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 system for measuring a property of borehole environs comprising a formation penetrated by a borehole, said system comprising:
(a) a tool conveyable within said borehole;
(b) an acoustic transmitter disposed within said tool, wherein
(i) said transmitter transmits an asymmetric acoustic pulse into said borehole environs, and
(ii) said acoustic pulse generates a plurality of acoustic modes within said borehole environs; and
(c) an acoustic receiver array disposed within said tool and axially displaced from said transmitter; wherein
(i) said receiver array comprises at least two receiver stations at different axial spacings from said transmitter,
(ii) each said station responds, at a plurality of azimuthal positions, to said modes generated by said transmitter,
(iii) said responses are combined using angular decomposition to simultaneously identify said modes, and
(iv) said identified modes are used to determine said property.
2. The system of claim 1 wherein said transmitter is operated repetitively transmitting 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.
3. The system of claim 1 ; wherein
(a) each said receiver station comprises a plurality of broad band receiver element responsive to acoustic energy of frequency in the range of about 500 Hertz to about 50 kilohertz; and
(b) a single said receiver element at each said azimuthal position and near the periphery of said tool.
4. The system of claim 1 wherein
(a) each said receiver station comprises a single broad band receiver element responsive to acoustic energy of frequency in the range of about 500 Hertz to about 50 kilohertz; and
(b) said single receiver element is rotated through each said azimuthal position to measure said responses at each said azimuthal position.