Abstract:
A borehole logging tool system includes a surface system, a logging array, and a logging cable providing power supply and data paths connecting the logging array to the surface system, wherein the logging array includes a series of discrete sondes connected together. The sondes in the logging array, for example a borehole seismic logging array, are connected to their neighbours by means of lengths of logging cable. Such cable can be the same as that connecting the logging array to the surface system. The logging array can also include a master controller module which communicates with the surface system and which includes a first controller module which connects to the surface system and a second controller which controls operation of the sondes in the logging array independently of any other borehole logging tools connected to the surface system. The master controller can include a data buffer for handling data from the array and a clock which can be synchronised with a clock at the surface and which can be used in the control of the sondes in the array. Adopting such an arrangement with a borehole seismic logging array allows the sondes to continue acquiring data continuously under control of the master controller module irrespective of the transmission of data to the surface by the telemetry system.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a borehole logging system and to a communication system for use in such logging systems. In particular, the invention concerns borehole logging systems which include a number of discrete logging sondes connected together to form an array, for example a borehole seismic array tool or to muliple measuring entities connected to form a string. 
     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 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 for use in logging boreholes 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 surface, 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. Both uplink and downlink are half duplex. A suitable protocol for implementing such a system is described in U.S. Pat. No. 5,191,326 and U.S. Pat. No. 5,331,318, the contents of which are incorporated herein by reference. The FTB signal path is typically constituted by a pair of coax cables or a twisted pair conductor running along the length of the tool string. 
     The tools T 1 , T 2  . . . in the tool string are typically a series of sondes which make physical measurements of the formation surrounding the borehole, for example electrical, nuclear and acoustic measurements. The sondes are usually connected together to form a rigid tool string with electrical connectors permitting data and power connection between or through the sondes. In use, the operator must configure the FTB from the surface system to indicate the number of nodes (i.e. number of tools or sondes) such that the system can allocate addresses for each node. Once this is set, it is fixed and must be completely reconfigured to change the number of nodes. 
     Certain borehole tools are commonly found in the form of arrays, in which a number of similar (or identical) sondes which make the same measurement are connected together. Such an approach is often found in borehole seismic logging tools and examples can be found in SEISMIC APPLICATIONS Vol. 1, CROSSWELL SEISMOLOGY &amp; REVERSE VSP by Bob A. Hardage, Geophysical Press Ltd., London 1992. Because of the necessity to couple the measurement sondes closely to the borehole wall in such cases in order to improve the acoustic detection ability, and the difficulty of achieving such coupling with a very long tool string, it is often proposed to join the sondes together with lengths of flexible cable, often called “bridles”. The Array Seismic Imager ASI tool of Schlumberger, the SST 500 tool of CGG and other examples of such “array” or “multi-level” tools are found in U.S. Pat. No. 5,157,392. 
     One problem encountered with multi-level borehole seismic tools is that the large quantity of data recorded for each shot is greater than can be handled by current wireline telemetry systems. The tool described in U.S. Pat. No. 5,157,392 attempts to overcome this problem by providing memory in each sonde and in a downhole cartridge which is connected to the logging cable. In use, a signal is sent from a surface system to the cartridge to instruct activation of the measuring devices in each sonde for a predetermined time after the signal is received. This signal is coordinated with the firing of the surface source so that the sondes are active when the signal arrives. In order to overcome the limitations of the telemetry system, the sondes and the downhole cartridge are provided with buffers or memories which store the recorded signals. The stored signals are then telemetered to the surface over the logging cable when the sensors are not recording and when the tool is being moved in the borehole. 
     U.S. Pat. No. 5,585,556 describes a measurement while drilling system for making seismic measurements. In order to overcome the limitations of the telemetry system, signals are recorded downhole when drilling has stopped and a surface source is activated and stored. Some processing is performed on these signals and the processed data transmitted to the surface. The downhole tool must be retrieved in order to download all of the stored signals. In order to operate, the system is described as having synchronised clocks in the surface and downhole systems. 
