JPH0664154B2 - Submarine seismic sensor - Google Patents

Description

The present invention relates to the use of fiber optic pressure sensors to determine the direction of propagation of seismic pressure waves in water.

A long cylindrical plastic streamer that extends for one or two miles during subsea seismic surveys
Put multiple pressure sensors inside. The streamer is towed underwater to the desired depth by the ship. The surface layer below sea level is acoustically probed by any suitable means. The sound waves are reflected from the surface layer below and return to the water surface in the form of pressure waves. The pressure wave is detected by the pressure sensor and converted into an electric signal. The electrical signal is transmitted to the towing vessel via a transmission line in the streamer.

The reflected sound wave returns directly to the pressure sensor and is detected there for the first time, but not only that, but also the detected sound wave is reflected by the water surface and returns to the pressure sensor. The sound wave reflected on the water surface is delayed by a time proportional to twice the depth of the pressure sensor, and appears as a secondary signal or a ghost signal. The sound wave that directly returns and the sound wave that is reflected on the water surface approach each other in time-several ms, and therefore tend to interfere with each other. Therefore, in determining the propagation direction of a sound wave, it is desirable that the waves propagating in the upward and downward directions can be easily discriminated during data processing.

Two separate sensors can be fixedly arranged in a vertical array. Therefore, of course, the propagation direction of a sound wave can be easily identified by measuring the time difference of a specific wave reaching each sensor forming a vertical array. For example, US Patent No.
See 3,952,281. However, this method
Requires a separate hydrophone cable for books. This prescription is obviously uneconomical, as each cable costs about $ 500,000.

Given a sufficiently compact sensor, it would be possible to load nearly vertical arrays of sensors separated by several inches in the same streamer. However, earthquake streamer cables can twist and bend while being towed underwater. With conventional detectors, if a nearly vertical array of sensors is loaded into the streamer, there is no way to determine which sensor in the array is on top.

It is also important to be able to identify unwanted horizontally propagating waves due to scatterers in or near the bottom of the water layer.

As is well known, there is a hydrostatic pressure gradient between two vertically separated points in water. Therefore, if the hydrostatic pressure gradient between two vertically aligned detectors can be measured, the upper detector in the array can be identified.

A conventional underwater detector or hydrophone uses a piezoelectric ceramic wafer as a detection portion. Generally, the wafer is loaded to operate in bend mode. Transient pressure changes due to acoustic waves bend the wafer and generate an AC current.
Wafers are also sensitive to hydrostatic pressure. However, hydrostatic pressure DC charges quickly leak through attached circuits. Therefore, the differential DC component due to the hydrostatic pressure difference of the detector signal cannot be detected.

It is an object of the present invention to provide multiple arrays of sensors in an inexpensive streamer capable of detecting transient AC pressure signals due to seismic waves, and DC bias due to vertical hydrostatic pressure gradient in the direction of seismic wave arrival in 3D space. Is to identify with respect to the vertical direction detected.

According to the invention, a plurality of optical fibers and a sensor array are attached to a plurality of sensor stations (sensor loading positions) spaced along the streamer inside an earthquake streamer. Each sensor array consists of a set of at least three and preferably four single mode fiber optic coils that act as pressure sensors. 90 ° on the inside of the streamer skin when four coils are used
Four coils are installed separately from each other. A laser or LED for each set of sensor coils emits monochromatic coherent light via the input transmission line. Transient and steady pressures in the sensor coil modulate the light. 1
The modulated output light from each sensor coil of the set is sent to a multi-input photodetector, where the light from each sensor coil is each combined with standard light. The resulting optical beat signal is converted by the photodetector into an AC electrical signal that represents the polarity and intensity of the transient seismic signal entering the sensor coil.

If the different modulated output lights are combined with each other,
The phase difference between the output lights depends on the hydrostatic pressure gradient between the sensor coils.
It is preferably done in a photodetector that converts to a DC electrical signal with an intensity that represents a DC bias. Due to an earthquake
The AC and DC bias signals are transmitted to a data processor, which analyzes the propagation direction of the incoming seismic wave.

