Volumetric and non-volumetric sources-based seismic survey and method

A seismic survey system for surveying a subsurface. The system includes a volumetric land source buried underground for generating P-waves; a non-volumetric land source buried underground for generating P- and S-waves; plural receivers distributed about the volumetric and non-volumetric land sources and configured to record seismic signals corresponding to the P- and S-waves; and a controller connected to the volumetric land source and the non-volumetric land source and configured to shot them in a given pattern.

BACKGROUND

1. Technical Field

Embodiments of the subject matter disclosed herein generally relate to devices and methods for generating seismic waves underground and, more particularly, to mechanisms and techniques for generating seismic waves with volumetric and non-volumetric seismic sources.

2. Discussion of the Background

Land seismic sources may be used to generate seismic waves in underground formations for investigating geological structures. A seismic source may be located on the ground or it may be buried in the ground. The seismic source, when activated, imparts energy into the ground. Part of that energy travels downward and interacts with the various underground layers. At each interface between these layers, part of the energy is reflected and part of the energy is transmitted to deeper layers. The reflected energy travels toward the surface of the earth, where it is recorded by seismic sensors. Based on the recorded seismic data (traces), images of the underground layers may be generated. Those skilled in the art of seismic image interpretation are then able to estimate whether oil and/or gas reservoirs are present underground. A seismic survey investigating underground structures may be performed on land or water.

Current land seismic sources generate a mixture of P-waves and S-waves. A P-wave (or primary wave or longitudinal wave) is a wave that propagates through the medium using a compression mechanism, i.e., a particle of the medium moves parallel to a propagation direction of the wave and transmits its movement to a next particle of the medium. This mechanism is capable of transmitting energy both in a solid medium (e.g., earth) and in a fluid medium (e.g., water). An S-wave, different from a P-wave, propagates through the medium using a shearing mechanism, i.e., a particle of the medium moves perpendicular to the propagation direction of the wave and shears the medium. This particle makes the neighboring particle also move perpendicular to the wave's propagation direction. This mechanism is incapable of transmitting energy in a fluid medium, such as water, because there is not a strong bond between neighboring water particles. Thus, S-waves propagate only in a solid medium, i.e., earth.

The two kinds of waves propagate with different speeds, with P-waves being faster than S-waves. They may carry different information regarding the subsurface and, thus, both are useful for generating a subsurface image. However, when both of them are recorded with the same receiver, the strong S-wave content may obscure the P-wave content in certain portions, rendering the final image inaccurate.

Thus, there is a need to record both types of waves, with the ability to separate, at the emission stage, the two kinds of waves as needed. However, current use of land seismic sources does not offer this possibility. Currently, P- and S-waves generated by a land seismic source are simultaneously recorded by plural receivers, and during the processing stage, various strategies are employed for separating the two. However, this process may be time-intensive and inaccurate.

BRIEF SUMMARY OF THE INVENTION

According to an exemplary embodiment, there is a seismic survey system for surveying a subsurface. The system includes a volumetric land source buried underground for generating P-waves; a non-volumetric land source buried underground for generating P- and S-waves; plural receivers distributed about the volumetric and non-volumetric land sources and configured to record seismic signals corresponding to the P- and S-waves; and a controller connected to the volumetric land source and the non-volumetric land source and configured to shot them in a given pattern.

According to another exemplary embodiment, there is a method for combining traces related to a surveyed subsurface for enhancing clarity of the subsurface. The method includes receiving first traces corresponding to a volumetric source; receiving second traces corresponding to a non-volumetric source, wherein the first and second traces correspond to the surveyed subsurface; extracting from the first traces, third traces that correspond to near offset reflections and transmissions and the third traces contain substantially P-waves; replacing with the third traces, in the second traces, fourth traces that correspond to the near offset reflections and transmissions, wherein the fourth traces include both P- and S-waves; and using the obtained combination of second traces and third traces to generate a final image of the subsurface.

According to still another exemplary embodiment, there is a method for conducting a surveying a subsurface. The method includes deploying plural receivers above and/or below land; burying a volumetric source underground; burying a non-volumetric source underground; shooting the volumetric and non-volumetric sources; and combining first traces corresponding to the volumetric source with second traces corresponding to the non-volumetric source to generate a final image of the subsurface.

DETAILED DESCRIPTION OF THE INVENTION

The following description of the exemplary embodiments refers to the accompanying drawings. The same reference numbers in different drawings identify the same or similar elements. The following detailed description does not limit the invention. Instead, the scope of the invention is defined by the appended claims. The following embodiments are discussed, for simplicity, with regard to the terminology and structure of a land seismic source used to perform a seismic survey to evaluate the structure of a solid formation. However, the embodiments are not limited to this structure, but they may be used for reservoir characterization, e.g., 4-dimensional surveying.

