MXPA06008525A - Marine seismic acquisition system - Google Patents

Abstract

A marine cable for seismic surveys is described with a plurality of ceramic pressure sensors (901-904) arranged in groups of at least two pressure sensors with a group output being representative of the vertical pressure gradient at the group location, and an inclinometric system including one or more transducers for determining the orientation of the sensors of the group in order to determine their true vertical separation.

Description

For two-leuer codes and other abbreviations, referto the "Guidance Notes on Codes and Abbreviations" appearing at the beginning-ning ofeach regular issue of the PCT Gazette.

MARINE SEISMIC ACQUISITION SYSTEM FIELD OF THE INVENTION This invention relates to methods and apparatus for obtaining seismic data in a seismic acquisition system, with a type cable or as a conductor, with the use of secondary arrangements of seismic sensors.

BACKGROUND OF THE INVENTION In seismic exploration of the sea, a plurality of seismic sensors are housed in long tubular plastic cables, which may extend for several miles. In accordance with the type of seismic research, these cables are known as ocean floor cables or conductors (OBC). A driver is towed by a seismic collector vessel, through water and to a desired depth. A marine seismic source, such as an air gun, is used to generate acoustic waves. The acoustic waves are reflected from the lower layers of the earth, to return to the surface of the water in the form of pressure waves. The pressure waves are detected by the pressure sensors and converted into electrical signals. A towed conductor comprises a plurality of pressure-sensitive hydrophone elements housed within a waterproof bushing and electrically coupled with the recording equipment on board the craft. Each element of the hydrophone within the conductor is designed to convert the mechanical energy present, in pressure variations surrounding the hydrophone element, into electrical signals. This conductor can be divided into a number of separate modules or sections, which can be decoupled from one another and which are individually waterproof. Individual conductors can be towed in parallel, through the use of floats to create a two-dimensional arrangement of the hydrophone elements. Common data conductors run through each of the driver modules and carry the signal from the hydrophone elements to the recording equipment (called acoustic data). A hydrophone can produce electrical signals in response to pressure variations of the acoustic waves through the hydrophone. Several hydrophones may be coupled together to form an active section or a group of conductors or acoustic sensors. The electrical signals from multiple hydrophones of an active section typically combine to provide an average signal in response and / or to increase the ratio of noise to signal. Recently, a new generation of drivers was introduced, with the use of so-called receiving points. In these conductors, the signals can be recorded by individual hydrophones. The details of the designs of the new drivers, compared to conventional drivers, are described on pages 16 to 31 of the "Summer 2001" edition of the Oi If ield.

For the purpose of the present invention, it is important to note that in both the receiving point conductor and the conventional conductor, the hydrophones are arranged essentially in linear arrays in the direction of the conductor. Reflected sound waves not only return directly to the pressure sensors where they were first detected, but the same reflected sound waves are reflected a second time from the surface of the water and return to the pressure sensors. Of course, the sound waves reflected to the surface are delayed by an amount of time proportional to twice the depth of the pressure sensors and appear as "phantom" or secondary signals. Because sound waves reflected to the surface and direct waves arrive almost together in time, they tend to interfere with each other or with other signals that propagate through the earth and share the same time of arrival. It is therefore desirable, determine the direction of the propagation of sound waves, so that sound waves propagated up and down, can be separated during data processing. The so-called double sensor towed conductors, the driver carries a combination of pressure sensors and speed sensors. The pressure sensor is typically a hydrophone and the motion or speed sensors are geophones or accelerometers. In U.S. Patent 6,512,980, a driver is described, as a carrier of pairs of pressure sensors and motion sensors combined with a third sensor, a noise reference sensor. The noise reference sensor is described as a variant of the prior art of the pressure sensor. In practice, towed dual-sensor conductors have difficulty using them like the geophones deployed in the conductor, which generates signals proportional to the driver's vibrations. Also, it is often not easy to correlate the respective outputs of the hydrophones and the geophones. Additionally it is known to place two individual hydrophones in a vertical arrangement. Of course, it should be relatively easy, to identify the propagation direction of the sonic waves, from the difference measured in the time in which a particular wave reaches the respective sensors that make up the vertical array, as described, for example. , in U.S. Patent Number 3,952,281. However, that method requires two separate hydrophone cables. The cost of such cables is close to half a million dollars each, this measure is hampered by the relative complexity of the deployment and the high costs involved in doubling the number of drivers for an investigation. In U.S. Patent Nos. 4,547,869 and 4,692,907 it has been suggested to mount an essentially vertical arrangement of sensors, within the same conductor, with some separation. But a seismic conductor cable twists and turns as it is towed in the water. The twisting and turning of the driver makes it difficult to distinguish between the sensors in the vertical array. The '907 patent, suggests the use of sensors with differential flotation in liquid-filled chambers. Patent '869, describes an acquisition system based on a single model of optical fibers, by using the difference in phase shifts due to the hydrostatic pressure of the optical signal of the diametrically opposite pairs, to the fiber sensors as means to identify the orientation of the fiber sensors. A similar conductor is described in EP 0175026 A1. Outside the seismic fields, arrangements of hydrophone groups for linear antennas have been suggested (WO-03/019224 A1) and in light of the foregoing, an objective of this invention is to provide an improved seismic acquisition system, including arrangements of hydrophones in a cable or in a plurality of cables, transported by a seismic vessel.

