A new ultrasound-based technique is proposed to assess the arterial stiffness: the radiation force of an ultrasonic beam focused on the arterial wall induces a transient shear wave (� 10 ms) whose propagation is tracked by ultrafast imaging. The large and high-frequency content (100 to 1500 Hz) of the induced wave enables studying the wave dispersion, which is shown experimentally in vitro and numerically to be linked to arterial wall stiffness and geometry.The proposedmethod isappliedinvivo.Byrepeatingtheacquisition upto 10timesper second(theo- retical maximal frame rate is � 100 Hz), it is possible to assess in vivo the arterial wall elasticity dynamics: shear modulus of a healthy volunteer carotid wall is shown to vary strongly during the cardiac cycle and measured to be 130 ± 15 kPa in systole and 80 ± 10 kPa in diastole. (E-mail: mathieu.couade@gmail.com) 2010 World Feder- ation for Ultrasound in Medicine & Biology.

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Quantitative Assessment of Arterial Wall Biomechanical Properties Using Shear Wave Imaging

An acoustic MWD system comprises at least one transmitter array and at least one receiver array spaced axially along an essentially tubular downhole subassembly. Each transmitter and receiver array includes a plurality of elements circumferentially disposed on the outside of the assembly. The downhole subassembly also comprises a processor, data storage, telemetry elements, power supplies and control circuits as well as other types of sensors. In one embodiment of the invention, the transmitters are operated as a quadrupole in orthogonal orientations and signals from two receiver elements processed to attenuate common modes (including body waves through the tool, monopole waves and Stoneley waves). In another embodiment, the transmitters are used as crossed dipoles and signals received by cross-dipole receivers may be used to determine shear wave birefringence in the formation. In yet another embodiment, the transmitters are used as crossed dipoles and a single dipole receiver produces information relating to the birefringence: rotation of the drillstring makes it possible to operate the transmitters in two orthogonal orientations.

Measurement-while-drilling acoustic system employing multiple, segmented transmitters and receivers

A casing sensor and methods for sensing using a casing sensor are disclosed. The casing sensor includes a casing shoe and a sensor coupled to the casing shoe. A casing data relay includes a downhole receiver coupled to a well casing and a transmitter coupled to the receiver. The casing sensor may be coupled to the transmitter. A drill string actuator may be controllable through the downhole receiver.

Casing mounted sensors, actuators and generators

The present invention provides a system for drilling boreholes having a downhole subassembly which contains an acoustic measurement-while-drilling system It uses the acoustic velocity through the formations surrounding the borehole and an acoustic transmitter and a set of receivers for determining the bed boundaries surrounding the borehole formation An acoustic isolator on the tool may be used to attenuate body waves traveling between the transmitter and the receivers Additional attenuation of the body waves is provided by a threshold filter based upon the absolute maximum of the received signals Semblance of the data is determined in a slowness/intercept-time domain Coherence and semblance filtering methods are used to differentiate between reflection signals and noise The position and orientation of the bed boundary relative to the tool are determined A further processing step uses the relative position and orientation determined for a number of tool positions to further discriminate against noise and obtain an absolute position and depth of the bed boundaries

Semblance processing for an acoustic measurement-while-drilling system for imaging of formation boundaries

We develop a rock physics model based on nonlinear elasticity that describes the dependence of the effective stiffness tensor as a function of a 3D stress field in intrinsically anisotropic formations. This model predicts the seismic velocity of both P- and S-waves in any direction for an arbitrary 3D stress state. Therefore, the model overcomes the limitations of existing empirical velocity-stress models that link P-wave velocity in isotropic rocks to uniaxial or hydrostatic stress. To validate this model, we analyze ultrasonic velocity measurements on stressed anisotropic samples of shale and sandstone. With only three nonlinear constants, we are able to predict the stress dependence of all five elastic medium parameters comprising the transversely isotropic stiffness tensor. We also show that the horizontal stress affects vertical S-wave velocity with the same order of magnitude as vertical stress does. We develop a weakanisotropy approximation that directly links commonly measured anisotropic Thomsen parameters to the principal stresses. Each Thomsen parameter is simply a sum of corresponding background intrinsic anisotropy and stressinduced contribution. The stress-induced part is controlled by the difference between horizontal and vertical stresses and coefficients depending on nonlinear constants. Thus, isotropic rock stays isotropic under varying but hydrostatic load, whereas transversely isotropic rock retains the same values of dimensionless Thomsen parameters. Only unequal horizontal and vertical stresses alter anisotropy. Since Thomsen parameters conveniently describe seismic signatures, such as normal-moveout velocities and amplitudevariation-with-offset gradients, this approximation is suitable for designing new methods for the estimation of 3D subsurface stress from multicomponent seismic data.

