Abstract:
An apparatus and a method for providing three dimensional seismic images using directional sensing rotation within a geological structure&#39;s complete vector field produced by seismic energy emanating from seismic scatterers in the sub-surface of the geological structure, which significantly reduces the need for distribution of seismic energy sources and receivers over the entire surface of the geological structure. The apparatus includes an omni-azimuthal source of seismic energy  22  positioned adjacent a surface of a geological structure  18  for emitting a signal  27  of sufficient energy and bandwidth to produce seismic energy  28  and  97  from the seismic scatterers in the geological structure  18.  A plurality of arrays of sensors  20  are also provided. Each array  20  has directional sensing receivers  30  aligned in the geological structure  18  for receiving and measuring the seismic energy  28  and  97  to create a complete vector field. The complete vector field is processed using sensing rotation with uphole summing and triangulation to generate a three dimensional seismic image and provide directional measurements that precisely locate the seismic scatterers  24 . The method includes establishing an acquisition system  19  having a plurality of arrays  20  below a surface containing the geological structure  18;  energizing the seismic scatterers  24  with a seismic disturbance  27 ; and recording a complete vector field response of the seismic scatterers  24  to the seismic disturbance  27.

Description:
This application is a continuation of application Ser. No. 08/950,726, filed Oct. 15, 1997 now U.S. Pat. No. 6,023,657. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates generally to seismic surveying, and more particularly to three dimensional imaging based upon the use of omni-azimuth seismic energy sources and directional sensing of seismic scatterers. 
     To investigate a salt dome or like configuration, both a seismic source and seismic receivers or detectors, such as hydrophones, three component geophones, or three component accelerometers, are suspended in a single borehole. Then, seismic signals are sent from the suspended source, such as an airgun, and received by the receivers to define a more accurate map of the salt dome&#39;s flank configuration than possible with surface-located seismic sources and detectors. 
     Presently, three-dimensional (3-D) seismic surveys are based upon recording a vertical component of seismic motion reflected from sub-surface seismic reflectors. The 3-D surveys rely on the measurements of travel time from source to reflector, to receiver, and the geometry of source-receiver positions on the surface. This technique requires an even distribution of seismic energy sources and receivers over the entire surface of the geological field. The seismic data is acquired separately at each receiver and it is processed by corrected summing to create an image of the sub-surface. 
     What is needed is an apparatus and a method for conducting 3-D seismic surveys using directional sensing rotation within a geological structure&#39;s complete vector field that is produced by seismic energy emanating from seismic scatterers in the sub-surface of the geological structure, thereby significantly reducing the need for distribution of seismic energy sources and receivers over the entire surface of the geological structure. 
     SUMMARY OF THE INVENTION 
     The present invention, accordingly, provides an apparatus and a method for conducting 3-D seismic surveys using directional sensing rotation within a geological structure&#39;s complete vector field. Seismic energy emanating from seismic scatterers in the sub-surface of the geological structure produces sufficient energy for the seismic survey. This technique significantly reduces the need for distribution of seismic energy sources and receivers over the entire surface of the geological structure. To this end, an apparatus for providing a three-dimensional seismic image includes an omni-azimuthal source of seismic energy positioned adjacent to a surface of a geological structure. The source emits a signal of sufficient energy and bandwidth to produce seismic energy from a seismic scatterer in the geological structure. A plurality of arrays of sensors are also provided. Each array has directional sensing receivers aligned in the geological structure for receiving and recording measurement of diffracted seismic energy, to produce a complete vector field. The complete vector field is processed to generate the three-dimensional seismic image. 
