Abstract:
An Oil Based Mud Imaging (OBMI) sonde adapted to be disposed in a wellbore includes a first imaging tool and at least one additional imaging tool connected to the first imaging tool, the additional imaging tool having a rotational offset and a significant vertical offset with respect to the first imaging tool when the OBMI sonde is disposed in the wellbore. The first imaging tool is connected to the additional imaging tool via a special adapter disposed between the first imaging tool and the additional imaging tool. The bottom of the first imaging tool plugs into one end of the special adapter and the top of the additional imaging tool plugs into the other end of the special adapter. The special adapter is made in a special way such that, when the bottom end of the first imaging tool is plugged into the one end of the special adapter and the top end of the additional imaging tool is plugged into the other end of the special adapter, the additional imaging tool is offset both vertically and rotationally with respect to the first imaging tool. The rotational offset requires that four pads of the additional imaging tool be offset azimuthally by an angle of approximately 45 degrees with respect to four pads of the first imaging tool. As a result, the OBMI sonde generates an output record medium having eight tracks instead of the traditional four tracks thereby giving a user a better view of a formation penetrated by the wellbore.

Description:
BACKGROUND OF INVENTION  
         [0001]    The subject matter of the present invention relates to a dual oil based mud imaging (OBMI) sonde adapted to be disposed in a wellbore, and, more particularly, to two oil based mud imaging (OBMI) sondes used in combination and joined together by a special adaptor, the second OBMI sonde having sensors which are offset azimuthally by a predetermined angle relative to the sensors of the first OBMI sonde. As a result, the second OBMI sonde will survey areas of the wellbore which are not being surveyed by the first OBMI sonde.  
           [0002]    It has always been a challenge for Petroleum Geologists worldwide to find a means to examine and understand the geological characteristics of subsurface lithologic formations. Technological advances in the petroleum industry have made it possible to acquire measurements of the physical properties of subsurface rocks, including micro-resistivity measurements which can be processed into electrical images. A problem area has been wells drilled using oil-base and synthetics-base mud systems. Wells are drilled using oil-base and synthetics-base mud systems in order to minimize any economic risks and maximize drilling efficiency. These mud systems are extremely resistive. Conventional borehole imaging sensor-arrays cannot acquire images in these non-conductive fluids. To make possible borehole resistivity image acquisition in these non-conductive fluids, specialized sensors have been developed to obtain high-resolution images of the borehole. just as image data from conventional imaging devices can be used in studies for structural and stratigraphic interpretation, including thin-bed detection, compartmentalization, high-resolution net-pay calculation, well correlation, etc., so can image data from oil-base and synthetics-base mud systems. However, there is a limitation in the circumferential coverage of the borehole using these specialized tools. That is, with respect to the borehole circumferential coverage limitation, due to physical problems in the well during image acquisition, there are intervals in the image where the image is highly distorted due to the tool-string getting stuck in the well and subsequently pulling free, or due to poor hole conditions, or drilling mud anisotropy, or even merely electrical noise. The aforementioned circumferential coverage of the borehole can be greatly increased and the above referenced problems can be corrected by connecting one or more additional imaging tools to a first imaging tool in the tool string, the additional imaging tools having a fixed preset rotational offset and a significant vertical offset with respect to the first imaging tool in the tool string.  
         SUMMARY OF INVENTION  
         [0003]    Accordingly, an imaging sonde includes a first imaging tool and at least one additional imaging tool connected to the first imaging tool, the additional imaging tool having a fixed preset rotational offset and a significant vertical or longitudinal offset with respect to the first imaging tool in the tool string.  