Lasers, photodetectors, data processors and other optical and electronic circuits may be installed on the towing vessel. Input light and modulated output light are transmitted to the sensor coil through a bundle of optical fibers.

A laser or LED, a photodetector and a beam splitter providing standard light may be provided at the sensor station as a module, one for each set of sensor coils. The modulated output light is converted into an electrical signal AC and
DC components are analyzed. Electrical signals are transmitted by wires to the data processor.

Referring now to FIG. 1, a ship 10 towing an earthquake streamer 12 in water 14 is shown. The streamer 12 is towed by an exterior lead-in lead 16 including a pressure resistant member for protection, and one or two lead-in leads 16 are provided.
It contains a bundle of one or more optical fibers. The lead-in lead 16 and the streamer 12 are stored on the reel 18 at the stern of the ship 10 when not in use. The streamer 12 has several fiber optic sensor coil sets 20, one for each sensor station. As will be seen later, each sensor coil set 20 has three, preferably four sensor coils. A measuring device package 22 such as a laser, a photodetector, an optical coupler and a data processing device is installed on the ship 10.
The measuring device package 22 will be described in detail later.
The buoy 24 shows the rear end of the streamer 12. A known system using one sensor per sensor station is shown in U.S. Pat. No. 4,115,753.

The streamer 12 consists of a long cylindrical skin of polyvinyl chloride, polyurethane or similar plastic, approximately 3 inches in diameter and closed at both ends. A complete streamer can be thousands of feet long, but can be split into many removable parts for easier handling. The streamer is filled with a substantially incompressible fluid, which is well permeable to seismic waves so that the internally loaded sensor and the external pressure can be coupled. The pressure-resistant member 28 is usually made of a stainless steel cable, and is passed through the entire streamer to prevent breakage due to towing force.

2 is a longitudinal sectional view of the cable portion of the sensor station, and FIG. 3 is a sectional view taken along line 3-3 of FIG. A set of at least 3 and preferably 4 sets of sensor coil / sat 20 optical fiber sensor coils
30, 32, 34, 36, each sensor coil having an elongated shape and mounted on the inner surface of the outer skin 26 of the streamer 12 parallel to its longitudinal axis. For illustration purposes it is assumed that four sensor coils are used. Thus, 30 and 32 and 34
There will be two sets of sensor coils like and 36. The sensor coils of each set are placed radially opposite one another at 90 ° intervals, and as far as possible from and parallel to the longitudinal axis of the streamer. The sensor coil is preferably held in place by a plastic spider such as 38. Since the longitudinal axis of the streamer during towing is approximately horizontal, the sensor coil set will form a two-dimensional array with a spread in the vertical plane comparable to the inner diameter of the skin 26.

The sensor coil is made by winding a single mode glass fiber many times with less light loss per unit length. The size and number of turns of the sensor coil depends on the total length of optical fiber required.

As is well known, when an optical fiber is subjected to compressive force, its refractive index and / or its elongation changes. The phase shift between the light transmitted through the standard fiber and the light transmitted through the detection fiber under compressive force is the length of the fiber and / or the increment of the index of refraction and / or extension, or It is a function of both. See, eg, US Pat. No. 4,320,475. An actual pressure sensor requires a fiber of about 100 m as a detection fiber. An elongated fiber coil loop approximately 2 m long and 2-3 cm wide requires about 25 turns. The sensor coil must be mounted so that bending or movement of the streamer skin does not distort the coil shape. Such distortion will, of course, give a false signal to the system.

Two fiber optic bundles 40, 42 run through each spider and streamer carrying a sensor coil at each sensor station. The fiber optic bundle 40 is an outgoing transmission link through which input light from a laser (not shown in FIG. 2) is emitted to the sensor coil. The optical fiber bundle 40 may consist of a single fiber for coupling the transmitted light to each sensor coil, or it may consist of a single fiber bundle,
In this case, individual fibers are assigned to each sensor coil. In reality, the sensor coils receive a common light input. For example, the sensor coil 36 is an input fiber lead.
35 and output fiber lead 37, other sensor coils also have similar input and output fiber leads. The small size and light weight of fibers allows hundreds of fibers to be combined into a bundle without unnecessarily bulking.