     A principal advantage of the present invention is that the sub-surface geological strata is energized (“illuminated”) by the seismic energy source. The energization causes elastic discontinuities (“seismic scatterers”) to diffract the seismic energy as if they were the source of such seismic energy. The receivers measure all diffractions. A recording unit records all measured diffractions. A processing system processes the recorded information to locate the seismic scatterers, thus creating a three-dimensional image of the sub-surface, which image can be interpreted for geological significance. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates a geological structure with a sub-surface seismic scatterer. 
     FIG. 2 illustrates a directional sensing vertical array with triphones. 
     FIG. 3 is an enlarged view of a portion of FIG.  2 . 
     FIG. 4 illustrates a vectorial rotation of the array of FIG. 2, for calibration. 
     FIG. 5 illustrates uphole summing using one vertical array along a wavefront at a right angle to the vertical. 
     FIG. 6 is an enlarged view of a portion of FIG.  5 . 
     FIG. 7 illustrates uphole summing using one vertical array along a wavefront emerging at an angle φ to the vertical. 
     FIG. 8 is an enlarged view of a portion of FIG.  7 . 
     FIG. 9 illustrates the preferred embodiment of the present invention, which uses uphole summing in conjunction with sensing rotation between three vertical arrays along a unique wavefront. 
     FIG. 10 illustrates bending of a monofrequency wavepath. 
     FIG. 11 illustrates the range and resolution power of the vertical array shown in FIG.  1 . 
     FIG. 12 is an aerial view of a field with omni-azimuthal sources and directional sensing vertical arrays. 
     FIG. 13 is a flowchart illustrating the process of establishing a plurality of arrays in a geological structure to measure the response of seismic scatterers to seismic disturbances. 
     FIG. 14 is a flowchart illustrating the process of sensing rotation and uphole summing to produce a three dimensional seismic image. 
     FIG. 15 illustrates triangulation techniques utilizing two directional sensing vertical arrays. 
     FIG. 16 is a flowchart illustrating the process of sensing rotation and triangulation using secondary arrival measurements. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1, within a geological structure  18 , an acquisition system  19  includes a plurality of directional sensing vertical arrays (DSVA), each designated  20 , an omni-azimuthal (i.e. no azimuthal variation) seismic source (OSS)  22  and a seismic scatterer  24 . The seismic scatterer  24  is an elastic discontinuity which when energized by a passing seismic wave, acts as a new and independent source of seismic energy. The geological structure  18  is part of a geological field. Although FIG. 1 illustrates only one scatterer  24 , the geological structure  18  has multiple seismic scatterers  24 . Each DSVA  20  is located at a near-surface distance, typically within 500 feet of a surface of the geological structure  18 . The OSS  22  is located on the surface or within the near-surface of the geological structure  18  in a horizontal or vertical arrangement. The OSS  22  has single or multiple elements, having sufficient energy, bandwidth, and beam-angle to adequately energize exploration objectives, such as the geological structure  18 . The OSS  22  is either an impulsive source (such as explosives, impactors, and the like), a coherent vibratory source, or a random vibratory source. The OSS  22  is designed to produce seismic energy that is repeatable in order to overcome random ambient noise interferences, as discussed below. The OSS  22  emits a seismic energy  27  at a beginning time. The seismic energy  27  is omni-azimuthal, with sufficient energy and bandwidth to energize geological objectives, such as the geological structure  18 . The seismic energy  27  has a signature that is repeatable for a full spectrum of frequencies. 