           [0004]    An Oil Based Mud Imaging (OBMI) sonde adapted to be disposed in a wellbore includes four pads which are adapted to extend radially when the sonde is in the wellbore, each of the four pads touching a wall of the wellbore with the pads extended radially in the wellbore. The OBMI sonde is then pulled upwardly to the ground surface at the wellbore, and each of the pads generate a “track” that is adapted to be displayed and/or recorded on an output record medium. A “track” is comprised of a plurality of resistivity curves as a function of depth in the wellbore (five resistivity curves for the OBMI). Since there are four pads on the OBMI sonde, four “tracks” will be recorded and/or displayed on the output record medium. However, since there are four pads on the OBMI sonde, there are four “regions” disposed in between each of the four adjacent pads. As noted earlier, the four pads will survey four portions of the wellbore. However, there are no pads on the OBMI sonde in each of the four “regions”. As a result, since there are no pads on the OBMI sonde in each of the four “regions”, those portions of the wellbore will not be surveyed by the OBMI sonde. As a result, in order to solve this problem, the OBMI sonde includes a first imaging tool and at least one additional imaging tool connected to the first imaging tool via a special adapter, the additional imaging tool having a rotational offset and a significant vertical or longitudinal offset with respect to the first imaging tool in the OBMI tool string. That is, the first imaging tool will, for example, have four pads. The four pads on the first imaging tool will, for example, have a first pad at approximately zero (0) degrees azimuthally, a second pad at approximately ninety (90) degrees azimuthally with respect to the first pad, a third pad at approximately one-hundred eighty (180) degrees azimuthally with respect to the first pad, and a fourth pad at approximately two-hundred seventy (270) degrees azimuthally with respect to the first pad. The additional imaging tool is connected to the first imaging tool via the special adapter. The additional imaging tool will be offset vertically or longitudinally in the wellbore with respect to the first imaging tool by a distance “d” (i.e., the vertical offset). In addition to the vertical or longitudinal offset, the additional imaging tool will also have a rotational offset with respect to the first imaging tool. That is, the additional imaging tool will also have, for example, four pads. However, in addition to the vertical offset, the four pads of the additional imaging tool will, for example, have a first pad at approximately fourty-five (45) degrees azimuthally with respect to the first pad of the first imaging tool, a second pad at approximately one-hundred thirty five (135) degrees azimuthally with respect to the first pad of the first imaging tool, a third pad at approximately two-hundred twenty five (225) degrees azimuthally with respect to the first pad of the first imaging tool, and a fourth pad at approximately three-hundred fifteen (315) degrees azimuthally with respect to the first pad of the first imaging tool. As a result, the four pads of the first imaging tool of the OBMI sonde will survey the four portions of the wellbore that are adjacent the four pads of the first imaging tool. However, in addition, the four pads of the additional imaging tool of the OBMI sonde will also survey the four portions of the wellbore that are adjacent the four “regions” which are located in between the four pads of the first imaging tool. As a result, an output record medium generated by the OBMI sonde of the present invention will include eight tracks instead of the traditional four tracks of a prior art OBMI sonde.  
           [0005]    As noted earlier, the first imaging tool is connected to at least one additional imaging tool via the special adapter disposed between the first imaging tool and the additional imaging tool. The first imaging tool plugs into one end of the special adapter, and the additional imaging tool plugs into the other end of the special adapter. The special adapter is made in a special way such that, when the first imaging tool is plugged into the one end of the special adapter and the additional imaging tool is plugged into the other end of the special adapter, the additional imaging tool is “offset rotationally” with respect to the first imaging tool; that is, there is a “rotational offset” or “azimuthal offset” or “angular offset” of the additional imaging tool with respect to the first imaging tool.  
           [0006]    As a result of the use of the special adapter disposed between the first imaging tool and the additional imaging tool in the wellbore, the additional imaging tool is “vertically offset” with respect to the first imaging tool. However, in addition, the additional imaging tool is “rotationally offset” with respect to the first imaging tool. When the additional imaging tool is “rotationally offset” with respect to the first imaging tool, the four pads on the first imaging tool will, for example, have a first pad at approximately zero (0) degrees azimuthally, a second pad at approximately ninety (90) degrees azimuthally, a third pad at approximately one-hundred eighty (180) degrees azimuthally, and a fourth pad at approximately two-hundred seventy (270) degrees azimuthally. However, in addition, the four pads on the additional imaging tool will, for example, have a first pad at approximately fourty-five (45) degrees azimuthally, a second pad at approximately one-hundred thirty five (135) degrees azimuthally, a third pad at approximately two-hundred twenty five (225) degrees azimuthally, and a fourth pad at approximately three-hundred fifteen (315) degrees azimuthally.  
           [0007]    Further scope of applicability of the present invention will become apparent from the detailed description presented hereinafter. It should be understood, however, that the detailed description and the specific examples, while representing a preferred embodiment of the present invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become obvious to one skilled in the art from a reading of the following detailed description.  
