Abstract:
The invention is an electric sweep type seismic vibrator source of the type used in seismic prospecting for hydrocarbons. The source uses an engine and generator combination to create electric power to drive a frame of linear electric motors that direct a rod or piston to contact the ground in a recurring fashion. Preferably, a foot is arranged on the bottom end of the rod or piston for contact with the ground and by engaging the grid of motors to push down against the ground in a rapid progression, acoustic energy is created and delivered into the ground for geophones to sense and record.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a non-provisional application which claims benefit under 35 USC §119(e) to U.S. Provisional Application Ser. No. 61/640,196 filed Apr. 30, 2012, entitled “ALTERNATIVE VIBRATOR ACTUATOR SOURCE,” which is incorporated herein in its entirety. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
     None. 
     FIELD OF THE INVENTION 
     This invention relates to vibratory seismic sources and particularly to seismic sources that are held to the ground to deliver vibratory impulses into the earth for seismic prospecting of hydrocarbons and other subsurface resources. 
     BACKGROUND OF THE INVENTION 
     In the process of acquiring seismic data, seismic energy is delivered into the earth. Over the years, the preferred attributes of the seismic energy delivered into the earth have been honed to include a broad spectrum of wavelengths and sufficient power across the spectrum to be recorded at the surface. In general, a suitable land source must be able to deliver seismic energy waves in a spectrum of wavelengths from about 8 Hz up to 60-80 Hz. The source must have sufficient power across the spectrum so that the seismic waves have measurable amplitude at the surface after transiting deep into the earth, reflecting from or refracting through layers in the earth and transiting back to the surface. A last major characteristic of a desirable seismic source is that the energy from the source is distinguishable in the data record from seismic energy from other sources whether from background sources or other seismic prospecting. 
     Explosive charges have long been used as seismic sources although the intense release of energy is typically not permitted except in remote locations. Explosive sources, however, provide a wide array of wavelengths with considerable power across the spectrum. 
     Hydraulic reciprocating seismic vibrators or vibes have been in use for many years using a baseplate connected to hydraulic rams that cause a reaction mass to reciprocate up and down to shake the ground through the baseplate. The hydraulic rams are operated to move the reaction mass through a sweep of the desired frequencies. However, the hydraulic systems are limited in their ability to provide sufficient power at high frequencies due to limitations of hydraulic flow in and out of the hydraulic cylinders. At very high hydraulic velocities, the hydraulic fluid is subject to cavitation when reversing directions that limits the amplitude of the movement of the reaction mass and thus the energy input in to the earth. At low frequencies it is difficult for the hydraulic vibe to have enough travel to generate a low frequency wave into the ground. For example, consider how one would generate a one Hz wave with a hydraulic vibe. A very long throw of the reaction mass is needed to generate that wavelet because the mass has to be moving down and up for the full one second. 
     BRIEF SUMMARY OF THE DISCLOSURE 
     The invention more particularly relates to a vibratory seismic source for delivering acoustic energy into the earth for seismic prospecting including a generator for creating electric power and a plurality of linear motors arranged in a grid and each including a rod that is arranged to move generally vertically to contact the ground with a lower end of the rod The source also includes a control system for controlling movement of the rods such that the rods vibrate the ground and deliver acoustic energy into the ground. 
     The invention also relates to a process for delivering acoustic energy into the earth for seismic prospecting wherein electric power is created and the electric power is used to move a rod of a linear electric motor generally vertically to contact the ground with a lower end of the rod. The movement of the rods is controlled such that the rods vibrate the ground and deliver acoustic energy into the ground. 
     “Generally vertical” or “generally vertically” should be interpreted as meaning “with an axis of movement sufficiently nearly vertical with respect to the ground so as effectively to impart energy to the ground.” Normally, the axis of movement would be less than 20 degrees to vertical, or in another embodiment less than 10 degrees to vertical. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete understanding of the present invention and benefits thereof may be acquired by referring to the follow description taken in conjunction with the accompanying drawings in which: 
         FIG. 1  is an elevation view of a discrete electric seismic source unit; 
         FIG. 2  is an enlarged fragmentary view of an electromechanical linear motor assembly for delivering seismic energy into the ground; and 
         FIG. 3  is an enlarged perspective fragmentary view of a grid of electro mechanical linear motor assemblies for cooperatively delivering seismic energy into the ground. 
