Abstract:
A seismic signal generating system comprises a hammer positionable to impact a baseplate assembly. An actuator acts cooperatively with the hammer to urge the hammer to impact the baseplate assembly. A friction brake is actuated to impart a friction force to the hammer. The friction force restrains motion of the hammer until the brake is released. A method of generating a seismic signal comprises coupling a hammer to an actuator. The hammer is restrained from motion using a friction brake. The friction brake is released such that the actuator urges the hammer into contact with a baseplate assembly generating a seismic signal.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims priority from U.S. Provisional Application Ser. No. 60/772457 filed on Feb. 10, 2006, which is incorporated herein by reference. 
     
    
     BACKGROUND  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to the field of seismic data acquisition and more particularly to seismic signal generating devices and their methods of use.  
         [0004]     2. Background Information  
         [0005]     Seismic geophysical surveys are used in petroleum, gas mineral and water exploration to map the following: stratigraphy of subterranean formations, lateral continuity of geologic layers, locations of buried paleochannels, positions of faults in sedimentary layers, basement topography, and others. Such maps are deduced through analysis of the nature of reflections and refractions of generated seismic waves from interfaces between layers within the subterranean formation.  
         [0006]     A seismic energy source is used to generate seismic waves that travel through the earth and are then reflected by various subterranean formations to the earth&#39;s surface. As the seismic waves reach the surface, they are detected by an array of seismic detection devices, known as geophones, which transduce waves that are detected into representative electrical signals. The electrical signals generated by such an array are collected and analyzed to permit deduction of the nature of the subterranean formations at a given site.  
         [0007]     An impact source is a weight striking the surface of the earth directly or impacting a plate placed on the earth&#39;s surface, yielding seismic energy. A weight-drop is an example of a type of impact source. The actuation time of common impact sources varies between actuations This variation may cause problems in synchronizing a source with seismic receivers to obtain the most useful data. In addition, the use of multiple sources is desirable to increase the generated seismic signal. The variation of actuation times of multiple units may degrade the transmitted signal such that the received data is of marginable use.  
       SUMMARY  
       [0008]     In one aspect of the present invention, a seismic signal generating system comprises a hammer positionable to impact a baseplate assembly. An actuator acts cooperatively with the hammer to urge the hammer to impact the baseplate assembly. A friction brake is actuated to impart a friction force to the hammer. The friction force restrains motion of the hammer until the brake is released.  
         [0009]     In another aspect, a method of generating a seismic signal comprises coupling a hammer to an actuator. The hammer is restrained from motion using a friction brake. The friction brake is released such that the actuator urges the hammer into contact with a baseplate assembly generating a seismic signal.  
         [0010]     In yet another aspect, a seismic acquisition system comprises a plurality of seismic signal generating systems disposed proximate each other. A friction brake is disposed with each of the plurality of seismic signal generating systems for releasing a hammer to generate a seismic signal. A plurality of controllers are associated with the plurality of seismic signal generating system. Each controller controls the release of the friction brake in the associated seismic signal generating system. Each controller stores in a memory disposed therein a system response time of the associated seismic signal generating system. A master controller is spaced apart from the plurality of seismic signal generating systems, and receives data related to the response time of each seismic signal generating system. The master controller determines a delay time for actuating each seismic signal generating system such that each of the seismic signal generating systems generates the seismic signal within a predetermined time period.  
         [0011]     Non-limiting examples of certain aspects of the invention have been summarized here rather broadly, in order that the detailed description thereof that follows may be better understood, and in order that the contributions they represent to the art may be appreciated. There are, of course, additional features of the invention that will be described hereinafter. 
     
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0012]     For a detailed understanding of the present invention, references should be made to the following detailed description of the exemplary embodiment, taken in conjunction with the accompanying drawings, in which like elements have been given like numerals, wherein:  
         [0013]      FIG. 1  shows system components of one example of the present invention with a partial section elevation view of an illustrative example of a seismic impact source;  
         [0014]      FIG. 2  shows a section view of a brake assembly;  
         [0015]      FIG. 3  shows a section view of a baseplate assembly;  
         [0016]      FIG. 4  shows a view of a tiltable seismic source according to one embodiment of the present invention;  
         [0017]      FIG. 5  shows an illustrative example of a seismic data gathering system including multiple seismic impact sources; and  
         [0018]      FIG. 6  shows a section view of a seismic source and baseplate assembly positioned on an inclined surface.  
     
    
     DETAILED DESCRIPTION  
       [0019]     The following description presents non-limiting examples of embodiments of the present invention. Referring initially to  FIG. 1-3 , in one illustrative embodiment of the present invention, a seismic signal generating system  5  comprises an impact source  10  located on earth surface  75 , hydraulic source  92  supplies hydraulic fluid through hydraulic manifold  94  to seismic impact source  10  under control of a controller  90 .  