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0008]    A full understanding of the present invention will be obtained from the detailed description of the preferred embodiment presented hereinbelow, and the accompanying drawings, which are given by way of illustration only and are not intended to be limitative of the present invention, and wherein:  
         [0009]    [0009]FIGS. 1 through 4 illustrate a prior art Oil Based Mud Imaging (OBMI) sonde;  
         [0010]    [0010]FIG. 4A illustrates an output record medium generated by the OBMI sonde of the prior art, the output recording medium having four tracks corresponding, respectively, to the four pads on the OBMI sonde;  
         [0011]    [0011]FIG. 5 illustrates a dual Oil Based Mud Imaging sonde (hereinafter referred to as a “dual OBMI sonde”) of the present invention including a first imaging tool and a second additional imaging tool connected to the first imaging tool, the second additional imaging tool being rotationally and vertically offset with respect to the first imaging tool;  
         [0012]    [0012]FIG. 6 illustrates a top view of the first imaging tool of the dual OBMI sonde of FIG. 5 taken along section lines  6 - 6  of FIG. 5;  
         [0013]    [0013]FIG. 7 illustrates a top view of the second imaging tool of the dual OBMI sonde of FIG. 5 taken along section lines  7 - 7  of FIG. 5;  
         [0014]    [0014]FIG. 8A illustrates another view of the dual OBMI sonde of FIG. 5;  
         [0015]    [0015]FIG. 8B illustrates a view of the first imaging tool of the dual OBMI sonde of FIG. 8A;  
         [0016]    [0016]FIG. 8C illustrates a view of the second additional imaging tool of the dual OBMI sonde of FIG. 8A;  
         [0017]    [0017]FIG. 9 illustrates a top view of the prior art OBMI sonde of FIGS. 1 and 3, this top view showing an OBMI sonde having four pads, each pad adapted to touch a side wall of the wellbore;  
         [0018]    [0018]FIG. 10 illustrates another top view of the first imaging tool of the dual OBMI sonde of FIG. 5 taken along section lines  6 - 6  of FIG. 5 (this is similar to the top view shown in FIG. 6);  
         [0019]    [0019]FIG. 11 illustrates a construction of the “special adapter” which interconnects the second additional imaging tool to the first imaging tool of the dual OBMI sonde of FIG. 5 of the present invention;  
         [0020]    [0020]FIG. 12 illustrates a comparison of an output record medium generated by the prior art OBMI sonde of FIGS. 1 through 4 showing four tracks against the output record medium generated by the dual OBMI sonde of the present invention showing eight tracks; and  
         [0021]    [0021]FIGS. 13 and 14 illustrate a more detailed view of the output record medium generated by the dual OBMI sonde of the present invention showing eight tracks including four tracks generated by the four pads on the first imaging tool and four additional tracks generated by the four pads on the second additional imaging tool of the dual OBMI sonde of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0022]    Referring to FIGS. 1 and 2, a first prior art Oil Based Mud Imaging (OBMI) sonde  40   a  is illustrated.  
         [0023]    In FIG. 1, the first OBMI sonde  40   a  includes four pads  10   a - 10   d  adapted to touch a wall of the wellbore when the OBMI sonde is pulled upwardly to a surface of the wellbore. The OBMI sonde  40   a  of FIG. 1 is owned and operated by Schlumberger Technology Corporation of Houston, Tex. The four pads include a first pad  10   a  (not shown in FIG. 1) mounted on a central shaft  12 , a second pad  10   b  mounted to the central shaft  12 , a third pad  10   c  and a fourth pad  10   d  both mounted to the central shaft  12 . In FIG. 1, the four pads  10   a - 10   d  are shown in their extended position, the pads extending radially outward until the pads touch a wall  14  of the wellbore. When the pads touch the wall  14  of the wellbore, the OBMI sonde  40   a  of FIG. 1 is pulled upwardly to a surface of the wellbore and, responsive thereto, an output record medium (see FIG. 4A) is generated having four tracks corresponding, respectively, to the four pads  10   a - 10   d  on the OBMI sonde. The four tracks each represent resistivity curves as a function of depth in the wellbore. The four tracks will be discussed later in this specification.  
         [0024]    In FIG. 2, a top view of the first OBMI sonde  40   a  of FIG. 1, taken along section lines  2 - 2  of FIG. 1, is illustrated. In FIG. 2, the first OBMI sonde  40   a  includes the four pads including pad  10   a  and pad  10   b  and pad  10   c  and pad  10   d . The four pads  10   a - 10   d  are each connected to the central shaft  12 , the pads  10   a - 10   d  being shown in their extended position. That is, the pads  10   a - 10   d  have been extended radially outward until the pads  10   a - 10   d  each touch a wall  14  of the wellbore. In this position, the first OBMI sonde  40   a  of FIG. 2 is ready to be pulled upwardly to a surface of the wellbore and, responsive thereto, the output record medium including the four tracks of FIG. 4A will be generated (one track for each pad  10   a - 10   d ).  
         [0025]    Referring to FIGS. 3 and 4, a second prior art Oil Based Mud Imaging (OBMI) sonde  40   b  of FIGS. 1 and 2 is illustrated. However, in FIGS. 3 and 4, the pads are rotationally offset.  
