Abstract:
A seismic energy source includes a base plate having a first electromagnet thereon. A reaction mass has a second electromagnet thereon. A switchable current source is connected to the first and second electromagnets such that the first electromagnet repels the second electromagnet. At least one damper is functionally connected between the reaction mass and the base plate. The damper is configured to enable faster movement of the reaction mass away from the base plate than toward the base plate.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    Not applicable. 
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    Not applicable. 
       BACKGROUND OF THE INVENTION 
       [0003]    1. Field of the Invention 
         [0004]    The invention relates generally to the field of seismic energy sources. More particularly, the invention relates to impulsive seismic energy sources using electromagnets to move a mass. 
         [0005]    2. Background Art 
         [0006]    Seismic surveying includes imparting seismic energy from a seismic energy source into rock formations below the land surface, or below the bottom of a body of water in marine environments. The seismic energy travels from the source, through the rock formations and is reflected from acoustic impedance boundaries in the subsurface. Such acoustic impedance boundaries are typically at the interfaces of layers of different rock formations. The reflected seismic energy is detected by a plurality of seismic receivers disposed at the surface, on the water bottom or in the water. The detected seismic energy is interpreted to infer, among other things, structure and composition of the rock formations below the surface or water bottom. 
         [0007]    Seismic energy sources known in the art include “impulsive” sources. Impulsive sources impart seismic energy into the subsurface in short duration events, wherein in each event substantially all the seismic energy is generated at the same time. Impulsive sources include, among others, air guns, water guns, dynamite and weight drop devices. 
         [0008]    Another type of impulsive seismic energy source known in the art uses electromagnets to lift a moveable reaction mass towards a top block composed of laminated magnetic steel, which rests on top of a frame. The reaction of lifting the mass towards the top block is transferred via the frame to a base plate in contact with the ground. The movable mass is caused to move when the electromagnet therein is actuated to cause attraction between the electromagnet and the top block above it. A limitation to the foregoing type of mass lift impulsive seismic source known in the art is that the magnetic attractive force between the electromagnet and the top block is inversely related to the distance between them. Thus, on actuation, the attractive force between the electromagnet on the movable mass and the steel top block is smallest. Therefore, the maximum distance that the movable mass may be disposed from the top block when the movable mass is at rest is limited. Since no force can be transmitted to the ground once the attracting surfaces of the moving mass and top block meet, the above distance limitation will impose an absolute limit on the active stroke of the device, and thus the maximum attainable ground movement caused by the base plate. Further, because the attractive force between electromagnet and top block increases as the distance therebetween is reduced, the movable mass tends to accelerate during its travel toward the top block, limiting the accuracy of timing of seismic impulses generated by the source. 
         [0009]    There continues to be a need for improved mass lift seismic energy sources. 
       SUMMARY OF THE INVENTION 
       [0010]    A seismic energy source according to one aspect of the invention includes a base plate having a first electromagnet thereon. A reaction mass has a second electromagnet thereon. A switchable current source is connected to the first and second electromagnets such that the first electromagnet repels the second electromagnet. At least one damper is functionally connected between the reaction mass and the base plate. The damper is configured to enable faster movement of the reaction mass away from the base plate than toward the base plate. 
         [0011]    A method for seismic surveying according to another aspect of the invention includes actuating a first electromagnet associated with a reaction mass and a second electromagnet associated with a base plate in contact with a ground surface so as to cause the reaction mass to lift away from the base plate. The first electromagnet and the second electromagnet are deactivated such that gravity draws the reaction mass toward the base plate. Motion of the reaction mass toward the base plate is damped sufficiently to substantially prevent generation of an impulse when the reaction mass contacts the base plate. 
         [0012]    Other aspects and advantages of the invention will be apparent from the following description and the appended claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1  shows an example seismic source and seismic receivers. 