     
    
    
     DETAILED DESCRIPTION 
     Turning now to the detailed description of the preferred arrangement or arrangements of the present invention, it should be understood that the inventive features and concepts may be manifested in other arrangements and that the scope of the invention is not limited to the embodiments described or illustrated. The scope of the invention is intended only to be limited by the scope of the claims that follow. 
     As shown in  FIG. 1 , an alternative vibrator actuator source  10  is shown comprising a chassis  12 , four wheels  15  and a driver&#39;s cab  18 . The alternative vibrator actuator source  10  uses a diesel engine  21  to drive a hydraulic pump system  22  and to also turn an electric generator  23 . The hydraulic pump system  22  may be used to drive the source  10  from location to location and to operate other equipment on the source  10  or a conventional vehicle drive train may be used to drive the wheels  15 . For the source  10 , the electric generator  23  provides the electric power to deliver the acoustic energy into the ground. 
     Referring more specifically to  FIGS. 2 and 3 , the acoustic energy delivery system  30  is carried under the chassis  12  and comprises a frame  32  that carries a number of linear motors  35 . Each linear motor  35  includes a form of a tubular body  36  and a rod or actuation bar  38  that extends telescopically from the tubular body  36 . A replaceable foot  39  is attached to the bottom end of the rod  38  for contacting the ground. The frame  32  includes mounts for a grid of linear motors  35 . In one envisioned embodiment, the frame  32  has approximately 112 linear motors  35  that are arranged in a grid of perhaps 8 by 14. Each linear motor is capable of outputting a peak acceleration force of approximately 2400 Newtons (N) or approximately 540 pounds-force while using 34 amps RMS (Arms) at 240 volts AC. The 112 linear motors would then be capable of outputting 268,800 N or 60,480 pounds-force using approximately 914 kilowatts of power. An additional advantage to the linear motor is that they come in varying sizes and force outputs that that can be tuned to achieve a desired acceleration, force and sustained velocity of motion. Also, the electronic control for the linear motor is well understood because of their wide use in manufacturing applications. 
     In operation, the frame  32  is lowered into proximity to the ground G and the linear motors  35  are operated to lower the replaceable feet  39  into contact with the ground G. Once all of the replaceable feet  39  are in contact with the ground G, the linear motors  35  are activated to thrust the rods  38  toward the ground G and deflect the ground G and thereby deliver an impulse into the earth. The linear motors  35  are quickly operated to recoil the rods  38  under control without disengaging contact with the ground G by the replaceable feet  39 . By successive thrusts and recoils, a sweep of acoustic energy is effectively delivered into the earth while the feet remain in contact with the ground G. It should be noted that the undulations and irregularities of the ground G may be accommodated avoiding decoupling across the dimension of the frame  32 . This method may be arranged to automatically compensate for surface topographic variations along with soft and hard spots on the ground surface like rocks or logs. While it is recognized that ground typically does not deflect much, it does not take much deflection with a 60,000 pound vibrator holding the replaceable feet  39  to the ground G to deliver very useful acoustic energy. In this procedure, all of the linear motors  35  would be operated at the same time using electrical power created by the electric generator  22 . The impulses would be repeated in a sequence where the impulse would occur with progressively increasing or decreasing rapidity such that a progression of frequencies of impulse forces would effectively deliver acoustic energy into the earth. The acoustic energy being characterizeable as a progressive sweep of frequencies covering a spectrum from about 1 Hz up to at least 80 Hz and preferably up to at least 120 Hz. 
     The electric linear motors  35 , working in conjunction, would not suffer the limitations of the hydraulic pumping systems at high frequency. Cycling electric power to the linear motors  35  to cause controlled movement of the rods  38 , and with such instant response, the impulse frequency range is greatly expanded. By using electrical control circuits that are commonly available for diesel electric train locomotives and hybrid cars, the power can be applied instantly with a very high degree of control and stabilization. 