According to an exemplary embodiment, a combination of a volumetric source and a non-volumetric source is used to perform a seismic survey. The two different land seismic sources may be shot sequentially or simultaneously to generate both P- and S-waves. The reflected waves are recorded by plural receivers. While the non-volumetric source produces strong S-waves for near offset reflections and transmission (i.e., the waves that travel directly from the source to the receivers) and they hide the reflected and transmitted waves for long offsets, the volumetric source produces, essentially, only P-waves, which do not hide the near offset reflections and transmissions. Thus, by recording P-waves generated by the volumetric source and also P- and S-waves generated by the non-volumetric source over a same subsurface, it is now possible to separate the S-waves from the P-waves for near offset reflections and transmissions as discussed next.

Some examples of volumetric sources are now presented. A first volumetric source may be driven in an impulsive mode or in a vibratory mode. For example,FIG. 1Aillustrates a seismic source10configured to operate in an impulsive mode. The seismic source10includes a spherical tank12filled with fluid14(e.g., mineral oil or water) buried underground16and in close contact with the ground. At the surface18, a pump20is used to feed fluid into the tank12, and valves22and24are used to control the out-flow and in-flow of the fluid between the tank12and the pump20. The pump20may include a power pack and controllers. With these controls, which may be operated remotely via telemetry unit26from a central control and recording station28, it is possible to build up pressure in the tank that will expand its volume and then quickly release it, causing a pressure pulse and generating P-wave seismic energy.

Although the tank12is illustrated inFIG. 1Aas being spherical, it may have a cylindrical shape. Note that a spherical shape minimizes S-wave production because a spherical shape source10is acting like a monopole, i.e., generating only spherical waves40, as schematically illustrated inFIG. 1B. However, even a cylindrical tank having a length comparable to the cylinder's diameter can be considered a volumetric source. From this point of view, a source is considered to be volumetric when most of the generated energy is carried by P-waves and not S-waves. Thus, although an ideal volumetric source is considered to generate no S-waves, in practice, a volumetric source also generates some S-waves.

Optionally, a clean-out line equipped with valve30may be used to drain the fluid from the tank12. A cement plug32may be provided on top of the tank12for burying the source, and a seismic sensor34(e.g., hydrophone) may be placed in the tank12for measuring the seismic waves produced. Also, a pressure transducer36may be provided inside the tank12for measuring the fluid pressure acting on the walls in contact with the earth. This configuration is best suited when the tank12is buried at a shallow depth, because if the inlet and outlet lines are too long, the high frequency output of the system may be compromised due to the fluid inertance imposed by long passageways. The fluid inertance will tend to limit the rate at which pressure can change.

Alternatively, the seismic source may be vibratory as illustrated inFIG. 2. The source100has a tank102that includes a cavity104. The same considerations discussed above regarding the shape of the tank12apply to tank102. An actuation mechanism (e.g., piston arrangement)105is provided inside the cavity104and may include two back-to-back actuators106and108, which may be electromagnetic. The actuation mechanism may be fixed relative to the tank102with a support element109, which may be a bracket. In one application, one or more than two electromagnetic actuators are used. Each actuator may include a coil106aor108aconfigured to electromagnetically displace a corresponding piston106bor108b. Alternatively, the piston may be driven by a motor and cam system at a frequency geared to the motor speed.

The piston motion causes an increase and decrease in the pressure110of a working fluid112inside the tank102, causing an increase and decrease in pressure on the ground120. These pressure changes cause a seismic P-wave signal to propagate from the source into the ground. The frequency of the generated P-wave may be controlled by controlling the movement of the pistons106band108b. Note that electromagnetic actuators have a larger displacement than conventional piezoelectric units.

To transform the displacement of the pistons106band108bfrom a low force into a large force with smaller displacements, as desired for the present volumetric source, a fluid may be used for coupling, as discussed next. The volumetric source100, as already noted above, is configured to change one or more dimensions and, thus, its volume when actuated. However, because the tank102is made of steel or other similar material, the source100cannot accommodate overly large dimensional changes. Thus, it is desirable that displacement of the pistons with low force be transformed into a small displacement with high force to act on the walls102aof the tank102.

According to the exemplary embodiment illustrated inFIG. 2, the piston arrangement105is immersed in the working fluid112so that the working fluid112couples the pistons106band108bto the walls102aof the tank102. At the same time, the working fluid also cools the coils106aand108a. The back sides of the pistons106band108bform an inner cavity114. This inner cavity114may be configured to trap another fluid116(e.g., air). Thus, the back sides of the pistons106band108bwork against the fluid116. In this case, the fluid116works to counteract the hydrostatic pressure in the first fluid112. In other words, the fluid116works as a spring. Other volumetric sources exist but are not discussed herein.

