Abstract:
A seismic system has a plurality of seismic sources. Each of the seismic sources includes a reaction mass movable with respect to a geological structure and includes an actuator moving the reaction mass. The movement of the reaction mass emits a source signal as seismic energy into the geological structure. In the system, each of the source signals is generated from a base vibratory signal segmented into a plurality of segments, where the base vibratory signal sweeps from an initial frequency to a final frequency. Each of the segments has a partition of sweep frequencies different from the other segments. Each of the source signals has a distinguishing arrangement of the segments different from the other source signals.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is divisional of U.S. patent application Ser. No. 11/856,386, filed Sep. 17, 2007, which is incorporated herein by reference in its entirety and to which priority is claimed. 
    
    
     BACKGROUND 
     1. Technical Field 
     The present disclosure generally relates to seismic prospecting and in particular to methods and apparatus for acquiring and processing geophysical information. 
     2. Background Information 
     In the oil and gas exploration industry, geophysical tools and techniques are commonly employed in order to identify a subterranean structure having potential hydrocarbon deposits. Seismic vibratory energy sources, or simply seismic vibrators, have been used in the field many years for generating source signals. A seismic vibrator in its simplest form is merely a heavy vehicle that has the ability to shake the ground at a predetermined range of frequencies to impart vibratory seismic signals into the subsurface of the earth over a specified period of time, which allows for an instantaneous energy level less than impulsive generators such as dynamite. 
     The imparted energy, termed “seismic source signals” herein, travels through the subsurface and reflects some of the energy from subsurface geological structures or layers. The reflected energy is then transmitted back to the earth&#39;s surface where it is recorded using an earth motion detector. The recorded data is processed and interpreted to yield information about location and physical properties of subsurface structures and layers. 
     The seismic source signal is typically a sweep signal, or simply sweep. Sweeps are sinusoidal vibrations and may have duration times on the order of about 5 to longer than 20 seconds depending on the terrain, the subsurface lithology, economic constraints and physical capabilities of the vibrator. The sinusoidal sweep can be increased in frequency overtime, which is called an “upsweep.” The upsweep is the signal used typically in modern seismic exploration. Also, the sinusoidal sweep can be decreased in frequency overtime, which is called a “downsweep.” The end products of the vibrator sweep are waves that propagate through the earth to return information about the subsurface. 
     The seismic waves travel through the ground and reflect off subterranean formations. Boundaries between formations of differing material, density or structure often reflect seismic waves, and seismic information relating to these waves is collected, processed interpreted to generate a representation or “pictures” of the subsurface. Any number of exploration systems may be used to gather the desired information for processing. Receiver sensors such as velocity geophones, accelerometers and/or hydrophones may be laid out in lines, or optionally towed in the case of hydrophones, for measuring the amplitude of seismic waves due to the seismic source, reflected off subsurface boundaries, and then returning to the deployed sensors. Multiple source point acquisition surveys provide a method of reducing the time to acquire a complete survey area of data. Traditional single source point acquisition acquires one source point of data at an exclusive time. Multiple source point acquisition is used to acquire many source points of data at an exclusive time, providing for faster acquisition of the data over traditional single source methods. In order to separate each source form an acquired multiple source record, contemporary multiple source methods employ methods that emit longer signals or increase the number of vibratory signals emitted in comparison to the tradition single source emission. 
     SUMMARY 
     The following presents a general summary of several aspects of the disclosure in order to provide a basic understanding of at least some aspects of the disclosure. This summary is not an extensive overview of the disclosure. It is not intended to identify key or critical elements of the disclosure or to delineate the scope of the claims. The following summary merely presents some concepts of the disclosure in a general form as a prelude to the more detailed description that follows. 
     Disclosed is a method and apparatus for generating a seismic source signal. In one aspect, a method includes generating energy in the form of a plurality vibratory signals being time sequence signals partitioned as a function of time, wherein each of the plurality of vibratory signals comprises a distinguishing signature. The vibratory signals are emitted into a terrain of interest as seismic source signals. 
     In another aspect, a method for geophysical information acquisition includes positioning a plurality of seismic receivers in a terrain of interest and receiving a plurality of seismic source signals with the plurality of seismic receivers, the seismic source signals being time sequence signals partitioned as a function of time and/or frequency, wherein each of the plurality of seismic source signals comprises a distinguishing signature. 
     In another aspect, a method for processing geophysical information includes receiving recorded seismic information, the recorded seismic information including a plurality of seismic signals, the seismic signals being time sequence signals partitioned as a function of time and/or frequency, wherein each of the plurality of signals comprises a distinguishing signature. The method further includes processing the seismic information to separate the seismic information with respect to each of the plurality of signals. 
