Dipole Transducer Research Articles

Joint inversion of formation radial shear-velocity profiles by dipole acoustic logging while drilling

Formation properties near a borehole are frequently altered by mechanical damage from drilling, leading to variations of formation elastic velocities in the radial direction. Inversions of radial shear profiles are necessary for a real-time evaluation of the drilling conditions. In this study, we investigate acoustic wave propagation in a logging-while-drilling (LWD) model excited by a dipole transducer in cylindrically layered media. It is revealed that the dispersion characteristics of the dipole waves of the LWD are sensitive to the radial formation velocity variation. Accordingly, we develop a joint inversion method that evaluates radial shear-velocity profiles using the dipole modes of the first and second orders. The propagation of these two modes is strongly influenced by formation alterations, but the inversion results obtained using a single dipole mode indicate ambiguity caused by the nonuniqueness problem. Through our joint inversion, the additional second-order dipole wave can effectively help address the nonuniqueness issue in dipole acoustic logging, although its dispersion features differ in fast and slow formations. It is validated that the joint inversion has considerably improved convergence and accuracy in evaluating the thickness and shear velocity of the altered zone, compared with the estimation by a single acoustic mode. Furthermore, we develop numerical results based on synthetic logging data to demonstrate the feasibility of this inversion in a continuously varying profile, regardless of whether the formation surrounding the borehole is fast or slow. Even when strong drilling noises of low frequencies are added to the receiver signal arrays, the means of joint inversion can remain stable, and the shear-velocity profiles can be evaluated with good precision.

Studies on a Piezoelectric Cylindrical Transducer for Borehole Dipole Acoustic Measurements

In this article, a novel design of a piezoelectric dipole transducer is proposed for formation acoustic velocity measurement in the vicinity of a borehole with a frequency range of 0.4–6 kHz. The transducer which actuates a cylindrical shell to generate a pure dipole mode wave by using multiple piezoelectric bender bars is analyzed theoretically and simulated numerically by using the finite element method (FEM). Moreover, the transducer is fabricated and tested to compare with the numerical simulation results, which shows that the test and simulation results are in good agreement. Finally, compared with numerical simulation results of the traditional dipole transducer, it is shown that the proposed dipole transducer has higher transmitting sensitivities than commonly used ones, especially in low frequency responses. This work lays a foundation for the new development of the transducer in borehole dipole acoustic shear wave measurements. Especially, in a slow formation where the shear wave velocity is lower than that of compressional wave in the borehole fluid, the transducer could be used for highly efficient shear wave velocity measurements.

Eliminating the azimuth ambiguity in reflected S-wave imaging logging based on the azimuthal receiver mode

Due to the symmetrical directivity of a dipole transducer, dipole reflected S-wave imaging logging results in ambiguous azimuth measurements of reflectors. Based on an orthogonal dipole source and an eight-element azimuthal receiving station, we proposed an improved method for measuring the azimuth of a reflector and applied it to a borehole azimuthal acoustic reflection tool in our laboratory. Based on the dipole transmitting and receiving mode, to determine the strike of a reflector, this method superposes the echo amplitude and azimuth distribution curves of X and Y dipole transmitting and azimuthal receiving and compares the amplitude-superposed value of the echo received by two receiving elements close to the reflector azimuth and a virtual image azimuth. It accurately obtained the true azimuth of the reflector. The feasibility of this method was verified by numerical simulation, a physical experiment, and a field test. The results show that the amplitude of the P–P, P–S/S–P, and S–S echoes received by the different receiving elements were different when there was a reflector in place. For any type of echo, the amplitude superposition value of the echo received by a receiving element close to the reflector azimuth was significantly greater than that of the echo received by a receiving element close to a false azimuth. Therefore, based on the azimuthal receiving mode, the azimuth ambiguity of dipole-reflection S-wave logging can be eliminated.

Properties of PMN-PT single crystal piezoelectric material and its application in underwater acoustic transducer