         [0020]     Seismic impact source  10  comprises a frame  23  having a friction brake assembly  15  and a baseplate assembly  20  described in more detail below. Friction brake assembly  15  is mounted atop frame  23  and restrains the motion of hammer  25  until breaking action is released. When released, hammer  25  is driven downwards by a gas spring  32  and forced to strike impact surface  60 . The impact signal generated by the striking of impact surface  60  by hammer  25  is transmitted through the components of baseplate assembly  20  into the surface  75  of the earth  76 . This impulse signal is then transmitted through the earth strata and detected by seismic receivers on the surface of the earth. Alternatively, seismic receivers may be positioned either temporarily or permanently in a subterranean well. Such a well may be vertical, inclined, or horizontal.  
         [0021]     In one embodiment gas spring  32  comprises spring rod  30  attached to hammer  25  and cylinder  31  attached to caliper  22 . High pressure gas is stored in cylinder  31  at a predetermined pressure. The high pressure gas is compressed to an even higher pressure when spring rod  30  is pushed upward to a cocked position. In one embodiment, the high pressure gas is dry nitrogen. Alternatively, any suitable substantially inert gas may be used, such as argon. The pressure in the cocked position may reach several thousand pounds per square inch. This pressure acts on spring rod  30  to impart a force acting downward on spring rod  30 . When the brakes in friction brake assembly  15  are released, the gas pressure on spring rod  30  and the force of gravity act to accelerate hammer  25  toward baseplate assembly  20 . While described herein is using a gas spring, it is intended that the present invention encompass any suitable spring type. Such springs include, but are not limited to, hydraulic springs, coil springs, and elastomer springs.  
         [0022]     Hammer  25  is made of a metallic material such as steel and provides the weight used to generate a portion of the impact force. Hammer  25  may weigh several hundred to several thousand pounds. Additional weight may be added to the hammer by attaching add on weight  26  to hammer  25 . The motion of hammer  25  is closely guided by guide plates  28 . Guide plates  28  may be made from a suitable plastic material. Such plastic materials include but are not limited to: nylon, teflon, and any other suitable material.  
         [0023]     After hammer  25  generates a seismic signal by striking impact surface  60 , hammer  25  may be returned to its cocked position by the action of hydraulic cocking cylinder  50  and cocking rod  45 . Cocking cylinder  50  may be operated under control of controller  90 .  
         [0024]     Baseplate assembly  20  comprises a striker pivot  62  contacting a lower pivot  65 . Lower pivot  65  is mounted on intermediate plates  71  which is in turn mounted on baseplate  70 . Striker pivot  62  has an upper impact surface  60  which is contacted by hammer  25 . Striker pivot  62  has a lower concave surface  63  that substantially mates with convex surface  64  of lower pivot  65 . Both concave surface  63  and convex surface  64  may each be substantially spherical. The substantially spherical shape of these surfaces is advantageous in the transmission of the seismic signal from impact surface  60  to baseplate  70 . In one aspect, the substantially spherical nature of the mating surfaces provides an increased contact area for the transmission of the seismic signal. Both striker pivot  62  and lower pivot  65  may be made from metallic materials including, but not limited to: aluminum bronze, aluminum, steel, and beryllium copper.  
         [0025]     As shown in  FIG. 6 , in another aspect, the substantially spherical nature of the mating surfaces allows a certain amount of angularity between the baseplate  70  and the top plate  61  due, in one example to uneven ground. As shown, the earth&#39;s surface  75  is at an angle, α, with respect to true horizontal. The substantially spherical nature of striker pivot  62  and lower pivot  65  allow the hammer  25  to be operated in a substantially vertical orientation thereby maximizing the gravitational acceleration on hammer  25  during the hammer strike.  
         [0026]     Multiple air bags  55  are attached between baseplate  70  and top plate  61 . Air bags  55  operate to isolate the frame mounted components from the shock associated with the hammer strike. Such airbags are commercially available and are not discussed here further. Chain  67  acts to restrain the downward motion of baseplate  70  with respect to top plate  61  during a hammer strike.  
         [0027]     In one embodiment, sensor  80  is attached to baseplate  70  and may be used to characterize the seismic signal transmitted through baseplate  70 . Sensor  80  may also be used to characterize the response time of seismic impact source  10  with respect to an initiation signal from controller  90 . Sensor  80  may be an accelerometer or any other device having suitable amplitude and frequency range to characterize the seismic signal transmitted through baseplate  70 . Such accelerometers are commercially available and will not be discussed here in detail.  
         [0028]     As shown in  FIG. 1  and  2 , friction brake assembly  15  comprises caliper  20 , brake pistons  16 , and brake pads  17 . As shown in  FIG. 2 , opposing sets of brake pistons  16  and brake pads  17  are employed in the present example. When hammer  25  is in the upward cocked position, hydraulic fluid in reservoir  18  is pressurized to force brake piston  16  against brake pad  17  which in turn contacts hammer  25  creating a friction force to restrain motion of hammer  25 . When hydraulic pressure in reservoir  18  is released, gas spring  32  and the force of gravity accelerate hammer  25  to impact with baseplate assembly  20 . This technique results in a quick, reliable, and repeatable release mechanism. The present embodiment employs two pairs of opposed brake pads  17  and brake pistons  16 , acting against opposite sides of hammer  25 . Other numbers of pairs of opposed brake pads and brake pistons may be used. Alternatively, a floating caliper may be employed wherein pistons are on only one side of the caliper.  
