Abstract:
A formation logging tool having a substantially continuous central mandrel with regularly spaced mass blocks disposed thereon, at least some of the mass blocks carrying sensors such as receivers. By adopting this structure, the tool can be made to behave as a mass-spring structure and its flexural and extensional behaviour controlled such that its dispersion curve does not extend into the dispersion curve of the formation to be logged. The structure can be applied to the whole of the logging tool or just to the receiver section and/or any spacer section between the receiver and the transmitter section.

Description:
FIELD OF THE INVENTION 
   The present invention relates to structures for use in receiver arrays and spacers for use in sonic borehole logging tools. In particular, the invention relates to a structure having a particular flexural behavior which is designed to reduce the impact of interference with sonic logging measurements. 
   BACKGROUND OF THE INVENTION 
   The field of sonic logging of boreholes in the oil and gas industry involves making acoustic measurements in the borehole at frequencies typically in the range 500 Hz-20 kHz. Below this range is typically considered as the seismic domain, above it the ultrasonic domain. A summary of the general techniques involved in borehole acoustic logging can be found in GEOPHYSICAL PROSPECTING USING SONICS AND ULTRASONICS, Wiley Encyclopedia of Electrical and Electronic Engineering 1999, pp 340-365. 
   One example of a sonic logging tool used by Schlumberger is the Dipole Sonic Imaging tool (DSI), shown in schematic form in  FIG. 1 . The DSI tool comprises a transmitter section  10  having a pair of (upper and lower) dipole sources  12  arranged orthogonally in the radial plane and a monopole source  14 . A sonic isolation joint  16  connects the transmitter section  10  to a receiver section  15  which contains an array of eight spaced receiver stations, each containing two hydrophone pairs, one oriented in line with one of the dipole sources, the other with the orthogonal source. An electronics cartridge  20  is connected at the top of the receiver section  15  and allows communication between the tool and a control unit  22  located at the surface via an electric cable  24 . With such a tool it is possible to make both monopole and dipole measurements. The DSI tool has several data acquisition operating modes, any of which may be combined to acquire waveforms. The modes are: upper and lower dipole modes (UDP, LDP)—waveforms recorded from receiver pairs aligned with the respective dipole source used to generate the signal; crossed dipole mode—waveforms recorded from each receiver pair for firings of the in-line and crossed dipole source; Stoneley mode—monopole waveforms from low frequency firing of the monopole source; P and S mode (P&amp;S)—monopole waveforms from high frequency firing of the monpole transmitter; and first motion mode—monopole threshold crossing data from high frequency firing of the monopole source. One frequently observed problem in dipole logging is the propagation of a flexural signal from the source to the receivers along the tool itself. This signal, often known as a “tool arrival”, interferes with the detection of the corresponding signal that has propagated in the formation and so is highly undesirable. Approaches that have been taken to remove or reduce the occurrence of tool arrivals include the provision of a device or structure between the source and receivers which prevents propagation of the tool arrival (an “isolator”), and adoption of a receiver structure which delays or attenuates the tool arrival. 
   One form of isolator is found in tools in which the sources and receiver are found in two separate bodies connected by a relatively flexible connector such as a cable or flexible tube. An example of this is found in U.S. Pat. No. 5,343,001. Such an approach is effective in preventing the tool arrival from passing directly along the tool body from the source to the receiver but has the problem in that the tool cannot be used in any borehole which is not vertical, or nearly so. Since boreholes that are deviated from vertical are very common, such a tool has limited application. This structure also does not address the problem of a flexural signal coupling into the receiver structure from the borehole and then propagating along the receiver. 
   For tools in which the sources and receiver are connected in a relatively rigid structure (i.e. one which can operate in deviated boreholes), the approach has been to interpose an isolator between the source and receiver which interrupts the tool arrival path with a structure which delays and/or attenuates the flexural signal propagating along the tool body. In the DSI tool described above, the sonic isolation joint includes stacks of rubber and steel washers located around connecting members. This structure is the only connection between the transmitter and receiver, there being no continuous housing or tool body between the two. The sonic isolation joint is disclosed in more detail in U.S. Pat. No. 4,872,526. 
   Another form of isolator is a segmented structure in which the isolator is made up from a series of segments, each of which is connect only to its neighbors, there being some resilient or absorbent material at each joint. Examples of such structures are found in U.S. Pat. No. 5,229,553 which has a series of shells and spools, or in U.S. Pat. No. 5,728,978 which has a number of tubular members joined by interlocking lobes (see also SPE 56790 A Dipole Array Sonic Tool for Vertical and Deviated Wells, Lucio N. Tello, Thomas J. Blankinship, Edwin K. Roberts, Computalog Research. Rick D. Kuzmiski, Computalog Ltd., 1999 SPE Annual Technical Conference and Exhibition, Houston, Tex. 3-6 Oct. 1999). 