BRIEF DESCRIPTION OF THE INVENTION According to a first aspect of the present invention, a marine seismic research system is provided, with a marine cable with a plurality of pressure sensors of ceramic pieces, wherein the plurality of pressure sensors, is arranged in groups of at least two pressure sensors with an output group, which is representative of the vertical pressure gradient in the group location, the system also comprises one or more electromechanical transducers to determine the relative position of the at least Two pressure sensors in order to determine their vertical separation. The cable of the present invention may be an ocean floor cable or a vertical seismic cable, such as that used for the vertical seismic profile (VSP). However, it is preferable that the cable is one of a plurality of conductors towed behind a seismic survey vessel through the body of water. A group is defined by the proximity between (a) and (b), by the processing of the outputs of the hydrophones. The hydrophones of a group are essentially close neighbors. In a conductor, the internal hydrophones of a group are typically separated by 1 to 10 cm, while the distance between the groups is 0.5 or 1 meter to 7.5 meters. In a preferred embodiment, the at least two pressure sensors that contribute to the output of the group, representative of the vertical pressure gradient, are located in an internal section of the cable to less than 6 cm or even up to 3 cm in length, which allows them to be mounted on a single hydrophone holder on a conductor. The vertical separation between the hydrophones of a group of preference is less than 6 cm. In a preferred embodiment, the hydrophones of a group are spaced equidistantly apart. Preferably, most or all of the hydrophones are arranged in a plane perpendicular to the main axis of the cable. However, for a complete waveform recording, involving the acquisition of vertical, in-line and cross-line seismic signals, it is important to have at least one pressure sensor located outside the plane. Or alternatively, a sensor of a neighboring group, provides the additional measure of out-of-plane pressure. In one embodiment of the invention, a group can be formed of four hydrophones in a tetrahedron array. It is advantageous to combine or wire the output signals of the hydrophones and / or amplify them, before a digitization process, since the pressure difference between two separated hydrophones can be extremely small. Another additional feature of the invention is to provide an inclinometric system for determining the orientation of the hydrophones within a group, in particular the vertical distance between those hydrophones used to determine the vertical pressure gradient. The measurement of the orientation or angle of rotation is necessary, since when the cable is suspended or is towed in the water, it is subject to twisting and turning. In a preferred embodiment the inclinometer system, characterized by one or more electromechanical or electroacoustic equipment that is not hydrophone. The devices are operated to cause a response indicative of the orientation or angle of rotation of the hydrophones. In a first embodiment, the electromechanical or electroacoustic devices of the inclinometric system are formed by one or more acoustic sources, to emit acoustic signals or pulses and a system to measure the arrival times of the pulses or the signals to the hydrophones. Preferably, existing sonic positioning systems are used, such as acoustic sources. However, it is possible to use easy-to-identify features and events generated by seismic sources for such measurements. The source or sources are located preferentially, in a conductor towed in parallel with the conductor that carries the hydrophone groups, to produce a cross-line angle of the incidences of the sonic signals. Alternatively, electromechanical or electroacoustic devices may take the form of a plurality of small inclinometers. It has been found that the precision of the measurements can be achieved by using small, robust inclinometers, preferably of solid state. By distributing a sufficient number of such known sensors along the cable, their orientation can be measured, with respect to the vertical and horizontal directions. In both variants, the measurement is independent of the hydrostatic pressure, that is, the height of the water column on the sensors. This and other additional aspects of the invention are described in detail in the following examples and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS Now the invention will be described, only by way of example, with reference to the accompanying drawings, of which: Figure 1 shows a schematic illustration of a ship carrying conductors and seismic sources. Figure 2 is a vertical cross section of the cable of a conductor with two hydrophones. Figure 3 shows a vertical cross-section of the cable of a conductor with three hydrophones. Figure 4 is a vertical cross section of the cable of a conductor with two hydrophones and a unit that generates the sum and difference of the outputs of the two hydrophones, as output signals. Figure 5 is a plot of the relative amplitude of the pressure gradient measurement, as a function of the signal frequency. Figure 6 illustrates the relative amplitude of a measurement of the pressure gradient, as a function of the signal frequency, for the different angles of rotation of a conductor cable. Figure 7 shows another vertical cross section of a conductor cable with three hydrophones with and without an inclinometer. Figure 8 shows a vertical cross section of a conductor cable with five hydrophones arranged in a plane. Figure 9 shows a perspective view of a cable section of a conductor, with two neighboring groups of three hydrophones. Figure 10 shows a diagram of a perspective view of a cable section of a conductor, with a group arranged in the form of a tetrahedron of four hydrophones.

