Abstract:
In accordance with an illustrative embodiment of the present invention, there is shown a substantially rigid member having interruptions in the longitudinal continuity of the member to provide tortuous paths for the passage of acoustic energy along the member. A plurality of masses are periodically spaced along the interior of the member and are each mechanically integral with opposite sides of the member at locations chosen to enable the member and masses to cooperate as a mechanical filter. By so doing, the structure made of the member and masses will have good acoustic delay and attenuation characteristics as well as good mechanical characteristics.

Description:
This is a continuation of application Ser. No. 256,074 filed May 23, 1972, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to acoustic well logging apparatus and more particularly to apparatus for attenuating and delaying acoustic energy. 
     An acoustic well logging apparatus is usually cylindrically shaped and suitably sized for passage through a fluid filled borehole. In the present day form of acoustic well logging apparatus, a transmitter is energized with a pulse of energy to emit acoustic energy into the borehole fluid surrounding the investigating apparatus for passage to the formation surrounding the borehole. The acoustic energy travels through the formation and a portion thereof is detected by one or more nearby acoustic receivers after passing from the formation back into the borehole. In some presently used systems, there are a plurality of acoustic transmitters and receivers. 
     In the most common type of acoustic logging apparatus, the travel time for acoustic energy to travel between the transmitter and receiver is measured. This is accomplished by generating a pulse which has a known time relationship with the firing of the transmitter and generating a second pulse coincident with the detection of one of the initial half-cycles of the acoustic energy arriving at the receiver. These pulses can then be processed to obtain numerical indications of the travel time between the transmitter and receiver. When making such measurements of the travel time of the formation adjoining a borehole, it is imperative that the acoustic energy travelling through the formation arrive at the receiver before the energy travelling through the borehole. Usually the velocity of acoustic energy  is significantly less in a borehole fluid than in the formation. However the acoustic velocity in a longitudinal support member constructed of steel will be much greater than in the formations. Of course it would be possible to construct the longitudinal support member of a low velocity material such as rubber but this would severely decrease the mechanical strength and rigidity of the support member and is undesirable for this reason. 
     This problem has been solved to a large extent by constructing the support member in a manner to delay the acoustic energy. Such a construction is shown in U.S. Pat No. 3,191,141 granted to N. A. Schuster on June 22, 1965. In this Schuster structure, a support member having a generally tubular configuration has a plurality of void or open spaces arranged about the periphery of the tubular member and along its length so as to provide a tortuous path for acoustic energy attempting to pass longitudinally along the support member. This arrangement is particularly useful for delaying acoustic energy at low frequencies. Other apparatus for providing such a delay for acoustic energy can be found in U.S. Pat. No. 3,381,267 granted to W. E. Cubberly, Jr., et al on Apr. 30, 1960; U.S. Pat. No. 3,191,142 granted to Maurice P. Lebourg on June 22, 1965, and U.S. Pat. No. 3,190,388 granted to E. I. Moser et al on June 22, 1965. As stated in the above mentioned patents, this acoustic delay structure will also provide some attenuation of acoustic energy because of the constantly varying shape and size of the acoustic path. As shown in the above-mentioned Moser et al patent, weights can be added to the periphery of this tubular member to enhance this effect. 
     Another prior-art technique for preventing passage of acoustic energy along the support member has been to construct the support member as a so-called mechanical filter which operates to attenuate acoustic energy above a selected frequency. An example of such construction can be found in U.S. Pat. No. 2,757,358 granted to J. O. Ely on July 31, 1956. In this Ely patent, the support member consists of a plurality of spaced weights connected to one another by a plurality of tubes. While this structure will operate to attenuate high-frequency acoustic energy, there will be a direct path for low-frequency acoustic energy through the tubes such that this low-frequency energy will arrive at the receiver before the acoustic energy which travels through the formations. 
     It is recognized that a support member of an acoustic logging apparatus should be able to both delay and attenuate acoustic energy travelling longitudinally of the support member. At the same time, the support member should have a significant amount of rigidity and strength because of the harsh downhole conditions to which it will be subjected. Thus in designing such a support member, there are a number of conflicting considerations which must be taken into account. To enhance the delay characteristic of the support member, it would be desirable to provide as many slots as possible therein. However, the more slots that are placed in the support member, the weaker will be the structural characteristics thereof. If a plurality of weights are placed in the skin of the tubular member thus improving its filtering characteristics, its structural characteristics will also be worsened. Thus to provide the required structural characteristics for the support member, it has in the past been necessary to restrict the efficiency of the support member in delaying and attenuating acoustic energy travelling longitudinally of the member. 
