WO2002004985A2

WO2002004985A2 - Parametric shear-wave seismic source

Info

Publication number: WO2002004985A2
Application number: PCT/US2001/021827
Authority: WO
Inventors: Loran Ambs
Original assignee: Westerngeco, L.L.C.
Priority date: 2000-07-11
Filing date: 2001-07-11
Publication date: 2002-01-17
Also published as: WO2002004985A3; AU2001280513A1

Abstract

A system and method for generating low frequency seismic source signal by the non-linear combination of two higher frequency energy signals to form a shear-wave signal in geologic formations. The differential between the higher frequencies operates in a nonlinear medium to generate a highly directive, low frequency seismic energy source. A shear-wave signal is produced in the geologic formations by the interaction of two P-wave beams focused on a volume of such geologic formations. Each beam propagates the same signal but in opposite polarity to the other beam. At the beam intersection the vertical component cancels and the horizontal component is amplified. The system and method facilitates application of P-wave and shear-wave energy to subsurface geologic formations underlying soil or water.

Description

PARAMETRIC SHEAR-WAVE SEISMIC SOURCE

The present invention relates to the field of source energy generation for seismic exploration. More particularly, the invention relates to a system and method for efficiently generating a shear-wave signal within subsurface geologic formations. Seismic energy sources impart seismic energy into subsurface geologic formations underlying topsoil or water. Land-based source energy is generated with vibrators, explosives, and other devices. Marine seismic vessels tow vibrators, air guns, and other acoustic projector devices through the water column. The seismic energy travels downwardly through subsurface geologic formations and is reflected and refracted from interfaces between geologic features. The reflected signal impulses return to the surface and are detected with geophones, hydrophones, or a combination of both.

Various techniques have been developed to enhance seismic source energy transmission. In United States Patent No. 3,811,111 to Barbier et al. (1974), seismic signals were generated from a number of offset sources in a selected sequence. In another example, various techniques for pulsing source energy over selected frequency ranges were disclosed in United States Patent No. 3,968,471 to Savit (1976), in United States Patent No. 4,545,039 to Savit (1985), and in United States Patent No. 4,675,851 to Savit et al. (1987). Side lobe amplitudes were reduced by using time-variant techniques, by cross-correlating, and by stacking successively occurring sweeps. In United States Patent No. 5,467,320 to Maki (1995), a drill stem provided an acoustic source for propagating source energy waves.

High frequency parametric acoustic generation techniques are used in marine sonar applications to identify the existence and location of submerged objects. Such techniques focus energy on a desired target as disclosed in United States Patent No. 4,596,007 to Grail et al. (1986) where an interferometric sonar permitted high resolution observation of submerged objects. Two high frequencies Fl and F2 were transmitted simultaneously in the range of 200 kHz and hydrophones received the relatively low frequency difference (F1-F2) at 20 kHz.

A similar approach was described in United States Patent No. 3,786,405 to Chramiec et al. (1974), which used highly directive high-frequency transmission for sonic examination of pipelines and other objects buried in shallow seafloor trenches. A first high frequency and a second high frequency were directed into a nonlinear transmission medium for generating a different frequency radiation beam suitable for locating the buried objects. However, due to the relatively high frequencies employed, such techniques were useful only for surveying several meters of sediment and were not useful for other applications. Numerous sonar systems have used acoustical heterodyning and parametric sonar techniques to control acoustic energy transmission through water. United States Patent No. 4,081,783 to Honda (1978) disclosed a fish identification system using two high frequency ultrasonic waves. United States Patent No. 3,613,069 to Cary et al. (1971) disclosed a parametric sonar system using two ultrasonic waves having different frequencies. United States Patent No. 5,889,870 to Norris (1999) described the emission of sonic or subsonic compression waves from a resonant cavity. United States Patent No. 3,964,013 to Konrad (1976) disclosed a cavitating parametric acoustic source for generating acoustic energy at low and medium frequencies. In United States Patent No. 4,320,474 to Huckabay et al. (1982), the farfield amplitude was selectively increased by controlling the center-to-center spacing of parametric sonar sources. Beam steering by delaying or varying the signal phase was disclosed in United States Patent No. 4,190,818 to Follin et al. (1980), and United States Patent No. 3,824,531 to Walsh (1974) disclosed a technique of randomly arranging radiating elements to minimize the magnitude of side lobes.