         [0026]    In FIG. 3, the second OBMI sonde  40   b  includes four pads  20   a - 20   d  adapted to touch a wall  14  of the wellbore when the OBMI sonde is pulled upwardly to a surface of the wellbore. The OBMI sonde  40   b  of FIG. 3 is owned and operated by Schlumberger Technology Corporation of Houston, Tex. The four pads include a first pad  20   a  mounted on a central shaft  12 , a second pad  20   b  mounted to the central shaft  12 , a third pad  20   c  and a fourth pad  20   d  both mounted to the central shaft  12 . In FIG. 3, the four pads  20   a - 20   d  are shown in their extended position, the pads extending radially outward until the pads touch a wall  14  of the wellbore. When the pads touch the wall  14  of the wellbore, the second OBMI sonde  40   b  of FIG. 3 is pulled upwardly to a surface of the wellbore and, responsive thereto, an output record medium (see FIG. 4A) is generated having four tracks corresponding, respectively, to the four pads  20   a - 20   d  on the OBMI sonde. The four tracks each represent resistivity curves as a function of depth in the wellbore. The four tracks will be discussed later in this specification. In FIG. 3, however, the pads  20   a - 20   d  have been “rotationally offset”; that is, the pads  20   a - 20   d  have been rotated azimuthally until the pads  20   a - 20   d  are offset azimuthally by an angle of approximately 45 degrees relative to the position of the pads  10   a - 10   d  in FIGS. 1 and 2. This “rotationally offset” feature is best illustrated in FIG. 4.  
         [0027]    In FIG. 4, a top view of the second OBMI sonde  40   b  of FIG. 3, taken along section lines  4 - 4  of FIG. 3, is illustrated. In FIG. 4, the second OBMI sonde  40   b  includes the four pads including pad  20   a  and pad  20   b  and pad  20   c  and pad  20   d . The four pads  20   a - 20   d  are each connected to the central shaft  12 , the pads  20   a - 20   d  being shown in their extended position. That is, the pads  20   a - 20   d  have been extended radially outward until the pads  20   a - 20   d  each touch a wall  14  of the wellbore. In this position, the OBMI sonde  40   b  of FIG. 3 is ready to be pulled upwardly to a surface of the wellbore and, responsive thereto, the output record medium including the four tracks of FIG. 4A will be generated (one track for each pad  20   a - 20   d ). In FIG. 4, the first pad  20   a  has been “offset rotationally” or “offset azimuthally” by an angle of approximately 45 degrees with respect to the position of pad  10   a  of FIG. 2. Similarly, the second pad  20   b  has been “offset rotationally” by an angle of approximately 45 degrees with respect to the position of pad  10   b  of FIG. 2. The third pad  20   c  has been “offset rotationally” by an angle of approximately 45 degrees with respect to the position of pad  10   c  of FIG. 2. The fourth pad  20   d  has been “offset rotationally” by an angle of approximately 45 degrees with respect to the position of pad  10   d  of FIG. 2. However, the second OBMI sonde  40   b  of FIGS. 3 and 4 is identical to the first OBMI sonde  40   a  of FIGS. 1 and 2, even though the pads  20   a - 20   d  in FIG. 4 have been “rotationally offset” or “azimuthally offset” or “angularly offset” relative to the position of pads  10   a - 10   d  in FIG. 2.  
         [0028]    Referring to FIG. 4A, the output record medium produced by the OBMI sonde  40   a  and  40   b  of FIGS.  1 - 4  is illustrated. In FIG. 4A, the output record medium includes four tracks, a first track  30   a  corresponding to pad  10   a  or  20   a , a second track  30   b  corresponding to pad  10   b  or  20   b , a third track  30   c  corresponding to pad  10   c  or  20   c , and a fourth track  30   d  corresponding to pad  10   d  or  20   d . When the OBMI sonde  40   a  or  40   b  of FIGS.  1 - 4  is pulled upwardly to a surface of the wellbore, an output record medium is generated which includes the four tracks  30   a - 30   d . Each track  30   a - 30   d  includes a plurality of resistivity curves as a function of depth. That is, each pad  10   a - 10   d  and  20   a - 20   d  includes a plurality of button pairs (typically five button pairs in OBMI). When the OBMI sonde  40   a  or  40   b  is pulled upwardly to the surface of at the wellbore, the plurality of button pairs generate a corresponding plurality of resistivity curves as a function of depth in the wellbore. Since there are typically five button pairs on each pad  10   a - 10   d / 20   a - 20   d , five resistivity curves will be generated for each pad, one resistivity curve as a function of depth in the wellbore for each button pair on each pad. The five button pairs on each pad comprise a “track”. Therefore, for each pad, the five resistivity curves generated by each pad will comprise a “track”. In FIG. 4A, four “tracks” are illustrated, tracks  30   a - 30   d . Each “track”  30   a - 30   d  will provide an indication of resistivity as a function of depth in the wellbore for each corresponding pad  10   a - 10   d / 20   a - 20   d  on the OBMI sonde  40   a  or  40   b.    
         [0029]    Referring to FIGS. 5, 6, and  7 , the dual Oil Based Mud Imaging Sonde (dual OBMI sonde)  41 , in accordance with the present invention, is illustrated.  
