Method and apparatus for determining properties of anisotropicelastic media

A method for determining properties of earth formations surrounding a borehole, comprising the steps of:

• establishing acoustic waves in said formations;
• measuring, at longitudinally spaced locations along the borehole, at least two orthogonal components of acoustic waves which have propagated through the formations;
• computing, for positions corresponding to a plurality of adjacent ones of said locations, model composite waves which would result from the superposition of a plurality of model acoustic waves each having selected model parameters, said model acoustic waves including a plurality of slow and fast model acoustic shear waves travelling in opposite directions and said model parameters including model velocities and model polarizations for each of said model acoustic shear waves;
• determining an error value which depends on the differences, at each of said plurality of adjacent ones of said locations, between measured wave components and the model composite waves;
• modifying said model parameters;
• iteratively repeating said computing, determining, and modifying steps to reduce said error;
• and
• storing the ultimately modified model parameters as being indicative of properties of said formations in the region of at least one of said plurality of adjacent ones of said locations.

A method for determining properties of earth formations surrounding a borehole in which acoustic waves were established in said formations and, at longitudinally spaced locations along the borehole, there were measured at least two orthogonal components of acoustic waves which have propagated through the formations, comprising the steps of:

• computing, for positions corresponding to a plurality of adjacent ones of said locations, model composite waves which would result from the superposition of a plurality of model acoustic waves each having selected model parameters, said .model acoustic waves including a plurality of slow and fast model acoustic shear waves travelling in opposite directions and said model parameters including model velocities and model polarizations for each of said model acoustic shear waves;
• determining an error value which depends on the differences, at each of said plurality of adjacent ones of said locations, between measured wave components and the model composite waves;
• modifying said model parameters;
• iteratively repeating said computing, determining, and modifying steps to reduce said error;
• and
• storing the ultimately modified model parameters as being indicative of properties of said formations in the region of at least one of said plurality of adjacent ones of said locations.

Apparatus for determining properties of earth formations surrounding a borehole:

• means for establishing acoustic waves in said formations;
• means for measuring, at longitudinally spaced locations along the borehole, at least two orthogonal components of acoustic waves which have propagated through the formations;
• means for computing, for positions corresponding to a plurality of adjacent ones of said locations, model composite waves which would result form the superposition of a plurality of model acoustic waves each having selected model parameters, said model acoustic waves including a plurality of slow and fast model acoustic shear waves travelling in opposite directions and said model parameters including model velocities and model polarizations for each of said model acoustic shear waves;
• means for determining an error value which depends on the differences, at each of said plurality of adjacent ones of said locations, between measured wave components and the model composite waves;
• means for modifying said model parameters;
• means for iteratively repeating said computing, determining, and modifying steps to reduce said error;
• and
• means for storing the ultimately modified model parameters as being indicative of properties of said formations in the region of at least one of said plurality of adjacent ones of said locations.