     The systems described above have certain limitations. It is not possible to acquire data continuously and the surface system must be closely associated with the source firing system. This is often not possible, especially in marine environments. It is also not possible with this system to decide after the fact which data is to be telemetered to the surface and which can be discarded. 
     SUMMARY OF THE INVENTION 
     The present invention provides novel methods for recording data in borehole logging systems, novel borehole logging systems and novel borehole seismic logging tools and systems. 
     A method of recording data in a borehole logging system according to a first aspect of the invention comprises recording data at multiple measuring elements (such as seismic sensors) in a downhole system in a substantially continuous manner; storing the recorded data in a memory downhole; determining a data time window and a data sampling rate; and communicating, from the memory to the surface system, data falling in the determined time window and sampled at the determined sampling rate. 
     Preferably, time stamp data is associated with the recorded data in the memory. The time stamp data can be generated with a clock in the downhole system. In such a case, a synchronisation signal can be generated with a clock in the surface system, the synchronisation signal being sent to the downhole system and used to synchronise the clock in the downhole system with the clock in the surface system. The clock in the surface system can be synchronised with a time signal from a GPS system. 
     The time window and sampling rate can be communicated to the downhole system in a signal from the surface system. Alternatively, the time window and sampling rate can be determined in response to a detected event. 
     It is also convenient to transmit to the surface system data relating to the operating of the signal source which creates the signals sensed downhole. 
     The downhole system preferably includes a downhole telemetry cartridge and a sensor network cartridge, the recorded data being stored in the sensor network cartridge and the data being communicated to the surface via the downhole telemetry cartridge. 
     It is particularly preferred to assemble the downhole system at the surface and connecte it to the surface system and lower it into the borehole. By providing power to the downhole system, data can be recorded as the downhole system is lowered into the borehole. 
     A borehole logging system according to a second aspect of the invention comprises a surface system; and a downhole system, connected to the surface system, and including: a series of measuring elements; a memory; means for passing data from the measuring elements to the memory; and means for communicating data in a predetermined time window and at a predetermined sampling rate from the memory to the surface system. 
     A borehole seismic logging system according to a third aspect of the invention comprises a surface unit; a downhole seismic detector array connected to the surface unit and including a control module including a memory; and a series of shuttles, each of which has a sensor, the shuttles being connected to the control module and operating so as to record seismic signals and transmit data to the control module in a substantially continuous manner; wherein the control module communicates to the surface system data in a predetermined time window and at a predetermined sampling rate. 
     Preferably, the downhole system is connected to the surface system by means of a logging cable providing a power and data communication path. 
     The downhole array can further comprise a telemetry cartridge to which the control module is connected and via which it communicates with the surface system. Furthermore, the array can include a clock which provides time data to be associated with seismic signals recorded in the control module memory. The clock is preferably synchronised with a clock in the surface unit by means of control signals sent from the surface unit. 
     Where the system also includes a seismic source, the surface unit can receive time signals indicating operation of the source, the time signals being used to determine the time window and the sampling rate. 
     A borehole seismic logging tool according to a fourth aspect of the invention comprises a control module including a memory; and a series of shuttles, each of which includes a sensor and is connected to the control module such that, when supplied with power, it records seismic signal substantially continuously and transmits the recorded signals to the control module where they are recorded in the memory. 