DETAILED DESCRIPTION OF THE INVENTION A marine seismic acquisition system is illustrated in Figure 1. Four instrumented cables or conductors 10, are towed by a ship 11. A frontal network 12 and a similar network in the tail of the network (not shown) , it is used to connect the ship and the drivers. Embedded in the front of the network are the seismic sources 13, typically in an array of air pistols. Each conductor 10 is typically assembled from many hydrophone fastening segments that are coupled to form the conductor. Between the segments, the drivers carry controllable baffles 111 (often known as fins or "birds") and other aids, to direct the driver along the desired trajectory in the water body. The exact position of modern drivers is controlled by a satellite based positioning system, such as a GPS or differential GPS, with GPS receivers on the front and on the driver's tail. In addition to positioning based on GPS, it is known to monitor the relative positions of the conductors and conductor sections, through a network of sonic transceivers 112, which transmit and receive acoustic or sonar signals. Such systems are available for sale at Sonardyne. The main purpose of a conductor 10 is to carry a large number of seismic sensors 101, which are distributed along its length. In Figure 1, the hydrophones are schematically represented as marked boxes. The hydrophones of the present invention are known as the tube type of ceramic pieces. As the geometric arrangement of the hydrophones is a feature of the present invention, the details of various possible arrays of the hydrophones within a driver's helmet are described in the following figures. In Figure 2 a cross-section of a hydrophone holder 21 is shown within a conductor cable 20. Two hydrophones 201, 202 are arranged diametrically and opposite each other within the openings 203, 204 of the fastener 21. An outer flexible cover 22 protects the hydrophone from direct contact with water. Each hydrophone consists of a hollow tube of material of ceramic pieces. The pressure leads to a deformation of the tube, which in turn generates an electrical signal that when amplified and calibrated properly, serves as a measure of pressure. According to the type of driver, one or more braided wire tension elements 23, run along the conductor through the length of the conductor segments or along their entire length. A central data transmission cable, comprising a plurality of electrical conductors and / or optical fibers 24, communicates the data along the length of the conductor, as well as for the towing ship. The fastener 21 of the hydrophone shown is one of a large number of fasteners distributed along the conductor. Typically, a conductor further includes chambers (not shown) between the fasteners, to be filled with any liquid (such as Kerosene) and / or solid flotation material (such as foam). Therefore, it is possible to tune the driver's float in the water. Typically, the hydrophones used in marine seismic conductors are cylindrical devices with their main axis (X) parallel to the main axis of the conductor, so the accelerations of the conductor in the crossing line (Y) and the vertical direction (Z), are cancel The Y axis and the vertical Z axis are shown in Figure 2 with the X axis (not shown), (the driver axis) pointing away from the paper plane. It is known that the vertical pressure gradient, at a location x along the conductor can be measured by using two hydrophones, with a known vertical distance between them. The pressure gradient vertical P,, can be calculated from two hydrophone records separate verticals, by subtracting the two measurements. dP (?) = (P1 { x) -P2 (x)) dz dz [1] N where P ?. { ?) and P2 (x), indicate the pressure measured by the upper hydrophone 201 and the lower hydrophone 202, respectively. The total pressure can be found from the output of one of the hydrophones or by the average of the two measurements of the hydrophones. In the arrangement of the vertical hydrophone of Figure 2, the measurement of the vertical pressure gradient is not sensitive to in-line accelerations or in-line pressure gradients, since the two pressure sensors have the same coordinate (X) in line. Therefore, the pressure gradient data is less contaminated with wave noise. In Figure 3, a modality of the upper group of hydrophones is shown. In the example, the fastener 31 includes an additional hydrophone 303 located in the center, which is added to the group of two separate vertical hydrophones 301 and 302, as described. Up to this point the other elements of Figure 3 have been described in Figure 2, and equivalent numbers have been used and the additional description of those elements has been omitted. In the embodiment of Figure 3, it has been seen as an advantage that, a measure of the pressure gradient can be achieved effectively, through an electrical connection of direct wiring of the pole (+) with the pole (-) of the other and vice versa. The potential difference between the two connections produces the pressure differential dP. The third additional hydrophone 303 is used for the average pressure measurement P. Because the difference between the two signals of the two hydrophones is very small, this subtraction, carried out locally from the sensors before the digitization, is much more accurate than in the arrangement of Figure 2. In the example of Figure 4, two hydrophones 401, 402, are used to determine the pressure differential P, -, and the sum P? + P2, by using an appropriate electrical circuit or network 44 of conductors. The two hydrophones 401, 402 are connected in such a way that one output of the circuit 44 is proportional to the difference between the hydrophones and thus to the pressure gradient, while the other is proportional to the sum and the average pressure between the two hydrophones, that is, for P \ -P2 and P? P < respectively. Up to this point, other elements of Figure 4 have already been described in Figure 2, and equivalent numbers have been used and therefore the description of these elements has been omitted. It is worth mentioning that a large dynamic range of the recording system is needed to achieve the required accuracy of the pressure gradient measurement. The theoretical amplitude response of two hydrophones in a given vertical separation for a pressure wave propagating vertically as a function of the frequency and separation of the sensor can be expressed as: F (?) ~ | Exp (-z'fe) - exp (z? Z) | where z, is half of the separation of the vertical sensor. This response F (?) Has been modeled and plotted in Figure 5, to 6 different separations of the sensors: 2 cm (51), 6 cm (52), 20 cm (53), 1m (54), 2m (55) and 5m (56). For example, in the case of a separation of 6 cm between the hydrophones, which reflect an upper limit for the vertical separation between the hydrophones of a group within a conductor cable, the curve 52 predicts a signal of the pressure gradient, with respect to to pressure at that frequency of -57 dB at 5 Hz, - 38dB at 50 Hz and -32 dB at 100 Hz. The amplitude of the signal of the pressure gradient decreases with the decrease in frequency, at 5Hz is 0.001412 times weaker than the pressure signal (-57 dB). With respect to the digitized output, it means that the first 10 most important bits of a pressure record are not used (for example, they are zeros). When the hydrophones are subtracted before recording, this bit loss does not occur, although an additional preamplifier may be required to stimulate the weakened gradient signal. With a measurement and knowledge of the pressure gradient P,, several known methods can be applied to attenuate or Remove ghosts from seismic data. Such methods are descd, for example, in the published International Patent Application WO 02/01254 and the United Kingdom patent GB 2363459. For example, it is known to use the vertical pressure gradient determined by: where Pu (x) is the wave field without continuous phantoms at position x, along the conductor, P (x) is the unprocessed recording of pressure and k. It is the vertical wave number. This equation can be solved in the wave-frequency number or the FK domain using the driver's data and the relationship between the horizontal line and the vertical wave number, ignoring the wave number on the crossover line: k2 =? 