In order to further understand the characteristics of PMN-PT piezoelectric single crystal materials, the piezoelectric constant d33, impedance and frequency characteristics of PMN-PT piezoelectric single crystal elements under different pressures are studied in this paper, and the performances of PMN-PT piezoelectric single crystal elements are compared with PZT-4 piezoelectric ceramic elements. The non prestressed PMN-PT piezoelectric single crystal sandwich dipole transducer is studied and compared with PZT-4 piezoelectric ceramic sandwich dipole transducer of the same size. The matching layer broadband PMN-PT piezoelectric single crystal circular composite rod transducer is studied. The stress distribution of the transducer under 10 N pressure is calculated, and the problem of local crack of the piezoelectric single crystal element is solved from the structural design. The results show that the PMN-PT piezoelectric single crystal material has obvious advantages in low frequency performance, acoustoelectric and electroacoustic conversion efficiency, and can improve the emission and reception performance of acoustic transducer at the same time. The maximum transmitting voltage response and receiving sensitivity of the non prestressed sandwich dipole transducer can reach 144 dB and −166 dB respectively. Compared with piezoelectric ceramic transducer, the resonance frequency is reduced by about 3 kHz, and the transmitting and receiving performance are improved by 3 dB and 6 dB respectively. The matching layer broadband PMN-PT piezoelectric single crystal circular composite rod transducer has a transmitting voltage response of more than 136 dB, fluctuation of less than 5 dB, receiving sensitivity of more than −176 dB and fluctuation of less than 6 dB in the frequency range from 20 kHz to 40 kHz. The research results of this paper have important reference value and guiding significance for the popularization of PMN-PT piezoelectric single crystal material and its application in acoustic transducer.

Interior sound field control using generalized singular value decomposition in the frequency domain.

The problem of controlling a sound field inside a region surrounded by acoustic control sources is considered. Inspired by the Kirchhoff-Helmholtz integral, the use of double-layer source arrays allows such a control and avoids the modification of the external sound field by the control sources by the approximation of the sources as monopole and radial dipole transducers. However, the practical implementation of the Kirchhoff-Helmholtz integral in physical space leads to large numbers of control sources and error sensors along with excessive controller complexity in three dimensions. The present study investigates the potential of the Generalized Singular Value Decomposition (GSVD) to reduce the controller complexity and separate the effect of control sources on the interior and exterior sound fields, respectively. A proper truncation of the singular basis provided by the GSVD factorization is shown to lead to effective cancellation of the interior sound field at frequencies below the spatial Nyquist frequency of the control sources array while leaving the exterior sound field almost unchanged. Proofs of concept are provided through simulations achieved for interior problems by simulations in a free field scenario with circular arrays and in a reflective environment with square arrays.

Study of UHF Frequency Response Characteristics of the Semicircle Dipole Inner Sensor for Partial Discharge Detection in GIS

The paper designed a semicircle dipole inner sensor for UHF partial discharge detection in GIS. By comparing the symmetric semicircle dipole sensor and asymmetric semicircle dipole sensor characteristics, the symmetric semicircle dipole sensor’s coupling property is better than that of asymmetric semicircle dipole sensor. At the same time, by changing the symmetric semicircle dipole transducer radius, the position of the feeder pole , the height of the feeder pole, analyze its influence on sensor features.

Investigation of low-frequency broadband dipole transducer driving by PMN-PT transverse shear mode

A low-frequency broadband dipole transducer with a standard dipole directivity pattern is a crucial factor in the research and development of a sonic logging tool. A new dipole transducer design based on cylindrical shell oscillating is proposed in this paper, in which transverse shear polarization PMN-PT crystal stacks are used to provide a driving force to excite a cylindrical-shell oscillation to generate dipole acoustic field. The resonant frequency of the transducer is decided by the dimensions of crystal stacks. Piezoelectric coefficient d 15 of the PMN-PT crystals can reached 7000 pC/N, which means there is enormous potential in high power transducer design; Several ceramic plates are connected in a series to reduce the resonant frequency of the exciting source and increase the driving amplitude at the same time so as to enhance the radiation power further; several stacks of different sizes are connected in a parallel to broaden the bandwidth. Finite element method is adopted to optimize the structure of transducer. According to the optimized results, a prototype is produced and then tested in anechoic tank. Test results show that the transmitting response is larger than traditional dipole transducer and the directivity pattern is better in the operating frequency range.

The research on piezoelectric cylindrical oscillating transducer for dipole acoustic logging

Acoustic transducer, which is a core component of acoustic logging tool, determines the development of acoustic logging technology. In order to realize the goal of dipole acoustic logging transducer with the features of low frequency, broadband and high radiation power, a cylindrical oscillating dipole acoustic transducer using an actuator of a group of different trilaminar piezoelectric bending bars is designed and studied. Modal and harmonic response analyses of the whole transducer are conducted by using the finite element method, including analyses of input conductance of transducer in air and water, transmitting voltage response and directivity patterns. Besides, the proposed transducer is fabricated and tested to compare with numerical simulation results. Moreover, the numerical results of the proposed transducer are compared with that of traditional dipole transducer. The comparison result indicates the proposed dipole transducer is characterized by low frequency, broadband, and high radiation power, which is feasible in the application of dipole acoustic logging.

Dipole transducer enhancement from a passive resonator.