     When the memory is full, it is preferred that new signals received from the shuttles are overwritten on old data already in the memory. The control module can also include a clock which provides time data to be associated with the recorded seismic signals. The control module preferably includes a first controller which can be connected to a surface system and a second controller which controls operation of the shuttles independently of any other borehole logging tools connected to the surface unit. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a schematic view of a telemetry system for borehole use; 
     FIG. 2 shows a borehole seismic logging tool embodying an aspect of the invention; 
     FIG. 3 shows the network topology of the tool of FIG. 2; 
     FIG. 4 shows more detail of the cartridge used in the tool of FIG. 2; 
     FIG. 5 shows more detail of the shuttle electronics used in the tool of FIG. 2; and 
     FIG. 6 shows detail of the network interface of the shuttle electronics shown in FIG.  5 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The invention will be described in the context of a multi-shuttle borehole seismic logging tool as is shown schematically in FIG.  2 . The tool comprises a surface unit  100  from which a tool string  110  is suspended in a borehole  120  by a conventional logging heptacable  130 . The connection between the cable  13 Q and the tool string  110  is provided by a downhole telemetry cartridge (DTC)  140  which communicates with the surface unit  100  in the manner described above in relation to FIG. 1. A tool cartridge  150  is connected below the telemetry cartridge  140 . An array of tool shuttles  160   1 ,  160   2 ,. . . ,  160   n , are connected to the tool cartridge  150 , and an array terminator  180  is provided at the lower end of the array connected to the last shuttle  160   n . Each shuttle  160  comprises a shuttle body  162 , and anchoring arm  164  and a three-axis geophone package  166 . The shuttles  160  are connected in an end to end arrangement with bridles  168  formed from logging heptacable. The number of shuttles in the array can vary from one to 20 depending on requirements. Also, other tool elements (sondes) can be connected to the downhole telemetry cartridge  140  above the tool cartridge  150 . 
     The tool cartridge  150  and the shuttles  160  define a network, the topology of which is schematically shown in FIG.  3 . The connections between the cartridge  150  and the first shuttle  160 , and between adjacent shuttles  160   n ,  160   n+1  is provided by heptacable bridles  168 . The cable has eight electrically conducting paths, conductors # 1  −# 7  and the armour. The cartridge  150  includes a controller module  152  which communicates with the telemetry system via an interface package such as those found in other downhole tool telemetry systems to the surface, and with the network of shuttles  160  below, and AC and DC power supplies  154 ,  156 . Each shuttle  160  includes a shuttle module  162 . with command and data interfaces as well as AC and DC power supplies  164 ,  166 . Command line signals CMD are implemented on conductors # 2 , # 3 , # 5  and # 6  of the cable using T 5  mode transformers. Data line signals DATA, are implemented on conductors # 2 , # 3 , # 5  and # 6  of the cable using T 2  mode transformers. Conductors # 1  and # 4  and the armour are used for power supply along the network. The command line is implemented in a daisy-chained, point to point configuration with re-timing and repeating in each shuttle. 
     The data line is implemented in daisy-chained, point to point configuration with re-timing and bidirectional data transmission in each shuttle. 
     The use of logging cable for the bridles offers a number of advantages. Logging cable is essentially cheap and plentiful at the well site which means that bridles can easily be made to measure according to requirements at the well site allowing greater flexibility in inter-shuttle spacing. In fact, the inter-shuttle spacing need not be regular across the array. Furthermore logging cable provides a good electrical power supply path across the array so as to allow faster and more reliable operation of the shuttles. Using mode transformers (e.g. T 5  or T 7 ) on the conductors for data communication means that this power can be supplied without compromising data quality or rate. 
     FIG. 4 shows the cartridge  150  in more detail. The cartridge connects to the tool bus (FTB) of the tool string by means of an interface package IP which functions in essentially the same manner as the IP found in other downhole tools, and forms part of the controller module CM which communicates with the telemetry system and tool string to send data up hole and receive commands sent down hole from the surface. 
     The cartridge  150  also includes a sensor network master SNM which transmits and receives command CMD+, CMD−and data DATA+, DATA−signals to and from the network using the logging cable bridles  168  as a signal path. The master SNM includes shuttle network controller SNC functions, a protocol handler PH and transmit/receive TX/RX functions. An AC/DC power supply PS 1  provides an electric power source for the cartridge electronics and for the shuttle electronics and sensors. An AC/AC inverter PS 2  provides power for motors powering the shuttle anchoring mechanism. Buffer memory MEM is provided for the controller and sensor network master modules CM, SNM and a clock CLK which can be synchronised with a clock in the surface unit via the telemetry system provides time information to the network. 
     The clock CLK is implemented as an oscillator in a phase locked loop under the control of a dedicated DSP, and outputs a VSI Clock value which is increased incrementally by the action of the oscillator. 