2 / c2 = k2 + k: [4] One of the biggest problems to overcome when implanting the present invention is caused by the rotation of the conductor around the longitudinal axis (x). It is known that the conductor cable can be twisted and twisted, thereby removing the vertically arranged hydrophones. This rotation of the conductor around its main axis introduces an error in the measurement of the vertical pressure gradient since it changes the vertical separation of the hydrophones. In practice, the rotation of the driver may occur during deployment or in operation. Rotation angles of the conductor up to 360 ° have been observed. At a 90 ° angle a pair of hydrophones, such as descd above in Figures 2-4, do not have a vertical separation and the vertical pressure gradient can not be measured. But even at angles less than 90 degrees, an error is introduced in the measurement of the pressure gradient. The error is due to the small rotation angles (rotation YZ), which is shown in Figure 6, with dashes illustrating the attenuation of the signal gradient signal at 6 cm vertical separation, for a rotation angle of 5o (61), 10 ° (62) and 15 ° (63). At an angle of rotation of 10 °, for example, curve 62 gives an error of -36.2 dB, which is almost constant with frequency. In order to reduce the error generated by the rotation of the conductor, the present invention includes a means for determining the angle of rotation of one or more groups of the hydrophones within the deployed conductor. In a first mode, the angle of rotation of the cable can be measured by using one or more inclinometers (or gyroscopes) that measure the angle of the crossing line with the horizontal. Such an inclinometer device has recently been used in cables at the bottom of the ocean (OBCs). Another one of the preferred embodiments makes novel use of the existing acoustic positioning systems 112, as descd above with reference to Figure 1. Such acoustic positioning system comprises high frequency sonic transceivers (1500 - 4500 Hz), placed inside each driver. Commonly, the signals emitted from those sources are captured by other transceivers in the driver's arrangement, which provides information of a relative position. In the present invention, the hydrophones are used to receive signals from the sonic transceivers. For acoustic sources located near nearby conductors with the same elevation as a group of hydrophones, the arrival time of the direct acoustic signal, for the two vertical hydrophones, is identical. This changes as the group hydrophones rotate around the driver's axis. One of the hydrophones moves near the sonic source, while the other moves away from it. An accurate measurement of the respective arrival times, when combined with the known relative positions of the hydrophones, then produces the angle of rotation. Such measurements extended to other geometric arrangements within a group of hydrophones, provided that the hydrophone maintains a fixed distance from each other. As an alternative to use the direct signal, the reflections of the ocean floor of the signal of the sonic transceiver or even the easily detectable signals generated by the seismic sources 110, can be used to determine the differences of the travel time, between the hydrophones of a group and in this way its angle of rotation. By using, for example, the positions of the receiver and the seismic source, and the depth of the water, the reflection of the bottom of the sea can be calculated at an angle and compared with the estimated pressure gradient. Instead of using a controlled sonic or seismic source, differences in normal hydrostatic pressure can be exploited to determine the relative depths of the hydrophones. As the hydrophones rotate, the height of the column of water on them changes and with it, also the static pressure. In U.S. Patent Number 4,547,869, a method is used for fiber pressure sensors, which are more commonly common for slow or near static pressure changes than ceramic-based hydrophones. Once the angle of rotation a, with respect to the vertical, is known, its effect on the measurement of the pressure gradient can be corrected by means of: dP / dz = (Px - P2) / (dz eos) [5] This method is best applied to the angles of rotation close to the vertical axis, while for angles close to the horizontal axis, the vertical gradient is not measured, since the difference P¡-P becomes zero. This has been recognized as a weakness of the foregoing modalities and the following embodiments and examples of the invention demonstrate that they avoid this weakness. In a first of the preferred embodiments of the present invention, as illustrated in Figure 7A, three hydrophones 701, 702, 703 are included in a group, each hydrophone is located in a corner of a triangle, which in turn is oriented in the plane of the vertical crossover line, for example, perpendicular to the longitudinal axis of the cable. The same group of hydrophones 701, 702, 703, is shown in Figure 7B with a solid state inclinometer 71 of the MEMS type. The inclinometer determines the rotation of the surrounding section of the wire of the conductor and therefore the orientation of the three hydrophones 701, 702, 703. The inclinometers 71 may be placed at the location of each hydrophone or they may be distributed scattered along the driver. In the latter case, the mechanical models of the driver are used to interpolate the rotation of the conductor sections between two inclinometers. An equidistant triangle with d 12 = d13 is shown in Figure 7c to illustrate geometric relationships and distances between hydrophones. The embodiment of this Figure 7 has the advantage that the vertical pressure gradient can be obtained by any angle of rotation of the conductor, including that of 90 degrees. An additional benefit is that the seismic interference noise from other acoustic sources can be reduced, as described in more detail below. Once the orientation is known, the vertical gradient can be calculated. The pressure measurement can be averaged over the three pressure measurements. For an equilateral triangle configuration as shown in Figure 7, the vertical pressure gradient can be calculated as a function of the angle of rotation a of the conductor by: dP / dz = (P-P2) / (2ducts (30 +)) + (Pi-P3) / (2ducts (30-)) [6] where dl2 and d13 are the distances between the hydrophones 701 and 702, and between, 701 and 703, respectively, as indicated in Figure 7B. Instead of recording hydrophone signals directly, the arrangement of Figure 7 can be increased with the use of an electrical circuit, as shown in Figure 4. Then, the outputs representing several linear combinations can be generated (sums / subtraction) from the measurements of the hydrophones. For example, it is possible to emit the average output pressure Pv P2 + P2 and the differences between P-? - P2 and Pi - P3. For a known angle of rotation of a conductor, then, the vertical pressure gradient can be calculated by means of equation 6. Another alternative configuration is shown in Figure 8, with two orthogonal pressure gradient sensors, each consisting of two wired hydrophones 801-804, in combination with a fifth hydrophone 805 single. This configuration is an extension of the hydrophone group shown in Figure 3, and similar to the example of Figure 4, the central hydrophone 805 of the group of Figure 8 can be omitted when the two hydrophone pairs are summed and subtracted, use an electrical circuit before being digitized.