A passive resonator model for enhancing the low frequency performance of dipole transducers is presented using a spherical wave function model. It is shown that because the near field response of a dipole has the same on-axis low frequency dependence as an equivalent monopole, it can effectively excite a nearby monopole passive resonator and do this with enough strength to significantly modify the response of the system. Sample results are presented to demonstrate the performance enhancement and to illustrate the conversion of the dipole beam pattern to a nearly omni-directional beam pattern.

Numerical simulation of an acoustic field generated by a phased arc array in a fluid-filled cased borehole

Transducers that are widely applied in cement bond evaluation tools, such as cement bond logs and variable density logs, cannot radiate acoustic energy directionally because of the characteristics of monopole sources. A phased arc array transmitter, which is a novel transducer that differs from monopole and dipole transducers, is presented in this study. To simulate the acoustic field generated by a phased arc array in a fluid-filled cased borehole with different channelings, a 3D finite-difference time-domain method is adopted. The acoustic field generated by a traditional monopole source is also simulated and compared with the field generated by the phased arc array transmitter. Numerical simulation results show that the phased arc array radiates energy directionally in a narrow angular range in the borehole, thereby compressing the acoustic energy into a narrow range in the casing pipe, the cement, and the formation. We present the analyses of first-arrival waveforms and the amplitudes of casing waves at different azimuthal angles for the two different sources. The results indicate that employing a directional source facilitates azimuthal identification and analysis of possible channeling behind the casing pipe.

Alternative tonpilz and bender transducer designs

Tonpilz transducer designs with half-wavelength water-sized radiating-pistons are commonly used in SONAR arrays. And here the tonpilz piston normally radiates in the broadside direction with reduced output as the array is steered to the end-fire direction. Bender transducer designs, used in low frequency applications, can take the form of a dipole transducer which, as a result of partial self-cancellation, can lead to a low source level output. We present two alternative transducer designs, which solve these problems. In the case of the tonpilz array, a cylindrical-shaped leveraged-transducer design with one-half water wavelength size and modal performance is proposed. This transducer is shown to operate in the first three modes allowing the formation of an element beam that may be steered in the general directions the array is steered, with full output in a single end-fire direction. In the case of the dipole transducer, advantage is taken of the strong near-field dipole acoustic pressure which is used to energize a nearby compliant parasitic resonator yielding a dominant monopole source of greater output. [Work supported in part by ONR.]

Moderately reverberant learning ultrasonic pinch panel

Tactile sensing is widely used in human-computer interfaces. However, mechanical integration of touch technologies is often perceived as difficult by engineers because it often limits the freedom of style or form factor requested by designers. Recent work in active ultrasonic touch technologies has made it possible to transform thin glass plates, metallic sheets, or plastic shells into interactive surfaces. The method is based on a learning process of touch-induced, amplitude-disturbed diffraction patterns. This paper proposes, first, an evolution in the design with multiple dipole transducers that improves touch sensitivity or maximum panel size by a factor of ten, and improves robustness and usability in moderately reverberant panels, and second, defines a set of acoustic variables in the signal processing for the evaluation of sensitivity and radiating features. For proof of concept purposes, the design and process are applied to 3.2- and 6-mm-thick glass plates with variable damping conditions. Transducers are bonded to only one short side of the rectangular substrates. Measurements show that the highly sensitive free lateral sides are perfectly adapted for pinch-touch and pinch-slide interactions. The advantage of relative versus absolute touch disturbance measurement is discussed, together with tolerance to abutting contaminants.

On time reversal mirrors

The concept of time reversal (TR) of a scalar wave is reexamined from basic principles. Five different time-reversal mirrors (TRMs) are introduced and their relations are analyzed. For the boundary behavior, it is shown that for a paraxial wave only the monopole TR scheme satisfies the exact boundary condition while for the spherical wave only the MD-mode TR scheme satisfies the exact boundary condition. The asymptotic analysis of the near-field focusing property is presented for two dimensions and three dimensions. It is shown that to have a subwavelength focal spot, the TRM should consist of dipole transducers. The transverse resolution of the dipole TRM is linearly proportional to the distance between the point source and the TRM. The mixed mode TRM has the similar (linear) behavior in three dimensions, but in two dimensions the transverse resolution behaves as the square root of the distance between the point source and the TRM. The monopole TRM is ineffective in focusing below the wavelength. Contrary to the matched field processing and the phase processor, both of which resemble TR, TR in a weak- or non-scattering medium is usually biased in the longitudinal direction, especially when TR is carried out on a single plane with a finite aperture. This is true for all five TR schemes. On the other hand, the TR focal spot has been shown repeatedly in the literature, both theoretically and experimentally, to be centered at the source point when the medium is multiple scattering. A reconciliation of the two seemingly conflicting results is found in the random fluctuations in the intensity of the Green function for a multiple scattering medium and the notion of scattering-enlarged effective aperture.