         [0030]    In FIG. 5, the dual OBMI sonde  41  includes the first OBMI sonde  40   a  connected to the second OBMI sonde  40   b  via a special adapter  50 . The first OBMI sonde  40   a  of FIGS. 1 and 2 including pads  10   a - 10   d  is connected to the second OBMI sonde  40   b  of FIGS. 3 and 4 including pads  20   a - 20   d  via a special adapter  50 . That is, the first OBMI sonde  40   a  of FIGS. 1 and 2 is connected to an upper end  50   a  of the special adapter  50 , and the second OBMI sonde  40   b  of FIGS. 3 and 4 is connected to a lower end  50   b  of the special adapter  50 . When the special adapter  50  interconnects the first OBMI sonde  40   a  of FIGS. 1 and 2 at its upper end  50   a  to the second OBMI sonde  40   b  of FIGS. 3 and 4 at its lower end  50   b , the second OBMI sonde  40   b  including pads  20   a - 20   d  is “rotationally offset” by a predetermined angle (in this embodiment, approximately 45 degrees) relative to the first OBMI sonde  40   a  including pads  10   a - 10   d . In addition, when the special adapter  50  interconnects the first OBMI sonde  40   a  of FIGS. 1 and 2 at its upper end  50   a  to the second OBMI sonde  40   b  of FIGS. 3 and 4 at its lower end  50   b , the second OBMI sonde  40   b  including pads  20   a - 20   d  is “vertically offset” or “longitudinally offset” by a distance “d” from the first OBMI sonde  40   a  including pads  10   a - 10   d . For example, in FIG. 5, note that the second OBMI sonde  40   b  is spaced by a vertical or longitudinal distance “d” from the first OBMI sonde  40   a . The term “vertically offset” refers to the distance “d” in FIG. 5 when the first and second OBMI tools  40   a  and  40   b  are disposed in the wellbore. However, in any event, the second OBMI tool  40   b  is “longitudinally offset” from the first OBMI tool  40   a  along the longitudinal axial length of the dual OBMI sonde  40  of FIG. 5 because the second OBMI tool  40   b  is spaced by a distance “d” from the first OBMI tool  40   a  along the longitudinal axial length of the dual OBMI sonde  41 . The “rotationally offset” feature can best be seen in FIGS. 6 and 7 of the drawings.  
         [0031]    In FIG. 6, a top view of the dual OBMI sonde  41  of FIG. 5, taken along section lines  6 - 6  of FIG. 5, is illustrated. In FIG. 6, recall that the first OBMI sonde  40   a  included pads  10   a ,  10   b ,  10   c , and  10   d  (see FIG. 2). In FIG. 6, the first pad  10   a  of the first OBMI sonde  40   a  is azimuthally located at approximately zero (0) degrees, the second pad  10   b  is azimuthally located at approximately ninety (90) degrees relative to pad  10   a , the third pad  10   c  is azimuthally located at approximately one-hundred eighty (180) degrees relative to pad  10   a , and the fourth pad  10   d  is azimuthally located at approximately two-hundred seventy (270) degrees relative to pad  10   a . However, in FIG. 6, recall that the second OBMI sonde  40   b  included pads  20   a ,  20   b ,  20   c , and  20   d  (see FIG. 4). In FIG. 6, the second OBMI sonde  40   b  is “rotationally offset” relative to the first OBMI sonde  40   a  because the pads  20   a - 20   d  of the second OBMI sonde  40   b  are rotated clockwise by an angle of approximately 45 degrees with respect to the pads  10   a - 10   d  of the first OBMI sonde  40   a . That is, in order to fully understand the “rotationally offset” feature, note the following angular dimensions: in FIG. 6, the first pad  20   a  of the second OBMI sonde  40   b  is azimuthally located at approximately fourty five (45) degrees relative to pad  10   a  of the first OBMI sonde  40   a , the second pad  20   b  is azimuthally located at approximately 45 degrees relative to pad  10   b , the third pad  20   c  is azimuthally located at approximately 45 degrees relative to pad  10   c , and the fourth pad  20   d  is azimuthally located at approximately 45 degrees relative to pad  10   d.    
         [0032]    In FIG. 7, a top view of the second OBMI sonde  40   b  of FIG. 5 taken along section lines  7 - 7  of FIG. 5 is illustrated. In FIG. 7, the second OBMI sonde  40   b , including pads  20   a - 20   b  (of FIG. 4), is shown as having pads  20   a - 20   d  that are “rotationally offset” by an angle of approximately 45 degrees with respect to the pads  10   a - 10   d  of the first OBMI sonde  40   a . In particular, in FIG. 7, pad  20   a  is rotated clockwise by an angle of approximately 45 degrees with respect to pad  10   a  of the first OBMI sonde  40   a . Similarly, pad  20   b  is rotated clockwise by an angle of approximately 45 degrees with respect to pad  10   b  of the first OBMI sonde  40   a . Pad  20   c  is rotated clockwise by an angle of approximately 45 degrees with respect to pad  10   c  of the first OBMI sonde  40   a . Pad  20   d  is rotated clockwise by an angle of approximately 45 degrees with respect to pad  10   d  of the first OBMI sonde  40   a.    
