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

• US 9,766,119 B2
• Filed: 09/08/2016
• Issued: 09/19/2017
• Est. Priority Date: 12/15/2011
• Status: Active Grant

• Current Assignee: Shell USA Incorporated (Shell Plc)
• Original Assignee: Shell Oil Company (Shell Plc)
• Inventors: Den Boer, Johannis Josephus, Mateeva, Albena Alexandrova, Pearce, Jeremiah Glen, Mestayer, Jeffery Joseph, Birch, William, Lopez, Jorge Louis, Hornman, Johan Cornelis, Kuvshinov, Boris Nikolaevich
• Primary Examiner(s): Petkovsek, Daniel
• Application Number:
  US15/259,348
• Publication Number:
  US 20160377476A1
• Time in Patent Office: 376 Days
• CPC Class Codes: G01H 9/004 using fibre optic sensors light guides per se G02B6/00, acousto-optical devices specially adapted for gating or modulating in optical wave guides G02F1/125
• CPC Class Codes: G01H 9/006 the vibrations causing a variation in the relative position of the end of a fibre and another element
• CPC Class Codes: G01V 1/226 Optoseismic systems
• CPC Class Codes: G01V 1/40 specially adapted for well-logging

Abstract

A method of distributed acoustic sensing includes providing a fiber optic distributed acoustic sensing system having a cable. A straight optical fiber extends parallel to a longitudinal axis of the cable along the cable length. A helically wrapped optical fiber extends along the cable length. The method includes transmitting optical signals into and receiving backscattered signals out of the optical fibers consisting of a component of said optical signals which component has been backscattered from impurities or inhomogeneities in the optical fibers, observing changes in the backscattered signals caused by axial stretching and compressing of the optical fibers caused by an incident wave, comparing the backscattered signals of the straight optical fiber and the helically wrapped fiber, and determining, based on the comparing of the backscattered signals, a direction of wave propagation of the incident wave with respect to the fiber axis for detecting broadside waves and axial waves distinguishably.

Claims

1. A method of distributed acoustic sensing, the method comprising the steps of:

providing a fiber optic distributed acoustic sensing system, the 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 one helically wrapped optical fiber extending along the cable length and having a first predetermined wrap angle;

transmitting optical signals into each of the optical fibers;

receiving backscattered signals out of each of the optical fibers consisting of a component of said optical signals which component has been backscattered from impurities or inhomogeneities in each of the optical fibers;

observing changes in the backscattered signals caused by axial stretching and compressing of each of the optical fibers caused by an incident wave;

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

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

2. The method according to claim 1, further comprising reconstructing a seismic wave amplitude from the backscattered signals.

3. The method according to claim 1, wherein the incident wave includes acoustic signals travelling in a formation induced by seismic or microseismic events.

4. The method according to claim 3, comprising the step of arranging the cable on a surface of the earth.

5. The method according to claim 3, comprising the step of arranging the cable within the formation.

6. The method according to claim 3, comprising the step of arranging the cable within a borehole in the formation.

7. The method 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°

.

8. The method according to claim 1, further including providing a first sheath layer on the outside of the elongated body and covering the elongated body and the first straight optical fiber.

9. The method according to claim 8, the cable including at least a second helically wrapped optical fiber wrapped around the outside of the first sheath layer.

10. The method according to claim 9, wherein the first helically wrapped optical fiber and the second helically wrapped optical fiber define different wrap angles.

11. The method according to claim 10, 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°

.

12. The method according to claim 11, the cable further including a third helically wrapped optical fiber disposed on the outer surface at a wrap angle between 90°

and 45°

.

13. The method of claim 1, comprising the step of measuring seismic signals along the cable as a function of incidence angle of the seismic signals.

14. The method of claim 1, the step of comparing the backscattered signals of the at least one straight optical fiber and the at least one helically wrapped optical fiber comprising using the backscattered signals of the at least one straight optical fiber to calibrate partitioning of the backscattered signals of the at least one helically wrapped optical fiber.

15. The method of claim 1, the at least one straight optical fiber being arranged in the cable.

16. The method according to claim 1, comprising the steps of towing the cable and using the cable as towed streamer cable.

17. The method according to claim 1, comprising the step of measuring arrival times and waveforms of acoustic signals, the incident wave comprising the acoustic signals.

18. A method of distributed acoustic sensing, the method comprising the steps of:

providing a fiber optic distributed acoustic sensing system, the 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;

transmitting optical signals into each of the optical fibers;

receiving backscattered signals out of each of the optical fibers consisting of a component of said optical signals which component has been backscattered from impurities or inhomogeneities in each of the optical fibers;

observing changes in the backscattered signals caused by axial stretching and compressing of each of the optical fibers caused by an incident wave;

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

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