     It is therefore an object of the present invention to provide new and improved acoustic logging apparatus wherein the support member has high-strength qualities and improved attenuation and delay characteristics for minimizing transmission of acoustic energy along the support member. 
     The above discussion has been concerned with the type of acoustic well logging apparatus wherein the acoustic transducers are mounted on the longitudinally extending support member. In addition to this form of acoustic logging apparatus, it is also possible to mount some or all of the acoustic transducers on a pad member which is pressed against the wall of the borehole for investigating the adjoining formations. Such apparatus can be found in U.S. Pat. No. 3,406,776 granted to L. Henry on Oct. 22, 1968. 
     In constructing such a pad member, it is sometimes important that it maintain good strength characteristics and have the ability to attenuate or delay acoustic energy travelling along the length of the pad member. One might suggest that the attenuating and delaying structure of the patents discussed above could be applied to such pad structures. However, the small size of the pad member makes this very difficult. 
     It is therefore another object of the present invention to provide a new and improved pad structure for acoustic well logging apparatus which has high strength characteristics as well as the ability to attenuate acoustic energy. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, apparatus for attenuating and delaying acoustic energy comprises a substantially rigid member having interruptions in the longitudinal continuity of the wall of the member to provide only tortuous paths for the passage of acoustic energy along the member. The apparatus further comprises a plurality of masses periodically spaced along the interior of the member and arranged such that each mass is mechanically integral with opposite sides of the interior of the member at locations chosen to enable the member and masses to cooperate together as a mechanical low-pass filter. By so doing, most of the acoustic energy attempting to pass through the member will be attenuated and delayed. 
     For a better understanding of the present invention, together with other and further objects thereof, reference is had to the following description taken in connection with the accompanying drawings, the scope of the invention being pointed out in the appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows acoustic well logging apparatus constructed in accordance with the present invention; 
     FIG. 2 is a symbolic representation of a mechanical filter for purposes of explaining its operation; 
     FIG. 3 is an enlarged view of a portion of the support member shown in FIG. 1 constructed in accordance with one embodiment of the present invention; and 
     FIGS. 4 and 5 are enlarged views of a portion of the support member of FIG. 1 constructed in accordance with other embodiments of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Now referring to FIG. 1, a downhole investigating apparatus 10 is supported in a borehole 11 filled with a suitable drilling mud 12 for investigating subsurface earth formations 13. The investigating apparatus 10 includes a longitudinally extending support member 14 which includes upper and lower offcentering arms 15 and 16 which are maintained in an extended position by suitable springs 17 and 18. The arms 15 and 16 can be withdrawn to the side of the support member 14 by suitable hydraulic means (not shown). A pair of standoffs 20 and 21 are provided on the opposite side of the support member 14 from the eccentering arms 15 and 16 for maintaining the support member 14 a fixed distance from the wall of the borehole 11. The standoffs 20 and 21 include rotating wheels 22 and 23 coupled to the support member by linking arms 24 and 25 which are maintained in an extended position by suitable springs 26 and 27. Suitable stop means (not shown) within the support member 14 are provided for limiting the extent to which the standoff means 20 and 21 can be extended outward. Also included within the support member 14 are suitable hydraulics (not shown) for retracting the standoffs 20 and 21. 
     The investigating apparatus 10 also includes an acoustic transmitter 30 located in a cavity 31 which has an opening to the mud within the borehole. This opening is situated along the same generatrix of the borehole as are the standoffs 20 and 21. 