     As seismic energy  27  travels through the geological structure  18 , it energizes the seismic scatterer  24  and all other seismic scatterers (not shown), located within the geological structure  18 . Once energized, the seismic scatterer  24  acts as an independent source of seismic energy and produces a diffracted seismic energy  28 , i.e. a seismic energy radiated by an elastic discontinuity that has been energized by a seismic disturbance. The seismic scatterer  24  emits the diffracted seismic energy  28  in all directions. The diffracted seismic energy  28  travels back to each DSVA  20 , which measures the diffracted seismic energy  28  as a first arrival measurement for the seismic scatterer  24 . A recording unit (not shown) is coupled to each DSVA  20  to record the measured energies. As the diffracted seismic energy  28  travels back to each DSVA  20 , the diffracted seismic energy  28 , acting as an independent source of seismic energy, energizes secondary seismic scatterers (not shown) surrounding the seismic scatterer  24 . The energized secondary seismic scatterers produce secondary diffracted seismic energies that travel back to each DSVA  20 . Each DSVA  20  measures and the recording unit records, in addition to the first arrival measurements, secondary diffracted seismic energies produced by the secondary seismic scatterers. Therefore, each DSVA  20  measures a complete vector field. The complete vector field is used to produce a directional measurement and resolution of the geological structure  18  to accurately locate all seismic scatterers  24 . 
     Referring now to FIG. 2 ad FIG. 3, the DSVA  20  includes a plurality of triphones, each designated  30 . A triphone is a three-component geophone with identical orthogonal elements, each making an angle of 54 degrees, 35 minutes with the vertical (also called a Gal&#39;perin geophone), which are commercially available from Input/Output, Inc., having a place of business at 1104 West Airport Blvd., Stafford, Tex. 77477-2416, as part number CA 2729. The length of the DSVA  20  equals a longest wavelength component, which is to be measured and recorded, of the diffracted seismic energies produced by the seismic scatterers  24 . Increasing the length of the DSVA  20  improves its resolution power. Additionally, a spacing interval  29  separates each triphone  30  from adjacent triphones  30 . The user selects the interval  29  to equal half of the shortest wavelength component to be measured and recorded. Shortening the interval  29  enhances resolution of the DSVA  20 . The longest and shortest wavelength components are calculated using a compressional-wave (p-wave) velocity of the geological structure  18  surrounding the DSVA  20 . The p-wave velocity is a function of the velocity field. If the p-wave velocity is 1500 meters per second, and the desired measurement bandwidth is 10 through 100 Hz, then the DSVA  20  would have twenty of the triphones  30 . The interval  29  between adjacent triphones  30  would be 7.5 meters. Thus, the length of the DSVA  20  would be 150 meters. 
     The user secures each DSVA  20  in position within a borehole using a material of equal or slightly lesser propagation velocity than the formation, which is part of the geological structure  18 , surrounding the borehole. The user secures each DSVA  20  in a different borehole. The user surveys each borehole containing the DSVA  20  to determine the borehole&#39;s precise coordinates and inclination, to calibrate each DSVA  20 . The user calibrates each DSVA  20  using XYZ coordinates, orientation, and interconnecting travel times for each triphone  30  of each DSVA  20  with respect to other triphones  30  in all the other DSVAs  20 . 
     Referring now to FIG. 4, XYZ coordinates  31  are defined by loading each DSVA  20  into the borehole in such a manner that the user measures and records the exact depth of each triphone  30  of each DSVA  20 . The user orients each DSVA  20  to a C axis, true North axis  33 , using vectorial rotation of data for each triphone  30  in relation to a multi-element up-hole calibration source of known coordinates, conveniently located in the vicinity of each DSVA  20 . The user determines the vectorial rotation according to the following equations: 
     