An example of a non-volumetric source is discussed next.FIG. 3Aillustrates a non-volumetric source300(a similar source is described, for example, in U.S. Pat. No. 7,420,879 to Meynier et al., the entire content of which is incorporated herein by reference) that includes plural vibrators (electromechanical, electromagnetic, hydraulic, piezoelectric, magnetostrictive, etc.) forming a pillar301in contact with plates302and303. A force is applied to the pillar301to displace the plates302and303, thereby generating a seismic wave. Because the ground around the source is displaced unsymmetrically, strong S-waves are generated.FIG. 3Bschematically illustrates lobes320representing the S-waves and waves330representing the P-waves. Note that a volume of the source does not necessarily increase when the plates302and303move apart, contrary to a volumetric source, because the ground between these two plates may move toward the pillar301.

Pillar301, which may be covered with a deformable membrane304, is connected by a cable305to a signal generator306. Source300is placed in a cavity or well W, for example, of 5 to 30 cm in diameter, at a desired depth under the weather zone layer WZ, for example, between 5 and 1000 m. A coupling material307, such as cement or concrete, is injected into the well to be in direct contact with pillar301over the total length thereof and with plates302and303. To allow the coupling material307to be homogeneously distributed in the space between plates302and303, the plates may have perforations308. The diameter of plates302and303substantially corresponds to the diameter of the cavity or well W so as to achieve maximum coupling surface area.

The signal generator306generates an excitation signal in a frequency sweep or a single frequency, causing elements forming the pillar301to expand or contract temporarily along the pillar's longitudinal axis. Metal plates302and303are mounted on the pillar ends to improve the coupling of pillar301with coupling material307. Coupling material307intermediates the coupling between the source and the formation. For example, plates302and303have a thickness of 10 cm and a diameter of 10 cm. Pillar301may have a length exceeding 80 cm. The membrane304may be made of polyurethane and surround pillar301to decouple it from the coupling material (cement)307. Thus, only the end portions of pillar301and plates302and303are coupled with the coupling material (cement)307. Upon receiving an excitation (electrical signal) from the signal generator306, source300generates forces along the pillar's longitudinal axis. This conventional source provides good repeatability and high reliability, once a good coupling is accomplished.

A typical pillar has a cylindrical shape with a radius of 5 cm and a length of 95 cm. This pillar may consist of 120 ceramics made, for example, of lead-zirconate-titanate (PZT) known under the commercial name NAVY type I. Each ceramic may have a ring shape with 20 mm internal diameter, 40 mm external diameter and 4 mm thickness. The maximum length expansion obtainable for this pillar in the absence of constraints is 120 μm, corresponding to a volume change of about 1000 mm3. The electrical signals fed to the pillars have 5-300 Hz, 2500 V peak maximum and 2 A peak maximum. The numbers presented above are exemplary and those skilled in the art would recognize that various sources have different characteristics. Other non-volumetric sources exist but are not presented herein.

However, the novel embodiments discussed next apply to any kind of volumetric and non-volumetric sources. According to an exemplary embodiment illustrated inFIG. 4A, a land seismic surveying system400includes sources402a-band receivers404i. Sources402a-bmay be located inside a well406, underground. Source402amay be volumetric and source402bmay be non-volumetric, as discussed above. In another embodiment, the non-volumetric source is at a greater depth than the volumetric source, i.e., opposite what is shown inFIG. 4A. This arrangement has the advantage that a single well accommodates both sources.FIG. 4Bschematically illustrates the P- and S-waves generated by a combination of volumetric and non-volumetric sources402a-b.

However, as illustrated inFIG. 5, multiple wells may be dug to accommodate individual sources402a-b. Receivers404iare distributed at the surface410and/or below the surface. In one exemplary embodiment, the receivers are buried in the ground as discussed with regard toFIG. 4A. Also, the depths of the various sources may change with the survey. In one application, all the sources are buried at the same depth H as illustrated inFIG. 6. In another exemplary embodiment, the volumetric sources402aare located at a first depth H1, and the non-volumetric sources402bare located at a second depth H2, different from H1.FIG. 7illustrates the case when H1is greater than H2. Note that the sources may be located in a well as shown inFIG. 4Aor completely buried underground.

Returning toFIG. 4A, each source is linked to a corresponding cable420aand420bthat connects the sources to one or more controllers430, a controller including a processor432and a storage device434. The processor432may be programmed to shoot the sources simultaneously, sequentially, using the slip-sweep technique, or any other known technique. Receivers404imay be distributed according to various configurations. For example, the receivers may be located above or below the ground. If below ground, they may be located vertically above the sources, between the volumetric and non-volumetric sources, below the sources or based on a combination of these arrangements. In one application, receivers404iare distributed in another well407. The depth distribution of the receivers inside this additional well may be similar to that used when the receivers are not placed in the well. Receivers404imay be linked to a controller440that includes a processor442and a storage device444. When in use, the receivers may send the seismic data, through a wireless or wired interface, to the storage device444and the processor442may be configured to process the data as discussed later. The controller may be located in the field or at a remote location, for example, in a processing center.

