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FIELD OF THE INVENTION 
     The present invention relates to a telemetry system for use in borehole logging tools. In particular, the invention relates to a system for communicating between a borehole tool when it is located in the borehole and a surface system. The invention also provides a system for communication between different tools connected to the same surface system while in the borehole. 
     BACKGROUND OF THE INVENTION 
     In the logging of boreholes, one method of making measurements underground comprises connecting one or more tools to a cable connected to a surface system. The tools are then lowered into the borehole by means of the cable and then drawn back to the surface (“logged”) through the borehole while making measurements. The cable, often having multiple conductors (7 conductor “heptacable” is common). The conductors of the cable provide power to the tool from the surface and provide a route for electric signals to be passed between the tool and the surface system. These signals are for example, tool control signals which pass from the surface system to the tool, and tool operation signals and data which pass from the tool to the surface system. A schematic view of a prior art telemetry system is shown in FIG.  1 . The system shown comprises a digital telemetry module DTM which is typically located at the surface, a cable C, a downhole telemetry cartridge DTC at the head of a tool string which includes a number of downhole tools T 1 , T 2 , . . . each containing a respective interface package IP 1 , IP 2 , . . . through which they are in communication with the DTC via a fast tool bus FTB. This system is configured to handle data flows in opposite directions, i.e. from the tools, via the respective IPs and FTB, to the DTC and then to the DTM over the cable (“uplink”), and the reverse direction from the DTM to the DTC and tools over the same path (“downlink”) Since the principal object of the system is to provide a communication path from the tools to the surface so that data acquired by the tools in use can be processed and analysed at the surfaces the protocol used favours the uplink at the cost of the downlink to optimise data flow from the tools. The communication path is split into two parts, the cable C and the tool bus FTB, and operation of these two are asynchronous to each other. In the FTB, the uplink and downlink both comprise biphase modulation using a half duplex systems of identical instantaneous data rate and frequency synchronised to a clock in the DTC. The difference between the uplink and the downlink is that the uplink uses CRC error detection with retransmission of detected bad packets while the downlink always sends twice. On the other hand, in the cable C the uplink and downlink systems are quite different. Uplink communication uses quadrature amplitude modulation with T5 and T7 cable modes being used, whereas downlink uses biphase modulation and T5 cable mode only. Both uplink and downlink are half duplex with CRC error detection and retransmission of detected bad packets. The result of this is that the uplink will often have an effective data rate of 500 Kbps compared to an effective downlink rate of 40 Kbps. Thus, when running at 6 Hz, such a system will have a period of 166.7 ms of which approximately 150 ms is allocated to uplink. A suitable protocol for implementing such a system is described in U.S. Pat. Nos. 5,191,326 and 5,331,318, the contents of which are incorporated herein by reference. 
     Recent developments in downhole tools have resulted in tools including functional components which can be accessed via the tool telemetry system and reprogrammed. An example of this is described in WO 97/28466. While it is possible to effect reprogramming of such a tool using the telemetry system described above, the relatively large amount of data to be transmitted downhole and the relatively low data rate of the downlink mean that the amount of time taken to achieve reprogramming of a single tool is likely to be in the order of hours. This effectively means that downhole reprogramming is impractical and even reprogramming at the surface is a slow and intensive process. 
     The present invention has the object of providing a telemetry system which maintains the priority given to uplink data flow in logging use, but which can be configured to allow increased downlink data flow when required. 
     SUMMARY OF THE INVENTION 
     One aspect of the present invention provides a borehole telemetry system comprising a surface telemetry module, a downhole telemetry module, and a multiplexed datalink between the surface and downhole modules capable of transferring data alternately between an uplink in which date is transferred from the downhole module to the surface module and a downlink in which data is transferred from the surface module to the downhole module; wherein the data link can be switched between a first configuration in which a relatively long time is assigned to the uplink and a relatively short time is assigned to the downlink, and a second configuration in which a relatively long time is assigned to the downlink and a relatively short time is assigned to the uplink. 
     The modulation of the uplink and the downlink can be the same or different. In a preferred embodiment, the uplink uses quadrature amplitude modulation and the downlink uses biphase modulation. Where a multiconductor cable is used to provide the datalink different modes can be used for uplink and downlink. For example, T5 and T7 modes can be used for uplink and T5 for downlink. Other modes or forms of datalink can be used if appropriate. 