     It should be recognized that higher frequencies than typically delivered may be achievable by the source  10 . Perhaps frequencies as high as 200 Hz or higher may become useful in the field of seismic prospecting. There is no recognized reason that source  10  cannot deliver such high frequency acoustic energy into the ground G. And it is generally understood that high frequency energy provides high resolution data. Unfortunately, high frequency energy attenuates in the earth more rapidly than low frequency energy. With a large number of linear electric motors, whether 200, more than 200, possibly more than 2000 or less than 200 possibly less than 100, will be able to deliver high energy at high frequency. The size of the linear motors may be reduced or increased to adjust and adapt to ideal energy delivery system to create an optimal frequency range with high energy across the spectrum. The selection of the specific linear motors is an engineering issue at production time because they can be sourced that have a large thrust force but with short strokes as compared to those that have longer strokes with less thrust, but higher speeds. The alternative vibrator actuator source  10  invention is the substitution of electric power for hydraulic power in a vibrator source. In a preferred embodiment, the alternative vibrator actuator source  10  is comprised of the following main components: (1) a vehicle component comprising an articulated vehicle chassis  12  with drivers&#39; cab  18  capable of supporting the equipment and sustaining the rigors of a vibrator source; (2) a mechanical generator system that is made up of a diesel engine  21 -electric generator  23  package, an electric power accumulator, power conditioner, power distribution center, and all necessary cables and switches; (3) an electromechanical acoustic energy delivery system  30 ; and (4) a control system for controlling the delivery of acoustic energy by the electromechanical acoustic energy delivery system  30 . 
     The vehicle component or chassis  12  is a vehicle similar to the standard “buggy” type vibrator vehicles used for many all-terrain 60,000 lb industry vibrator source trucks. The chassis  12  will have four large tires  15  with a suitable propulsion system. The chassis  12  includes a hinge behind the cab  18  to provide the steering by articulating the chassis at the hinge or articulation point. 
     The mechanical generator system is similar to that used in many hybrid vehicles where a motor, generator, and battery pack are used to power the vehicle but on a larger scale. The mechanical generator system is comprised of an industry standard diesel engine-electric generator package that is capable of producing at minimum 800 kilowatts of electric power. The engine-generator package typically comes mounted on a steel beam base and will be installed as a kit that can be easily installed, removed and replaced as needed. Typical engine-generator packages of this kilowatt size have a weight of approximately 20,000 lb. The engine-generator package of the present invention would replace a conventional engine-hydraulic pump utilized in standard vibrators. 
     In order to smooth out the electrical power load an electric power accumulator is utilized that is comprised of a bank of batteries with the appropriate charging and control circuits. A power conditioner system is utilized to assure proper voltages are provided and to limit current draw. 
     The electromechanical system&#39;s displacement apparatus contains the acoustic energy delivery system  30  including the multiple linear motors  35 . It is the component that actually imparts force on to the earth to create deformations. The acoustic energy delivery system  30  replaces the reaction mass and associated base plate on a standard hydraulic vibrator. 
     The electronic control system controls the functions of the acoustic energy delivery system  30  including the individual linear motors  35 . The rate at which each rod moves downwardly and is allowed to retract is under the control of the electronic control system. The frequency of each pulse of seismic energy being delivered into the earth is dictated by the rate at which each rod is directed by the control system to move downwardly. 
     In closing, it should be noted that the discussion of any reference is not an admission that it is prior art to the present invention, especially any reference that may have a publication date after the priority date of this application. At the same time, each and every claim below is hereby incorporated into this detailed description or specification as an additional embodiment of the present invention. 
     Although the systems and processes described herein have been described in detail, it should be understood that various changes, substitutions, and alterations can be made without departing from the spirit and scope of the invention as defined by the following claims. Those skilled in the art may be able to study the preferred embodiments and identify other ways to practice the invention that are not exactly as described herein. It is the intent of the inventors that variations and equivalents of the invention are within the scope of the claims while the description, abstract and drawings are not to be used to limit the scope of the invention. The invention is specifically intended to be as broad as the claims below and their equivalents.