     An apparatus according to one aspect includes a baseplate that is coupled to a surface of the earth, a reaction mass that is moved with respect to the baseplate, and a feedback-controlled actuator coupled to the reaction mass, the feedback controlled actuator providing a force to the reaction mass that induces linear movement of the reaction mass, the linear movement of the reaction mass causing seismic energy propagation into the earth, wherein the seismic energy is in the form of a plurality of time sequence seismic signals, the seismic signals being partitioned as a function of time, wherein each of the plurality of seismic signals comprises a distinguishing signature. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a detailed understanding of the present disclosure, reference should be made to the following detailed description of the several non-limiting embodiments, taken in conjunction with the accompanying drawings, in which like elements have been given like numerals and wherein: 
         FIG. 1  illustrates a non-limiting example of a method for generating and propagating seismic energy into the earth; 
         FIGS. 2A-2C  illustrate a non-limiting example of generating a vibrator system input signal having a unique signature; 
         FIGS. 3A-3D  illustrate a non-limiting example of a method for generating unique input signals for a four-source seismic source signal generating scheme where each input signal has a unique signature; 
         FIG. 4  schematically illustrates a non-limiting example of a land seismic vibrator system for generating and propagating compressional wave energy into the earth; 
         FIG. 5  is another schematic illustration of a non-limiting land seismic vibrator system for generating and propagating transverse wave energy into the earth&#39;s surface; 
         FIG. 6  illustrates an example of a land seismic vibrator system having a drive external to a reaction mass; and 
         FIG. 7  illustrates an example of a land seismic vibrator system having a drive internal to a reaction mass. 
     
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     The present disclosure uses terms, the meaning of which terms will aid in providing an understanding of the discussion herein. For example, the term information processing device as used herein means any device that transmits, receives, manipulates, converts, calculates, modulates, transposes, carries, stores or otherwise utilizes information. In several non-limiting aspects of the disclosure, an information processing device includes a computer that executes programmed instructions for performing various methods. 
     Geophysical information as used herein means information relating to the location, shape, extent, depth, content, type, properties of and/or number of geologic bodies. Geophysical information includes, but is not necessarily limited to marine, transition zone, ocean bottom cable and land seismic information. Seismic information as used herein includes, but is not limited to, one or more or any combination of the following, analog signals, digital signals, recorded data, data structures, database information, parameters relating to surface geology, source type, source location, receiver location, receiver type, time of source activation, source duration, source frequency, energy amplitude, energy phase, energy frequency, wave acceleration, wave velocity and/or wave direction. 
     Geophysical information may be used for many purposes. In some cases, geophysical information such as seismic information may be used to generate an image of subterranean structures. Imaging, as used herein includes any representation of a subsurface structure including, but not limited to, graphical representations, mathematical or numerical representation, strip charts or any other process output representative of the subsurface structure. 
     The term “reaction mass” is used herein in conjunction with a seismic vibrator. Land seismic vibrators according to the disclosure may use a heavy weighted structure that reciprocates to impart controlled energy into the earth. This heavy weighted structure is generically termed a “reaction mass” herein. A “reaction mass” may move relative to other vibrator structures, such as a baseplate. 
     Disclosed is a simultaneous or asynchronous seismic vibrator multiple sourcing apparatus and method that operates in an acquisition time frame significantly shorter than current seismic vibrator multiple sourcing methods. Vibration duration using the disclosed methods and apparatus are comparable to conventional seismic vibrator single source duration, while providing data separation comparable or superior to current multiple source techniques, by emitting a set of low distortion time sequences that are uniquely partitioned in frequency and/or phase as a function of time such that each source within the set emits a distinct signature. The new partitioning methods disclosed will allow approximately the same sweep times (6-12 seconds) and sweep effort as conventional single source point acquisition. Time efficiency advantages will be realized by the disclosed methods using single source protocols or using multi-source protocols. 
       FIG. 1  illustrates a non-limiting example of a method for generating separable seismic source signals. In general, the method  100  includes receiving an input signal at a seismic vibrator  102 , generating energy in the form of a plurality of time sequence vibratory signals  104 , and emitting the vibratory signals into a terrain of interest  106  as seismic source signals. 
     The “input signals” in several non-limiting embodiments are the desired signal forms for a particular seismic survey. In an ideal case, the seismic source signal traveling in earth is identical to the input signal. The input signals may be created off site of the survey location or the input signals may be generated at the survey location. Once generated, the input signals are loaded onto the vibrator or vibrators for use by the vibrator electronics, e.g. the controller, to control the vibrator system. The vibrator system converts the input signals to kinetic energy in the vibrator reaction mass. The energy in the vibrator system reaction mass will be referred to herein ss the vibratory signal. 
     In several non-limiting embodiments, several seismic vibrators are positioned on or near the terrain of interest. The individual seismic vibrators may be of any type seismic vibrator that is capable of controlling distortion to a level that enables signature separation during processing in field units or in a seismic information processing facility. Suitable non-limiting vibrators may include a hydraulic actuator system having a feedback control circuit for controlling harmonic distortion. One example of such a vibrator is disclosed in U.S. patent application Ser. No. 11/691,925 for “Apparatus and Method for Generating a Seismic Source Signal,” the entire specification of which is hereby incorporated herein by reference. Other non-limiting examples of suitable vibrators include seismic vibrators having controlled linear electric motor actuators. Some examples of which are described below. 