   As well as providing an isolator between the source and receiver, modifications to the structure of the receiver section itself have been proposed. In the DSI tool, for which the receiver housing provides the main structural strength for the tool, a combination of slots and apertures and mass loading rings are used to modify the acoustic behavior of the housing to reduce or delay to flexural (and other) tool arrivals. Further examples of this approach can be found in U.S. Pat. No. 4,850,450 and U.S. Pat. No. 5,036,945. In U.S. Pat. No. 5,731,550 the segmented structure applied to the isolator in U.S. Pat. No. 5,229,553 is also applied to the receiver section. However, since this is not a rigid structure, it may be necessary to also provide a housing or sleeve to make the tool able to operate in deviated boreholes. Other approaches to addressing this problem are discussed in PCT Application No. PCT/EB98/00646, published as WO99/56155, and incorporated herein by reference. 
   To date, no approach has been completely successful in removing or preventing flexural tool arrivals. It is an object of the present invention to provide a tool structure in which the problem of flexural tool arrival can be handled in a way which does not compromise the ability of the tool to make dipole measurements of the formation. 
   SUMMARY OF THE INVENTION 
   The present invention provides a structure for a logging tool which comprises a substantially continuous central mandrel having regularly spaced mass blocks disposed thereon, at least some of the mass blocks carrying sensors such as receivers. 
   By adopting this structure, the tool can be made to behave as a mass-spring structure and its flexural and extensional behaviour controlled such that its dispersion curve does not extend into the dispersion curve of the formation to be logged. The structure can be applied to the whole of the logging tool or just to the receiver section and/or any spacer section between the receiver and the transmitter section. 
   A tool incorporating the invention will include at least one acoustic signal source and a receiver section and/or a spacer section having the mandrel-mass block structure. Preferably the tool comprises a transmitter section with monopole and orthogonal dipole sources, a spacer with the mandrel as the load bearing member, and a receiver section with further monopole sources and a receiver array formed from the mandrel-mass block structure. 
   Where the mass block structure is applied to the receiver section of a logging tool, some of the blocks are used to carry acoustic receiver elements such as hydrophones. The blocks act as receiver mountings and are connected to each other only via the mandrel. By locating receiver elements on a number of adjacent blocks, a receiver array can be formed. The array will typically comprise a number of receiver stations (mass blocks), for example eight, twelve or sixteen stations, each of which has several receiver elements arranged in a regular manner around the periphery of each station, for example four or eight receiver elements. 
   Front end electronics can be associated with each receiver element so as to provide a digital output from each one. The required circuitry can be located around the mandrel adjacent to the respective receiver mountings. In this manner, communication of signals along the tool in the digital domain can be achieved. 
   The receiver elements are preferably provided with appropriate electronics so that the output is in digital form. 
   Further monopole sources can be located at either end of the receiver array. 
   Where the mandrel-mass block structure is applied to a spacer section, it is preferably disposed between the transmitter and receiver sections of the tool. The mandrel acts as a continuous load-bearing structure and can be hollow to define a wiring conduit between the two parts of the tool. A non-load bearing outer sleeve made from a material such as teflon can be provided (a similar sleeve can also be provided for a receiver structure). 
   The invention is described below in relation to the drawings by way of example. It will be appreciated that variations can be made in implementation while remaining within the scope of the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a prior art sonic logging tool; 
       FIG. 2  shows a logging tool incorporating embodiments of the present invention; 
       FIG. 3  shows a more detailed view of a transmitter module of the tool of  FIG. 2 ; 
       FIGS. 4   a ,  4   b  and  4   c  show more detailed views of a spacer section of the tool of  FIG. 2  and the mass block used therein; 
       FIG. 5  shows a general view of the interior of the receiver sonde of the tool of  FIG. 2 ; 
       FIG. 6  shows a partial view of the physical elements of the receiver and near transmitter section of the receiver sonde; 
       FIGS. 7   a ,  7   b  and  7   c  show side, cross-section, and isometric views of a mass block used in the receiver sonde; 
       FIG. 8  shows an alternative block design; 
       FIG. 9  shows a PCB mounting; 
       FIG. 10  shows acquisition electronics at the sensing element level; 
       FIG. 11  shows acquisition electronics at the receiver station level; 
       FIG. 12  shows the architecture of the receiver communication bus; 
       FIG. 13  shows a first stage amplifier circuit design; and 
       FIG. 14  shows a second stage amplifier circuit design. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring now to  FIG. 2 , there is shown therein a borehole logging tool including a receiver section and a spacer section according to embodiments of the invention. The tool shown in  FIG. 2  comprises an acoustic transmitter module  110  including a centraliser  112  and a standoff  114 . The transmitter module  110  is shown in more detail in  FIG. 3  and comprises an electronics section  120  with appropriate electronics and drive circuitry for the acoustic sources, an oil volume compensator section  122 , a first dipole source  124  (nominal “Y” direction), a second dipole source  126  (orthogonal to the first source  124 , nominal “X” direction) and a monopole source  128 . The dipole sources  124 ,  126  are substantially as described in the applicants&#39; copending U.S. patent application Ser. No. 09/537,836 entitled “Dipole Logging Tool”, filed Mar. 2, 2000 (incorporated herein by reference) and the monopole source  128  is substantially as described in U.S. Pat. No. 5,036,945 (incorporated herein by reference). A feed-through section  130  is provided to allow power and signalling wiring to be connected to the portion of the tool above the transmitter module  110 . 
