the transformed spectrum is subjected to the 2D inverse Fourier transform to get the time-domain image again. The algorithm is computationally attractive due to the use of 2D FFT. The performance of the proposed frequency-domain algorithm and the classical time-domain SAFT are compared in the paper using simulated and real ultrasonic data.

Acoustic behavior of ordered droplets in a liquid: a phase space approach

The transmission of an acoustical signal through a spatial arrangement consisting of a bidimensional crystal of droplets (liquid spheres) immersed into another liquid is analyzed. As a first approximation, the paraxial case is solved by considering a set of acoustical lenses which allow us to model the effect of each droplet on the signal. An expression for the Wigner distribution function that lets us evaluate the corresponding image, diffraction pattern, and even the output signal of any given paraxial input signal to that crystalline substrate is obtained, with particular emphasis on the case of an incoming plane wave. To solve the nonparaxial situation, a generalization of the concept of focal distance interpreting every sphere as a superposition of concentric rings of different radius, which permits us to find a general expression for the Wigner distribution function is proposed.

Nondestructive imaging of shallow buried objects using acoustic computed tomography

The nondestructive three-dimensional acoustic tomography concept of the present investigation combines computerized tomography image reconstruction algorithms using acoustic diffracting waves together with depth information to produce a three-dimensional (3D) image of an underground section. The approach illuminates the underground area of interest with acoustic plane waves of frequencies 200-3000 Hz. For each transmitted pulse, the reflected-refracted signals are received by a line array of acoustic sensors located at a diametrically opposite point from the acoustic source line array. For a stratified underground medium and for a given depth, which is represented by a time delay in the received signal, a horizontal tomographic 2D image is reconstructed from the received projections. Integration of the depth dependent sequence of cross-sectional reconstructed images provides a complete three-dimensional overview of the inspected terrain. The method has been tested with an experimental system that consists of a line array of four-acoustic sources, providing plane waves, and a receiving line array of 32-acoustic sensors. The results indicate both the potential and the challenges facing the new methodology. Suggestions are made for improved performance, including an adaptive noise cancellation scheme and a numerical interpolation technique.