Abstract:
The invention relates to acquiring seismic data from the earth using geophones that are tuned to have differing frequency sensitivity ranges. The differing frequency sensitivity ranges cover a broader effective frequency range so that low frequency energy and high frequency energy are less attenuated in the raw data record. Two separate geophones would be used at the same node and three or more geophones may be used in combination at each node. When three or more geophones are used in combination, each may have a separate but overlapping frequency sensitivity range or two or more of the geophones may have the same frequency range sensitivity.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a non-provisional application which claims benefit under 35 USC §119(e) to U.S. Provisional Application Ser. No. 61/509,451 filed Jul. 19, 2011, entitled “MULTIPLE FREQUENCY GEOPHONE STRINGS,” which is incorporated herein in its entirety. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
       [0002]    None. 
       FIELD OF THE INVENTION 
       [0003]    This invention relates to seismic prospecting and especially to delivering seismic energy into the earth and receiving and recording the reflected and refracted seismic energy at the surface. 
       BACKGROUND OF THE INVENTION 
       [0004]    In the process of acquiring reflected and refracted seismic energy from the earth, listening and recording equipment is typically arranged in long lines to form an array with locations or nodes along the lines where geophones are connected to the lines at the nodes. It is fairly common to have more than one geophone connected to a node or digitizing box and not uncommon to have two, three, four or five and sometimes many more at each node. For example, there was a survey in the Middle East that used 288 geophones at each node. For the purposes of this discussion, a node will represent a controlling box that could be an autonomous node or a cable connected analog to digital conversion box in the case of a cable based recorder. Typically, the geophones are stabbed into the ground in the same formation or arrangement around each node. The data coming from all the geophones is combined at the node and digitized so as to provide data from the node without separately identifying what each individual geophone received. The data is separated by node so that each node is viewed as a single receiver point and all the receiver points form a large three dimensional array for receiving and recording seismic energy emanating from the earth. 
         [0005]    An individual geophone typically includes a magnet and an electrically conductive coil where one or both are arranged to move vertically or horizontally in the case of a three component geophone along an axis, one relative to the other. As the magnet and coil move relative to one another, the magnet induces a small electrical current in the coil that can be measured and recorded. Typically, springs having very minimal resistance to movement are used to bias the moving elements into a central location or neutral position where vibrations from the earth cause the magnet to deflect from the neutral position. The spring strength is typically arranged to be very slight so that slight vibrations of the ground due to the arrival of a seismic wave will cause movement of the magnet within the coil. 
         [0006]    For hydrocarbon prospecting, geophysicists are interested in data having a broad frequency spectrum. Low frequency energy is used for the analysis of rock densities and wave velocity through the substructures while high frequencies provide higher resolution on smaller scale substructures. Geophones for hydrocarbon prospecting are designed to accurately sense and record a broad frequency spectrum of seismic energy with magnets size and weights to be within tight tolerances along with precisely engineered springs to permit movement in response to a broad frequency array. Most geophones used for hydrocarbon prospecting are rated for 10 Hz and greater although data is acquired below 10 Hz that is used. However, data below 8 Hz is typically absent in the data record. 
       BRIEF SUMMARY OF THE DISCLOSURE 
       [0007]    The invention more particularly relates to a process for acquiring seismic data for prospecting for hydrocarbons. An array of receiver locations is created in the field where each receiver location includes a node and at least a first and second geophone is connected to each node in the array of receiver locations. The first geophone is configured to have a first vibrational frequency sensitivity range and the second geophone is configured to have a second and different vibrational frequency sensitivity range. Seismic energy is delivered into the ground to create an upcoming seismic wave field to be sensed by the geophones at the various receiver locations of the array. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    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: 
           [0009]      FIG. 1  is perspective view of a simplified seismic acquisition system showing a seismic vibrator delivering seismic energy into the ground while lines of receivers are laid out in an array for sensing and recording seismic energy reflecting from the subsurface; and 
           [0010]      FIG. 2  is a close up view of three geophones attached to a single node of a seismic cable; 
           [0011]      FIG. 3  is a close up view comparable to  FIG. 2  showing three geophones connected to an autonomous recorder to provide data for a receiver point comparable to the  FIG. 2  set up without using conventional long seismic cables; and 
           [0012]      FIG. 4  is a cross sectional view showing the inside of a basic geophone used as a receiver for seismic prospecting. 
       
    
    
     DETAILED DESCRIPTION 
       [0013]    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. 
         [0014]    As shown in  FIG. 1 , a seismic acquisition generally indicated by the number  10  is shown where a seismic vibrator  15  is positioned to shake the ground and deliver seismic energy into the earth. Lines  21 ,  22  and  23  are laid out with a series of nodes  25  along the length thereof with a geophone installation  30  attached to each node  25 . Each node  25  includes circuitry to recognize what is sensed by the geophone installation  30  and communicate such signals to a recorder truck  28 . 