   As shown in  FIG. 2 , connected immediately above the transmitter module  110  is a spacer section  132 . Two options are shown in the figure, a long section  132   a  and a short section  132   b . The length of the spacer section can be selected according to the expected acoustic behaviour of the formation to be logged. The spacer section  132  is described in more detail in relation to  FIGS. 4   a ,  4   b  and  4   c , and comprises an inner mandrel  200  formed from a titanium alloy pipe having a series of stainless steel mass structures  210  comprising blocks with a cylindrical outer surface  212  and a shaped inner surface  214  defining a cavity  216  mounted securely at regular intervals along the length of the mandrel  200 . The masses  210  are secured to the mandrel  200  by heating each mass  210  to cause it to expand and sliding it into place over the mandrel  200  using a bore  220  defined by the inner surface  214  of each mass  210 . The mass  210  is then allowed to cool and shrink around the mandrel  200 . By careful selection of the material and structure of the mandrel  200  and masses  210 , and appropriate positioning of the masses  210  along the mandrel  200 , the spacer can be configured to behave acoustically like a mass-spring structure which does not interfere with the acoustic signals used for evaluation of the formation surrounding the borehole, while still providing suitable physical structure and support for the other parts of the tool. Since there is no sleeve or housing around the spacer, and the mass blocks  210  are hollow and not sealed to each other, it is possible for borehole fluids to enter the cavity  216  in the mass blocks  210  and mud to build up inside the blocks and affect their acoustic behaviour. In order to allow cleaning of the cavity  216 , bores  218  are provided through the sidewall  212  of the blocks  210 . The mandrel  200  is hollow and connected to feed throughs  230 ,  240  at either end of the spacer section  132  such that wiring (not shown) can pass through the spacer  132  between the transmitter module  110  and the receiver sonde  134 . 
   The top of the spacer section  132  is connected to a receiver sonde  134  comprising a receiver and near monopole transmitter section  136 , an oil volume compensator  138  and a sonde electronics section  140 , and which is provided with rubber standoffs  142 ,  144 . A general view of the internal structure of the receiver sonde  134  is shown in  FIG. 5 . The receiver and near monopole transmitter section  136  of the sonde  134  comprises an array  145  of receiver stations  146  ( 16  in this example although other numbers are possible) spaced along a central mandrel  148 , each station  146  comprising a receiver mounting block  150  connected to the mandrel  148  and having a number of sensing elements  152  (hydrophones) arranged equiangularly around the circumference of the block  150 . In the present case, eight elements  152  are provided but other numbers, e.g. four, can also be used. Front end electronics boards are associated with each receiver station  146  and are described in more detail below. Monopole transmitters  154 ,  156  are mounted at either end of the receiver array  145 . The receiver and near monopole section  136  is encased in an armoured teflon sleeve  158  and is filled with oil for pressure compensation. The oil volume compensator  138  is connected above the receiver and near monopole transmitter section  136  and connected to the interior thereof. The sonde electronics section  140  is connected above the oil volume compensator  138  and includes front end power supplies and step up transformers (not shown) for the monpole sources. Feed throughs  160  are provided to allow wiring communication between the various sections of the sonde  134 . The upper part of the sonde  134  is also provided with feed throughs  162  for connection to a master electronics cartridge  164  which also has a centraliser  166 . The cartridge  164  is provided with standard connectors  168  which allow connection to other tools in a logging tool string or to a telemetry cartridge which communicates with a surface system via a wireline logging cable (not shown). 