         [0033]    In FIGS. 5 and 6, when the dual OBMI sonde  41  of FIG. 5 is pulled upwardly to a surface of the wellbore, the pads  10   a ,  10   b ,  10   c , and  10   d  of the first OBMI sonde  40   a  will survey the wall  14  of the wellbore at the following azimuthal or angular locations relative to the location of pad  10   a : zero (0) degrees using pad  10   a , ninety (90) degrees using pad  10   b , one-hundred eighty (180) degrees using pad  10   c , and two-hundred seventy (270) degrees using pad  10   d . However, the pads  20   a ,  20   b ,  20   c , and  20   d  of the second OBMI sonde  40   b  will survey the wall  14  of the wellbore at the following azimuthal or angular locations relative to the location of pad  10   a : fourty five (45) degrees using pad  20   a , one-hundred thirty five (135) degrees using pad  20   b , two-hundred twenty five (225) degrees using pad  20   c , and three-hundred fifteen (315) degrees using pad  20   d . The term “survey the wall  14  of the wellbore” means that the pads  10   a - 20   d  will touch and rub-against the wall  14  of the wellbore when the dual OBMI sonde  40  is being pulled upwardly to a surface of the wellbore; and, responsive thereto, an output record medium will be generated (such as a well log or other graphical chart) where the output record medium will display a plurality of “tracks” (such as the eight tracks seen in FIG. 13) which correspond, respectively, to the plurality of pads  10   a - 10   d / 20   a - 20   d  used by the dual OBMI tool  41  of FIG. 5.  
         [0034]    Referring to FIGS. 8A, 8B, and  8 C, another more realistic view of the dual OBMI sonde  41  in accordance with the present invention is illustrated. In FIG. 8A, the dual OBMI sonde  41  includes the first OBMI tool  40   a  connected to the second OBMI tool  40   b  via a special adapter  50 . The first OBMI tool  40   a  includes pads  10   a ,  10   b ,  10   c , and  10   d . The second OBMI tool  40   b  includes pads  20   a ,  20   b ,  20   c , and  20   d . The pads  10   a ,  10   b ,  10   c , and  10   d  of the first OBMI tool  40   a  are shown in their extended position (extended radially outward) for touching the wall  14  of the wellbore. The angular or azimuthal position of the pads  10   a ,  10   b ,  10   c , and  10   d  on the first OBMI tool  40   a  relative to pad  10   a  of the first OBMI tool  40   a  are: 0 degrees for pad  10   a , 90 degrees for pad  10   b , 180 degrees for pad  10   c , and 270 degrees for pad  10   d . The pads  20   a ,  20   b ,  20   c , and  20   d  of the second OBMI tool  40   b  are shown in their extended position (extended radially outward) for touching the wall  14  of the wellbore. The angular or azimuthal position of the pads  20   a ,  20   b ,  20   c , and  20   d  on the second OBMI tool  40   b  relative to pad  10   a  of the first OBMI tool  40   a  are: 45 degrees for pad  20   a , 135 degrees for pad  20   b , 225 degrees for pad  20   c , and 315 degrees for pad  20   d . As a result, the pads  20   a - 20   d  of the second OBMI tool  40   b  will survey (i.e., develop tracks like those shown in FIG. 13) the azimuthally oriented regions of the wellbore which are disposed in-between adjacent pads (i.e., in-between adjacent pads  10   a - 10   b ,  10   b - 10   c ,  10   c - 10   d , and  10   d - 10   a ) of the first OBMI tool  40   a . Therefore, instead of generating four tracks similar to the four tracks shown in FIG. 4A generated by the prior art OBMI tool of FIGS.  1 - 4 , the dual OBMI sonde  41  of the present invention will generate eight tracks similar to the eight tracks shown in FIG. 13. In FIG. 8B, the four pads  10   a ,  10   b ,  10   c , and  10   d  of the first OBMI tool  40   a  are shown in their extended position, pad  10   a  being at 0 degrees, pad  10   b  being at 90 degrees relative to pad  10   a , pad  10   c  being at 180 degrees relative to pad  10   a , and pad  10   d  being at 270 degrees relative to pad  10   a . In FIG. 8C, the four pads  20   a ,  20   b ,  20   c , and  20   d  of the second OBMI tool  40   b  are shown in their extended position, pad  20   a  being at 45 degrees relative to pad  10   a , pad  20   b  being at 135 degrees relative to pad  10   a , pad  20   c  being at 225 degrees relative to pad  10   a , and pad  20   d  being at 315 degrees relative to pad  10   a.    
         [0035]    Referring to FIG. 9, a more realistic top view of the prior art OBMI sonde  40   a  of FIG. 1, taken along section lines  2 - 2  of FIG. 1, is illustrated. Note that the pads  10   a - 10   d  are in their extended position adapted to touch an internal wall  14  of the wellbore. Pad  10   a  is located at an azimuthal angle of 0 degrees relative to pad  10   a , pad  10   b  is located at 90 degrees relative to pad  10   a , pad  10   c  is located at 180 degrees relative to pad  10   a , and pad  10   d  is located at 270 degrees relative to pad  10   a.    