Numerical simulation of an acoustic field generated by a phased arc array in a fluid-filled borehole

Transducers that are widely applied in cement bond evaluation tools, such as cement bond logs and variable density logs, cannot radiate acoustic energy directionally because of the characteristics of monopole sources. A phased arc array transmitter, which is a novel transducer that differs from monopole and dipole transducers, is presented in this study. To simulate the acoustic field generated by a phased arc array in a fluid-filled cased borehole with different channelings, a 3D finite-difference time-domain method is adopted. The acoustic field generated by a traditional monopole source is also simulated and compared with the field generated by the phased arc array transmitter. Numerical simulation results show that the phased arc array radiates energy directionally in a narrow angular range in the borehole, thereby compressing the acoustic energy into a narrow range in the casing pipe, the cement, and the formation. We present the analyses of first-arrival waveforms and the amplitudes of casing waves at different azimuthal angles for the two different sources. The results indicate that employing a directional source facilitates azimuthal identification and analysis of possible channeling behind the casing pipe.

Effects of boundary conditions on vibrating mode of acoustic logging dipole transducer

Acoustic transducer is an important part of acoustic well logging tool. In this paper, ANSYS software package is used to design acoustic dipole transducer and simulate vibrating mode of the dipole transducer in different mechanical boundary conditions. The results show that boundary conditions influence the number of vibrating mode in the same frequency band and the frequency value of the same vibrating mode. Several acoustic dipole transducers are designed according to the results of numerical simulation and laboratory measurements. The basic frequency of vibrating mode of experiment has good agreement with that of simulation. The numerical simulation plays a good guidance role in designing, producing and correctly installing the acoustic dipole transducer.

Shear slowness measurement by dispersive processing of the borehole flexural mode

In sonic well logging, the flexural mode generated by a dipole transducer allows measurement of shear slowness in slow formations, that is, formations in which the shear slowness is slower than the borehole fluid speed. The flexural mode is a dispersive borehole mode, but many current processing methods do not account for this dispersion. The approximate semblance equation is derived to explain what happens when a dispersive wave is processed nondispersively. The processing described here is a dispersive analog of the slowness‐time coherence (STC) processing used commonly for array sonic waveforms. It back‐propagates waveforms according to model dispersion curves and calculates semblance. It is a specialized version of the maximum likelihood (ML) and least‐mean‐squared‐error (LMSE) estimator for formation shear slowness. The dispersive analog of STC is too slow for depth‐by‐depth logging but may be a helpful analytic tool. A high‐speed variant, dispersive STC (DSTC), eliminates this difficulty by a new window‐positioning technique and can be faster than STC. DSTC results based on data from a real‐axis integration model show that the technique measures the formation shear slowness with small bias because of windowing and other arrivals. Results are presented from two wells logged with a dipole sonic tool. A log from a fast formation shows that shear slownesses measured from the flexural mode by DSTC agree with monopole results. A log from a slow formation is also presented.

Applications of Terfenol-D in China

Applications of Terfenol-D in China are reviewed. Cylindrical, compound-bar, flextensional transducers for underwater acoustics, dipole transducers for acoustic oil-well logging, controllers and actuators are described in this paper. In addition, this paper presents the fundamentals of design for Terfenol-D transducer, such as measurement of electromechanical coupling coefficient k 33, estimation of prestress and calculation of resonant frequency, etc. But even getting some solutions for these problems, we still require further work to pay more attention to the applications of Terfenol-D.

Rare-earth iron ’’square ring’’ dipole transducer

A rare-earth iron magnetostrictive transducer in the form of a ’’square ring’’ has been constructed to operate in a dipole mode as well as in an omni mode. The outer diameter of the transducer is 6.5 in. (0.1651 m) and it resonates at 2.0 kHz in the omni mode and 2.4 kHz in the dipole mode.

Dipole characteristics of a magnetostrictive rare earth iron “square‐ring” transducer

A magnetostrictive transducer has been constructed of rare earth Terfenol D (Tb0.3Dy0.7Fe2) rods in a “square‐ring” configuration, which is essentially cylindrical. The transducer, 6 in. in diameter and 2 in. high, is resonant at 2.0 kHz in the omnidirectional mode. Some underwater detection systems also require directional information. A means of operating the “square‐ring” in a dipole mode is developed. The physical mechanism of the dipole transducer behavior is described. The modification to the transducer and the driving circuit are presented. The acoustic calibration of the transducer in the dipole mode shows acceptable performance from 1.5 to 5 kHz. The dipole resonance is 2.4 kHz. [Work supported by NSWC in cooperation with NADC.]