Patent EA-003846-B1 (EA)
Title: Method and system for acoustic frequency selection in acoustic logging tools
Inventor: IKEGAMI TORU
IPC: G01V1/46
Filed: 2001-05-16
Abstract: 1. A method for transmitting sonic waves in a logging system having a sonde in a borehole with one or more dipole sources of the sonde to optimally excite dipole flexural mode sonic waves in the borehole, comprising the steps of : (a) determining the Airy frequency of the borehole; and (b) firing said dipole sources at a dipole firing frequency selected based on the Airy frequency. 2. The method of claim 1, wherein the dipole sources are fired at the dipole firing frequency over a frequency range, wherein the dipole firing frequency and frequency range are selected to be as close as possible to a firing frequency range from half to twice the Airy frequency. 3. The method of claim 1, further comprising the step of : (c) receiving said transmitted sonic waves with an array of sonic receivers. 4. The method of claim 3, further comprising the step of : (d) determining physical properties of underground formations around said borehole based on said sonic waveform measurements. 5. The method of claim 1, wherein step (a) comprises the steps of: (1) transmitting initial sonic waves with said dipole sources at an initial firing frequency; (2) receiving said initial sonic waves with an array of sonic receivers; and (3) determining the Airy frequency of the borehole based on said received initial sonic waves. 6. The method of claim 5, wherein step (a) (3) comprises the step of determining the Airy frequency with a processor located in the sonde in the borehole. 7. The method of claim 5, wherein step (a) (3) comprises the steps of : transmitting to a processor at the surface said received sonic wave information and determining, with the processor, the Airy frequency. 8. The method of claim 5, wherein step (a) (3) comprises the steps of: determining the flexural arrival time of the flexural mode of the initial sonic waves, determining the shear slowness based on the flexural arrival time, and determining the Airy frequency based on the shear slowness and the diameter of the borehole. 9. The method of claim 1, wherein step (b) comprises the steps of : inputting the Airy frequency into a look-up table (LUT); and receiving from the LUT, in response to the input Airy frequency, an output index for selecting a selected one of a plurality of excitation waveforms corresponding to a selected dipole firing frequency having a frequency range. 10. The method of claim 1, wherein said one or more dipole sources comprise a pair of dipole sources. 11. A sonic logging system for measuring physical properties of underground formations around a borehole, comprising: (a) a processor for determining the Airy frequency of the borehole; and (b) a sonde having one or more dipole sources for transmitting sonic waves in the borehole at a selected one of a plurality of dipole firing frequencies, wherein the selected dipole firing frequency is selected based on the Airy frequency. 12. The system of claim 11, wherein the sonde fires the dipole sources at the dipole firing frequency over a frequency range, wherein the dipole firing frequency and frequency range are selected to be as close as possible to a firing frequency range from half to twice the Airy frequency. 13. The system of claim 11, the sonde comprising an array of sonic receivers for receiving said transmitted sonic waves. 14. The system of claim 13, further comprising a processor for determining physical properties of underground formations around said borehole based on said sonic waveform measurements. 15. The system of claim 11, further comprising: means for transmitting initial sonic waves with said dipole sources at an initial firing frequency; an array of sonic receivers for receiving said initial sonic waves; and a processor located in the sonde for determining the Airy frequency of the borehole based on said received initial sonic waves. 16. The system of claim 15, wherein the processor determines the Airy frequency by: determining the flexural arrival time of the flexural mode of the initial sonic waves, determining the shear slowness based on the flexural arrival time, and determining the Airy frequency based on the shear slowness and the diameter of the borehole. 17. The system of claim 11, further comprising: a look-up table (LUT) for receiving the Airy frequency as an input and for providing, in response to the input Airy frequency, an output index for selecting a selected one of a plurality of excitation waveforms corresponding to a selected dipole firing frequency having a frequency range. 18. The system of claim 11, wherein said one or more dipole sources is a pair of dipole sources. 19. A sonde for transmitting sonic waves to optimally excite dipole flexural mode sonic waves in a borehole, the sonde comprising: (a) a processor for determining the Airy frequency of the borehole; and (b) one or more dipole sources for transmitting sonic waves in the borehole at a selected one of a plurality of dipole firing frequencies, wherein the selected dipole firing frequency is selected based on the Airy frequency.