Abstract:
A system for detecting source acoustic energy in seismic operations. The system is operable on land and in marine applications. On land, a piezoelectric film is positioned between a vibrator or other source baseplate and the soil. Compression of the piezoelectric film generates an electrical signal proportional to the total ground force, including force due to flexural or resonance of source components, exerted by the seismic source. In marine applications, a source displaces water to produce a pressure pulse in the water, and a piezoelectric film detects the resulting pressure pulse and the flexural movement of the source to produce an electrical signal proportional to the movement of the active component.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to the field of seismic source detection in geophysical operations. More particularly, the invention relates to a seismic source sensor for detecting operation of a seismic source in water or on land. 
     Seismic energy sources transmit acoustic energy into land or water surfaces which propagates downwardly through underlying geologic formations. The interfaces and features of subsurface geologic formations reflect a portion of the acoustic source energy which moves upwardly to the land or water surface where it is recorded. On land, explosives, vibrators, and other devices generate the seismic energy. The receiver elements are deployed at or near the surface of the Earth. In water, the acoustic source energy is generated using air guns or other water displacing devices. The seismic cables and associated receiver elements are towed behind marine seismic vessels or are laid on the seafloor. 
     The reflected seismic source energy is detected with hydrophone and geophone sensors located at a distance from the seismic source. Pressure sensitive type films have been used in U.S. Pat. No. 4,789,971 to Powers et al. (1988), which disclosed an acoustic hydrophone formed with polyvinylidene flouride (PVDF). Another acoustic sensor was disclosed in U.S. Pat. No. 5,361,240 to Pearce (1994) wherein a flexible piezoelectric film was wrapped several times around a mandrel. A hollow space between the film and the mandrel provided a pressure compensation chamber to permit activation of the film. Another acoustic sensor was disclosed in U.S. Pat. No. 5,774,423 to Pearce et al. (1998) wherein a flexible piezoelectric material was encapsulated within a segmented housing. Two or more clam shell type housings were fastened to a cable to form a hydrophone and a hollow space permitted flexure of the piezoelectric material. 
     Because of the depth of the geologic formations under investigation, hydrophone and geophone sensors are typically located at a significant distance from the seismic source. Therefore, the energy detected by the hydrophone and geophone sensors represents a “far-field” energy pulse resulting from transmission of the seismic source. The transmitted seismic pulse obtained from a point on or near the source does not accurately characterize the far-field signal observed at the seismic sensors. Conventional sensors used to detect the energy output of the source are located at discrete points on or near the energy source. Because the sensors occupy discrete positions they cannot sense the total energy output of the energy source. Current practice involves estimates of the far-field signal from either the weighted sum of the vibrator and reaction mass signals, known as ground force, of a land vibrator or near-field hydrophone measurements made near a marine source. Although such estimates represent an improvement over previous methods, such estimates do not reflect the total energy output of the seismic source and do not adequately represent the far-field signature of the source. 
     A need exists for a system capable of predicting the far-field acoustic signal initiated by a seismic source. This prediction can be made by assessing movement of the seismic energy source during source activation. However, tests of seismic sources such as slotted cylinders, vibrators and similar sources demonstrate that the active components of such devices do not move uniformly during activation. For the seismic frequency range of interest, generally between two and one hundred-twenty Hertz, land sources exhibit a very complex motion. The source baseplate often flexes due to the force being used to vibrate the Earth surface. The source may rock back and forth due to uneven ground. Resonances within the source structure may be transmitted through the vibrator baseplate into the ground. Because all of these motions cause energy to be input into the ground, such motions should be represented in the far-field signature so that they are treated as part of the signal rather than undesirable noise. 
     The current practice of utilizing discrete sensors to measure the source output ignores these energy sources. The acoustic pulse of a wavetrain produced by a marine seismic source depends on the total displacement of water by the transducer. Local variations in component motion have some effect on the far-field acoustic signal. The total range of displacement of the seismic source actuator affects low frequency signal generation. Attempts to characterize the far-field acoustic signal from such marine seismic sources have required numerous sensors on or near the source exterior and have not been possible with a single sensor. 
     Various systems have been proposed to predict far-field signal transmission from a seismic source. For example, one system was described in U.S. Pat. No. 4,184,144 to Rickenbacker, which measured the output force of a seismic vibrator. U.S. Pat. No. 4,646,274 to Martinez disclosed a method and apparatus for correcting distorted seismic data. U.S. Pat. No. 4,670,863 to Sallas and Trevino disclosed a vibrator seismic source having a distortion limiting control system. U.S. Pat. No. 4,750,157 to Shei disclosed a seismic vibrator impedance determination and compensation system. U.S. Pat. No. 4,755,976 to Edelmann disclosed a method and apparatus for controlling and analyzing energy transfer to soil, and U.S. Pat. No. 5,790,473 to Allen described a high fidelity vibratory source seismic method for use with a plurality of seismic sources. 