     The investigating apparatus 10 also includes a pad member 35 coupled to the support member 14 along the same generatrix as the standoffs 20 and 21 and transmitter 30 by a pair of linkage arms 36 and 37. The pad 35 is maintained in an extended position by a suitable spring 38. Suitable hydraulics (not shown) are contained with the support member 14 for retracting the pad 35 to a closed position relative to the support member 14 when desired. The pad member 35 also includes a pair of rotating wheels 38 and 39 which operate to maintain the pad a fixed distance from the borehole wall and also to minimize acoustic noise which might otherwise result from the pad scraping against the borehole wall. The pad member 35 also includes a pair of acoustic receivers 40 and 41 situated in a pair of cavities 42 and 43 respectively. The cavities 42 and 43 have openings toward the borehole wall along the same generatrix as the opening of the cavity 31 for the transmitter 30. 
     The transmitter 30 and receivers 40 and 41 are desirably cylindrical transducers having their axes parallel to one other and perpendicular to the longitudinal axis of the support member 14. A more detailed explanation of the theory of operation of the acoustic transducers shown in FIG. 1 can be found in copending application Ser. No. 256,075  filed by J. C. Trouiller on May 23, 1972 (now U.S. Pat. No. 3,978,939, issued Sept. 7,1976). For a more detailed explanation of the mechanical construction of the eccentering arms, standoffs and retractable pad, reference is made to copending application Ser. No. 256,107 filed by J. Planche on May 23, 1972 (now U.S. Pat. No. 3,795,414, issued Mar. 5, 1974). 
     In operation, suitable electronic circuitry (not shown) contained within a fluid-tight cartridge in the support member 14 operates to energize the transmitter 30 for emitting acoustic energy into the adjacent formation 13, as represented by the energy ray 45. This energy is detected by the receivers 40 and 41 and converted into electrical signals which are processed by the electronic circuitry within the support member 14 for transmission to the surface of the earth. Typically, the travel time of acoustic energy between the two receivers 40 and 41 will be measured. If desired, another transmitter like the transmitter 30 could be suitably placed on the support member 14 below the pad member 35 and on the same generatrix thereas. 
     A measurement of acoustic travel time will of course be upset if acoustic energy is allowed to travel along the support member 14 since it will arrive at the receivers 40 and 41 much sooner than the energy travelling through the formation 13. Likewise, the measurement will be upset (although to a lesser extent) if the acoustic energy is allowed to pass through the pad member 35. 
     To delay the passage of acoustic energy along the length of the support member 14, a plurality of open spaces or slots 50 which each extend longitudinally around a portion of the periphery of the support member 14 are located in a staggered relationship to one another along the length of the support member. As set forth in the above-mentioned Schuster patent, these slots operate to delay the travel of acoustic energy along the length of the support member 14 by providing a tortuous path for such energy. As stated earlier, the number and extent of such are limited by the desire to maintain the strength characteristics of the tubular support member 14. Moreover, by locating a plurality of slots along the periphery of a tubular support member, the constantly varying shape and size along this member will provide a degree of acoustic attenuation. 
     As stated earlier, it is also possible to construct the support member in the form of a mechanical low-pass filter. Turning now to FIG. 2, there is shown a symbolic representation of such a filter. The weights or masses are represented by the elements M and the portions of the support member between weights are represented by the springs K. This structure of FIG. 2 will operate when subjected to vibrations arising from acoustic energy to provide a virtual low-pass filter having a cutoff frequency f c  such that ##EQU1## where k and m are respectively the return constant stiffness of the springs K and the weight of the masses M. As can be seen from equation (1), the greater is the weight m and the less is the return constant k of the spring, the lower will be the cutoff frequency f c . The optimum condition would be for total attenuation of energy at all frequencies which would require the impossible condition that the mass m should be extremely large and/or the spring constant be extremely small. 
     The slotted delay line construction will be particularly useful for delaying acoustic energy at low frequencies. Moreover, a degree of attenuation will be attained with such a structure. As we have seen, by placing weights or masses in the periphery of such a slotted member acoustic attenuation can be obtained with such a slotted member. Unfortunately, because of very important structural considerations, there is a limit on the amount of weights which can be placed at selected points on the periphery of a member already weakened by slots, thus increasing the cutoff frequency f c . Moreover, the larger the percentage of the slotted support member taken up by slots or void spaces, the weaker will be the support member and the greater the delaying effect. If thus follows that with a lower limit on the mechanical strength of the support member, the cutoff frequency f c  and/or the delaying effect will have to be comprised--perhaps to an undesirable extent in some cases. 