       
           c= ( c   1   , c   2   , c   3 )= a×e    equation 1(a)  
       
     
     
       
         δ=cos −1 ( a·e )   equation 1(b)  
       
     
     
       
         
           
             
               
                 
                   
                     
                       
                         
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     where δ is a rotation angle, α is an observed vector, c is an axis of rotation, and e is an emergence vector. From the elements of the up-hole calibration source, the user obtains interconnecting travel times to each triphone  30 . The user determines differential travel times based on the relationship between the position of the OSS  22  and depth of each triphone  30 . Using the differential travel times, the user generates a velocity field for an area of the geological structure  18  surrounding each DSVA  20 . Later, the user uses the velocity field to adjust the diffracted seismic energies  28  along wavefronts traveling through the area of the geological structure  18  surrounding each DSVA  20 . 
     The user selects a layout and a position for each DSVA  20  and the OSS  22  depending on the nature and extent of the exploration objectives. Once the user selects the layout and the position, acquisition of field data can commence. The acquisition involves recording the complete vector field. The OSS  22  generates the seismic energy  27  as illustrated in FIG.  1 . Each of the DSVAs  20  measures the diffracted seismic energy  28  for each of the seismic scatterers  24 . The recorded information relating to the seismic energy  27  has a real component and an imaginary component, together called a complex trace. The complex trace has attributes that includes information about instantaneous phase, reflection strength representing the envelop of a given wavelet, instantaneous bandwidth, instantaneous polarity, and instantaneous frequency, along with other properties that can be calculated for the complete vector field. 
     Ambient noises produced during the recording of the complete vector field must be sufficiently attenuated. Ambient noises are random while the seismic energy  27  is repeatable. Accordingly, the diffracted seismic energy  28  is also repeatable. In order to sufficiently attenuate ambient noises, the OSS  22  repeats the seismic energy  27  to record successive complete vector fields and hence successive complex trace attributes. Successive seismic energies  27  are produced until the complex trace attributes of two successive complete vector fields indicate that ambient noises are sufficiently attenuated. For example, a zero instantaneous phase differential of the complex trace attributes of two successive complete vector fields indicates that ambient noises have been sufficiently attenuated. 
     The complete vector field comprises multiple signals received from multiple directions, with ambient noises sufficiently attenuated. Each triphone  30  measures the diffracted seismic energy  28  from any given direction. Thus, each DSVA  20  can precisely locate the seismic scatterer  24  using a combination of uphole summing, as discussed below, and sensing rotation. Sensing rotation is used because each directional measurement is represented by a three-component orthogonal signal. The three-component orthogonal signal is referenced to the azimuth of the true North axis  33 , and also referenced to a declination from the horizontal using the vectorial rotation calibration data established by equations 1(a) through 1(d). The vectorial rotation involves summing the three-component orthogonal signal in such manner that the resulting signal corresponds to the one that would have been received from the direction of the seismic scatterer  24 . This is done by summing the three-component orthogonal signal in proportion to the spherical coordinate transform that corresponds to the azimuth and inclination in the direction of the seismic scatterer  24 . The process is represented by the equation:              ρ   =         X     sin                 φcos                 θ       +     Y     sin                 φ                 sin                 θ       +     Z     cos                 φ         3             equation                   (   2   )                                  
     where ρ is the scalar value of the sum, θ is the azimuth, φ is the inclination, X is an x-component of the three-component orthogonal signal, Y is a y-component of the three-component orthogonal signal, and Z is a z-component of the three-component orthogonal signal. Thus, the triphone  30  can be focused in any given direction using vectorial rotation, without physical rotation of the triphone  30 . 
     Referring now to FIG.  5  and FIG. 6, triphones  30   a ,  30   b  and  30   n  are part of each DSVA  20 . A processing system (not shown) performs uphole summing by taking the diffracted seismic energy  28  received at the triphone  30   a , the deepest triphone of the DSVA  20 , and summing it to the diffracted seismic energy  28  received at the triphone  30   b , which is immediately above the triphone  30   a , with an uphole time delay designated ΔT. The processing system repeats the summing process up to, and including, the diffracted seismic energy  28  received at the uppermost triphone  30   n . Summing with the delay ΔT along a vertical axis  34  of the DSVA  20  enhances an emerging wavefront  35  at zero degrees from the vertical axis  34 . 