     Variations in the allowable distortion level of generated signals may be accommodated by separation processing methods where the emitted signal signatures are distinct enough for recognition within the band of distortion. In general, the term “low distortion” as used herein relates to total harmonic distortion (THD). THD adds risk to the ability to separate recorded information, and the acceptable level of THD may vary and may depend on one or more factors. Some factors that may allow for higher or lower acceptable THD include the particular seismic record, recording methods used, filtering methods used, subtraction methods used and information processing methods. Therefore, the term “low distortion: should be considered herein as meaning an acceptable level of THD. In most cases, a THD of less than about 5% will be acceptable. In some embodiments, acceptable THD may be 5% or more. Source separation in any particular seismic information acquisition operation will be a function of the source geometry, signal strength, and uncorrected vibrator distortion. 
     In several embodiments, the input signals are frequency partitioned with respect to time in order to form input signals having distinct signatures. In another embodiment, the input signals are phase partitioned with respect to time in order to form signals having distinct signatures. In other non-limiting embodiments, the input signals may be partitioned by a combination of frequency and phase with respect to time. 
     Several seismic vibrators may be operated to emit respective partitioned vibratory signals as seismic source signals closely matching respective input signals at the about the same time or in a substantially simultaneous manner. In another embodiment, the partitioned vibratory signals may be emitted as seismic source signals in a partially overlapping manner with respect to time. Alternatively, the partitioned vibratory signals may be emitted asynchronously in series. 
       FIGS. 2A-2C  and  FIGS. 3A-3C  illustrate a non-limiting method of generating input signals having a unique signal signature. Those skilled in the art will recognize that depicting actual signals using graphically unscaled data would not be practical. Thus, the figures are scaled and shifted for simplicity to illustrate the concept.  FIG. 2A  shows a base signal  200  having amplitude (vertical axis) scaled to ±1. The horizontal axis illustrates 4000 data points within a sweep of 5 to 25 Hz. In  FIG. 2B , the base signal is segmented using a tapering process performed by a sweep synthesizer program or other signal generating program. Here, the base signal is shown segmented into four segments  202   a ,  202   b ,  202   c  and  202   d , but more or less than four segments may be used. Note that each segment has a unique frequency characteristic. Phase characteristics of several source signals will be described later with reference to  FIGS. 3A-D . The segments of the signal  202  are then spectrally compensated using the program to arrive at the input signal having unique signal signature  204 . 
     In one non-limiting embodiment, a seismic information acquisition operation is performed by conducting a seismic survey using four seismic sources where each source emits a partitioned vibratory signal into the earth as a seismic source signal having a unique vibratory signal signature closely matching the respective input signal. The seismic information acquired using seismic sensors may then be separated relative to the several source signals using information processing methods that separate the information based in part on the signal signatures. In the four-source example, the respective input signals may be partitioned as shown in  FIGS. 3A-3D , although other signatures are within the scope of the disclosure. Here,  FIG. 3A  shows an input signal  204  having a signature as described above and shown in  FIG. 2C .  FIG. 3B  illustrates an input signal  302  used by a second vibrator where the second vibrator input signal  302  has a signature unique with respect to the signal  204  loaded in the first vibrator. Likewise,  FIGS. 3C and 3D  illustrate respective unique input signals  304 ,  306  loaded into the electronics of the respective vibrators and that are unique with respect to the other emitted signals  204 ,  302 . Each unique signature input signal may be generated by a circulatory time shift of signals using the segmenting and compensating process discussed above with respect to a base signal  200 . 
       FIG. 4  schematically illustrates a non-limiting example of a land seismic vibrator system  400  for generating and propagating seismic energy signal into the earth in the form of a time sequence seismic source signal that is partitioned as a function of time, wherein the signal comprises a distinguishing signature as discussed above and shown in  FIGS. 2C and 3A . The seismic vibrator system  400  includes a reaction mass  404  and a support member  406  mechanically coupled to a baseplate  408 . The baseplate  408  maintains physical contact with the earth&#39;s surface  410  during operation. The seismic vibrator system  400  includes an electric power generator  414  that provides electrical power to an electric driver  412 . A driver interface  416  provides mechanical, electrical or magnetic communication between the electric driver  412  and the reaction mass  404 . The electric driver  412  in response to the input signal causes the reaction mass  404  to reciprocate along one or more axes during operation, thereby converting the input signal to kinetic energy in the reaction mass  404 . The kinetic energy is in the form of a vibratory signal that closely matches the input signal. 
     The seismic vibrator system  400  may further include a controller  424  for controlling the electric driver  412  and movement of the reaction mass  404 . Sensors  418 ,  420 ,  422  may be used to monitor operating parameters of the system  400 . In one embodiment, an output signal from the sensors  418 ,  420 ,  422  may be incorporated into a feedback control circuit  426  for providing control of the reaction mass  404  movement and thus better control of the system  400  output energy imparted into the earth to control and maintain distortion levels to an acceptable THD. 
     The electric power generator  414  may be any suitable system that generates sufficient power for the seismic vibrator system  400 . The electric power generator  414  may include any suitable device or combination of devices for supplying electrical power to the seismic vibrator system  400 , including, but not limited to a gasoline engine, a diesel engine, and propane or natural gas powered generators. In one embodiment, the electric power generator  414  can be an industrial or commercial electrical supply network. In one embodiment, the electric generator can be a 250 kW three phase generator driven with a 300 to 400 HP power unit. In several embodiments, the electric power generator  414  provides electric power to the electric driver  412 . 
     The electric driver  412  may include any number of drive types suitable for moving a heavy mass, such as the reaction mass  404 . The electric driver  412  converts the electrical energy supplied by the electric power generator  414  to mechanical or magnetic energy sufficient to reciprocate the reaction mass  404 . In one embodiment, the electric driver  412  includes rotary electric motor or several rotary electric motors. In one example, a “squirrel-cage” type motor may be used. In one embodiment, the electric driver  412  includes a linear electric motor or several linear electric motors. In some cases, a stator is rigidly coupled to a reaction mass  404  support structure and one or more armatures may be rigidly attached to the reaction mass  404 . In other examples, a stator is rigidly coupled to a reaction mass  404  and one or more armatures may be rigidly attached to a support structure. In several embodiments, the electric driver  412  may include electromagnetic devices disposed on or about the reaction mass  404 . 
     Continuing with the example of  FIG. 4 , the driver interface  416  may include any number of devices or structures suitable for interfacing the electric driver  412  to the reaction mass  404 . The driver interface  416  may include a gearbox for converting energy from the electric driver  412  reciprocating motion in the reaction mass  404 . In one embodiment, the electric driver  412  includes a linear induction motor, and the driver interface  416  can be an electromagnetic interface. In several embodiments, the energy to reciprocate the reaction mass  404  can be supplied using one or more electromagnetic devices disposed internal to the reaction mass  404 . In one or more embodiments, the energy to reciprocate the reaction mass  404  can be supplied using one or more electromagnetic devices disposed external to the reaction mass  404 . 
     The reaction mass  404  can be a structure fabricated from any resilient metallic material, such as steel, steel alloys, or any suitable composite material. The reaction mass  404  may be formed of a monolithic member or may include several members suitably coupled to form a reaction mass  404 . The weight of the reaction mass  404  and the weight of the baseplate  408  may be selected according to any suitable ratio. The reaction mass  404  can be equipped with an internal cooling system, external cooling system, or any combination thereof, to maintain the electrical or electromagnetic driver  412  at a suitable operating temperature ranging from about 0° C. to about 150° C. In some embodiments, the weight of the reaction mass may be selected to be a minimum of twice the weight of the baseplate  408 . The reaction mass  404  can weigh from about 2,000 lbs to about 15,000 lbs, about 4,000 lbs to about 13,000 lbs, or about 6,000 lbs to about 12,000 lbs., or about 15,000 lbs. to about 40,000 lbs. 
     The reaction mass  404  according to the example of  FIG. 4  can be moved along on a vertical axis to generate the vibratory signal according to the input signal used. Movement of the reaction mass  404  in this manner imparts a force to the support member  406  once per cycle using the driver  412  and driver interface  416 . The support member  406  can be rigidly attached to the baseplate  408  to permit transfer of the kinetic energy from the reaction mass  404  to the baseplate  408  via the support member  406 . The support members  406  may be fabricated from any resilient metallic material, such as steel, steel alloys, or any suitable composite material capable of withstanding the force imparted from the reaction mass  404  and transferring the kinetic energy from the reaction mass  404  to the baseplate  408 . The support member  406  can be rigidly affixed to the baseplate  408  using any suitable mechanical attachment including welding, bolting, pinning, or the like. In some cases, isolators such as air bags are used to isolate the baseplate  408  from vibrations caused by vibrator truck systems and motors. 
     In this manner, substantially all of the energy imparted to the baseplate is that of the reciprocating reaction mass  404 . 
     The baseplate  408  can be fabricated from any resilient metallic material, such as steel, steel alloys, or any suitable composite material capable of supporting the weight of the reaction mass  404 . In one embodiment the weight of the baseplate is approximately one-half the weight of the reaction mass  404 . In one or more embodiments, the weight of the baseplate can range from about 1,000 lbs to about 8,000 lbs, about 2,000 lbs to about 6,000 lbs, or about 3,000 lbs to about 6,000 lbs. 
     In one embodiment, the baseplate  408  can be lowered from the transporter and placed into contact with the earth&#39;s surface  410  using either a hydraulically or electrically actuated lift system. The lift system can be configured such that the entire weight of the transporter rests upon the baseplate  408 , thereby typically providing 60,000 pounds of down force to maintain contact between the baseplate  408  and the earth&#39;s surface while the seismic vibrator system  100  is in operation. Additional external weight can be added, if necessary, to ensure contact between the baseplate  408  and the earth&#39;s surface  410  is maintained at all times while the seismic vibrator system is in operation. 
     In one embodiment, one or more sensors  418 ,  420 ,  422  can be located on or in close proximity to the seismic vibrator system  400  to provide real time monitoring of system performance. In one embodiment, the one or more particle motion sensors  418  can include, but are not limited to single or multiple axis accelerometers, or geophones mounted proximately to the system  400  to monitor ground movement imparted to the surface of the earth  410  by the system  400 . In one embodiment the one or more particle motion sensors  420  can include, but are not limited to micro electromechanical systems (MEMS) sensors, analog accelerometers with suitable A/D conversion and/or vibration sensors mounted on the system  400 . In one embodiment, the one or more displacement sensors  422  can include laser, inductive and/or other types of proximity sensors to measure the displacement of the reaction mass  404  relative to the baseplate  408 . 
     The sensors  418 ,  420 ,  422  can be used to provide signal inputs to the controller  424 . In one embodiment, the controller  424  can be used to provide adjustments to operating variables such as stroke, acceleration, and frequency of the seismic vibrator system  400  in response system parameters monitored using the sensors  418 ,  420 ,  422 . In one embodiment, pulse width modulation can be used to adjust the current supplied to the driver  412  to optimize system performance based upon sub-surface conditions. Control adjustments may be made in real time using feedback obtained during vibrator  400  operation. 
     The reaction mass  404  can reciprocate during operation along a vertical axis normal to the surface of the earth  408 . A reciprocating cycle can begin with the reaction mass in physical contact with the support member  406 , and the reaction mass  404  may be raised approximately 2″ to 6″ (5 cm to 15 cm) above the support member  406 . At the top of a stroke, the reaction mass  404  is accelerated downward using gravity, electrical energy, mechanical energy, or any combination thereof and then up again in a reciprocating motion that imparts energy to the support member  406 . The reciprocating motion transfers kinetic energy from the reaction mass  404 , through the support member  406  to the baseplate  408 . The energy imparted to the baseplate  408  by the reaction mass  404  establishes a compressional seismic wave (“P-Wave”) which propagates into and through the surface of the earth  410 . The time required for the reaction mass  404  to travel through one complete oscillatory cycle (“cycle time”) determines the frequency of the seismic waves generated by the seismic vibrator system  400 . In one or more embodiments, the cycle time for the reaction mass  404  can be varied from about 2 to about 300 cycles per second (Hertz). 
     The electrical power used to reciprocate the reaction mass  404  may be based upon the desired frequency and amplitude of the seismic waves imparted to the earth&#39;s surface. In one illustrative example, with a reaction mass of 8,000 pounds (2985 kg), a frequency of 25 hertz (i.e. 25 cycles per second), a displacement of approximately 4 inches (11 cm), and 2 G of acceleration at the point where the reaction mass  404  imparts a force to the support member  406 , a minimum energy input of about 270 kW may be used for moving the reaction mass  404 . In this illustrative example, the reciprocation of the reaction mass  404  will impart a series of seismic waves into the earth&#39;s surface  410 , each containing approximately 5,400 foot-pounds (7,000 joules) of energy. 
       FIG. 5  is another non-limiting example of a seismic vibrator system  500  for generating and propagating seismic energy into the earth&#39;s surface. Seismic shear waves (“S-Waves”) can be introduced into the earth&#39;s surface using the seismic vibrator system  500  shown. Similar to  FIG. 4 , the seismic vibrator system  500  can include a reaction mass  504  and support member  506  mechanically coupled to a baseplate  508 . The baseplate  508  maintains physical contact with the earth&#39;s surface  410  during operation. The seismic vibrator system  500  includes an electric power generator  414  that provides electrical power to the driver  412 . A driver interface  416  provides mechanical, electrical or electromagnetic communication between the driver  412  and the reaction mass  504 . The electric driver  412  causes the reaction mass  504  to reciprocate along one or more axes during operation. Similar to the system depicted in  FIG. 4 , the seismic vibrator system  500  may further include a controller  424  for controlling the electric driver  412  and movement of the reaction mass  504 . Sensors  418 ,  420 ,  422  can be incorporated into a feedback control circuit providing control of the reaction mass  504  movement and thus enhanced control of the system  500  seismic output. 
     Similar to  FIG. 4 , the system depicted in  FIG. 5  can include an electric generator  414 , in one example a 250 kW, 3 phase generate driven by a 300 to 400 HP power unit, for supplying electrical power to the electric driver  412 . Also similar to  FIG. 4 , the driver  412  can include, but are not limited to, rotary electric motors, linear electric motors, electromagnetic devices, or any combination thereof disposed on or about the reaction mass  504 . The system depicted in  FIG. 5  can include a driver interface  416  including but not limited to mechanical 
     Similar to  FIG. 4 , the reaction mass  504  can be fabricated from steel, steel alloys, stainless steel, stainless steel alloys or other metallic composites. In one embodiment, the reaction mass  504  can be fabricated using one or more composite materials. In one embodiment, the reaction mass  504  can be solid member. In one embodiment, the reaction mass  504  can have one or more inductive or magnetic devices disposed within or about the reaction mass  504 . In one embodiment, the weight of the reaction mass  504  can be a minimum of twice the weight of the baseplate  508 . In one embodiment, the reaction mass  504  can weigh from about 2,000 lbs to about 15,000 lbs, about 4,000 lbs to about 13,000 lbs, or about 6,000 lbs to about 12,000 lbs. 