         [0029]     Controller  90  may comprise circuits  96 , a processor  97 , and computer readable medium  98 . Computer readable medium  98  may be any suitable storage medium including, but not limited to, RAM, ROM, CD, hard disk, DVD, flash memory, and any other suitable medium not yet developed. Instructions may be stored in computer readable medium  98  for execution by processor  97  for controlling the operation of seismic impact source  10 . Controller  90  may be programmed to control power source  92  and valve manifold  94  to control the operation of seismic impact source  10 . Such control may be used to operate the friction brake  15  and cocking cylinder  50  during operation. Controller  90  may also include suitable circuits and hardware, such as antenna  93 , for transmitting and receiving data and instructions from a remote master controller as described below.  
         [0030]     Controller  90  may comprise suitable circuits  96  and instructions stored in computer readable medium  98  for processing signals from sensor  80 . In one illustrative example, signals from sensor  80  may be used to characterize the impact seismic signal generated during operation a seismic source  10 . Such signals may be analyzed or both amplitude and frequency content and monitored over time to determine changes in system operation. In another illustrative example, signals from Sensor  80  may be used to characterize the response time of each seismic source  10 . For example, the components of each seismic impact source  10  may vary in their individual response. In order to determine the system response, the time between initiation signal from controller  90  until the hammer impacts the baseplate assembly may be determined. It is anticipated that each seismic source  10  will have a slightly different response time. This system response time may be used to coordinate multiple sources as described below with regard to  FIG. 5 .  
         [0031]     Referring also to  FIG. 4 , in one illustrative example seismic impact source  10  has frame  100  attached thereto. Frame  100  may be attached to seismic source  10  using a mechanical technique known in the art. Frame  100  as arm  105  attached thereto. Arm  105  has pivot axle  120  that facilitates attachment of frame  100  to support vehicle  110 . Arm  105  also has crank arm  124  integral thereto. Cylinder  115  with associated cylinder rod and  116  are attached between support vehicle  110  and pivot point  125  on crank arm  124 . Cylinder  115  may be actuated to extend and retract cylinder rod  116  such that seismic source  10  moves through an angle θ with respect to the vertical as shown. Such angular movement may be used to accommodate uneven ground as shown in  FIG. 6 . Alternatively, such angular movement to be used to impart a shear wave through baseplate assembly  20  into the earth. As discussed previously, the spherical surfaces of striker pivot  62  and lower pivot  65  are well suited to accommodate such angular movement.  
         [0032]     Referring also to  FIG. 5 , in one embodiment, a seismic system  300  may comprise multiple seismic signal generating systems  5  in synchronous operation to generate a larger seismic signal. Each of the seismic signal generating systems  5 , as described previously comprises its own controller  90 . As shown in  FIG. 5 , a master controller  205  may comprise circuits  206 , a processor  207 , and computer readable medium  208 . Multiple seismic receivers  200  may be located in suitable patterns away from the seismic source for detecting the seismic signal transmitted through the earth.  
         [0033]     As described previously, computer readable medium  208  may be any suitable storage medium including, but not limited to, RAM, ROM, CD, hard disk, DVD, flash memory, and any other suitable medium not yet developed. Instructions may be stored in computer readable medium  208  for execution by processor  207  for controlling the operation of seismic system  300 .  
         [0034]     Master controller  205 , may be remotely located from the cluster of seismic signal generating systems  5 . Transmission of data and command signals between controllers  90  and master controller  205  may be by wired or wireless communication techniques. Wireless communication techniques include but are not limited to radio frequency transmission, infrared transmission, optical transmission, and microwave transmission. Wired communication techniques include electrical conductor and fiber optic transmissions. Master controller  205  may also transmit data and receive commands from another remote location.  
         [0035]     As one skilled in the art will appreciate, when actuating multiple impact devices such as seismic signal generating systems  5 , it is desirable that the signals from each device be generated at substantially the same time. In real-world operation, sufficient received signal resolution may be achieved if the multiple impact devices generate seismic signals within less than a predetermined time interval of no more than about 2 ms. In one example of the present invention, master controller  205  uses data related to the response time of each seismic signal generating system  5  to synchronize the signal generated by each seismic signal generating system  5  within the predetermined time interval. Each seismic signal generating system  5  may determine its response time after each generated signal. Controllers  90  may then transmit the latest determined response time to master controller  205  for use in the next generated signal. In one example, master controller  205  may determine the largest response time and determine a delay time for actuating each of the other signal generating systems such that they all generate a seismic signal at substantially the same time within the predetermined interval. Alternatively, controller  90 , on each individual seismic signal generating system  5 , may only transmit changes in the response time to master controller  205 . Master controller  205  will then adjust the delay time of a particular seismic signal generating system  5  based on its changed response time.  
         [0036]     While the foregoing disclosure is directed to the non-limiting illustrative embodiments of the invention presented, various modifications will be apparent to those skilled in the art. It is intended that all variations within the scope of the appended claims be embraced by the foregoing disclosure.