     Referring now to FIG.  7  and FIG. 8, an emerging wavefront  36  travels along an axis  38 , at an angle φ to the vertical axis  34 . Uphole summing with an angle uphole delay ΔT*cos φ, along the angle axis  38 , enhances the emerging wavefront  36  traveling at the angle φ. Summing with the angle uphole delay ΔT*cos φ, and sensing rotation of the triphones  30   a  through  30   n  to any given azimuth and declination, allows the DSVA  20  to focus in a desired direction to isolate the diffracted seismic energies  28  received from the desired direction. Therefore, the DSVA  20  is made highly directional to precisely locate the seismic scatterer  24 , using sensing rotation in conjunction with uphole summing. 
     Referring now to FIG. 9, in the preferred embodiment of the present invention, a DSVA  20   a , a DSVA  20   b , and a DSVA  20   c  record an emerging wavefront  54 , i.e. a wavefront orthogonal to a diffraction direction propagating away from a given seismic scatterer. The DSVA  20   a , the DSVA  20   b , and the DSVA  20   c  are shown with identical azimuths and vertical inclinations. Thus, vision lines of the DSVA  20   a , the DSVA  20   b , and the DSVA  20   c  are parallel and in unison with proper rotation. As the emerging wavefront  54  cuts across the DSVA  20   a , the DSVA  20   b , and the DSVA  20   c  along an emerging azimuth and an emerging declination, the processing system performs uphole summing using corresponding triphones and uphole delays. Uphole summing for the DSVA  20   a  occurs simultaneously to uphole summing for the DSVA  20   b  and the DSVA  20   c . The processing system uses uphole delays calculated for the emerging azimuth and the emerging declination, based on velocities of the velocity field obtained during the calibration process. Thus, by combining the uphole summing for the DSVA  20   a  with the DSVA  20   b  and the DSVA  20   c , the processing system enhances information recorded from the emerging azimuth and the emerging declination while attenuating signals from the other directions. For example, a triphone  56 , a triphone  58 , and a triphone  60  of the DSVA  20   a , the DSVA  20   b , and the DSVA  20   c , respectively receive the wavefront  54 . Thus, summing the recorded measurements of the triphones  56 ,  58 , and  60  is the same as uphole summing along one path of the emerging wavefront  54  at an instant in time. At another instant in time, the wavefront  54  reaches a triphone  62 , a triphone  64 , and a triphone  66 . Eventually, the wavefront  54  will propagate through the geological structure  18  to reach a triphone  68 , a triphone  70 , and a triphone  72 . Therefore, starting at the triphone  62 , and stopping when summing has reached the triphone  68 , a total of fifteen intervals are summed to enhance the diffracted seismic energy associated with the wavefront  54 . 
     Referring now to FIG. 10, monofrequency wave  73  travels along a monofrequency wavepath  74 . Monofrequency decomposition of the diffracted seismic energy  28  from the seismic scatterer  24  yields a plurality of Fresnel rings (i.e. a monofrequency response of the diffracted seismic energy from the seismic scatterer) of a certain shape, size, and distribution as measured by the multiple DSVA  20  of FIG.  9 . Each Fresnel ring has a first Fresnel zone  75 , i.e. the portion of a scatterer from which diffracted energy can reach a detector within one-half wavelength of the first diffracted energy. In general, smaller Fresnel rings correspond to higher frequency components and larger Fresnel rings to lower frequency components. For a given angle of incidence of the monofrequency event, the angle of refraction varies as a function of the wavelength, and thus velocity. Furthermore, variations in the angle of refraction cause bending in wavepaths. Wavepaths differ according to the velocities and time-distance between the seismic scatterer  24  and each DSVA  20 . Thus, a velocity function can be derived from analyses of the Fresnel rings as a function of time-distance. The velocity function acts as a velocity model, which comes from the monofrequency decomposition of the Fresnel rings. Accordingly, the velocity function is used to correct the bending and hence accurately locate the position of the seismic scatterer  24 . 
     Referring now to FIG. 11, the DSVA  20  has a top triphone  76 , a bottom triphone  78 , a range D, a scanning resolution d, an angular sampling rate  84 , and a vision angle φ. The DSVA  20  has a total vertical delay T and a temporal sample rate Δt. The angular sample rate  84  is defined by the equation:              Δφ   =         cos     -   1            (         T                 cos                 φ     -     Δ                 t       T     )       -   φ             equation                   (   3   )                                  
     From equation (3), the scanning resolution d of the DSVA  20  can be determined and expressed by the following equation: 
     