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Nonlinear rock physics model for estimation of 3D subsurface stress in anisotropic formations: Theory and laboratory verification

An analysis is carried out of axisymmetric waves propagating along fluid‐loaded cylindrical shells within the framework of linear elasticity and classical perfect‐slip boundary conditions at the solid–fluid interface. Numerical solutions are obtained for various axisymmetric eigenmodes for a cylindrical shell in vacuum; a cylindrical shell surrounded by a liquid of infinite radial extent; a hypothetical liquid column with both the stress‐free and displacement‐free boundary conditions; a cylindrical shell with a liquid core; and a cylindrical shell immersed in an infinite liquid. Numerical results are obtained for both the radiating (leaky) and nonradiating eigenmodes of the system by a careful search of the complex eigenfrequencies of the associated boundary value problem. In particular, attenuation of leaky modes due to radiation of energy into the surrounding medium is expressed in terms of the imaginary part of the eigenfrequency. Computational results are presented for the dispersion curves as well as the displacement and stress amplitude component distributions along the radial direction for various propagating modes of the system. Practical benefits from such analyses are discussed.

Axisymmetric wave propagation in fluid-loaded cylindrical shells. I: Theory

We show both experimentally and theoretically how stress concentrations affect the velocity field around a borehole, and how the velocity field influences dipole anisotropy measurements. At low frequencies the dipole mode is sensitive to the far-field stresses primarily, so standard sonic log interpretation correctly yields the direction of maximum stress. At higher frequencies, the dipole mode is sensitive to near-field stress concentrations such that the fast polarization direction is aligned with the direction of minimum tectonic stress. These effects combine to produce a crossover in the dipole dispersion curves measured in the fast and slow directions. With broad-band dipole data, the dispersion crossover can be used as an indicator of stress-induced anisotropy dominating over weak intrinsic anisotropy.

Effects of borehole stress concentrations on dipole anisotropy measurements

This paper discusses both theoretical and experimental aspects of axisymmetric wave propagation along fluid‐loaded cylindrical shells (excluding torsional modes). For a steel cylindrical shell with a fixed ratio of inner to outer radius, four different fluid configurations are considered: water inside and outside the steel shell; air inside and outside; water inside and air outside; and air inside and water outside. Calculations of the transient pressure response for the case of an axisymmetric ring source and a point receiver are made as a function of source–receiver separation. Experimentally, a PZT ring source and ring receiver are placed around a steel cylindrical shell with an outer radius of 9.53 mm and an inner radius of 7.94 mm. Waveforms are recorded for multiple source–receiver hydrophone spacings in the frequency band 50–240 kHz. Using a Prony’s method, the complex wave number as a function of frequency for each of the modes in the system is derived from both the theoretical and experimental wa...

Axisymmetric wave propagation in fluid‐loaded cylindrical shells. II: Theory versus experiment

Systems and methods for determining a formation property related to formation strength and stresses are disclosed. A method for determining a formation strength includes obtaining radial formation property measurements at different wellbore pressures; generating a radial stress profile based on a formation model; generating a radial stress function from the radial stress profile; and comparing the radial formation property measurements with the radial stress function to determine the formation strength. A method for determining a formation stress profile includes deriving formation parameters from a formation radial profiling; obtaining formation log data that comprise formation density data; estimating formation stresses from the formation log data; and deriving a radial stress profile based on a formation model, the derived formation parameters, and the estimated formation stresses.

Methods and systems for determining formation properties and in-situ stresses

Schlieren techniques are used to show that bounded beam reflection effects occur at liquid/solid‐plate/liquid (L/SP/L) interfaces which are analogous to the bounded beam reflection effects reported previously at liquid/solid (L/S) interfaces. At L/SP/L interfaces, nongeometric effects are shown to be present in both the reflected and transmitted beams when the incident angle of an ultrasonic beam corresponds to the Lamb angle. In addition, similarities between wave phenomena at L/SP/L interfaces and L/S interfaces are presented which suggest that the description of bounded beam reflection derived by Bertoni and Tamir for a L/S interface can be qualitatively applied to the L/SP/L interface case. This theory is shown to account for the lateral extent to which a leaky Lamb surface wave, excited by mode conversion, propagates.Subject Classification: [43]35.54; [43]20.40, [43]20.30.

Thomas J. Plona

Papers

Axisymmetric wave propagation in fluid-loaded cylindrical shells. I: Theory

Effects of borehole stress concentrations on dipole anisotropy measurements

Axisymmetric wave propagation in fluid‐loaded cylindrical shells. II: Theory versus experiment

Methods and systems for determining formation properties and in-situ stresses

Ultrasonic bounded beam reflection and transmission effects at a liquid/ solid‐plate/liquid interface