     The system can effect the change between the configurations by sending a control signal from the surface module to the downhole module. 
     A system according to the invention finds particular application in borehole logging using a string of downhole tools. In the first configuration, the system is optimised to send data from the downhole tools to the surface, and in the second configuration is optimised to allow programming of the tools in the tool string from the surface telemetry module. A wireline cable is a common form for the datalink between the surface and downhole telemetry modules. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a schematic view of a prior art telemetry system; 
     FIGS. 2 a  and  2   b  show schematically the configuration of a telemetry system for downlink and uplink respectively; 
     FIG. 3 shows the manner in which data is handled in the DTC for transmission over the cable to the surface; 
     FIG. 4 shows the manner in which data is handled in the DTC received over the cable from the surface for transmission to the tool string; 
     FIG. 5 shows the time allocation to uplink and downlink in the first configuration of the system according to the invention; and 
     FIG. 6 shows the time allocation to uplink and downlink in the second configuration of the system according to the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIGS. 2 a  and  2   b  show schematic diagrams of the two configurations of a telemetry system in accordance with the invention. The basic functional parts of the system comprise a surface telemetry module  10 , a cable  12  and a downhole telemetry cartridge  14 . The surface telemetry module  10  includes a downlink modulator  16  and an uplink demodulator  18 , either of which can be connected to the cable  12 . The downhole telemetry cartridge  14  likewise contains a downlink demodulator  20  and an uplink modulator  22 , either of which can be connected to the cable  12 . In FIG. 2 a , the system is configured for downlink and so has the downlink modulator  16  and the downlink demodulator  20  connected to the cable  12 . In use, signals pass from the surface telemetry module  10 , via the modulator  16 , cable  12  and demodulator  20  to the downhole telemetry cartridge  14  from which they are passed to the various tools in the tool string (not shown). FIG. 2 b  shows the situation for uplink in which the connections and data flow are reversed. 
     FIGS. 3 and 4 show schematically the manner in which data is in the uplink and downlink respectively. For the uplink (FIG. 3) the DTC receives a number of packets, each from one FTB frame F and originating with an IP in the tool string (not shown). Each packet comprises a packet sync word  100 , a packet core  102  and a CRC (cyclic redundancy check) word  104 . The DTC checks the CRC word and any other inconsistency in each packet core  102 . Depending on the error (if any), and error bit is set in a packet error word  106 , incomplete error word  108  or delay error word  110  (one bit for each packet core). Any packet core with an error is discarded and the status sent uphole and the user notified. An acknowledgement is sent to each IP at the next FTB frame start. If no acknowledgement is sent the IP considers this as a retransmission request for that packet. The DTC strips the CRC  104  and packet sync  100  words from each packet and combines the packet cores  102  to form a superpacket  112  to which a packet error word  106 , incomplete error word  108  and delay error word  110  are added. Each superpacket occupies one position in a multi-word buffer  116  in the DTC and is tagged with a superpacket index  118  indicating the position in the buffer  116 . Each buffer space is provided with an age pointer  120  to enable the oldest superpacket to be sent first, and an empty/full indicator  122  to indicate if a superpacket has been received without errors or needs to be retransmitted. When the cable uplink frame opens, each superpacket is sent uphole to the DTM one after the other as long as the cable uplink period permits (or until the buffer is empty if this is shorter). To each superpacket is added a series of control words or bits, such as a T5 superpacket sync  124 , T7 superpacket sync  126 , superpacket length  128 , DTC uplink control echo (DTC gains, status and transmit rates)  130 , DTC departure time  132 , DTC arrival time for the previous cable frame  134 , and CRC word  136 . 