         [0015]    Turning to  FIG. 2 , a geophone installation  30  is shown to include three separate geophones  35 ,  45  and  55 , each stabbed into the ground. This is the normal mode of deployment for a point receiver or a “potted” array. In the conventional arrangement, each of the geophones  35 ,  45  and  55  are the same design and configuration. The number of such geophones is shown as three, but in practice may be one, two, three, four or five and even more in some circumstances. The signals provided to the nodes  25  are conventionally added together to provide essentially a combined signal for each node  25  without breaking down what is sensed into the vibrations sensed by each separate geophone. Each geophone  35 ,  45  and  55  include a connecting wire  36 ,  46  and  56 , respectively, and a plug  37 ,  47  and  57 , respectively, for connecting the geophones to the node  25 . Each plug is arranged so that while the first plug  37  is plugged into the node  25 , the second plug  47  is connected into the back of the first plug  37  and the third plug is connected into the back of the second plug  57 . This plugging arrangement is conventional and the stacking of the respective plugs provides the node with signals from each geophone of what the geophone installation  30  is sensing, cumulatively. 
         [0016]    In a comparable installation shown in  FIG. 3 , an autonomous node  125  is connected to a first geophone  135 , a second geophone  145  and a third geophone  155 . The autonomous node receives the signals from the geophones, digitizes the signal and record the digitized signal for later download. 
         [0017]    Referring now to  FIG. 4 , a geophone  235  is shown with a body  238  and a spike  239  at the bottom thereof for being stabbed into the ground. Within the body  238  is a magnet  271  fixed on a vertical post  272 . Around the outside of the magnet  271  is an electric coil  276  that moves vertically with respect to the fixed magnet  271  The coil  276  is a common element in current geophone design and may include copper wire wrapped in a prescribed manner around a plastic frame that is inconsequential, magnetically and electrically. Springs  274  are attached at the top and bottom of the coil  276  to keep it in a vertically central location called a neutral position with respect to the magnet  271 . As the geophone is fixed into the ground by the spike  239 , vibrations in the ground cause the coil to move up and down along the magnet  271 . The relative movement of the coil with respect to the magnet  271  induces a small but measurable electric current in the coil. That current is provided down the connecting wire  236  to the node for communication to the truck  28 . 
         [0018]    Geophones used for prospecting are sized and engineered to provide reliable signals from about 10 Hz vibrations up to well over 200 Hz. Geophones for sensing earthquakes have magnets and coils that are necessarily larger in mass with corresponding springs to sense vibrations at less than 1 Hz. According to the current invention, each node is provided with at least two geophones, but the geophones rather than being the same, are constructed with at least a different magnet size, different coil size or structure or different spring strength to be sensitive to at least a slightly different frequency spectrums. In particular, each node  25  according to the present invention would include a first geophone  135  having sensitivity at less than 10 Hz and preferably down to about 4 Hz and more preferably down to about 2 Hz and even using a geophone that has sensitivity at the low end to about 1 Hz. The upper end of the frequency spectrum for such a geophone would likely be too low for use in a hydrocarbon prospecting seismic study alone but each geophone installation  30  would include a second geophone  145  having frequency sensitivity up to at least 120 Hz and where the frequency range sensitivities of the first and second geophones  135  and  145  overlap. For example, a first geophone may be rated for 2 Hz up to 55 Hz while the second may be rated from 10 Hz up to 140 Hz. The signals of the two geophones would be combined by the node and recorded as it if were one big broad spectrum geophone. 
         [0019]    In another aspect of the invention, two higher frequency sensitivity geophones may be arranged at each node along with one or two or three lower frequency sensitivity geophones. It is preferred that each node be provided with the same set up regarding the number of geophones, the number of low frequency geophones and the number of high frequency geophones and the arrangement of the high and low frequency geophones relative to the node in the array of receivers. 
         [0020]    In another aspect of the invention, three geophones may be used at each node where each geophone has a separate frequency sensitivity range such that a first is a low frequency range, the second is a medium frequency range and the third is a high frequency range. This arrangement may accommodate multiples of each range and may include more than three overlapping ranges. For example, each node may be provided with five geophones where one geophone has a very low range sensitivity, a second geophone has a medium low range sensitivity, a third geophone has a medium range sensitivity, a fourth geophone has a medium high range sensitivity and the fifth has a high range sensitivity where all geophones overlap or only a few overlap. 
         [0021]    By analogy, consider an electronic speaker for high fidelity or hi-fi stereo sound system. Most hi-fi speakers have component speakers comprising a tweeter for high frequency sound, a midrange speaker for midrange frequency sound and a woofer for low frequency sound. Most higher end systems include a subwoofer for really low frequencies. By properly choosing the individual component speakers and drivers and then managing the crossover signals, one can create a flat spectrum for the sound coming from the sound system. The present invention employs a concept that is, in a sense, the opposite of a component speaker. By carefully choosing our geophones and arranging them on the string with the proper wiring, the composite signal from the diverse geophones captures a much broader bandwidth then any one geophone could do on its own. 
         [0022]    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. 
         [0023]    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. 
       REFERENCES 
       [0024]    All of the references cited herein are expressly incorporated by reference. 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 data after the priority date of this application. Incorporated references are listed again here for convenience:
   1. U.S. Pat. No. 4,285,054 (McNeel), “Geophone” (Aug. 18, 1981)   2. U.S. Pat. No. 4,597,070 (Redecker)&#39; “Geophone”, (Jun. 24, 1986)   3. U.S. Pat. No. 5,475,652 (McNeel et al.); “Dual Gimbal Geophone” (Dec. 12, 1995).   4. U.S. Pat. No. 7,284,431 (Barger) “Geophone” (Oct. 23, 2007).   5. U.S. Re. 41,626 (Cheung et al.), “Multiple Magnet Transducer with Differential Magnetic Strengths,” (Sep. 7, 2010) . . .