The fiber optic bundle 42 is the return transmission link for the sensor coil output light. There is one output fiber for each sensor coil. Therefore, four output fibers are required to serve each sensor station. The free end of the fiber optic bundle 42 emerging from the streamer and the lead-in lead on the ship is coupled to the optical processing circuit described below.

A preferred method of operating the present invention is shown in FIG. 4, where a schematic of the optical processing circuitry is shown. In FIG. 4, the left side of the broken line 44 is entirely on the ship 10 and forms a part of the measuring device package 22. The right side of the broken line 44 is a part of the streamer 12.

A laser or LED 46, preferably operating near the infrared portion of the spectrum, directs coherent light 47 toward an optical coupler 48.
And the optical coupler 48 couples the light into the fiber or fibers forming the fiber optic bundle 40. Optical coupler 48 essentially serves as a common input to the fiber optic bundle. Light is transmitted to a fiber optic sensor coil where it is modulated by seismic transient pressure waves and ambient hydrostatic pressure. The modulated light returns from the sensor coil through the fiber optic bundle 42 to the measurement device package 22. Although only one typical sensor station is shown in FIG. 4 for simplicity, it should be understood that the optical fibers 40, 42 can be extended to serve other sensor stations.

In the measuring device package 22, a portion 53 of the coherent light 47 is directed by the beam splitter 50 to an appropriate optical delay module 52, from which the output becomes the standard light 54. Optical delay module 52 includes beam splitter 50
And sensor coils 30,3 at any sensor station
The light 53 is delayed by an amount corresponding to the optical path length between 2, 34 and 36. A differential delay module is associated with each of the plurality of sensor stations to compensate for differences in optical path length.

The modulated output light returns from the sensor coils 30, 32, 34, 36 through the corresponding optical fibers 30 ', 32', 34 ', 36'. The output light is combined with the standard light 54 by a suitable photodetector in each multi-input combining module 56. The resulting beat frequency is converted into a series of electrical AC waves that represent transient pressure changes due to seismic waves. The electrical signals from the four sensors can be multiplexed onto the data processor 58 via line 60.

The DC bias due to the hydrostatic pressure gradient between the light passing through the first pair of radially opposed sensor coils such as 30 and 32 is 2
It is measured by combining the two output lights at photodetector 62. The phase shift between the two output lights is converted to an electrical DC bias signal having a sign and strength and sent to the data processor 58 via line 64. Similarly, the DC bias between the output light of the second set of sensor coils 34 and 36 is measured by photodetector 66. The resulting electrical output is sent to data processor 58 via line 68. The orientation of the sensor coil with respect to the vertical plane can be analyzed from the strength of the two bias signals using well known mathematical algorithms. Since we already know the orientation of the sensor coils in the vertical plane, the data processor 58 then analyzes the direction of the seismic pressure wave propagation, which is the time it takes for each sensor coil in the array to arrive. This is done by measuring the difference.

The above discussion discloses means for analyzing the intensity and direction of seismic waves propagating in a fluid medium such as water in a vertical plane perpendicular to the cable. The direction of propagation in three-dimensional space can of course be determined, which measures the difference in the arrival times of the same seismic wave at two or more sensor stations following along the cable by methods known in the art. It is done by doing. The time difference in the longitudinal direction can be combined with the time difference in the vertical direction by simple vector addition, and the three-dimensional propagation direction is analyzed.

Although the present invention has been described in terms of certain specific embodiments, those skilled in the art will be able to contemplate other devices having similar effects without departing from the scope of the claims. For example, each sensor array could have its own laser, beam splitter, photodetector, etc., all contained in separate modules attached to each sensor station of the streamer. The measured phase-shifted electrical analog signals of modulated light and standard light are transmitted by wire to a data processor 58 on board the ship 10.

Conventionally, there was a drawback that it was not possible to identify which sensor was above in the vertical array of sensors due to twisting or bending during the use of a streamer cable for detecting seabed seismic waves, but as in the present invention, there was a change in hydrostatic pressure. Is configured to detect, the effect that it becomes possible to detect the upper sensor in the array in a compact form.

