Borehole apparatus and methods for simultaneous multimode excitation and reception to determine elastic wave velocities, elastic modulii, degree of anisotropy and elastic symmetry configurations
An acoustic borehole logging system for generation and detection of multipole modes used to determine elastic properties of earth formations characterized as inhomogeneous anisotropic solids. The system concurrently generates and senses monopole, dipole, quadrupole and any higher order pole in the borehole/formation system in order to characterize the elastic properties and stress state of material penetrated by the borehole. Multipole modes of all orders are induced simultaneously without the need for separate transmitter and receiver systems. Performance of the logging system is not compromised due to eccentering of the axis of the tool in the borehole, tool tilt with respect to the axis of the borehole, or mismatch of response sensitivity of multiple receivers within the tool. The system comprises apparatus for generating and sensing acoustic signals in a borehole in an earth formation, and further comprises a processing method by which the sensor response signals are processed and analyzed to obtain desired formation parameters of interest.
1. A method for determining a property of a borehole environs comprising a formation penetrated by a borehole, said method comprising the steps of:
(a) disposing an acoustic transmitter and an acoustic receiver array within a tool operating within said borehole;
(b) simultaneously generating a plurality of acoustic modes in said borehole environs with an asymmetric pulse of acoustic radiation emitted by said transmitter;
(c) measuring responses of said receiver array induced by said pulse, wherein
(i) said receiver array comprises at least two receiver stations at different axial spacings from said transmitter, and
(ii) each said receiver station comprises a plurality of receiver elements disposed azimuthally near the periphery of said tool;
(d) combining said responses to simultaneously identify said modes; and
(e) determining said property from said identified modes.
2. The method of claim 1 comprising the additional step of determining velocity of each said mode.
3. The method of claim 2 comprising the additional steps of determining amplitude and velocity of types of wave forms induced in said borehole environs.
4. The method of claim 3 comprising the additional step of using said velocities of said modes and said velocities and amplitudes of said types of waveforms to determine a parameter of interest: wherein
(a) said parameter of interest is determined in terms of local material coordinates; and
(b) said local material coordinates are related local borehole coordinates which define a location at which said parameter of interest is measured within said borehole.
5. The method of claim 4 comprising the additional steps of determining a plurality of parameters of interest comprising elastic moduli, degree of formation anisotropy, elastic symmetry and other geophysical properties of said formation.
6. The method of claim 4 wherein:
(a) said transmitter comprises two transmitter elements oriented to yield independent and orthogonal direct ultrasonic reflection measurements at a wall defining said borehole; and
(b) said reflection measurements are used to determine elastic constants of said formation.
7. The method of claim 4 comprising the additional step of transforming said local material coordinates of said parameter of interest into global coordinates.
8. The method of claim 1 comprising the additional steps of operating said transmitter repetitively to transmit a plurality of pulses at a repetition rate ranging from about 1 to about 10 pulses per second and at a frequency in the range of about 500 Hertz to about 20 kilohertz.
9. The method of claim 1 wherein a value of said property is independent of relative orientations of major axes of said tool and said borehole.
10. The method of claim 1 wherein a value of said property is independent of sensitivity mismatches in said receiver elements.