       
           d=D tanΔφ  equation (4)  
       
     
     In FIG. 12 a geological field  90  is illustrated with multiple DSVAs  20  and multiple OSSs  22  for large field exploration. In addition to the features noted above, layout of the field  90  allows collection and correction of near-horizontal data. A path traveled along a near-horizontal plane suffers from bending. If bending occurs, the seismic scatterer appears to be deeper than the seismic scatterer&#39;s true position, because velocity generally increases with depth. Thus, near-horizontal data relating to the location of a seismic scatterer  92  shows the seismic scatterer  92  located deeper than its true position. Consequently, the user corrects for bending in the near-horizontal data by warping the velocity field using near-vertical data. Bending of the line of vision is minimum in a vertical direction. Thus, seismic scatterers are located in near true position when using near-vertical data. Accordingly, the near-vertical data relating to the location of the seismic scatterer  92  is the most accurate measurement due to minimum velocity variations with depth in the vertical direction. Thus, the user determines a true position of the seismic scatterer  92  using the near-vertical data measured by a DSVA  94  located substantially above the seismic scatterer  92 . 
     The user compares the true position to a secondary position of the seismic scatterer  92  determined using the near-horizontal data measured by a DSVA  96  located some distance away from the seismic scatterer  92 . By comparing, the user determines an error and a correction factor needed to correct the near-horizontal data. The user corrects the near-horizontal data by adjusting to near-vertical data on adjacent and overlapping coverage in the geological field using the correction factor. The correction involves warping a seismic image in space. The amount and distribution of the warping is related to a velocity field causing the bending or distortion. In addition to the warping, a non-zero offset source (not shown) positioning can add valuable information for recording shallow data and defining the velocity field with greater precision. 
     Referring now to FIG. 13, the user begins in step  101 . In step  102 , the user establishes an acquisition system. Then in step  104 , the user triggers the OSS  22  to energize the seismic scatterers  24 . In step  106 , the recording unit records the response of the seismic scatterers  24  to the seismic energy  27 . In step  108 , the user corrects near-horizontal data. 
     In order for the user to establish the acquisition system in step  102 , in step  112 , the user selects the spacing interval between each of the triphones  30 . In step  114 , the user also selects the length of each DSVA  20 . In step  116 , the user secures the DSVA  20  in the borehole. In step  118 , the user aligns the axis of the DSVA  20  to other DSVAs  20 , so that all of the axes are parallel. In step  120 , as the user places each DSVA  20  into the borehole, the user determines the depth of each triphone. In step  122 , the user fires calibration shots in the vicinity of each DSVA  20 . In step  124 , the processing system establishes a time delay based on the response of each DSVA  20  to the calibration shot. In step  126 , the processing system cross-calibrates the DSVAs  20 . 
     In step  132 , the user selects the OSS  22 . In step  134 , the user places the OSS  22  at or near the surface of the geological structure  18 . In step  136 , the user triggers the OSS  22  and the recording unit records the response. In step  138 , the user determines whether ambient noises are sufficiently attenuated. If ambient noises are not sufficiently attenuated, then execution returns to step  136  and the user triggers the OSS  22  again. If ambient noises are sufficiently attenuated then the recording unit has recorded the complete vector filed in the step  106 . 
     Referring now to FIG. 14 in step  144 , the processing system senses rotation. In step  146 , the processing system generates components of the measured values in a sensing direction. Depending on the type of processing desired, the processing system performs step  148 , step  150 , or both in sequentially. It is noted that the processing system can perform step  148  and step  150  in any order. In step  148 , the processing system uses uphole summing to enhance the resolution of the three dimensional image in the sensing direction. In step  150 , the processing system uses uphole summing along a wavefront to enhance sensing along the wavefront. In step  152 , the processing system determines whether a three dimensional image has been generated. If more directions must be considered, then in step  154 , the processing system selects a new direction, and execution returns to step  144 . 
     In step  158 , the processing system selects a recorded measurement of the triphone  30   a , (FIG.  5 ), the bottom sensor, and in step  160 , performs uphole summing. In step  162 , the processing system selects the recorded measurement of the next highest triphone, and, in step  164 , checks to see if the triphone is the highest triphone. If the next highest triphone is not the triphone  30   n , then the processing system returns to the step  160  to perform uphole summing. If the next highest triphone is the triphone  30   n , then the processing system goes to step  150  if summing along the wavefront is required. If uphole summing is not required then the processing system goes to step  152  to determine is a three dimensional seismic image is generated. 
     To sum along the wavefront in step  150 , in step  170 , the processing system selects a starting time. In step  172 , the processing system sums along the wavefront corresponding to a path of the wavefront. In step  174 , the processing system determines if the time selected corresponds to a time when the wavefront reaches the top triphone of at least one of the DSVAs  20 , which lies in the path of the wavefront. If the top triphone is not reached, then in step  176 , another time is selected, and execution returns to step  172 . Otherwise, execution returns to step  152 , and continues until a three dimensional seismic image is generated, and execution ends at step  190 . 