Additionally, to be able to operate all angles of rotation, an additional advantage of the configuration is that the pressure gradient P, on the crossing line can be calculated and be used to reduce seismic interference. The pressure gradient in the crossing line will be dominated by seismic interference, since the energy related to the source will propagate dominantly in the vertical plane in line, which means that the seismic lines are predominantly lines of depression. Some remnant-related source-related signals can be removed using an FK filter applied to the common tripping group of the pressure gradient at the crossover line. The contribution of seismic interference noise to the recording of the pressure is given by: Ps¡ Equation [7] requires the wave number ky on the crossover line, which can be estimated when the relative angle of the source of seismic interference is known, with respect to the orientation of the conductor. The pressure wave field due to seismic interference is then subtracted from the total pressure field: PNOSXP'PS, [8] Instead of this simple subtraction, the seismic interference can also be removed with the adaptation of filters as described for example in the application of the international patent WO-97/25632.

In the example described above, the hydrophones forming a group are essentially arranged in a plane perpendicular to the main axis of the conductor cable. However, for many seismic applications, it is advantageous to register as many components of the pressure wave field as possible, within the constraints posed by the equipment. Such complete or almost complete acquisition, that of the wave field can complete its mission, by using at least one additional hydrophone that is located outside the plane defined by the other hydrophones. The additional hydrophone may be any part of the same group, for example, located near other hydrophones of the group, or be a member of a distant one, preferably a neighboring group of hydrophones. In the example of Figure 9, a perspective view of a section of a conductor with two fasteners 91, 92 of neighboring hydrophones is shown. The fasteners are made of a plastic material structure with holes to allow the passage of wire cables 93, 94, 95 through the length of the conductor section. The two fasteners have holes to mount six hydrophones 901-904, only four of which are visible in the view. Additionally, the fasteners have sealing rings 911, 912 and 921, 922 to slide an outer shield or cover (not shown) on the conductor. The typical space between two groups of hydrophones is 3.125 m.