   The receiver sonde is shown in more detail in  FIGS. 6 ,  7  and  8 . The basic structure of the receiver section  136  is a mandrel  148  and mass block  150  arrangement similar to that used in the spacer section. Monopole sources  154 ,  156 , essentially the same as that described in relation to the transmitter section above, are provided at either end of the receiver section  136 . The mandrel  148  extends between these sources  154 ,  156  and the series of mass blocks  150  are mounted on the mandrel  148  in the same way as in the spacer section. Sixteen adjacent blocks  150  define receiver mountings  170  each of which carries a circumferential array of receiver elements (hydrophones)  172  spaced around the periphery thereof. One diametrically opposed pair of elements in each station are aligned with a respective one of the dipole sources. In this embodiment, eight receiver elements  172  are provided. It will be appreciated that the number of stations and the number of receiver elements at each station can be selected according to requirements, for example, twelve stations, each with four receiver elements could be chosen. 
   The blocks  150  comprise a relatively elongated, tubular body  180  having a bore  182  extending through the middle. An end section  184  of the bore  182  has a region  186  of reduced diameter which embraces the outer surface of the mandrel  148 . The outer part  188  of the block  150  is formed into a mounting cavity  190  for the sensing element  172 . An alternative form of block  150  is shown in  FIG. 8 . These forms, or other similar structures can be used to define the acoustic behaviour of the receiver section, particularly in the flexural mode. Each block  150  is connected so that it does not contact the adjacent blocks directly. The only continuous structure in the receiver is the mandrel  148 . Dummy blocks (such as shown in  FIG. 8 ) can be provided at the ends of the receiver station array  145  to ensure consistent acoustic behaviour of the structure near the ends of the array. 
   The sensing element  172  is preferably a piezoelectric pressure sensor. The preferred form of sensor comprises a piezoelectric cylinder with end caps connected by a screw extending through the cylinder. Another form of sensor is a polarised stack of piezoelectric plates. These can be in the form of a stack with a screw extending through the centre of the stack to compress the plates. Alternatively, the plates can be located in a housing and separated from each other by electrodes to maximise the pressure effect on the plates. Whichever form of sensor is used, it is preferred that the axis of polarisation is parallel to the longitudinal axis of the tool. The exact manner in which the sensing element  172  is mounted in the block  150  will depend upon the form of the sensing element used. 
   Front end electronics are mounted on circuit boards (not shown) located on mountings  250  (see  FIG. 9 ) positioned around the outer part of each block  150 , one set of boards on a mounting  250  being associated with each receiver station. The mountings  250  comprise four surfaces  252  located between circular end fittings  254  which fit over the block  150 . The outer diameter of the end fittings is substantially the same as that of the mounting cavity  190 . 
   The basic electronic structure for the receiver front end is shown in  FIG. 10  and comprises the sensing element  172 , whose output is fed to a first stage  300  including a charge to voltage conversion amplifier with a first order high pass filter. The output from the first stage  300  passes to a second stage  302  which has a programmable gain amplifier and a ADC input buffer. The output from the second stage  302  passes to an ADC  304  with a 20 bit delta-sigma converter and decimation filter which provides serial data to a DSP  306 . When extended to an eight sensor station as described here, the front end electronics can be implemented as shown in  FIG. 11 , with separate first and second stages for each sensor and a two channel ADC  304  being provided to handle the outputs from two sensors at a time. It will be appreciated that the number of channels for the ADC will depend on the particular implementation of an ADC used. The resulting output from the DSP provides a digital output for each receiver station # 1  through # 16  which is passes along a common serial bus  308  in the receiver section to a master DSP  310  in the master electronics cartridge  164  ( FIG. 12 ). 
   The preferred implementation of the first stage amplifier is shown in  FIG. 13  and comprises a differential charge amplifier circuit. The sensor output signal Si is provided to an OPA404-type op amp modified by a test signal St and RC circuits RfCf, R1C1 to give a first stage output O 1 . Other implementations might also be appropriate, such as single-ended or balanced charge or voltage amplifiers. 
   The preferred implementation of the second stage amplifier is shown in  FIG. 14 . This takes as its input the first stage output O 1  and conditions the signal using two OPA404 type op amps (OPA404a, OPA404b) and a PGA with appropriate R and C elements. Again, other circuit designs might also be appropriate. The output of the second stage passes to the ADC and then to the receiver DSP which acquires serial data from the four ADCs per station through a parallel bus and converts it to serial data. The DSP also functions to provide signal processing for signal deconvolution when coded sequences are used from the acoustic signal sources (e.g. M-sequences), to provide controls to the devices such as the ADC and PGA on the circuit boards and to communicate with the master DSP in the electronics cartridge including transmission of the acquired data. 
   The above description is by way of example of various embodiments of the invention. Changes can be made while still utilising the inventive concept presented here. In particular, the physical size and shape of the mandrel and block structures can be varied to suit requirements. Also, the electronic designs presented here may be replaced by others in particular circumstances. None of these changes affect the inventive concept presented here. The invention can be applied to other tools in which it is desired to generate acoustic signals and make acoustic measurements.