     For the downlink (FIG.  4 ), the DTM sends a downlink string  200 , typically comprising a T5 superpacket sync word  202 , a superpacket length word  204 , DTC uplink control  206 , DTC slave clock adjustment  208 , training bit  210 , sequence bit  212 , superpacket acknowledgement  214 , N packet cores  216 , and CRC word  218 . On receipt  230 , the sync pulse  202  is stripped off, the CRC computed and any error sent back to the DTM requesting retransmission. The DTC takes note of the arrival time and stores it for transmission back to the DTM on the next uplink transmission for slave clock synchronisation. The DTC uplink controls  206  are applied to the DTC during the next uplink transmission. The DTC slave clock adjustment is used to keep the DTC slave clock  235  synchronised to the DTM&#39;s master time clock. The training bit  210  is used to set the DTC training mode and the sequence bit  212  toggles for every downlink sequence to detect missing frames  240 . The superpacket acknowledgement  214  confirms the previous uplink, each bit corresponding to one superpacket. Every superpacket indicated as having been received with an error will be retransmitted and every superpacket that is indicated as received without error has its position in the buffer  116  indicated as empty by flag  122  and available for re-use  250 . The remaining packet core data  260  waits until the next FTB downlink opens  270 , at which point CRC  272  and sync words  274  are added to each packet  276  to create FTB packets  278  each of which is repeated  278 ′ so that the relevant IP can select the best one by checking the CRC. The FTB packets are sent in the order in which they are received by the DTM (first in to DTM, first out of DTC). 
     FIG. 5 shows the time allocation for uplink t U  and downlink t D  in normal logging operation. For the uplink U, the time is determined by the data rate, the number of superpackets (a typical buffer will contain 32 superpackets) and the size of each superpacket (typically 1024 words to each superpacket, of which a core of up to 1016 words are packet cores and there is one packet core per tool in the string, for example up to 16 tools). As the downlink D only contains one string, the time is dependent on the data rate, the number of packets (typically one per tool in the tool string, therefore typically up to 16), and the number of words per packet (typically up to eight words). This is the manner in which the system of FIG. 1 operates at all times, and in the context of this invention can be considered as the first datalink configuration. 
     For on-board programming activities, the present invention adopts the second datalink configuration as shown in FIG. 6, in which more time t D′  is available to downlink D′ and less time t U′  to uplink U′. This can be achieved in the following manner: First, the number of packet cores and/or the number of words per packet core in each downlink string is increased. Typically the data rate and modulation type used for the downlink will remain the same as in the first configuration to simplify implementation. Second, the size of the superpackets in the uplink is reduced by reducing the size of the packet core data in each superpacket. In one embodiment, each core packet will consist of one word which is merely a confirmation of the receipt of the data from the previous downlink. Thus the size of the superpacket core data could be as small as 16 words in the typical tool string configuration mentioned above and consequently less time will be required to transmit the entire contents of the buffer. Again the data rate and modulation type need not be changed. 
     The switch between the two configurations requires modification of the control of both the DTM and the DTC. The DTM is under direct software control at the surface. The DTC control can be modified using the DTC uplink control part of the downlink string. Appropriate control words are included to change the superpacket size. Each FTB packet can include instructions for each tool to send only the one word receipt acknowledgement. 
     By adopting the approach described above, it is possible to arrange for reprogramming of a tool over the normal logging cable in a time which can be measured in minutes rather than hours as is the case with current telemetry systems. 
     When in the second configuration, the FTB packet differ in that they are longer than in the first configuration and more than one FTB packet can go to a given tool from each downlink string. In this case, each FTB packet will require specific addressing in order to be received by the IP for the tool in question. Clearly it is possible to intersperse periods of the two configurations to allow the tools to alternate between logging and reprogramming. 
     At the end of the programming, which can be confirmed by appropriate acknowledgement signals, the DTM sends another DTC control to instruct the DTC to resume its original telemetry behaviour (first configuration). 
     It will be appreciated that other aspects of the system can be changed in order to achieve the second configuration. For example, the number of positions available in the buffer can be reduced during the enhanced downlink configuration. Also, modulation types can be changed to favour one or other configuration. However, this may require significant modification of hardware from current versions of the DTM and DTC. 
     While the invention has been described in relation to the use of wireline heptacable, it is not restricted to this either for tool conveyance or data communication. The methodology of the invention can be applied to any suitable datalink which has to be optimised for data flow in different directions at different times.

Summary:
A borehole telemetry system including a surface telemetry module, a downhole telemetry module, and a multiplexed datalink between the surface and downhole modules capable of transferring data alternately between an uplink in which date is transferred from the downhole module to the surface module and a downlink in which data is transferred from the surface module to the downhole module; wherein the data link can be switched between a first configuration in which a relatively long time is assigned to the uplink and a relatively short time is assigned to the downlink, and a second configuration in which a relatively long time is assigned to the downlink and a relatively short time is assigned to the uplink. The first configuration can be used to transmit data when logging with downhole tools and the second configuration can be used to program logging tools.