Conventional seismic source energy generation systems do not effectively focus source energy to geologic formations located thousands of meters below the surface and do not efficiently produce shear waves within such formations. To provide sufficient energy at frequencies capable of penetrating to such depths and returning measurable reflected impulses, conventional seismic energy sources impart significant energy, however, almost two thirds of such energy is converted to surface waves transmitted laterally from the seismic energy source. Such energy is environmentally disruptive and produces seismic noise interfering with detection of seismic reflection signals returned from deep subsurface geologic formations. Conventional, purpose-built shear-wave seismic sources sometimes comprise large mechanical devices using hydraulically actuated vibrators to impart powerful horizontal motion to geologic formations as disclosed in United States Patents Nos. 4,321,981 to Waters (1982) and 4,632,215 to Farris (1986). United States Patents Nos. 4,050,540 to Cholet and 5,000,285 to Airhart (1991) disclose passive and active weight drops, and United States Patents Nos.

4,059,820 to Turpening (1977) and 4,316,521 to Chelminski (1982) disclose contained explosives. Each of these systems requires a complex baseplate to couple the machine to the geologic formations.

A need exists for an improved method for efficiently transmitting seismic source energy to subsurface geologic formations underlying water or topsoil and for generating subsurface shear waves in such geologic formations.

The invention provides a seismic source system and method for generating a shear-wave signal in geologic formations. The seismic source system comprises a first parametric transmitter for directing a first beam into the geologic formations in a selected direction, and a second parametric transmitter for directing a second beam into the geologic formations in a selected direction intersecting said first beam at a selected location within the geologic formations to form a shear-wave signal.

The method of the invention comprises the steps of operating a first parametric transmitter to direct a first beam into the geologic formations in a selected direction, and of operating a second parametric transmitter to direct a second beam into the geologic formations in a selected direction intersecting the first beam at a selected location within the geologic formations to form a shear- wave signal.

Figure 1 illustrates a schematic view showing the principal components of the seismic source equipment and subsurface parametric formation of a seismic signal.

Figure 2 illustrates a beam pattern for a 2 kHz carrier frequency.

Figures 3 illustrates a beam pattern for a 1kHz carrier frequency.

Figure 4 illustrates formation of a shear- wave from intersection of two parametric transmitters.

The invention provides a parametric energy source system and method for generating source energy for use in seismic exploration operations. The invention utilizes the non-linear acoustic properties of Earth materials to generate an efficient, focused seismic energy beam for acoustically probing subsurface geologic formations. In particular, the invention is capable of generating a shear-wave signal at a selected position within subsurface geologic formations underlying soil or water. As used herein, the term "soil" means Earth materials such as topsoil or subsoil, rock, sand, aggregate, marsh, peat, and other substantially solid materials.

Figure 1 illustrates a schematic representation of a parametric transmitter useful in generating a shear- wave signal. Signal generators 10 and 112 create two high frequency signals Fl and F2, and signals are merged by combiner 14 for transmission to seismic vibrator 16. Seismic vibrator 16 can comprise reaction mass 22, actuator or actuators 24, and baseplate 26 in contact with soil

18. Seismic vibrators of various types and operating principals are well known in the art and can comprise different configurations. As described below, actuator 24 can comprise any device suitable for inducing acoustic source energy into soil 18. Seismic vibrator 16 may comprise multiple vibrating units projecting Fl and 2.

Figure 2 illustrates a beam pattern for a 2kHz vibrator 16 having a two meter diameter baseplate. The primary beam is approximately 36 degrees in width and the magnitude of the first side lobe is approximately 80 dB lower. For transmit angles near 90 degrees (horizontal), the signal is attenuated by approximately 150 dB.

Figure 3 illustrates a beam pattern for a 1 kHz carrier frequency. As illustrated, the primary beam is approximately 75 degrees wide and attenuation for signals transmitted near horizontal is approximately 110 dB. The examples shown in Figures 2 and 3 illustrate that higher carrier frequencies produce a narrower beam pattern with improved focusing of energy downwardly in a selected direction. The effect of focusing seismic energy downwardly dramatically reduces the energy projected horizontally as surface waves such as ground roll, Rayleigh waves, Love waves, and others.

