Abstract: The present invention is directed to sonic logging while drilling. A transmitter and at least one receiver are mounted on a drill collar for performing sonic investigations of the formation traversing a borehole. It has been discovered that a drill collar has a natural stop band or notch where acoustic energy propagating in this frequency range is severely attenuated. Thus, to reduce drill collar acoustic coupling, the transmitter is operated within this natural stop band of the drill collar. An imperforate stop band filter is also included between the transmitter and receiver to amplify the natural stop band. The transmitter is mounted transvere to the longitudinal axis of the drill collar, and is preferably mounted within means that reduce drill collar acoustic coupling.

Abstract: The present invention is directed to sonic logging while drilling. A transmitter and at least one receiver are mounted on a drill collar for performing sonic investigations of the formation traversed by a borehole. It has been discovered that a drill collar has a natural stop band or notch where acoustic energy propagating in this frequency range is severely attenuated. Thus, to reduce drill collar acoustic coupling, the transmitter is operated within this natural stop band of the drill collar. An imperforate stop band filter is also included between the transmitter and receiver to enhance the natural stop band. The transmitter is mounted transverse to the longitudinal axis of the drill collar, and is preferably mounted within means that reduce drill collar acoustic coupling.

Abstract: An attenuator for use in attenuating tube waves in a borehole includes a body formed from a permeable material having a rigid matrix which can be saturated with fluid. In its simplest form, the attenuator is one or more cylindrical bodies as part of a borehole acoustic logging system. Alternatively, the shape, particularly at the end regions, can be modified to reduce the amount of reflection of tube waves in the borehole. Another attenuator includes a body for placement in a borehole formed from an attenuating material, the shape of the body being selected such that the change in impedance experienced by acoustic waves in the borehole is gradual rather than abrupt. One particularly preferred shape for the body comprises one in which the diameter is greatest towards the center of the body, for example the shape formed by two cones placed base to base, or a cylinder having conically tapered ends. A number of bodies can be used where appropriate.

Abstract: Apparatus and methods for determining an indication of a nonlinear property of a formation traversed by a borehole are provided. An acoustic tool is used in a borehole to generate first and second signals of different frequencies. As a result of nonlinearities in the formation, the mixing of the first and second signals results in a third signal having a frequency equal to the difference of the frequencies of the first and second signals. The amplitude of the third signal is measured by a detector, and the indication of nonlinearity of the formation is determined according to a relationship which relates the measured amplitude at the measured frequency to the amplitudes of the first and second signals, the frequency of measurement, the velocity of the measured wave, the distance of the receiver from the source, and a function of the nonlinear and linear parameters of the formation.

Abstract: Methods and apparatus are provided for determining an indication of a nonlinear property of the formation. A sonic tool is used in a borehole to generate a signal having multiple cycles of a wave of a predetermined frequency. The amplitude of a tube wave resulting from the signal is measured. The measured amplitude is then used to determine an indication of the nonlinearity of the formation according to a relationship which relates the measured amplitude to the amplitude of a second harmonic tube wave which is generated in the borehole by the tube wave of the predetermined frequency due to the nonlinear property of the formation. The indication of nonlinearity may then be utilized to give an indication of the relative consolidation of the formation surrounding the borehole.

Abstract: A method of investigating a formation traversed by a borehole includes, measuring at a plurality of different static borehole pressures the acoustic Stoneley and/or flexural wave velocities of waves propagating through the borehole and formation, and generating an indication of the nonlinearity of the formation by processing the velocity measurements. The velocity measurements are processed either by determining a fractional change in the measured acoustic velocity and dividing that fractional change by the change in borehole pressure to provide frequency dependent acoustoelastic coefficients, or by determining the fractional change in the measured acoustic velocity, and subtracting from the fractional change a component generated by the borehole fluid and a component due to linear aspects of the formation to provide a nonlinear formation component. By processing the velocity measurements at a plurality of frequencies, the nonlinear formation components are used to find nonlinear parameters of the formations.

Abstract: Low and high frequency flexural waves or their equivalents are generated with dipole or other source transducers. From measurements made at receiving transducers which are oriented at two orthogonal directions in a horizontal plane normal to the borehole axis, and via known processing techniques, the received signals are transformed into arrivals as a function of frequency such that the principal polarization directions and the magnitudes of the maximum and minimum wave velocities at those directions are determined at different frequencies. If the maximum velocity of the relatively low frequency flexural waves are in a first principal polarization direction, and the maximum velocity of the relatively high frequency flexural waves are in a second principal polarization direction which is substantially normal to the first principal direction, uniaxial stress in the formation is attributed to stress induced azimuthal anisotropy as opposed to an instrinsic anistropy in the formation.

Abstract: An apparatus for reducing or eliminating the effects of a quadrupole component of an acoustic signal received from a transversely mounted acoustic source in a drill collar of a logging while drilling tool. The apparatus includes a drill collar for supporting a drill bit, a transversely mounted transmitter situated in a first horizontal plane in the drill collar which transmits acoustic energy into an earth formation. The drill collar also houses at least one receiver mounted in a second horizontal plane in the drill collar at an angular displacement from the transmitter such that the effects of a quadrupole component of the acoustic energy detected by the receiver are rendered insignificant.