         [0036]    Referring to FIG. 10, a more realistic top view of the dual OBMI sonde  41  of the present invention of FIG. 5 taken along section lines  6 - 6  of FIG. 5 is illustrated. Compare FIG. 6 with FIG. 10 and note that the pads  10   a - 10   d ,  20   a - 20   d  are in their extended position adapted to touch an internal wall  14  of the wellbore. Pads  10   a - 10   d  belong to the first OBMI tool  40   a , and pads  20   a - 20   d  belong to the second OBMI tool  40   b . Pad  10   a  is located at an azimuthal angle of 0 degrees relative to pad  10   a , pad  20   a  is located at 45 degrees relative to pad  10   a , pad  10   b  is located at 90 degrees relative to pad  10   a , pad  20   b  is located at 135 degrees relative to pad  10   a , pad  10   c  is located at 180 degrees relative to pad  10   a , pad  20   c  is located at 225 degrees relative to pad  10   a , pad  10   d  is located at 270 degrees relative to pad  10   a , and pad  20   d  is located at 315 degrees relative to pad  10   a . Yet, pads  20   a - 20   d  of the second OBMI tool  40   b  are “vertically offset” or “longitudinally offset” from pads  10   a - 10   d  of the first OBMI tool  40   a  when the dual OBMI sonde  41  is disposed in a wellbore. As a result, the four pads  10   a - 10   d  of the first imaging tool  40   a  of the dual OBMI sonde  41  will survey the four portions of the wellbore that are adjacent to the four pads  10   a - 10   d . However, in addition, the four pads  20   a - 20   d  of the additional imaging tool  40   b  of the dual OBMI sonde  41  will also survey the four portions of the wellbore that are adjacent to the four “regions” which are located in between the four pads  10   a - 10   d  of the first imaging tool  40   a.    
         [0037]    Referring to FIG. 11, a construction of the special adapter  50  of FIGS. 5 and 8A is illustrated. In FIG. 11, the special adapter  50  includes a first end  50   a  adapted to receive a end of the first OBMI tool  40   a  and a second end  50   b  adapted to receive an end of the second OBMI tool  40   b . When the end of the first OBMI tool  40   a  is plugged into the first end  50   a  of the special adapter  50 , and when the end of the second OBMI tool  40   b  is plugged into the second end  50   b  of the special adapter  50 , the pads  20   a - 20   d  of the second OBMI tool  40   b  will automatically be “rotationally offset” or “azimuthally offset” or “angularly offset” relative to the pads  10   a - 10   d  of the first OBMI tool  40   a . This is because the special adapter  50  is specially manufactured in order to “rotationally offset” the pads  20   a - 20   d  of the second OBMI tool  40   b  relative to the pads  10   a - 10   d  of the first OBMI tool  40   a  (where the term “rotationally offset” is meant to indicate that pad  20   a  is rotated clockwise an azimuthal angle of 45 degrees with respect to pad  10   a , pad  20   b  is rotated clockwise an azimuthal angle of 45 degrees with respect to pad  10   b , pad  20   c  is rotated clockwise an azimuthal angle of 45 degrees with respect to pad  10   c , and pad  20   d  is rotated clockwise an azimuthal angle of 45 degrees with respect to pad  10   d ).  
         [0038]    Referring to FIG. 12, a comparison of output records is illustrated whereby an output record medium generated by the prior art OBMI sonde of FIGS. 1 through 4 showing four (4) tracks is being compared against the output record medium generated by the dual OBMI sonde  41  of the present invention showing eight (8) tracks. In FIG. 12, the presentation shows an image acquired by the dual OBMI sonde  41  of the present invention having eight (8) tracks (labeled “OBMI2 track”) and a standard prior art OBMI tool having four (4) tracks (labeled “Standard OBMI”). Notice the much more distinctly visible high apparent angle fractures (see the sinusoid in FIG. 12) in the “OBMI2 track” image.  
         [0039]    Referring to FIGS. 13 and 14, a more detailed view of the output record medium generated by the dual OBMI sonde  41  of the present invention is illustrated, FIGS. 13 and 14 showing eight tracks including four tracks generated by the four pads  10   a - 10   d  on the first imaging tool  40   a  and four additional tracks generated by the four pads  20   a - 20   d  on the second additional imaging tool  40   b  of the dual OBMI sonde  41  of the present invention.  