           [0014]      FIG. 2  shows an alternative example of the seismic source of  FIG. 1   
       
    
    
     DETAILED DESCRIPTION 
       [0015]    An example seismic acquisition system including a mass lift seismic energy source according to the invention is shown schematically in  FIG. 1 . The source  10  is disposed at a selected position on the Earth&#39;s surface  18  (or in a body of water) above subsurface rock formations  42  to be evaluated. A plurality of seismic receivers  14  such as geophones, hydrophones or accelerometers may be disposed at spaced apart locations on the surface, in the body of water or on the bottom of the body of water above the subsurface rock formations  42 . The receivers  14  generate electrical and/or optical signals in response to seismic energy detected from the subsurface formations  42 . The signals generated by the receivers  14  may be conducted to a recording system  12 . The recording system may include devices (not shown separately) for making a time indexed record of the signals detected by the receivers  14 . The recording time is typically indexed to the actuation time of the seismic energy source  10 . The source  10  may be actuated by a source control  43  that may be in signal communication with the recording system  12  using a radio link. Signals to actuate the source  10  may originate in the recording system  12  and be transmitted to the source control system  43  over the radio link. Using such configuration, the source  10  may be disposed remotely from the recording unit  12  without the need to connect the source control unit  43  to the recording unit with electrical cable. 
         [0016]    The source  10  may include a frame  72  which can be rigidly fixed to a reaction mass. In one example, explained further below, the reaction mass may consist of the vehicle (shown schematically at  76 ) which transports the source  10  to a desired location. In the example shown in  FIG. 1 , the mass, shown at  74 , may be a separate component which can move independently of the vehicle  76 . 
         [0017]    A first electromagnet  37  may be fixed under the base of the frame  72 . The frame  72  when it is in the rest position (electromagnets not activated) is in contact with one side of a base plate  77  disposed under the frame  72 . The other side of the base plate  77  is in contact with the ground  18  (or water or water bottom). A second electromagnet  39  may be disposed on top of the base plate  77 . The assembly consisting of the base plate  77  and the second electromagnet  39  can be laterally constrained to slide up and down relative to the frame  72 , but have substantially no freedom of movement side to side. The first  37  and second  39  electromagnets may each include a wire coil  36 ,  38 , respectively, disposed in magnetically permeable material,  37 A and  39 A. 
         [0018]    The speed of relative movement between the base plate  77  and the frame  72  may be limited in either or both directions by a plurality of dampers  71  which are connected between the frame  72  and the base plate  77 . The dampers  71  may enable the base plate assembly  77  to move downwardly relative to the frame  72  at a predetermined speed. In some examples the downward motion of the base plate  77  relative to the frame  72  may be unconstrained (that is, the dampers provide no damping in such direction). The dampers  71  are also configured to limit the speed at which the base plate assembly  77  can move upwardly relative to the frame  72 . The upward speed is preferably lower than that for the base plate  77  assembly to move downwardly relative to the frame  72 . The dampers  71  may in some examples be hydraulic, and may include a valve (not shown) with an internal piston which allows oil to flow at a certain speed one way through the piston, but to flow at a different speed in the opposite direction. In other examples, the dampers  71  may be electromagnetic. In still other examples, the dampers  71  may include an electrorheological fluid so that the damping can be electrically controlled, e.g., by the source control system  43 . 
         [0019]    In the present example, wherein the mass  74  is a separate component, there may be installed various connecting links and framing components, shown generally at  73  and referred to for convenience as a “structure” which enables the vehicle  76  to exert downward force on the base plate  77 . The structure  73  can be coupled to the base plate  77  to enable the base plate  77  to move upwardly and downwardly relative to the vehicle  76 . The foregoing feature may be provided by connecting the structure  73  to the base plate  77  using flexible, gas filled containers  75  known in the art as air bags. 