     In one embodiment, the reaction mass  504  can be reciprocated on a horizontal axis (i.e. along an axis parallel to the surface of the earth), imparting a force to the opposing side of the support member  506  once per cycle, thereby imparting a lateral motion to the baseplate  508 . The support member  506  can be rigidly attached to the baseplate  508  to permit the transfer of kinetic energy from the reaction mass  504  to the baseplate  508  via the support member  506 . In one embodiment, the support member  506  can be fabricated from any resilient metallic material, such as steel, steel alloys, or any suitable composite material capable of withstanding the force imparted to the reaction mass  504  and transferring the kinetic energy from the reaction mass  504  to the baseplate  508 . The reaction mass  504  can be equipped with an internal cooling system, external cooling system, or any combination thereof, to maintain the electrical or electromagnetic driver  512  at a suitable operating temperature ranging from about 0° C. to about 150° C. In one embodiment, the support member  506  can be rigidly affixed to the baseplate  508  using any means of mechanical attachment including welding, bolting, pinning, or the like. In one embodiment, the support member  506  can be fabricated integrally with the baseplate  508 . 
     The baseplate  508  can be fabricated from any resilient metallic material, such as steel, steel alloys, or any suitable composite material capable of withstanding the energy transferred from the reaction mass  504 . In one embodiment the weight of the baseplate  508  can be approximately one-half the weight of the reaction mass  504 . In several embodiments, the weight of the baseplate  508  can range from about 1,000 lbs to about 8,000 lbs, about 2,000 lbs to about 6,000 lbs, or about 3,000 lbs to about 6,000 lbs. 
     In one embodiment, the baseplate  508  can be lowered from the transporter and placed into contact with the earth&#39;s surface  410  using either a hydraulically or electrically actuated lift system. The lift system can be configured such that the entire weight of the transporter rests upon the baseplate  508 , thereby typically providing 60,000 pounds of down force to maintain contact between the baseplate  408  and the earth&#39;s surface while the seismic vibrator system  500  is in operation. Additional external weight can be added, if necessary, to ensure contact between the baseplate  508  and the earth&#39;s surface  410  is maintained at all times while the seismic vibrator system is in operation. 
     The sensors  418 ,  420 ,  422  can be located on or in close proximity to the seismic vibrator system  500  to provide real time monitoring of system performance. In one embodiment, particle motion sensors  418  can include, but are not limited to single or multiple axis accelerometers, geophones, or similar devices, mounted proximately to the system  500  to monitor ground movement imparted by the system  500 . In one embodiment, the particle motion sensors  420  can include, but are not limited to accelerometers and/or vibration sensors mounted on the system  500 . In one embodiment, the displacement sensors  422  can include laser, inductive and/or other types of proximity sensors to measure the displacement of the reaction mass  504 . 
     In one embodiment, the sensors  418 ,  420 ,  422  can be used to provide signal inputs to the feedback controller  424 . In one embodiment, the feedback controller is used to provide adjustments to operating variables such as stroke, acceleration, and frequency of the seismic vibrator system  500  in response to sub-surface conditions. In one embodiment, pulse width modulation can be used to accurately adjust the current supplied to the driver  412  to optimize system performance based upon sub-surface conditions. Control adjustments may be made in real time using feedback obtained during vibrator  500  operation. 
     The electric driver  412  can be used to reciprocate via the driver interface  416  the reaction mass  504  on an axis parallel to the surface of the earth  410 . In one or more embodiments, a reciprocating cycle can begin with the reaction mass in physical contact with the support member  506 . The reaction mass  504  can be axially displaced approximately 2″ to 6″ (5 cm to 15 cm) in a first direction along a horizontal axis bringing the reaction mass  504  into physical contact with the support member  506 . The reaction mass  504  can then be accelerated in a second direction, 180° opposed to the first direction, along the identical horizontal axis, until the reaction mass  504  once again contacts the support member  506 . 
     The force of the reaction mass  504  can be transmitted through the support member  506  to the baseplate  508 . Since the baseplate  508  contacts the earth&#39;s surface  410 , the energy imparted to the baseplate establishes a shear or S-wave which propagates through the surface of the earth  410 . The time required for the reaction mass  504  to travel through one complete reciprocating cycle (“cycle time”) determines the frequency of the S-waves generated by the seismic vibrator system  500 . In one or more embodiments, the cycle time can be varied from about 2 to about 300 cycles per second (Hertz). 
       FIG. 6  illustrates another non-limiting example of an illustrative apparatus for generating geophysical information used in imaging earth subsurface structures. In one or more embodiments, the seismic vibrator system  600  includes a reaction mass  604 , and a support member  606  mechanically connected or affixed to a baseplate  608 . The baseplate  608  maintains contact with the surface of the earth  410  during operation. In one embodiment, the seismic vibrator system  600  can include a linear electric motor  630 . In one embodiment, the one or more linear electric motors  630  can include one or more stators  634  coupled to a vibrator support structure and one or more armatures  632  coupled to the reaction mass  604 . The positions of the stator(s) and rotor(s) may be reversed in other embodiments. In one embodiment, the seismic vibrator system  600  includes sensors  418 ,  420 ,  422  in communication with a controller  424 . In one embodiment, the output from the controller  424  can be used to control the linear electric motor  630  thereby adjusting the seismic source signal generated by the seismic vibrator system  600 . 
