INCLINABLE VIBRATORY SEISMIC SOURCE

Abstract: An improved inclinable vibratory seismic source arranged to be vehicle mounted including a base plate mounted for movement relative to the vehicle frame and for engagement with the ground. Hydraulic cylinders are provided for raising and lowering the base plate. A shaft is connected to the base plate by a U-Joint and has a vibratory mass located thereon. Also, located on the shaft is a reaction mass that is resiliently coupled to the vibratory mass and tuned so that the reaction mass remains virtually vibration free. Control apparatus is connected to the reaction mass to position the shaft and vibratory mass at a preselected azimuth and inclination.

Description: TECHNICAL FIELD OF THE INVENTION This invention relates generally to apparatus for imparting seismic vibratory signals to the earth. More particularly, but not by way of limitation, this invention relates to improved apparatus for imparting seismic vibratory signals to the earth along a selected vector path. BACKGROUND OF THE INVENTION Recent advances in the ability of seismic data acquisition and processing equipment to handle large amounts of data have stimulated greater interest in the development of versatile high energy seismic sources adapted to produce both compressional and shear waves. One goal in the development of "multi-component" seismic sources is to enable the operator to more efficiently measure sub-surface formation properties such as porosity, fracture orientation, pore fluid type, or lithologic type. It is known that P, SH and SV waves travel with their own distinct velocities when passing through a particular geologic formation. Thus, changes in formation property, such as rock type, porosity and the like, may be detectable by changes in the full wave field (i.e., the P, SV and SH wave velocities). A strong correlation observed between lateral variations in the compressional to shear wave velocity ratio (Vp/Vs), with known changes in the geologic formation, suggests that these variations may be used to infer changes in the lithology, porosity, or pore fluid content of the sub-surface formation. By recording the full waveform of a seismic wave, including P, SH and SV waves, one can get far more detailed information concerning rock properties at depths than would otherwise be possible. There are two major types of surface seismic sources currently available which have been adapted to the production of combined P, SH, and S waves. One is the impulsive-type source wherein a heavy mass is propelled in a single downward stroke to impact a ground engaging target. The other is the reciprocating vibratory-type transducer which imparts a controlled, varying frequency signal into the earth, as exemplified by the Vibroseis ® reciprocating vibrator seismic source (a registered trademark of Conoco, Inc.). It is with the latter type of seismic source that the invention to be described is particularly concerned. The general concept of simultaneously imparting both compressional and shear wave vibrational energy to the earth with seismic exploration apparatus is taught in U.S. Patent No. 4,321,981 issued to K.H. Waters on March 30, 1982, entitled "Combination Shear Wave and Compressional Wave Seismic Energy Vibrator." In accordance with the above referenced patent, a combination of two or more vibrating masses may be operated along opposing non-vertical vibrational axes, oriented at a fixed angle to each other in parallel planes. The times of passage of the masses through a common central point as they vibrate along their respective axes can be controlled to establish a phase relation between the two vibratory motions. By varying the related phases of the two masses, one can effect desired inputs of compressional and shear waves simultaneously. The referenced Waters patent teaches that the parallel planes in which the respective vibration axes of the two masses lie are to be oriented either transversely to the line of seismic survey, i.e., the line between the location of the seismic source and the recording geophones, or along such survey line. With proper phasing, the above-referenced transverse axial orientation of the vibrating masses enables one to produce and record simultaneously P waves and shear SH waves. On the other hand, if the vibrational axes are aligned with the survey line, one can generate and record simultaneously P waves and shear SV waves. The vertical component of motion in both cases produces so called "Converted" shear SV waves, but such waves cannot be processed and interpreted with conventional seismic methods. Thus, Waters teaches in effect that only by operating the patented apparatus successively in these two modes can one generate and record at a single remote location all forms of a seismic wave, including P, SH and SV waves, in a manner which permits conventional processing. In the field of seismic exploration today, data acquisition and processing equipment has been developed to handle large amounts of data generated by apparatus constructed to produce both compressional and shear waves. Patents illustrating full wave field vibratory seismic sources include US patent