         [0020]    In the rest position the two electromagnets  37 ,  39  will be in contact. The source  10  is actuated by passing an electric current from the source control system  43  through the electromagnet coils  36 ,  38  in a direction such that the first  37  and second  39  electromagnets repel each other. The repulsion lifts the frame  72  and the mass  74 . Reaction to the foregoing motion causes the base plate  77  to be forced into the ground  18 . Forcing the base plate  77  into the ground  18  creates a seismic impulse. The fact that the impulse is caused by a precisely controllable electrical event means that this type of source can be readily synchronized in a fleet of similarly configured seismic sources in order to increase the magnitude of the impulse. It can likewise be readily synchronized with the seismic record made in the recording unit  12   
         [0021]    Once the electric current ceases to flow through the coils  36 ,  38 , the frame  72  and the mass  74  will drop toward the base plate  77  by the effect of gravity. The rate at which the frame  72  and mass  74  can drop is limited by the dampers  71 . The dampers  71  are preferably configured so that the movement of the frame  72  toward the base plate  77  after the electric current ceases does not result in an impact between the frame  72  and the base plate  77 . Such impact would adversely affect the quality of the impulse transmitted into the ground  18 . The dampers  71  may also be configured to limit any tendency for the base plate  77  to leave the ground after the impulse is generated. In the present example, the weight of the vehicle  76 , which is communicated to the base plate  77  through the structure  73  and air bags  75 , may also limit the tendency for the base plate  77  to leave the ground  18  after the impulse is generated. 
         [0022]    The base plate  77  may include thereon a motion sensor  16  such as a geophone or accelerometer to measure the motion of the base plate  77 . The signal generated by the motion sensor may be electrical or optical and may be conducted to the recording unit  12  using the radio link. 
         [0023]    Suitable configuration of the dampers  71  (i.e., suitably limiting the upward speed of the frame  72 ) may enable controlling the effective duration of the impulse transmitted into the ground  18  when the electromagnets  37 ,  39  are actuated. Controlling the duration of the impulse will have the effect of controlling the frequency content of the impulse. It is desirable to produce more energy at lower frequencies, because lower frequencies penetrate the subsurface better. 
         [0024]    In other examples, the mass can consist of essentially the entire transport vehicle Referring to  FIG. 2 , there may be installed a lift system  78 , which may be hydraulic or may be electric motors turning respective worm gears with ball nuts on the frame  72 , and which couples the frame  72  to the vehicle  76  chassis at a suitable location, and when operated causes the frame  72  to be lowered to the ground. The weight of the vehicle can then be transferred on to the frame  72 , such that the frame  72  and the vehicle  76  then effectively become a single rigid mass for the purposes of the subsequent activation of the source  10 . The electromagnets  37 ,  39  may be configured substantially as explained with reference to  FIG. 1 . 
         [0025]    In the rest position the two electromagnets  37 ,  39  will be in contact. The source  10  is activated by passing a current through the electromagnets&#39; coils  36 ,  38  in a direction such that they repel each other. The repulsion causes the base plate  77  to be forced into the ground and the mass to rise. Forcing the base plate  77  into the ground  18  creates an impulse which is used to investigate the subsurface structure. The fact that the impulse is caused by a precisely controllable electrical event means that this type of source can be synchronized in a fleet with similar sources in order to increase the magnitude of the impulse. It can likewise be synchronized with the seismic record. 
         [0026]    One significant advantage of a seismic energy source according to the present examples wherein only magnetic repulsion is used as contrasted with previous magnetic lift sources that use attraction between two electromagnets is that the source of the present invention avoids impact of the electromagnets with each other at great speed. In the present invention there is no physical impact between parts of the actuator. Such impact is undesirable since it wastes some energy by creating unwanted audio noise (known as “air blast”) which may degrade the reflected seismic signal detected by the seismic sensors. Further, the “air blast” may be objectionable in certain areas, such as environmentally sensitive or heavily populated areas. The latter advantage may enable use of a seismic source according to the invention in areas where the use of explosives and large, noisy seismic vibrators has been prohibited. 
         [0027]    While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims.