Detecting broadside acoustic signals with a fiber optical distributed acoustic sensing (DAS) assembly

1. A distributed fiber optic acoustic sensing system comprising:

a towed streamer cable having a cable length, the towed streamer cable comprising:

an elongated body having an outer surface;

at least one straight optical fiber extending parallel to a longitudinal axis of the towed streamer cable along the cable length; and

at least one helically wrapped optical fiber extending along the cable length and having a first predetermined wrap angle; and

light transmitting and receiving means optically connected to said at least one straight optical fiber and the least one helically wrapped optical fiber for, respectively, transmitting optical signals into each of the at least one straight and at least one helically wrapped optical fibers and receiving a backscattered component of said signals out of the at least one straight and at least one helically wrapped optical fibers.

2. The distributed fiber acoustic sensing system according to claim 1 , wherein the first predetermined wrap angle is measured with respect to a plane normal to the axis of the elongated body and wherein the first predetermined wrap angle is smaller than 90°.

3. The distributed fiber acoustic sensing system according to claim 1 , wherein the system further comprises a first sheath layer on the outside of the elongated body and covering the elongated body and the first straight optical fiber.

4. The distributed fiber acoustic sensing system according to claim 3 , wherein the towed streamer cable further comprises at least a second helically wrapped optical fiber wrapped around the outside of the first sheath layer.

5. The distributed fiber acoustic sensing system according to claim 4 , wherein the first helically wrapped optical fiber and the second helically wrapped optical fiber define different wrap angles.

6. The distributed fiber acoustic sensing system according to claim 5 , wherein the wrap angles are measured with respect to a plane normal to the axis of the elongated body and wherein the first predetermined wrap angle is 90° and the second predetermined wrap angle is less than 45°, or wherein the second predetermined wrap angle is 90° and the first predetermined wrap angle is less than 45°.

7. The distributed fiber acoustic sensing system according to claim 6 , wherein the towed streamer cable further comprises a third helically wrapped optical fiber disposed on the outer surface at a wrap angle between 90° and 45°.

8. The distributed fiber acoustic sensing system according to claim 1 , wherein the towed streamer cable is adapted to measure seismic signals along the towed streamer cable as a function of incidence angle of the seismic signals.

9. The distributed fiber acoustic sensing system according to claim 1 , wherein the at least one straight optical fiber is arranged in the towed streamer cable.

10. The distributed fiber acoustic sensing system according to claim 1 , further comprising:

means for transmitting optical signals into each of the at least one straight and at least one helically wrapped optical fibers;

means for receiving backscattered signals out of each of the at least one straight and at least one helically wrapped optical fibers consisting of a component of said optical signals which component has been backscattered from impurities or inhomogeneities in each of the at least one straight and at least one helically wrapped optical fibers;

means for observing changes in the backscattered signals caused by axial stretching and compressing of each of the at least one straight and at least one helically wrapped optical fibers caused by an incident wave;

means for comparing the backscattered signals of the at least one straight optical fiber and the at least one helically wrapped optical fiber; and

means for determining, based on the comparing of the backscattered signals, a direction of wave propagation of the incident wave with respect to the longitudinal axis of the towed streamer cable for detecting broadside waves and axial waves distinguishably.

11. A distributed fiber optic acoustic sensing system comprising:

a towed streamer cable having a cable length, the towed streamer cable comprising:

an elongated body having an outer surface;

at least one straight optical fiber extending parallel to a longitudinal axis of the towed streamer cable along the cable length; and

at least two orthogonal sinusoidal optical fibers extending along the cable length; and

light transmitting and receiving means optically connected to said at least one straight optical fiber and at least two orthogonal sinusoidal optical fibers for, respectively, transmitting optical signals into each of the at least one straight and at least at least two orthogonal sinusoidal optical fibers and receiving a backscattered component of said signals out of the at least one straight and at least at least two orthogonal sinusoidal optical fibers.

12. The distributed fiber acoustic sensing system according to claim 11 , further comprising:

means for transmitting optical signals into each of the at least one straight and at least at least two orthogonal sinusoidal optical fibers;

means for receiving backscattered signals out of the each of the at least one straight and at least at least two orthogonal sinusoidal optical fibers consisting of a component of said optical signals which component has been backscattered from impurities or inhomogeneities in each of the at least one straight and at least at least two orthogonal sinusoidal optical fibers;

means for observing changes in the backscattered signals caused by axial stretching and compressing of the each of the at least one straight and at least at least two orthogonal sinusoidal optical fibers caused by an incident wave;

means for comparing the backscattered signals of the at least one straight optical fiber and the at least two orthogonal sinusoidal optical fibers; and

means for determining, based on the comparing of the backscattered signals, a direction of wave propagation of the incident wave with respect to the longitudinal axis of the towed streamer cable for detecting broadside waves and axial waves distinguishably.

13. A distributed fiber optic acoustic sensing system comprising:

a cable having a cable length comprising:

an elongated body having an outer surface;

at least one straight optical fiber extending parallel to a longitudinal axis of the cable along the cable length; and

at least one helically wrapped optical fiber extending along the cable length and having a first predetermined wrap angle; and

light transmitting and receiving means optically connected to said at least one straight optical fiber and the least one helically wrapped optical fiber for, respectively, transmitting optical signals into each of the at least one straight and at least one helically wrapped optical fibers and receiving a backscattered component of said signals out of the at least one straight and at least one helically wrapped optical fibers;

wherein the cable is an ocean bottom cable (OCB) deployed on the seabed.

14. A distributed fiber optic acoustic sensing system comprising:

a cable having a cable length, the cable comprising:

an elongated body having an outer surface;

at least one straight optical fiber extending parallel to a longitudinal axis of the cable along the cable length; and

at least two orthogonal sinusoidal optical fibers extending along the cable length; and

light transmitting and receiving means optically connected to said at least one straight optical fiber and at least two orthogonal sinusoidal optical fibers for, respectively, transmitting optical signals into each of the at least one straight and at least at least two orthogonal sinusoidal optical fibers and receiving a backscattered component of said signals out of the at least one straight and at least at least two orthogonal sinusoidal optical fibers;

wherein the cable is an ocean bottom cable (OBC) deployed on the seabed.

15. A distributed fiber optic acoustic sensing system comprising:

a cable having a cable length, the cable comprising:

an elongated body having an outer surface;

at least one straight optical fiber extending parallel to a longitudinal axis of the cable along the cable length; and

a first sinusoidal optical fiber extending in a first plane along the cable length;

a second sinusoidal optical fiber extending along the cable length, wherein the second sinusoidal optical fiber defines a plane perpendicular to the first plane; and

light transmitting and receiving means optically connected to said at least one straight optical fiber and the first and second sinusoidal optical fibers for, respectively, transmitting optical signals into each of the at least one straight and first and second sinusoidal optical fibers and receiving a backscattered component of said signals out of the at least one straight and first and second sinusoidal optical fibers.

16. The distributed optic acoustic sensing system of claim 15 , wherein the cable is a towed streamer cable.

17. The distributed optic acoustic sensing system of claim 15 , wherein the cable is an ocean bottom cable (OCB) deployed on the seabed.