With this mixed arrangement of land seismic sources, an actual seismic survey has been performed and the following results have been obtained.FIG. 8illustrates traces recorded by the plural receivers using only volumetric sources402a. The number of receivers is represented on the X axis, and the time in seconds is represented on the Y axis. Note that good signals are obtained for the near offset reflections and transmissions800, but not-so-good signals are obtained for the far offset reflections and transmissions802. A near offset reflection means a reflected signal recorded by a receiver that is close (near) to the source while a far offset reflection is a trace recorded by a receiver that is far from the source. A near offset transmission means a signal that is transmitted directly from the source to a close by receiver while a far offset transmission is a signal that is transmitted directly from the source to a faraway receiver.

FIG. 9illustrates traces recorded with the plural receivers when non-volumetric sources are used. Note that the traces900corresponding to the near offset reflections and transmissions are very difficult to separate and process because of the strong S-waves, while the traces902corresponding to the far offset reflections and transmissions have better quality than the corresponding traces802. The traces shown inFIGS. 8 and 9may be obtained by sequentially shooting volumetric sources and non-volumetric sources. Alternatively, the volumetric and non-volumetric sources may be shot simultaneously in time, but with different frequencies, e.g., using sinusoids to drive the sources. In another embodiment, the sources may be fired simultaneously based on orthogonal signals.

Thus, according to an exemplary embodiment, traces800corresponding to the near offset reflections and transmissions may be extracted from the recordings corresponding to the volumetric source (P-waves) and then subtracted from traces900corresponding to the near offset reflections and transmissions corresponding to the non-volumetric source (P- and S-waves). In this way, for the near offset reflections and transmissions (not for the far offset reflections and transmissions), the traces corresponding to the S-waves may be separated. These traces can then be subtracted from traces900shown inFIG. 9to remove the S-waves contribution for the near offset reflections and transmissions, but not for the far offset reflections and transmissions.

In other words, as schematically illustrated inFIG. 10A, traces recorded with non-volumetric source have good quality (many wiggle lines) for the far offset reflections and transmissions (outside triangle1000) and low quality (few wiggle lines) for the near offset reflections and transmissions (inside the triangle1000). The traces recorded with the volumetric source, as illustrated inFIG. 10B, have poor quality for the far offset reflections and transmissions (outside triangle1000) and good quality for the near offset reflections and transmissions (inside the triangle1000). Thus, the volumetric data inside the triangle1000inFIG. 10Bis used to substitute the non-volumetric data inside the triangle1000inFIG. 10Aand, thus, as illustrated inFIG. 100, good quality traces are obtained for both the near offset reflections and transmissions (from the volumetric source) and the far offset reflections and transmissions (from the non-volumetric source). Note that far offset reflections and transmissions from both volumetric and non-volumetric data may be added together to enhance this portion of data as illustrated inFIG. 100.

Thus, as illustrated inFIG. 11, a method for combining traces related to a surveyed subsurface for enhancing clarity of the subsurface includes a step1100of receiving first traces corresponding to a volumetric source; a step1102of receiving second traces corresponding to a non-volumetric source, wherein the first and second traces correspond to the surveyed subsurface; a step1104of extracting from the first traces, third traces that correspond to near offset reflections and transmissions and the third traces contain substantially P-waves; a step1106of replacing with the third traces, in the second traces, fourth traces that correspond to the near offset reflections and transmissions, wherein the fourth traces include both P- and S-waves; and a step1108of using the obtained combination of second traces and third traces to generate a final image of the subsurface.

According to another exemplary embodiment illustrated inFIG. 12, there is a method for conducting a surveying a subsurface. The method includes a step1200of deploying plural receivers; a step1202of burying a volumetric source underground; a step1204of burying a non-volumetric source underground; a step1206of shooting the volumetric and non-volumetric sources; and a step1208of combining first traces corresponding to the volumetric source with second traces corresponding to the non-volumetric source to generate a final image of the subsurface. The step1208may include a step1210of extracting first traces corresponding to the volumetric source; a step1212of extracting second traces corresponding to the non-volumetric source, wherein the first and second traces correspond to the surveyed subsurface; a step1214of extracting from the first traces, third traces that correspond to near offset reflections and transmissions and the third traces contain substantially P-waves; and a step1216of replacing with the third traces, in the second traces, fourth traces that correspond to the near offset reflections and transmissions, wherein the fourth traces include both P- and S-waves.