Unipole and bipole acoustic logging while drilling tools

1. An acoustic logging while drilling tool, comprising:

a logging while drilling tool body;

a single unipole acoustic transmitter deployed on the tool body, the transmitter configured to transmit an acoustic waveform having a first frequency into a subterranean borehole;

a second unipole acoustic transmitter configured to transmit an acoustic waveform having a second frequency, wherein the second frequency is not equal to the first frequency; and

a single linear array of unipole acoustic receivers deployed on the tool body, the linear array including a plurality of longitudinally spaced apart unipole acoustic receivers, the acoustic receivers configured to receive said transmitted acoustic waveform, the acoustic receivers being longitudinally and azimuthally spaced apart from the unipole acoustic transmitter.

2. The logging while drilling tool of claim 1 , wherein the linear array comprises at least four unipole acoustic receivers.

3. The logging while drilling tool of claim 1 , wherein the linear array is azimuthally spaced apart from the unipole acoustic transmitter by an angle of about 180 degrees.

4. The logging while drilling tool of claim 1 , wherein the linear array is azimuthally spaced apart from the unipole acoustic transmitter by an angle of about 90 degrees.

5. The logging while drilling tool of claim 1 , wherein the linear array is azimuthally spaced apart from the unipole acoustic transmitter by an angle of at least 40 degrees.

6. An acoustic logging while drilling tool, comprising:

a logging while drilling tool body;

a single unipole acoustic transmitter deployed on the tool body, the transmitter configured to transmit an acoustic waveform having a first frequency into a subterranean borehole; and

a single linear array of unipole acoustic receivers deployed on the tool body, the linear array including a plurality of longitudinally spaced apart unipole acoustic receivers, the acoustic receivers configured to receive said transmitted acoustic waveform, the acoustic receivers being longitudinally spaced apart from the unipole acoustic transmitter and azimuthally spaced apart from the unipole acoustic transmitter by an angle of about 180 degrees.

7. The logging while drilling tool of claim 6 , further comprising a controller, the controller configured to make acoustic logging while drilling measurements, said measurements comprising (i) causing the unipole acoustic transmitter to transmit an acoustic waveform and (ii) causing the linear array of unipole acoustic receivers to receive corresponding acoustic waveforms.

8. The logging while drilling tool of claim 6 , further comprising a second unipole acoustic transmitter configured to transmit an acoustic waveform having a second frequency, wherein the second frequency is not equal to the first frequency.

9. The logging while drilling tool of claim 8 , wherein the second unipole acoustic transmitter is longitudinally spaced apart from the first unipole acoustic transmitter and deployed at the same azimuthal position as the first unipole acoustic transmitter.

10. The logging while drilling tool of claim 8 , wherein the second unipole acoustic transmitter is azimuthally spaced apart from the first unipole acoustic transmitter and deployed at the same azimuthal position as the linear array of unipole acoustic receivers.

11. An acoustic logging while drilling tool, comprising:

a logging while drilling tool body;

a single unipole acoustic transmitter deployed on the tool body, the transmitter configured to transmit an acoustic waveform having a first frequency into a subterranean borehole; and

a single linear array of unipole acoustic receivers deployed on the tool body, the linear array including a plurality of longitudinally spaced apart unipole acoustic receivers, the acoustic receivers configured to receive said transmitted acoustic waveform, the acoustic receivers being longitudinally spaced apart from the unipole acoustic transmitter and azimuthally spaced apart from the unipole acoustic transmitter by an angle of about 90 degrees.

12. An acoustic logging while drilling tool, comprising:

a logging while drilling tool body;

two unipole acoustic transmitters deployed on the tool body, the unipole acoustic transmitters being azimuthally spaced apart from one another by about 90 degrees, each unipole acoustic transmitter configured to transmit an acoustic waveform into a subterranean borehole;

two linear arrays of unipole acoustic receivers deployed on the tool body, the linear arrays being azimuthally spaced apart from one another by an angle of about 90 degrees, the unipole acoustic receivers configured to receive said transmitted acoustic waveform, the linear arrays being further longitudinally and azimuthally spaced apart from at least one of the unipole acoustic transmitters; and

a controller configured to make acoustic logging while drilling measurements, said measurements comprising (i) causing each of the two unipole acoustic transmitters to transmit an acoustic waveform, and (ii) causing each of the linear arrays of unipole acoustic receivers to receive corresponding acoustic waveforms, wherein the controller is further configured in (i) to cause the two unipole acoustic transmitters to transmit corresponding waveforms having opposite polarities.

13. The logging while drilling tool of claim 12 , wherein:

a first of the two unipole acoustic transmitters is deployed at the same azimuthal position as a first of the two linear arrays; and

a second of the two unipole acoustic transmitters is azimuthally spaced apart from a second of the two linear arrays by an angle of about 180 degrees.

14. The logging while drilling tool of claim 12 , wherein:

a first of the two unipole acoustic transmitters is azimuthally spaced apart from a first of the two linear arrays by an angle of about 90 degrees; and

a second of the two unipole acoustic transmitters is azimuthally spaced apart from a second of the two linear arrays by an angle of about 90 degrees.

15. The logging while drilling tool of claim 12 , wherein the controller is further configured (iii) to process the waveforms received in (ii) by a first of the two linear arrays of unipole acoustic receivers with a first polarity and the waveforms received in (ii) by a second of the two linear arrays of unipole acoustic receivers with a second polarity, the second polarity being opposite to the first polarity.

16. An acoustic logging while drilling tool, comprising:

a logging while drilling tool body;

a single bipole transmitter consisting of first and second unipole transmitters deployed at the same longitudinal position on the tool body, the first and second unipole transmitters being azimuthally spaced apart by an angle of about 90 degrees;

a single bipole array of unipole receivers consisting of first and second linear arrays of unipole receivers deployed at the same longitudinal position on the tool body, the first and second linear arrays being azimuthally spaced apart by an angle of about 90 degrees, the bipole array of unipole receivers being azimuthally spaced apart from the bipole transmitter by an angle of about 90 degrees; and

a controller configured to make acoustic logging while drilling measurements, said measurements comprising (i) causing the bipole acoustic transmitter to transmit an acoustic waveform, and (ii) causing the array of bipole acoustic receivers to receive corresponding acoustic waveforms.

17. The logging while drilling tool of claim 16 , wherein the controller is further configured in (i) to cause the two unipole acoustic transmitters to transmit corresponding waveforms having opposite polarities.

18. The logging while drilling tool of claim 16 , wherein the controller is further configured (iii) to process the waveforms received in (ii) by a first of the two linear arrays of unipole acoustic receivers with a first polarity and the waveforms received in (ii) by a second of the two linear arrays of unipole acoustic receivers with a second polarity, the second polarity being opposite to the first polarity.