         [0040]    In FIG. 13, this presentation shows images acquired by dual OBMI sonde  41  (i.e., the “OBMI2”) of the present invention. The static and dynamic tracks are labeled accordingly. The image segment acquired by each pad has been labeled as 1, 2, 3, 4 (acquired by the first tool  40   a ) and labeled as A, B, C, D (acquired by the second tool  40   b ). Looking at image segments from Pads 1, 2, 3 and 4 in the Static Track, it is observed that, at depth xx,x58 71 ft, the image segments are almost uniform in color. This corresponds to a time frame during data acquisition when the tool was stuck in the borehole, but continued to record data, and then pulled free. Once processed, this data appears as a “smear” on the image, as seen at depth xx,x58 71 ft in the image segments from Pads 1, 2, 3 and 4. When the first tool was stuck at the depth xx,x71 ft, the second tool (with fixed vertical offset from the first tool) was stuck at depth xx,x88 ft and caused a “smear” at depth xx,x75 88 ft (image segments from Pads A, B, C and D). However, when the first tool had passed this interval earlier, neither tool was stuck and the first tool had recorded a true data image (see image segments from Pads 1, 2, 3 and 4 at depth xx,x75 88 ft). Further, once the tools had broken free, the second tool passed through the zone that the first tool had “smeared” (depth xx,x58 71 ft) and the second tool recorded a true data image (image segments from Pads A, B, C and D). In this way, the second tool compensated for the loss of data by the first tool, and vice versa, and thus provided complete vertical coverage.  
         [0041]    In FIG. 14, this presentation also shows images acquired by the dual OBMI sonde  41  (i.e., the OBMI2) of the present invention. The static and dynamic tracks are labeled accordingly. The image segment acquired by each pad has been labeled as 1, 2, 3, 4 (acquired by the first tool) and labeled as A, B, C, D (acquired by the second tool). Looking at image segments from Pads 1, 2, 3 and 4 in the Static Track, it is observed that at depth xx,x45.5 61.5 ft the image segments have only slight variation in color. This corresponds to a time frame during data acquisition when the tool was stuck in the borehole, but continued to record data, and then pulled free. Once processed, this data appears as a “smear” on the image, as seen at depth xx,x45.5 61.5 ft in the image segments from Pads 1, 2, 3 and 4. When the first tool was stuck at the depth xx,x61.5 ft, the second tool (with fixed vertical offset from the first tool) was stuck at depth xx,x78.5 ft and caused a “smear” at depth xx,x62.5 78.5 ft (image segments from Pads A, B, C and D). However, when the first tool had passed this interval earlier, neither tool was stuck and the first tool had recorded a true data image (see image segments from Pads 1, 2, 3 and 4 at depth xx,x62.5 78.5 ft). Further, once the tools had broken free, the second tool passed through the zone that the first tool had “smeared” (depth xx,x45.5 61.5 ft) and the second tool recorded a true data image (image segments from Pads A, B, C and D). In this way, the second tool compensated for the loss of data by the first tool, and vice versa, and thus provided complete vertical coverage.  
         [0042]    A functional description of the operation of the dual OBMI sonde  41  of FIG. 5 of the present invention will be set forth in the following paragraph with reference to FIGS. 1 through 13 of the drawings.  
         [0043]    The dual OBMI sonde  41  of FIG. 5 is positioned in a wellbore as shown. The pads  10   a - 10   d  of the first OBMI tool  40   a  are located at the following angular positions relative to pad  10   a : 0 degrees, 90 degrees, 180 degrees, and 270 degrees; however, the pads  20   a - 20   d  of the second OBMI tool  40   b  are located at the following angular positions relative to pad  10   a:  45 degrees, 135 degrees, 225 degrees, and 315 degrees. An operator at the surface of the wellbore will now pull the dual OBMI sonde  41  of FIG. 5 upwardly to the surface. The pads  10   a - 10   d  and  20   a - 20   d  are actually touching the side walls of the wellbore  14  when the dual OBMI sonde  41  is pulled upwardly to the surface of the wellbore. Recalling that pads  10   a - 10   d  of the first OBMI sonde  40   a  of FIG. 5 will touch the side walls of the wellbore at the following angular degrees: 0, 90, 180, and 270; and recalling that the pads  20   a - 20   d  of the second OBMI sonde  40   b  of FIG. 5 will touch the side walls of the wellbore at the following angular degrees: 45, 135, 225, 315, when the dual OBMI sonde  41  of FIG. 5 is pulled upwardly to the surface of the wellbore, a new and novel output record medium will be generated and that new and novel output record medium will have the eight (8) tracks shown in FIG. 13 instead of the four tracks in FIG. 4A generated by the prior art OBMI tool of FIGS.  1 - 4 . As a result, more wellbore features can be seen on the eight-track output record medium of FIG. 13. That is, since there are eight tracks in FIG. 13 instead of the four tracks in FIG. 4A, more Earth formation features disposed on the side wall  14  of the wellbore of FIG. 5 will be visible on the eight tracks of the output record medium shown in FIG. 13.  
         [0044]    The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.