no. 4 662 473, issued May 5, 1987, to Robert M Betz, which utilizes a rotatable turntable with a single hydraulic ram thereon connected to the vibrator assembly so as to vary both the azimuth and inclination of the vibratory axis; US patent no. 4 655 314 issued April 7, 1987, to Tom P Airhart; US patent no. 4 660 675, issued April 28, 1987, to Tom P Airhart; and US patent no. 4 719 607 issued January 12, 1988 to Tom P Airhart. Each of these patents illustrates apparatus capable of producing vibratory seismic waves at varying inclinations and azimuths without repositioning the ground engaging base plate of the apparatus or without repositioning the vehicle utilized for carrying such apparatus. US patent no. 4 056 164 discloses a seismic signal source mounted on a bed of a vehicle including a restraining means which resists transverse rocking of the source. US patent no. 4 632 215 discloses a dual-mode vibrator for alternately injecting P waves and S waves into the earth. US patent no. 4 719 607 illustrates a general design for a multi-component vibratory seismic apparatus. This invention described hereinafter is an improvement to that apparatus. When conducting the seismic exploration of an area of interest, it is generally necessary that the vibratory seismic source be moved to a large number of locations. Accordingly, such apparatus is usually mounted on vehicles. These vehicles must include means for raising and lowering the ground engaging base plate and the vibratory source. They also usually include means for powering the vibratory source. An example of a vehicle mounted seismic source is illustrated in U.S. Patent No. 4,660,674 issued April 28, 1987, to Tom P. Airhart. It will be noted in that patent that the seismic source is not a vibratory seismic source, but rather utilizes a heavy mass that is propelled downwardly against the surface of the base plate to produce an instantaneous heavy shock wave thereto. Despite the differences in the type of seismic source illustrated, the patent does illustrate the mounting of a source on a vehicle such as is contemplated by this invention. From the listed patents, it can be seen that a number of attempts have been made to control the inclination and azimuth of the vibratory source relative to the base plate. While the foregoing systems work satisfactorily, it would be highly desirable to more precisely, and in a vibration-reduced arrangement, control the inclination and the azimuth of the seismic source. Accordingly, an object of this invention is to provide improved vibration-reduced seismic apparatus for imparting seismic vibratory signals to the earth at more precise inclinations and azimuths. SUMMARY OF THE INVENTION This invention provides improved vibratory apparatus for imparting seismic vibratory signals to the earth along a selected vector path, said apparatus being suitable for mounting on a vehicle frame for transportation and comprising: a ground-engaging base plate; a lifting frame connectable with the vehicle frame and said base plate whereby said base plate can be moved from a transport position out of engagement with the ground to an operating position engaging said ground; a vibrator for producing said vibratory signals, said vibrator including a shaft having an axial centerline, a first end moveably mounted on said base plate whereby said axial centerline can be located within preselected angles and at preselected azimuths, a second end, a vibratory mass mounted for limited reciprocating movement on said shaft intermediate said ends for imparting vibratory motion to said shaft control apparatus connecting said second end with said lift frame for positioning the axial centerline of said vibrator at desired inclinations and at desired azimuths, and characterised in that said control apparatus includes a plurality of variable length means extending in respective different directions from the second end of the shaft; means for selectively changing the length of each variable length means, a signal generator for transmitting a signal to cause said variable length means to change length to position said vibrator at a selected inclination and azimuth, and a substantially vibration free reaction mass moveably located on said shaft between said vibratory mass and said second end. BRIEF DESCRIPTION OF THE DRAWING The foregoing and additional objects and advantages of the invention will become more apparent when the following detailed description is read in conjunction with the accompanying drawing wherein like reference characters denote like parts in all views and wherein: FIGURE 1 is a view, partly in elevation and partly in cross-section illustrating vibratory seismic apparatus constructed in accordance with the invention, taken generally along the lines 1-1 of FIGURE 2. FIGURE 2 is a top view, partly in cross-section, of the apparatus of Fig. 1, taken generally along the lines 2-2 of FIGURE 1. FIGURE 3 is schematic diagram illustrating a typical circuit used for controlling the inclination and azimuth of vibratory apparatus that is constructed in accordance with the invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to the drawing and to