     In one embodiment, the linear electric motor  630  may include a linear induction motor (“LIM”) to achieve high acceleration of the reaction mass  604 . A LIM type design can have an active three-phase winding forming the one or more stators  334  and one or more passive conductor plates  632  in physical connection with, and affixed to, the reaction mass  604 . 
     In one embodiment, the linear electric motor  630  may include a linear synchronous motor (“LSM”) capable of achieving high speed and power at a lower acceleration than a comparable LIM. An LSM type design can have an active winding forming the one or more stators  634  and an array of alternate-pole magnets  632  in physical connection with, and affixed to, the reaction mass  604 . With an LSM type design, the one or more magnets  632  can be either permanent magnets or electromagnets. 
     As depicted in  FIG. 6 , the reaction mass  604  can reciprocate along a vertical axis normal to the surface of the earth  410 . Using the one or more linear electric motors  630 , the reaction mass  604  can be raised approximately 2″ to 6″ (5 cm to 15 cm) above a neutral position and accelerated downward using either gravity, the linear electric motor, or any combination thereof and then up again in a reciprocating motion that imparts energy to the support member  606 . The reciprocating motion transfers kinetic energy from the reaction mass  604 , through the support member  606  to the baseplate  608 . The energy imparted to the baseplate  608  from the reaction mass  604  establishes a compressional seismic wave (“P-Wave”), which propagates into the surface of the earth  410 . The time required for the reaction mass  604  to travel through one complete reciprocating cycle (“cycle time”) determines the frequency of the seismic waves generated by the seismic vibrator system  600 . The cycle time for the reaction mass can be varied from about 2 to about 300 cycles per second (Hertz). 
     Sensors  418 ,  420 ,  422  can be located on or in close proximity to the seismic vibrator system  600  to provide real time monitoring of system performance. In one embodiment, particle motion sensors  418  placed near the system can include, but are not limited to single or multiple axis accelerometers, or geophones to monitor ground movement imparted by the system  600 . In one embodiment particle motion sensors  420  coupled to the system  600  can include, but are not limited to accelerometers and/or vibration sensors to measure motion of the system components. In one embodiment the one or more particle motion sensors  420  can include, but are not limited to micro electromechanical systems (MEMS) sensors, analog accelerometers with suitable A/D conversion and/or vibration sensors mounted on the system  600 . Displacement sensors  422  can be used to measure the displacement or position of the reaction mass  604  during operation and may include laser, inductive and/or other types of proximity sensors. 
     In one embodiment, the sensors  418 ,  420 ,  422  can be used to provide signal inputs to the controller  424  via a feedback circuit  326 . In one embodiment, the feedback controller can provide one or more output signals to adjust operating variables such as stroke, acceleration, and frequency of the seismic vibrator system  600  in response to estimated operating parameters. In one embodiment, pulse width modulation can be used to adjust the current supplied to the driver  412  to optimize system performance based upon sub-surface conditions. Control adjustments may be made in real time using feedback obtained during operation of the seismic vibrator system  600 . 
       FIG. 7  illustrates yet another non-limiting example of an illustrative apparatus for generating and propagating a seismic source signal into the earth. A seismic vibrator system  700  according to this example includes a reaction mass  704 , and a support member  706  mechanically connected or affixed to a baseplate  708 . The baseplate  708  maintains contact with the surface of the earth  410  during operation. In one or more embodiments, the seismic vibrator system  700  can include one or more linear electric motors  430 . The linear electric motor  730  may include one or more stators  734  coupled to a vibrator support structure and one or more armatures  732  in physical connection with, and affixed to, the reaction mass  704 . The positions of the stators and armatures may be reversed in other embodiments. The seismic vibrator system  700  can include one or more sensors  418 ,  420 ,  422  in communication with a controller  424 . An output from the controller  424  may be used to control the electric motors  430  thereby adjusting the seismic source signal generated by the seismic vibrator system  700 . 
     The linear electric motors  730  may include a linear induction motor (“LIM”) or a linear synchronous motor (“LSM”). The linear electric motor  730  may be disposed internal to the reaction mass  704 . In one embodiment, the one or more stators  734  can be rigidly attached to the support member  706  that is further coupled to the vibrator support structure, and the armatures  732  can be attached to the reaction mass  704 ; thereby permitting the inductive forces between the stator  734  and the armatures to vibrate the reaction mass  704  along a vertical axis. 
     The reciprocating motion of the reaction mass  704  transfers kinetic energy from the reaction mass  704  through the support member  706  (when used) to the baseplate  708 . The energy transfer establishes a compressional seismic wave (“P-Wave”) which propagates into the surface of the earth  410 . The time required for the reaction mass  704  to travel through one complete cycle (“cycle time”) determines the frequency of the seismic waves generated by the seismic vibrator system  700 . In one or more embodiments, the cycle time for the reaction mass can be varied from about 2 to about 300 cycles per second (Hertz). 