Generator 16 can introduce carrier frequencies into soil 18 in a frequency range approximating 2 kHz. For seismic exploration, this frequency range is relatively high when compared to other conventional seismic sources. This carrier frequency is modulated by another signal which can sweep between 1994 Hz and 1880 Hz to produce a swept difference frequency ("DF") of 6 to 120 Hz. As shown in Figure 1, the nearfield spectrum includes Fl, F2, (F1+F2), and the difference frequency (F1-F2). Because soil 18 and geologic formations comprise nonlinear transmission media, high frequencies Fl and F2 interact to produce the difference frequency having a directivity pattern similarly shaped to that of the energy radiated by vibrator 16 at the higher frequencies.

By introducing seismic source energy into soil 18 at relatively high frequencies, energy transmission is more effective than at lower frequencies. Because geologic formations attenuate high frequencies more rapidly than low frequencies, the high frequency components of the energy do not penetrate deeply into the deeper geologic formations 20. Only the difference frequency components formed from non-linear interaction of the high frequency components are capable of deep penetration within the subsurface.

Source transducers formed by multiple elements can be phase-shifted or time-shifted to steer one or more difference frequency projection lobes. A primary or secondary lobe can be steered, and the character of a lobe can be modified by controlling the shift parameters. In addition to the attenuation of the high-frequency signals by the directivity of the parametric seismic array, high- frequency signals created at the surface are attenuated more rapidly than low- frequency signals due to preferential absorption of high-frequency seismic signals by subsurface geologic formations. This feature of the invention reduces or eliminates ground roll and focuses the compression wave energy downwardly for maximum effect.

The seismic sensors and electronics used to detect low-frequency seismic reflections are tuned to reject high frequency signals. The remaining high- frequency signals which may impinge upon the seismic sensors will not be recorded by the seismic instruments and so the ambient noise typically created by conventional low-frequency seismic sources will not be present to mask the low- frequency reflection and refraction signals. Hence the data quality expressed as signal-to-noise ratio will be improved by the absence of low-frequency ground roll. The absence of in-band ground roll will facilitate collection of high quality seismic data without specially designed receiver subarrays required to attenuate ground roll noise.

Figure 4 illustrates the application of the invention to shear- wave generation in subsurface geologic formations 20. Parametric transmitters 28 and 30 produce narrowly focused P-wave energy beams within a selected seismic band. Each beam can be focused on a selected volume within geologic formations 20, and a shear-wave is produced at the focal point by controlling the signal character of each beam. One beam propagates the same pilot signal as the other beam but has an opposite polarity. At the intersection between the two beams, the vertical components cancel and the horizontal components become additive. The resulting shear wave is polarized parallel to the line formed by parametric transmitters 28 and 30.

If the angle of each beam was directed away from vertical such as the forty-five degree angles illustrated in Figure 4, the P-wave energy has a vertical and horizontal component. The directivity of each beam from vertical permits the beams to intersect at a selected position within formations 20. Because of the strong directivity of the parametric transmission, seismic energy can be transmitted in selected directions from a horizontal baseplate. The directivity of parametric transmitters 28 and 30 is similar to that of an inclined, end-fire line array. Because of the strong directivity of parametric transmitters 28 and 30, seismic energy can be transmitted in different directions from a horizontal baseplate such as baseplate 26. The invention apparatus and method of the invention is applicable to land and marine environments because the invention focuses on the deep subsurface geologic formations 20 underlying the surface. On land, parametric transmitters 28 and 30 rest on the soil as the shear-wave propagating medium. In marine applications, parametric transmitters 28 and 30 can propagate the constituent P-wave beams in water at a distance above the water bottom and focus the beams in the subsurface sediments where the shear- wave result is propagated.