     Conventional source sensors do not effectively measure the far-field performance of a seismic source. The performance of land vibrators or other seismic sources is difficult to measure due to the complex nature of the movements of the source components. On land, prior efforts to characterize vibrator ground force at the baseplate-soil interface have been indirect. Single sensors on or near the moving components of the source have been used to infer the source output. These sensors could not, however, accurately measure such force because of baseplate flexibility and near-surface inhomogenieties. Similarly, accelerometer data from a relatively small number of points on the baseplate have been used to infer the force exerted on the ground, however such accelerometer data do not accurately represent the baseplate motion. 
     Accelerometers and other sensors located at the seismic source detect near-field representation at distinct points and do not accurately characterize the far field acoustic energy pulse. Previous attempts to directly measure the total ground force signal generated by the source have involved large, heavy force tiles which are difficult by their nature to use in a production mode. 
     The performance of air guns, slotted cylinders and other marine acoustic seismic sources is extremely difficult to measure, and the configuration and movement of each source often generate different source energy for each seismic event. 
     Because seismic sources generate different signals during each seismic event, differences in the far-field data will be affected by such source variations and complicate the assessment of reflected signals. Prediction and adjustment for variations in the source events is essential to accurate assessment and characterization of the reflected signals, and a need exists for an improved system capable of assessing seismic source operation. 
     SUMMARY OF THE INVENTION 
     The present invention provides an acoustic source sensor system for use in seismic operations. The system comprises a seismic source activatable to generate an acoustic energy event, a pressure sensitive film responsive to the acoustic energy event which is disposed at least partially about the source, a connector engaged with the film for detecting response of the film to the acoustic energy event and for transmitting an electric signal representing the film response, and a controller engaged with the connector for receiving the electrical signal. 
     One embodiment of the invention comprises a seismic source activatable to generate an acoustic energy event in water, a pressure sensitive film disposed at least partially about the source, a connector engaged with the film for detecting response of the film to the acoustic energy event and for transmitting an electric signal representing the film response, and a controller engaged with the connector for receiving the electrical signal. 
     Another embodiment of the invention comprises a seismic source activatable to generate an acoustic energy event discharging energy into soil, a pressure sensitive film disposed between the source and the soil, a connector engaged with the film for detecting response of the film to the acoustic energy event and for transmitting an electric signal representing the film response, and a controller engaged with the connector for receiving the electrical signal. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates a system having a piezoelectric film engaged with a seismic source. 
     FIG. 2 illustrates a film in cooperation with a marine slotted cylinder. 
     FIG. 3 illustrates discrete film elements on a slotted cylinder. 
     FIG. 4 illustrates a film in cooperation with a vibratory baseplate. 
     FIG. 5 illustrates a cover engaged with a film. 
     FIG. 6 illustrates a web integrated within a film for preserving the physical integrity of the film. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The invention provides a unique system for characterizing acoustic source energy and for predicting far-field acoustic signal produced by a seismic source. Referring to FIG. 1, seismic source  10  is activatable to generate an acoustic energy event. Pressure sensitive film  12  is disposed at least partially about source  10  and is responsive to the acoustic energy event. Connector  14  is engaged with film  12  for detecting response of film  12  to the acoustic energy event and for transmitting an electrical signal such as a voltage which represents the response of film  12 . Controller  16  is engaged with connector  14  for receiving the electrical signal. 
     FIG. 2 illustrates one embodiment of the invention wherein a seismic source such as slotted cylinder  18  is engaged with pressure sensitive film  12 . Connector  14  is attached between film  12  and controller  16 . When slotted cylinder  18  is activated to generate acoustic pulse energy by displacing water  20 , film  12  responds proportionally to the movement of slotted cylinder  18 . The magnitude, duration, and form of the film  12  response can be processed by controller  16  to characterize the acoustic pulse energy produced in water  20 . Such result can be stored for comparison with prior test data, or can be compared with calculated results to predict the resulting far-field acoustic signal. 
     Because film  12  stretches or compresses proportionally with the movement of slotted cylinder  18 , film  12  provides the equivalent sensing capability of multiple point sensors located on a surface of slotted cylinder  18 . Connector  14  and controller  16  can utilize the film  12  response to such movement to control the output of the source. 
     FIG. 3 illustrates another embodiment of the invention wherein piezoelectric film elements  22  are positioned on the surface of a slotted cylinder  18  to more closely characterize the movement of the slotted cylinder  18  structure. For example, a plurality of connectors  14  can be engaged with film  12  at various contact locations to detect movement of slotted cylinder  18  in one or more locations or directions. In the cases where multiple film elements are used to determine the total output, the responses of the individual film elements can be added to determine the total ground force output of the source. All but the largest sources will behave as a point source and will have little if any directivity. Higher frequency sources such as those used for sub-bottom profiling or engineering studies may benefit from characterizing the directionality of the source, and source directivity is more pronounced at high frequencies. If a separate sensor were placed on the top of the source the source ghost code should be detectable. This information could be used to estimate source depth or sea state. 
