DocumentCode :
2805194
Title :
Higher-order modes in a fluid-filled borehole
Author :
Sinha, Bikash K. ; Asvadurov, Sergey
Author_Institution :
Schlumberger-Doll Res., Ridgefield, CT, USA
Volume :
2
fYear :
1998
fDate :
1998
Firstpage :
1115
Abstract :
Sonic techniques in geophysical prospecting involve elastic wave velocity measurements that are performed by placing acoustic transmitters and receivers in a fluid-filled borehole. The signals recorded at the receivers are processed to obtain compressional and shear wave velocities in the surrounding formation. These velocities are generally used in seismic surveys for the time-to-depth conversion and other formation parameters, such as porosity and lithology. Depending upon the type of transmitter used and as a result of eccentering, one can excite axisymmetric (n=0), flexural (n=1), quadrupole (n=2), and hexapole (n=3) family of modes propagating along the borehole. We present a study of various propagating and leaky modes that includes their dispersion and attenuation characteristics caused by radiation into the surrounding formation. These propagation characteristics help in a proper selection of transmitter bandwidth for suppressing unwanted modes that create problems in the inversion for the compressional and shear velocities from the dispersive arrivals. Computational results for the axisymmetric family of modes in a fast formation with its shear velocity of 2032 m/s show the existence of Stoneley, pseudo-Rayleigh, and anharmonic cutoff modes. In a slow formation with its shear velocity of 508 m/s, we find the existence of Stoneley, and the first leaky-compressional mode which cuts in at approximately the same normalized frequency wa/VS=2.5 (a is the borehole radius), as that of the fast formation. The corresponding modes among the flexural family include the lowest-order flexural and anharmonic cutoff modes. For both the fast and slow formations, the first anharmonic mode cuts in at a normalized frequency wa/VS =1.5 approximately
Keywords :
acoustic dispersion; acoustic wave propagation; acoustic wave velocity; geophysical prospecting; seismic waves; 0 to 20 kHz; 2032 m/s; 508 m/s; Stoneley modes; anharmonic cutoff modes; attenuation characteristics; axisymmetric mode; compressional velocities; dispersion; dispersive arrivals; eccentering; elastic wave velocity; flexural mode; fluid-filled borehole; formation parameters; geophysical prospecting; hexapole mode; higher-order modes; leaky-compressional mode; lithology; porosity; propagation characteristics; pseudo-Rayleigh modes; quadrupole mode; shear wave velocities; sonic techniques; surrounding formation; time-to-depth conversion; transmitter bandwidth; unwanted modes; Acoustic measurements; Acoustic waves; Attenuation; Bandwidth; Frequency; Geophysical measurements; Performance evaluation; Signal processing; Transmitters; Velocity measurement;
fLanguage :
English
Publisher :
ieee
Conference_Titel :
Ultrasonics Symposium, 1998. Proceedings., 1998 IEEE
Conference_Location :
Sendai
ISSN :
1051-0117
Print_ISBN :
0-7803-4095-7
Type :
conf
DOI :
10.1109/ULTSYM.1998.765032
Filename :
765032
Link To Document :
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