One or more sensors 32 detect seismic energy reflected from interfaces and structures of subsurface formations 20. Controller 34 is engaged with sensors 32 to record data in the form of seismic records or traces, and is further capable of processing such data to generate maps representing formations 20. By providing P-wave and shear-wave seismic source energy with the same sources, one source system is capable of providing different data sets from each seismic source event. Although two parametric source generators are shown, three or more parametric source generators can be positioned in different configurations to change the propagation axis (polarization) for shear waves. If desired, P-wave generator 36 can introduce a P-wave signal into geologic formations 20. Generator 36 can comprise a separate device or can be integrated within one or both of parametric transmitters 28 and 30. Shear waves and P-waves can be generated simultaneously by using three or more parametric arrays. One or more pair of parametric arrays can generate shear waves in one or more polarization directions as described above while one or more additional parametric arrays can generate P-waves. For example, parametric arrays located at the corners of a square could simultaneously produce orthogonally polarized (or circularly polarized) shear waves. One or more parametric arrays located in or about the array could simultaneously operate in- phase to produce and steer a P-wave signal. Because the invention is more efficient than conventional source energy generation techniques, lighter weight vibrators and baseplates can be used. Reduced weight and equipment facilitates field operations and reduces overall cost. Additionally, fewer vibrators are required for the same energy transmission, thereby simplifying seismic operations and reducing the environmental impact of seismic source energy discharge. The invention can be performed with conventional seismic source equipment or can use transducers significantly different from conventional hydro-acoustic transducers. A parametric vibrator can comprise an electro-mechanical or piezoelectric device or combination of devices .

The invention simplifies the coupling of P-wave seismic sources to geologic formations and does not require contact with an ocean bottom to produce shear- waves in a marine environment. The dual use of parametric arrays facilitates generation of P-waves or shear-waves, and the invention furnishes a reliable, repeatable shear-wave source useful for 4C and 4D seismic surveys. Although the invention has been described in terms of certain prefeπed embodiments, it will become apparent to those of ordinary skill in the art that modifications and improvements can be made to the inventive concepts herein without departing from the scope of the invention. The embodiments shown herein are merely illustrative of the inventive concepts and should not be interpreted as limiting the scope of the invention.

Claims

1. A seismic source system for generating a shear-wave signal in geologic formations, comprising: a first parametric transmitter for directing a first beam into the geologic formation in a selected direction; and a second parametric transmitter for directing a second beam into the geologic formations in a selected direction intersecting said first beam at a selected location within the geologic formations to form a shear-wave signal.

2. A seismic source system as recited in Claim 1, wherein said first and second parametric transmitters are engagable with soil.

3. A seismic source system as recited in Claim 2, wherein said first and second parametric transmitters are engagable with water.

4. A seismic source system as recited in Claim 1, wherein said first and second beams comprise P-wave energy.

5. A seismic source system as recited in Claim 1, wherein said first and second beams are transmitted at substantially the same frequency.

6. A seismic source system as recited in Claim 1, wherein said first and second beams are transmitted at opposite polarity.

7. A seismic source system as recited in Claim 1, further comprising a sensor for detecting seismic energy reflected from the geologic formations.

8. A system for generating a shear- wave signal in geologic formations, comprising the steps of: operating a first parametric transmitter to direct a first beam into the geologic formations in a selected direction; and operating a second parametric transmitter to direct a second beam into the geologic formations in a selected direction intersecting said first beam at a selected location within the geologic formations to form a shear-wave signal.

9. A method as recited in Claim 8, further comprising the step of operating said first and second parametric transmitters to direct said first and second beams into soil.

10. A method as recited in Claim 8, further comprising the step of operating said first and second parametric transmitters to direct said first and second beams into water for transmission to geologic formations underlying the water.

11. A method as recited in Claim 8, further comprising the steps of operating said first parametric transmitter to direct said first beam into soil, and of operating said second parametric transmitter to direct said second beam into water for transmission to geologic formations underlying the water.

12. A method as recited in Claim 8, further comprising the step of operating said first and second parametric transmitters at substantially the same frequency.

13. A method as recited in Claim 8, further comprising the step of operating said first and second parametric transmitters to direct said first and second beams at opposite polarity.

14. A method as recited in Claim 8, further comprising the step of selecting the position of said first and second transmitters to control the direction of said first and second beams toward said selected location within the geologic formations.

15. A method as recited in Claim 8, further comprising the step of controlling the angle from horizontal of said first and second beams.

16. A method as recited in Claim 15, wherein the angle of said first and second beams is forty-five degrees from vertical.

17. A method as recited in Claim 8, further comprising the step of operating a sensor to detect P-wave and shear-wave energy reflected from the geologic formations.

18. A method as recited in Claim 17, further comprising the step of operating a controller to record said reflected energy detected by said sensor in the form of seismic traces.

19. A method as recited in Claim 18, further comprising the step of operating said controller to process said seismic traces and to generate data representing the geologic formations.

0. A method as recited in Claim 8, further comprising a third generator a P-wave in the geologic formations.