     FIG. 4 illustrates another embodiment of the invention wherein pressure sensitive film  24  is engaged with baseplate  26  of seismic source vibrator  28  positioned to insonify soil  30 . Film  24  is compressed by movement of baseplate  26  and a signal representing such response is detected with connector  14  for transmission to controller  16 . Film  24  provides the benefit of providing data from a single sensor responsive to the force (or motion) from a substantial area of the energy radiating surface. Film  24  is more reliable than multiple point source sensors and is less subject to damage caused by baseplate  26 . To further isolate film  24  from damage caused by abrasion, wear, tearing, puncture or other contact with baseplate  26 , cover  32  can be placed over film  24  to provide a shield between film  24  and soil  30  as shown in FIG.  5 . In another embodiment of the invention, web  36  can be integrated within film  24  as illustrated in FIG.  6 . Web  36  can comprise a structural member integrated adjacent to or into film  24 . Moreover, web  36  can be formed with a metallic, plastic, or composite material such as a fiber reinforced elastic material. 
     This embodiment of the invention provides a system for measuring ground force directly at the baseplate-ground interface. Ground force has been shown to be a very close approximation of the far-field signature of a land seismic source. A seismic source sensor such as piezoelectric film  24  is attached to the underside of baseplate  26 , either in its entirety or over a selected portion of baseplate  26 . Piezoelectric films such as film  24  are sensitive to compression, stretch, and bending. When compressed, piezoelectric films produce a voltage linearly proportional to the compression force. If the surface of film  24  is connected to common electrodes, the total force applied to the film surface is indicated by electrical signals. 
     In another embodiment of the invention, vibrator baseplate  26  can be engaged with at least two film elements positioned in a selected orientation as described for the marine source identified in FIG. 3. A two dimensional array can be formed with a series of line elements or patches of film elements. In another embodiment, piezoelectric material can be extruded into a selected shape such as a cable which can be positioned in one continuous length or in segments connected electrically. Strips or film elements can be laid in one or more directions to form a two-dimensional sensor capable of characterizing the force or movement of source. 
     Recently developed techniques use flexible piezoelectric film such as Kynar as the sensing element. Piezoelectric film produces an electric signal when stressed or strained. The sensitivity of the material is anisotropic so that the magnitude of the response will vary with the direction of the applied stress. The sensitivity of piezoelectric film due to applied stress in the thickness dimension is two or three times greater than that due to stress applied in the length or width dimension. 
     Although film  12  can be formed with different materials, one suitable material having good acoustic transductance characteristics for seismic operations comprises polyvinylidene flouride (“PVDF”). A stress applied to film  12  with a solid backing will respond to deformation in the thickness dimension and is proportional to the product of piezo-stress constant (in the thickness direction), thickness and pressure. If film  12  is backed with a compliant backing such as foam, the signal generated by the stress is proportional to the product of the piezo-stress constant (in the length direction), length and pressure. If the length of film  12  is more than three times the thickness, the signal produced by a compliant backed film is greater than that of a solid backed film when exposed to the same pressure. For this reason, a band of piezo-film wrapped about a compliant inner core produces a significantly larger signal than the same material wrapped about a solid core. 
     Although only two leads or connectors  14  are illustrated in FIG. 2, multiple leads (positive and negative) can be attached to different portions of film  12 . Leads such as connector  14  provide a means for communicating electric signals produced by film  12  to electronic components for further processing. Signal conditioner such as controller  16  provides temperature sensing and compensation, signal conditioning, filtering and amplification and can collect data from the film element or elements positioned on the surface of the seismic source. 
     The invention provides a marine seismic sensor having a surface area substantially larger than prior art sensors. Piezoelectric films produce a voltage when strained in any one of the three axes. In addition to producing an electric signal when a film is deformed by a pressure wave, film  12  produces a signal when stretched. Film  12  used as an acoustic pressure sensor may produce an unwanted signal when the sensor is stretched rather than compressed. This interfering signal can be reduced or eliminated by utilizing a combination of smaller film elements, which are less subject to stretching, and the appropriate combination of electrical signals generated from the film elements. 
     Depending on the way a piezoelectric film is fabricated and rolled, it may produce either a positive or negative electric signal when compressed. The undesired effect of signals produced by longitudinal stretching of film sensors could be removed by forming the sub-array of two layers of piezoelectric film where one produces a positive signal when compressed and the other produces an equal but negative signal. Both elements produce the same magnitude and polarity when deformed along the longitudinal direction. When the output of the two elements are electrically subtracted, the signal due to action normal to the film would be enhanced and the signal due to stretch would be cancelled. 
     The invention provides a superior system for instrumenting a seismic source so that the resulting sensor data can be evaluated to predict the resulting far-field acoustic signal. A two-dimensional motion sensor covering substantially all of the radiating surface or transducer shell provides adequate spatial sampling of the effective motion of a continuous wave source. A contiguous piezoelectric film facilitates characterization of the transducer with a single sensor, and provides the equivalent of a large number of point sensors distributed over the transducer active components. In land seismic operations, the system is suitable for vibrator sources and impactive sources such as elastic band, gas or vacuum accelerated weight drops, land airguns, weight drops or impactors. In marine seismic operations, the system is suitable for slotted cylinders, moving piston vibrators, constrained piston vibrators, flex tensional devices, or barrel-stave devices. 
     Although the invention has been described in terms of certain preferred 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.