Figures 1 and 2 in particular, shown therein and generally designated by the reference character 10 is apparatus for producing vibratory seismic signals. The vibratory apparatus 10 includes an upper lift frame 12 connected to a lower lift frame 14 by a plurality of guide posts or members 16. The lower ends of the posts 16 are connected to the lower lift frame 14 while the upper lift frame 12 is connected to the intermediate portion of the posts 16. The lower lift frame 14 is connected to a ground engaging base plate 18 by a plurality of spaced shock absorbing members 20. Only four of the members 20 are illustrated, but it should be understood that an adequate number of the members 20 will be provided to support the base plate 18 in tension when the apparatus is in the raised position. Each guide post 16 extends through a sleeve 22. The sleeves 22 may be attached to a vehicle frame 24 if the apparatus 10 is to be transportable by vehicle. Lift cylinders 26 may also be attached to the vehicle frame 24. The lift cylinders 26 are double-acting hydraulic cylinders having a piston rod 28 extending therefrom. The free end of the piston rod 28 is attached to the lift frame 14. The lift cylinders 26 are utilized to raise and lower the base plate 18 relative to the frame 24 as the guide posts 16 slide through the sleeves 22. Although not illustrated, apparatus is provided for connection with the conduits 30 and 32 connected with each cylinder 26 for controlling the raising and lowering of the base plate 18. The vibratory apparatus 10 also includes a vibrator generally designated by the reference character 34. A shaft 36 extends generally vertically through the vibrator 34 and has a first end connected whereby the vibrator 34 can be rotated relative to the base plate 18, such as by a universal joint 38 to the base plate 18. The other (second) end of the shaft is free. Just above the U-joint 38, the shaft 36 is provided with an enlarged flange portion 40 that supports a plurality of isolators 42 which in turn support a vibratory mass 44. The vibratory mass is of great weight and is arranged with the appropriate systems and controls for causing the mass 44 to vibrate or reciprocate the shaft 36 and, consequently, impart vibrations into the base plate 18. Above the vibratory mass 44 there is provided a plurality of vibration isolation members 46 that support a reaction mass 48 which encircles the shaft 36 near the free end thereof. To permit reciprocatory movement and to prevent relative rotation between the shaft 36, vibratory mass 44 and reaction mass 48, guide rods such as are illustrated at 50 and 52 extend therebetween. The airbag vibration isolators 42 and 46 are commonly used for isolation devices on seismic vibrators. Preferably, they are capable of being inflated so that they can be "tuned" to isolate such shocks and vibrations to protect the apparatus and so that the reaction mass 48 will remain stationary. To permit the vibrator 34 to be tilted to the proper inclination and azimuth, the upper side of the reaction mass 48 is provided with a plurality of ears 54. Each ear 54 is connected, respectively, with a cable 56, 57, 58, and 59. Each cable extends over a respective swivel-mounted pulley 61, 62, 63, and 64. The pulleys are carried by the upper lift frame 12. The opposite end of each of the cables is connected to a respective piston 65, 66, 67, or 68. The pistons are located in control cylinders 69, 70, 71, and 72, respectively. The control cylinders are each of the double-acting type and are mounted on the lower lift frame 14 so that they are raised and lowered, as the base plate 18 is raised and lowered relative to the vehicle frame 24. Mounted on the control cylinders are sensors 73, 74, 75, and 76, respectively. Each sensor is arranged to transmit a signal indicative of the position of the piston rods in the control cylinder therein so that the inclination and azimuth of the vibrator 34 can be controlled. As may be seen more clearly in FIGURE 2, an axial centerline 78 of the vibrator 34 may be positioned at any point within the circle described by line 80 which is the locus of the axial centerline 78 as the various control cylinders are actuated. The vibrator 34 can be tilted through an angle of about 22° relative to vertical. FIGURE 3 illustrates the controls utilized for positioning the vibrator 34 at the desired inclination and azimuth. As illustrated therein, the cylinders 69 and 70 are connected by conduits 84 and 86 with a control valve 88. The arrangement is such that cylinders 69 and 70 move the pistons 65 and 66 therein in opposite directions. Similarly, the cylinders 71 and 72 are connected by conduits 90 and 92 with a second control valve 94. The control valves 88 and 94 may be identical if desired. Again, the arrangement between the cylinders 71 and 72 and the control valve 94 is such that the pistons 67 and 68 therein are caused to move in opposite directions. It is desirable to be able to move the pistons in opposite directions simultaneously so that the cables tying the cylinders to the vibrator 34 are always in tension and, thus, maintain the vibrator 34 accurately at the desired inclination