     In this connection it would be desirable to provide a support member which will have a high degree of mechanical strength and at the same time provide a high degree of delay and attenuation for acoustic energy travelling along the length of the support member. In accordance with one feature of the present invention, this is accomplished by locating a plurality of weights at specified points along the interior of the support member 14 which weights are mechanically integral with opposite sides of the support member. These weights are represented in FIG. 1 by the dotted line elements 51. 
     Referring now to FIG. 3, there is shown an enlarged view of a portion of the support member 14 over the section which includes the slots 50 and weights 51. As seen in FIG. 3, the slots 50 are located in a staggered fashion around the periphery of the hollow tubular member 14. The weights 51 are attached as by welding to diagonally opposite sides of the tubular member 14 and thus are mechanically integral with opposite sides thereof. This structurally reinforces the support member. Alternatively, the weights 51 and tubular member 14 could be machined from one piece of metal. These weights 51 are connected on each side of the tubular member 14 at a location between the end points of two slots located along a common transverse plane relative to the tubular support member. 
     As shown in FIG. 4, it is also possible to construct this structure in accordance with the present invention by positioning weights 51 between two semi-circular cradles 66 having slots cut therein. 
     Turning now to FIG. 5, there is shown another embodiment of the present invention. A cylindrical metallic member 60 includes four longitudinally extending grooves 61 arranged along two planes symmetrical with relation to the diameter YY&#39; and parallel thereto. A first series of transverse openings 62 are formed at different points along the length of the member 60 and cut the diameter XX&#39; perpendicular to the diameter YY&#39; in cross-sectional planes of the member 60. These openings 62 extend through the member 60 to the grooves 61. A second series of transverse openings 63 are spaced along a plane defined by the Y-Y&#39; axis and the central longitudinally extending axis of the member 60 between the longitudinal grooves 61 and are located in the same cross-sectional planes as the openings 62. The openings extend completely through the member 60 parallel to the Y-Y&#39; axis. A double series of transverse notches 64a and 64b open into the longitudinal grooves 61 along cross-sectional planes which alternate with the planes containing the transverse openings 62 and 63. 
     The longitudinal grooves 61 along with the transverse openings 62 and 63 define the limits of a series of masses or weights 65 arranged between cradles 66. The transverse openings 62 and notches 64a and 64b provide the flexible connections between the masses 65, enabling the apparatus to operate as a mechanical low-pass filter. The transverse openings 62, 64a, and 64b along with the notches 61 provide the acoustic delay features of the structure. 
     It can thus be seen that with the structures shown in FIGS. 3, 4 and 5, a support member having notches cut therein to primarily provide a tortuous path for longitudinal or lengthwise travel of acoustic energy can operate in conjunction with a plurality of masses or weights to provide a mechanical low-pass filter. At the same time, the entire structure can be made very strong mechanically thus enabling the number and extent of the notches or openings which provide the delay characteristics and decrease the spring constant k to be substantially increased. 
     For structures of the type shown in FIGS. 3, 4 and 5, equation (1) takes the form of: ##EQU2## where E is the modulus of elasticity of the metal, m is the weight of each mass, S is the average section of a connecting spring, and a is the spacing between neighboring masses. Obviously, E and S should be as low as possible and m and a as large as possible to minimize the value of f c . However, S should not be decreased at the expense of mechanical strength. 
     It would be desirable if a high density material such as tungsten were used for the masses or weights and a material with a low modulus of elasticity such as titanium used in constructing the connecting springs. However, if this is not possible, a compromise material, such as steel, would have to be used for the entire structure. The spacing a should be much less than the wavelength of the acoustic energy in the fluid in which the structure is immersed. 
     The structures of FIGS. 3, 4 and 5 are shown as tubular in shape. While this is the most desirable shape for such a support member, it is to be understood that the invention is applicable to other shapes as well. For example, the support member could be elliptical or square in shape. 
     It can thus be seen that with the apparatus of the present invention, an acoustic logging apparatus can be constructed to provide very good structural characteristics coupled with very good attenuation and delay characteristics. In one form of the invention this has been accomplished by providing masses which extend from one side of a slotted support member to the other. 
     While there has been described what are at present considered to be preferred embodiments of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is, therefore, intended to cover all such changes and modifications as fall within the true spirit and scope of the invention.