     Sensors  418 ,  420 ,  422  can be located on or in close proximity to the seismic vibrator system  700  to provide real time monitoring of system performance. In one embodiment, particle motion sensors  418  can be placed on the ground proximate the system  700  to monitor ground movement imparted by the system  700 . The ground sensors  418  may include, but are not limited to single or multiple axis accelerometers, or geophones. Other particle motion sensors  420  may be coupled to the system  700  for estimating movement of vibrator components during operation. In one embodiment the one or more particle motion sensors  420  can include, but are not limited to micro electromechanical systems (MEMS) sensors, analog accelerometers with suitable A/D conversion and/or vibration sensors mounted on the system  700 . Position sensors  422  may be used to estimate the position of the reaction mass  704  during operation. These 
     In one embodiment, the sensors  418 ,  420 ,  422  can be used to provide signal inputs to the controller  424  via a feedback circuit  426 . In one embodiment, the controller  424  can provide one or more output signals to adjust operating variables such as stroke, acceleration, and frequency of the seismic vibrator system  700  in response to sub-surface conditions. In one embodiment, pulse width modulation can be used to adjust the current supplied to a driver, such as the driver  412  discussed above and shown in  FIG. 4 , to optimize system performance based upon sub-surface conditions. Control adjustments may be made in real time using feedback obtained during operation of the seismic vibrator system  700 . 
     In operation, methods according to the disclosure include generating a plurality of seismic source signals using several land seismic vibrators positioned on or near a terrain of interest. In reference to the several embodiments described above and shown in  FIGS. 1-7 , land seismic vibrators may be transported to a seismic survey area. A baseplate for each vibrator is then coupled to the earth&#39;s surface. In one embodiment, the transporter can be hydraulically or electrically lifted such that the weight of the transporter rests upon the baseplate. If necessary, additional external weight can be added to the transporter to ensure solid contact between the baseplate and the surface of the earth at all times while the vibrator is in operation. Safety sensors can be installed on the lift mechanism to warn of failure to lower the baseplate prior to operation or failure to raise the baseplate prior to moving the transporter. A reaction mass is moved in a reciprocating manner along one or more axes using a feedback-controlled electric linear motor or by a feedback-controlled hydraulic actuator. Kinetic energy from the reciprocating reaction mass is transferred to the baseplate, and a seismic source signal is propagated into the earth. Reciprocating the reaction may be accomplished using a feedback controlled linear induction motor or a linear synchronous motor or controlled hydraulic actuator as discussed above. When using electric motor embodiments, a drive interface may be used to convey energy from the electric motor to the reaction mass. 
     The actuator or motor and the motion of the reaction mass may be controlled using a controller to generate energy in the form of a plurality of time sequence vibratory signals, the vibratory signals being partitioned as a function of time, wherein each of the plurality of signals comprises a distinguishing signature, and operation parameters may be monitored according to several embodiments using sensors placed in, on or around each seismic vibrator. An output signal indicative of the estimated operating parameters may be conveyed to the controller via a feedback circuit, and the controller may be used to adjust acceleration, velocity, force, frequency, stroke or other parameters of the seismic vibrator in real-time during operation to reduce and control total harmonic distortion to about 5% or less THD. Pulse width modulation can be used to adjust the current flow to the seismic vibrator system in real-time to optimize system performance based upon observed sub-surface conditions. 
     Other embodiments include methods for acquiring seismic information and for processing seismic information. In one embodiment, a method for seismic information acquisition includes positioning a plurality of seismic receivers in a terrain of interest. The receivers are used to receive a plurality of time sequence vibratory signals that are generated as described above where the vibratory signals are partitioned as a function of time. Each signal includes a distinguishing signature that may be separated at the receiver, at a field processor or in a processing facility away from the terrain of interest. 
     In another embodiment, a method for processing seismic information includes receiving recorded seismic information. The recorded information is created by a seismic survey where a plurality of seismic vibrators generate signals as described above and which signals are received by seismic receivers and recorded on a recording medium. The recorded seismic information includes the time sequence vibratory signals generated at the vibrators and after the signals are emitted into the earth and have reflected off subterranean formations. The vibratory signals are partitioned as a function of time, wherein each of the plurality of signals comprises a distinguishing signature. The recorded seismic information is then processed to separate the seismic information with respect to each of the plurality of signals. 
     The present disclosure is to be taken as illustrative rather than as limiting the scope or nature of the claims below. Numerous modifications and variations will become apparent to those skilled in the art after studying the disclosure, including use of equivalent functional and/or structural substitutes for elements described herein, use of equivalent functional couplings for couplings described herein, and/or use of equivalent functional actions for actions described herein. Such insubstantial variations are to be considered within the scope of the claims below. 
     Given the above disclosure of general concepts and specific embodiments, the scope of protection is defined by the claims appended hereto. The issued claims are not to be taken as limiting Applicant&#39;s right to claim disclosed, but not yet literally claimed subject matter by way of one or more further applications including those filed pursuant to the laws of the United States and/or international treaty.