     Referring now to FIG. 15, in an alternate embodiment of the present invention, the processing system uses a secondary diffracted seismic energy  97  to located a secondary seismic scatterer  24   a . The diffracted seismic energy  28  re-energizes the secondary seismic scatterer  24   a , which occurs a predetermined time period after the initial energization caused by the seismic energy  27 . Re-energization continues for some time resulting in multiple energizations (not shown). Accordingly, multiple energizations will occur from multiple directions after the predetermined time period has lapsed. Each DSVA  20  measures the multiple energizations as secondary arrival measurements, after the first arrival measurements. 
     After the recording unit records the secondary arrival measurements, the processing system locates the seismic scatterers using directional sensing in conjunction with triangulation techniques. The processing system performs triangulation by comparing the secondary arrival measurements measured by at least two selected clusters of DSVAs  98  and  99  separated by a predetermined separation distance S. The separation Ad distance S is preferably in the range of one-half mile up to three miles. Initially the vision lines of each cluster of DSVAs  98  and  99  are parallel to one another. Triangulation is achieved by focusing the vision lines of the cluster of DSVAs  98  and  99 , using sensing rotation, so that the vision lines of each cluster moves from the parallel position toward each other, in search of coherency in the complete vector field. The coherency is determined by using some form of a pattern recognition process. Once the coherency is located, then the apparent position of the seismic scatterer  24   a , which produced the coherency measured by cluster of DSVAs  98  and  99 , can be accurately determined independent of time and velocity. Referring now to FIG. 16 a flowchart  200  illustrates the process for sensing rotation combined with triangulation using at least two DSVAs  20 , which process begins at step  201 . In step  210 , the user separates the DSVAs  20  by the separation distance S, FIG.  15 . In step  220 , the user triggers the OSS  22  to energize the seismic scatterers  24  to produce diffracted seismic energy. The diffracted seismic energy re-energizes the seismic scatterer  24   a . The re-energized seismic scatterer  24   a  produces the secondary diffracted seismic energy  97 . In step  240 , the recording unit records the secondary diffracted seismic energy  97  as the secondary arrival measurement, along with the first arrival measurements. In step  260 , the processing system focuses in a direction of a coherency. In step  280 , the processing system calculates the apparent position of the seismic scatterer  24   a . In step  290 , the user corrects near-horizontal data. The user repeats the process set forth in the flowchart  200  until all desired apparent positions of seismic scatterers are determined, and then ends execution in step  300 . 
     In another embodiment of the present invention, a non-repeatable random energy source (NRES) replaces the OSS  22 . The user monitors the output of the NRES and records a reliable measurement of the omni-azimuth signature for later correlation. 
     In yet another embodiment of the present invention the DSVA is replaced by a directional sensing array (DSA) that is substantially vertical. The DSA can deviate up to twenty degrees from the vertical. 
     In operation, the OSS  22  emits a seismic energy  27  into a geological field. The seismic energy  27  energizes seismic scatterers  24  in the geological field. The energized seismic scatterers  24  act as independent sources of seismic energy and emit diffracted seismic energies  28 . Each DSVA  20  measures the diffracted seismic energies  28  as first arrival measurements. As the diffracted seismic energies  28  travel through the geological structure  18 , the diffracted seismic energies  28  re-energize other seismic scatterers  24  to produce secondary diffracted seismic energies  97 . Each DSVA  20  measures the secondary diffracted seismic energies  97  as secondary arrival measurements. Thus, each DSVA  20  measures a complete vector field. The triphones  30  of the DSVA  20  can be focused in a desired direction using sensing rotation within the complete vector field. Sensing rotation is combined with uphole summing to precisely locate the seismic scatterers  24  in the three dimensional image using time-distance relations of the first arrival measurements. Alternatively, sensing rotation is combined with triangulation techniques to located the apparent position of the seismic scatterers  24  independent of time, using secondary arrival measurements. On the other hand, sensing rotation and uphole summing can be combined with sensing rotation and triangulation techniques to create the three dimensional seismic image. 
     The principle advantages of the present invention include the ability to measure and record a complete vector field; imaging seismic scatterers rather than just locating reflections; measuring direction vectors; requiring only partial surface coverage rather than 100% coverage; uphole summing of the data rather than individually acquiring data; and directional separation during processing. A geological survey of a geological structure can be accurately produced and the location of sub-surface elastic boundaries or seismic scatterers can be precisely determined. Less labor is needed, which reduces cost and increases security. Less channels are needed, and fewer source positions are required, to produce continuous coverage of the geological structure. Turn-around time for field acquisition data is reduced significantly. Higher resolution and an improved signal-to-noise ratio is achieved. Although illustrative embodiments have been shown and described, a wide range of modifications, changes and substitutions is contemplated in the foregoing disclosure. In some instances, some features of the embodiments may be employed without a corresponding use of other features. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the scope of the embodiments disclosed herein.