and azimuth. To provide power for moving the pistons in the control cylinders, each of the control valves is connected by a conduit 96 with a pump 98 which is driven appropriately to provide hydraulic fluid through the control valves 88 and 94 to the control cylinders. A conduit 100 extends from each of the control valves to a reservoir 102, which is connected to supply hydraulic fluid to the pump 98 by a conduit 104. To provide control for either positioning the vibrator 34 or for sensing the position thereof, a comparator 106 is connected by conductors 105 and 107 to the control valves. The comparator 106 is connected by a conductor 108 with the sensor 73, by a conductor 110 with the sensor 74, by a conductor 112 with the sensor 75 and by a conductor 114 with the sensor 76. Upon receipt of an input signal 116 into the comparator 106, appropriate signals are transmitted to the control valves to position the control cylinders. As the pistons move in the control cylinders, the sensors transmit a signal indicative of such position to the comparator 106 which continues to reposition the control cylinders until such time as the signal transmitted by the sensors corresponds to the input signal indicating the desired inclination and azimuth of the vibrator 34. OPERATION OF THE PREFERRED EMBODIMENT When the apparatus 10 is vehicle mounted, the vehicle (not shown) will be positioned so that the axial centerline 78 of the vibrator 34 is positioned over the precise point where it is desired to impart vibratory seismic signals into the earth. Appropriate controls (not shown) are actuated to extend the pistons 28 from the lift cylinders 26 until the base plate 18 comes into engagement with the ground. When this occurs, further movement of the pistons 28 is arrested, but the cylinders 26 lift the frame 24 upwardly until all or part of the weight of the vehicle is resting on the base plate 18. The vibrator 34 is then positioned in accordance with the desired angle of position of the vibratory signal through the base plate 18. Inspect signal 116 (see FIG. 3) is fed into the comparator 106 either manually or electronically. Upon receiving this signal, the comparator 106 transmits a signal through the conductors 105 and 107 to the control 'valves 88 and 94. The control valves are then positioned in accordance with the signal received. When the control valves move to the proper positions, hydraulic fluid is introduced into the cylinders 69, 70, 71 and 72 through the appropriate conduits. The fluid moves the pistons 65, 66, 67 and 68 to position the vibrator 34 by means of the cables 56, 57, 58 and 59 at the desired angle of inclination relative to the base plate 18 and at the desired azimuth. During the movement of the pistons in the control cylinders, signals are generated in the sensors 73, 74, 75 and 76. These signals are transmitted to the comparator 106 through the conductors 108, 110, 112, and 114. When the signals generated by the sensors are combined in the comparator 106 and compared with the input signal 106, which is in accordance with the desired position of the vibrator 34, continued movement of the vibrator 34 occurs until the compared signals are equal. When this occurs, movement is arrested and the vibrator 34 is in the desired position for transmitting the vibratory motion into the earth. As previously mentioned, no apparatus is illustrated for causing the vibrator 34 to oscillate. It should be understood that it is well-known in the art how such vibration is to be accomplished. An informative book on seismic vibrators is entitled Reflection Seismology - A Tool for Energy Resource Exploration . The author is Kenneth H. Waters and it was published by Wiley & Sons. The 3rd Edition was published in 1987. Vibrations induced by the vibrator 34 involve very large forces and, consequently, it is highly desirable to isolate the vibrator 34 from all the apparatus with the exception of the shaft 36 and the base plate 18. Also isolators 42 have been arranged to "counterbalance" mass 44 so vibrator 34 will have the same acceleration downward and upward, neutralizing the effect of gravity on mass 44. Also, the vibration isolators 46 that have been placed between the vibratory mass 44 and the reaction mass 48 have been tuned so that the reaction mass 48 remains essentially stationary. Accordingly, there is provided a plurality of vibration isolating couplings 20 between the vibrator 34 and the remainder of the apparatus, with the exception of shaft 36 and the base plate 18, which isolate the motion of the vibrator 34 from the supporting vehicle. The use of the cables 56, 57, 58, and 59 to connect the control cylinders with the vibrator 34 also aids in isolating the remainder of the apparatus from the vibration of the vibrator 34. The use of control cylinders disposed at substantially right angles to each other simplifies the problems involved in accurately locating the vibrator 34 at the desired inclination and azimuth. Having described but a single embodiment of the invention, it will be appreciated that many changes and modifications can be made thereto without departing from the scope of the invention as claimed.