The line pressure gradient P,, can be calculated with the combination of the output of a group of hydrophones, such as the three hydrophones in the fastener 91, with the output of a hydrophone of the neighboring group in the fastener 92. The seismic interference of the internal line can then be calculated by using the equivalence of the equation [7] for the address x, for example, by replacing yy ky for x and kx and subtract it when using the same procedure, as with the cross-line seismic interference described above. The line wave number kx can, for example, be estimated from the fk spectrum of the seismic interference. Alternatively, in-line interference can be removed by using known f-k filtering methods or other conventional filtering techniques. In Figure 10 an alternative embodiment is shown, wherein a group of three hydrophones 1001-1003, in a plane perpendicular to the axis of the conductor, is combined with a hydrophone 1004 out of additional plane. The four hydrophones 1001 - 1004 define a tetrahedral group of hydrophones that can be used to measure the entire acoustic wave field, for example, the gradient in the vertical directions, in the line and in the crossing line or any other of the three directions ( orthogonal). The gradients of pressure in line and in line of crossing can then be used to remove the seismic interference from all (the near) horizontal directions by following the procedure described above. The various configurations described herein can be used in an ocean bottom seismometer (OBS) or ocean floor cable (OBC). Since OBS are deployed in a scattered manner, the elimination of ghosts must be carried out of the domain of a common receiver. The modalities can be applied in recovery systems as well as in permanent systems. They will operate in a marine environment, as well as in transition zones. The elimination of ghosts by means of the use of pressure gradient measurements, can be beneficial in the acoustic telemetry under water, in order to eliminate the ghosts of the received signal before an additional process. It is also possible to improve the acoustic positioning systems, as described above through the use of pressure gradient measurements, by using phantom removal to remove the surface reflection of the sea from the sonic source signals.

