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 geophysical property of a material, said method comprising:
using an acoustic transmitter to emit an asymmetric pulse of acoustic radiation into the material;
finely sampling and digitizing a response of an acoustic receiver array to the asymmetric pulse of acoustic radiation in the material to avoid temporal and spatial aliasing;
from said response, simultaneously identifying, by angular decomposition, phases propagating over said receiver array into multiple acoustic modes;
calculating velocity of each said mode;
identifying types of wave form components for each said mode;
defining elastic constants for an elastic material having triclinic symmetry;
using results of the calculating, identifying, and defining steps to determine said elastic constants for said material thereby determining said geophysical property of said material.
2. The method of claim 1 comprising the additional steps of:
obtaining a set of measured values by calculating quasilongitudinal, quasishear wave S 1 , quasishear wave S 2 , and Stoneley wave velocities from said finely sampled and said digitized receiver array response;
obtaining a set of estimated wave velocities for quasilongitudinal, quasishear wave S 1 , quasishear wave S 2 , and Stoneley wave obtained by assuming a vertical transverse isotropic formation, a horizontal transverse isotropic formation, and and orthotropic formation;
combining the set of measured values and the set of estimated wave velocities to obtain a set of quality indicators;
iterating values of said elastic constants until said set of quality indicators stabilizes thereby defining a best set said elastic constants; and
using said best set of said elastic constants to determine said geophysical property.
3. The method of claim 1 wherein said geophysical property comprises elastic moduli or degree of anisotropy or elastic symmetry configuration.
4. The method of claim 1 wherein:
said material comprises earth formation penetrated by a well borehole;
said transmitter and said receiver array are disposed within a tool conveyed within said borehole.
5. The method of claim 4 comprising the additional step of defining location and orientation of said geophysical property with respect to a global coordinate system.