Claims: Vibratory apparatus for imparting seismic vibratory signals to the earth along a selected vector path, said apparatus being suitable for mounting on a vehicle frame for transportation and comprising: a ground-engaging base plate (18); a lifting frame (12,14) connectable with the vehicle frame and said base plate whereby said base plate can be moved from a transport position out of engagement with the ground to an operating position engaging said ground; a vibrator (34) for producing said vibratory signals, said vibrator including a shaft (36) having an axial centerline, a first end (38) moveably mounted on said base plate whereby said axial centerline can be located within preselected angles and at preselected azimuths, a second end, a vibratory mass (44) mounted for limited reciprocating movement on said shaft intermediate said ends for imparting vibratory motion to said shaft control apparatus (57,70 etc) connecting said second end with said lift frame (14) for positioning the axial centerline of said vibrator (34) at desired inclinations and at desired azimuths, and characterised in that said control apparatus includes a plurality of variable length means (56,57,58,59) extending in respective different directions from the second end of the shaft (36); means (69,70,71,72) for selectively changing the length of each variable length means, a signal generator (88,94,98) for transmitting a signal to cause said variable length means to change length to position said vibrator (34) at a selected inclination and azimuth, and a substantially vibration free reaction mass (48) moveably located on said shaft between said vibratory mass (44) and said second end. The apparatus of Claim 1 wherein said vibrator (34) includes a guide (50) for preventing relative rotation among said shaft (36), vibratory mass (44), and reaction mass (48). The apparatus of Claim 1 or 2, wherein said vibrator also includes: vibration isolator means (46) disposed between said reaction mass and vibratory mass; and, counterbalancing isolator means (42) disposed between the first end of said shaft and said vibratory mass. The apparatus of Claim 3, wherein the first end of said shaft has an enlargement (40) thereon and said counterbalancing isolator means includes at least one isolator member (42) located between the enlargement on said shaft and vibratory mass for supporting said vibratory mass on said shaft. The apparatus of Claim 3 or 4, wherein said counterbalancing isolator means includes a plurality of spaced isolator members (42) located between said shaft and vibratory mass. The apparatus of Claim 3, 4 or 5, wherein said vibration isolator means includes a plurality of vibration isolating members (46) located between said reaction mass (48) and vibratory mass (44) for preventing vibration of said reaction mass. The apparatus of any one of the preceding claims, wherein each said variable length means (56 - 59 etc) includes: a first end connected to said reaction mass; and a second end connected to said lift frame. The apparatus of Claim 7, wherein each said variable length means also includes: a powered cylinder (70) mounted on said lift frame and having a piston movable therein; and, cables (57) each having one end connected to a said piston and having another end connected to said reaction mass. The apparatus of any one of Claims 3 to 6, wherein said vibration isolator means includes at least one inflatable vibration isolating member (46) tuned to isolate the vibrations of said vibratory mass and shaft whereby said reaction mass is substantially vibration free. The apparatus of any one of the preceding claims, wherein each said variable length means (56 - 59) is located generally at a right angle to another of said variable length means when the axial centerline of said shaft is disposed substantially perpendicularly to an upper surface of said base plate.

Inventors: BAYS, MARVIN G.
Applicants: ATLANTIC RICHFIELD COMPANY
Application year: 1989
Main classification: G01V1
IPC codes: G01V1