Claims (15)

1. A seismic research system, which comprises a marine cable with a plurality of pressure sensors of ceramic pieces, characterized in that the plurality of pressure sensors is arranged in groups of at least two pressure sensors, with a group output which is representative of the vertical pressure gradient in the group location, the system also comprises one or more electromechanical transducers to generate signals to generate a response indicative of the orientation of the at least two pressure sensors.

2. The seismic investigation system according to claim 1, characterized by the group comprising at least three hydrophones.

3. The seismic research system according to claim 1, characterized in that the at least two pressure sensors of a group are located in a plane perpendicular to the main or longitudinal axis of the cable.

The seismic research system according to claim 1, characterized in that the at least one group of pressure sensors are located in a plane perpendicular to the main or longitudinal axis of the cable and where an output of said group is combined with an output of an additional hydrophone, located outside the plane.

5. The seismic research system according to claim 1, characterized in that the group comprises four hydrophones in a tetrahedral configuration.

The seismic survey system according to claim 1, characterized in that the at least two pressure sensors that contribute to the output of the group and are representative of the vertical pressure gradient, are placed within a section of the cable unless 10 cm long, measured in the main or longitudinal direction of the cable.

The seismic investigation system according to claim 1, characterized in that each pressure sensor of a group is arranged at a distance essentially equal to the other sensors of the group.

The seismic investigation system according to claim 1, characterized in that the pressure sensors of a group are connected to provide an output representative of a linear combination of the signals of an individual sensor before digitization.

9. The seismic survey system according to claim 1, characterized in that one or more electromechanical transducers are a plurality of nanometer distributed along the length of the cable.

10. The seismic investigation system according to claim 1, characterized in that one or more electromechanical transducers are one or more acoustic or sonic sources.

The seismic investigation system according to claim 10, characterized in that one or more electromechanical transducers are one or more acoustic sources, located within the cables towed in parallel, with the cable carrying the groups of at least two hydrophones .

The seismic survey system according to claim 1, characterized in that one or more electromechanical transducers are adapted to operate independently of the hydrostatic pressure.

13. A marine seismic cable with a plurality of pressure sensors of ceramic pieces, characterized in that the plurality of pressure sensors is arranged in groups of at least two pressure sensors, with a group output that is representative of the pressure gradient vertical in a group location, for use in the system according to claim 1.

14. A method to acquire an acoustic wave field having in-line, cross-line and vertical components, by using a seismic investigation system, characterized by a marine cable with a plurality of pressure sensors of ceramic pieces, wherein the plurality of pressure sensors is arranged in groups of at least two pressure sensors, with a group output that is representative of the vertical pressure gradient in the location of the group, in addition the system comprises one or more electromechanical transducers, to generate signals adapted for gene erar a response indicative of a relative position, of the at least two pressure sensors.

15. The seismic data obtained by using a method according to claim 14.