[Brief description of drawings]

1 shows a ship towing a streamer underwater with multiple fiber optic sensor coils at multiple sensor stations, and FIG. 2 shows a longitudinal cross-sectional view of the streamer at a typical sensor station. FIG. 3 is a sectional view of the streamer taken along line 3-3, and FIG. 4 shows a schematic of the optical circuit. [Explanation of symbols of main part] Pressure sensor …… 30,32,34,36 Array …… 20 Member …… 12 Monochromatic coherent light …… 47 Standard light …… 54 Transmission means …… 40,42 Signal recorder …… 22 Laser …… 46 Detection means …… 56 Photodetector …… 62,66

Claims (6)

[Claims] 1. A device for analyzing the magnitude and direction of a seabed seismic wave propagating in a fluid medium in a vertical plane, which emits a coherent beam of monochromatic light (47).
6) and at least three active fiber optic pressure sensors disposed substantially in the vertical plane for receiving and modulating the coherent beam in response to the propagation of pressure in the fluid medium to provide a modulated beam. A sensor array (20) comprising (30-34) and having a substantially common light beam input (12) and a separate light beam output (42), and receiving a modulated beam from said separate light beam output. Means (60) for deriving an AC signal component indicative of a transient pressure change due to a submarine seismic wave in the fluid medium by interferometrically combining each of the modulated light beams with a standard beam (60); Means (62, 66) for deriving a DC signal component indicative of the difference in hydrostatic pressure between the sensors of the array by interferingly combining the separated light beams with each other; Means for analyzing the direction and magnitude of the signal propagation by combining the DC signal component vector (5
8) A device comprising: 2. The device comprises an elongated streamer (12) which is closed at both ends and which has a tubular outer skin for accommodating a vertical array (20) of sensor coils therein, and a fluid pressure outside the streamer. And a volume of fluid contained within the streamer for coupling the sensor to the device according to claim 1. 3. The apparatus according to claim 2, wherein the device has a plurality of sets of sensor arrays (20) arranged along a longitudinal direction of the elongated streamer at a desired sensor station. The device according to. 4. The device comprises the fiber optic sensor (30, 32, 34, 36) adjacent an inner surface of the tubular outer skin (26).
A means (38) for supporting the streamer, wherein each sensor has an elongated configuration parallel to the longitudinal axis of the streamer. apparatus. 5. A method for analyzing the direction of a submarine seismic wave propagating in an aqueous medium, wherein the method is arranged substantially horizontally for receiving and modulating coherent and monochromatic light beams to provide a modulated light beam. Arranging a plurality of sets of active fiber optic sensors in and along the streamer at desired spacing with at least three fiber optic sensors substantially in a vertical plane; and the plurality of modulated beams. By individually and interferometrically combining the standard beams, an AC signal component indicating a transient pressure change in the water medium due to the ocean bottom seismic wave striking each sensor is derived, and the modulation for each pair of sensors is performed. By interfering with each other, the two light beams are interferometrically combined to derive a DC signal component indicating the magnitude of the relative hydrostatic pressure difference, and And measuring the time difference of arrival of the transient pressure change in the horizontal and vertical directions with respect to selected sensors arranged in the vertical direction, and analyzing the propagation direction of the seafloor seismic wave from the measured time delay. And a method comprising :. 6. A method of processing signals from a submarine earthquake, comprising underwater towing a tube filled with a fluid having a longitudinal axis and permeable to submarine seismic waves, each comprising at least three active lights. Arranging multiple sets of active fiber optic pressure sensor arrays with fiber pressure sensor coils within the tube and along its longitudinal axis, the coils of each array being within the tube at its longitudinal axis. A submarine seismic wave is detected by the optical fiber pressure sensor coil, which is provided in a plane vertical to the pipe and is evenly arranged so as to surround the pipe, and generates an output signal, and the output signal AC. And separating the DC signal component, and analyzing the AC and DC signal components in a vector to determine the arrival direction of the submarine seismic wave.