Sonic well logging for radial profiling

Apparatus for determining a radial profile of sonic shear velocity of formations surrounding a fluid-containing borehole, comprising:

• a logging device suspendible in the borehole;
• means for transmitting sonic energy from said logging device to establish flexural waves in the formation;
• means for receiving, at said logging device, sonic energy from said flexural waves, and for producing from the received sonic energy, measurement signals at a number of frequencies;
• means for determining, at each of said number of frequencies, the flexural wave velocity in the formation;
• means for deriving sonic compressional and shear velocities of the substantially undisturbed formation;
• means for deriving sonic compressional velocity of the borehole fluid;
• and
• means for determining the radial profile of sonic shear velocity from the derived compressional and shear velocities of the substantially undisturbed formation, the derived compressional velocity of the borehole fluid, and the flexural wave velocities at said number of frequencies.

A method for determining the radial variation in shear modulus of formations surrounding a fluid-containing borehole, comprising the steps of:

• suspending a logging device in the borehole;
• transmitting sonic energy from said logging device to establish flexural waves in the formation;
• receiving, at said logging device, sonic energy from said flexural waves, and producing from the received sonic energy, measurement signals at a number of frequencies;
• determining, at each of said number of frequencies, the flexural wave velocity in the formation;
• deriving sonic compressional and shear velocities of the substantially undisturbed formation;
• deriving sonic compressional velocity of the borehole fluid;
• and
• determining the radial variation in shear modulus from the derived compressional and shear velocities of the substantially undisturbed formation, the derived compressional velocity of the borehole fluid, and the flexural wave velocities at said number of frequencies.

For use in conjunction with a technique for investigating formations surrounding a fluid-containing borehole that includes the steps of:

• suspending a logging device in the borehole;
• transmitting sonic energy from the logging device to establish flexural waves in the formation;
• receiving, at the logging device, sonic energy from the flexural waves, and producing from the received sonic energy, measurement signals at a number of frequencies;
• a method for determining a radial profile of sonic shear velocity of the formation comprising the steps of:
• deriving sonic compressional and shear velocities of the substantially undisturbed formation;
• deriving sonic compressional velocity of the borehole fluid;
• determining, at each of said number of frequencies, the flexural wave velocity in the formation;
• and
• determining the radial profile of sonic shear velocity from the derived compressional and shear velocities of the substantially undisturbed formation, the derived compressional velocity of the borehole fluid, and the flexural wave velocities at said number of frequencies.