     The shuttle electronics are shown in more detail in FIGS. 5 and 6 and comprise two main functional blocks. A front end module  200  handles data acquisition and control at the shuttle level while a back end module  210  handles communication with the shuttle network. 
     As shown in FIG. 5, the shuttle includes a sensor package  220  which has a shaker  222  and three geophone accelerometers (GAC)  224   x ,  224   y ,  224   z  oriented in orthogonal directions, a motor  226  operating an anchoring arm (not shown) and various other auxiliary functions such as system check sensors (e.g. temperature)  228   a , anchoring arm force sensor  228   b , anchoring arm clutch position sensor  228   c , arm position sensor  228 d and anchor motor control  228   e.    
     The output from each GAC  224  is provided to an associated Σ-Δ ADC  230   x ,  230   y ,  230   z  which outputs a digital signal to a respective filter  232   x ,  232   y ,  232   z  in the back end module  210 . The outputs of the filters  232  are passed to a shuttle module  234  from where the signals are passed along the network to the cartridge and on to the surface. 
     The back end module  210  includes a network interface  236  which shows in more detail in FIG. 6 the connections to heptacable conductors # 2 , # 3 , # 5  and # 6  for command signals (CMD 1 , CMD 2 , CMDB 1 , CMDB 2 ) in T 5  mode, and data signals (DATAA+, DATAA−, DATAB+, DATAB−) in T 2  mode; and to conductors # 1 , # 4 , # 7  and ARMOR for AC and DC power for shuttle function and motor control (the connections between the network interface and the rest of the back end module are omitted for clarity in FIG.  6 ). 
     The back end module  210  not only receives the GAC outputs, it is also provided with a sych/clock recovery function  238  and an output to a test signal generator  240  in the front end module  200 . The test signal generator  240  can be used to drive the shaker  222  in the sensor package  220  or applied, via a switch  242 , to the GAC signal lines connecting to the pre-amps  225 . The back end module  210  also communicates with the auxiliary functions  228  of the front end module  200  via an appropriate A/D converter and front end multiplexer  244 . 
     In use, the tool string is assembled at the surface and if more than one type of tool is present in the string, an array tool such as that described above will typically be the bottom-most tool in the string. Once the array is placed in the well, a signal is sent from the surface to power up the tool, the signal being transmitted along the array of shuttles from the cartridge. On power up, each shuttle registers itself automatically in the network controlled by the cartridge. The network of shuttles then runs completely under control of the control module in the cartridge. 
     The clock in the cartridge is initially synchronised with the surface telemetry system clock via the digital telemetry system but runs independently of that clock apart from periodic resynchronisation. 
     Once the network has become active, it acquires data continuously, the GACs in each shuttle recording seismic signals without interruption. This data is time stamped in each shuttle using the network clock, and transmitted over the network to the cartridge where it is stored in the buffer memory. The data in the buffer memory is transmitted back to the surface over the digital telemetry system in the order in which it was received, but independently of the acquisition of the data by the shuttles. Should the buffer become full, newly acquired data overwrites the old data. Because of the provision of the network clock, it is possible to record data continuously and time stamp the data without being reliant on the digital telemetry system. Thus the acquisition of data is relatively independent of the performance of the telemetry system to the surface. The transmission of data to the surface can take place under the control of the digital telemetry system at whatever rate is available without compromising the ability of the array to acquire data at its optimum rate. 
     Since the sensors become active on power-up, it is possible to use them as descent monitors as the array is lowered into the borehole. The sensors will detect signals due to road noise as the tool is run into the borehole. If the sensors on one or more shuttles stop recording signal, it is an indication that the array is stuck at the sensors in question and running in can be stopped before the bridles or logging cable become tangled. 
     Once the desired depth is reached, the shuttles are anchored in the borehole by actuation of the anchoring arm mechanism. By measuring the anchoring arm force, the likely quality of data recorded at any given time can be evaluated. If the anchoring force is low, it is possible that the shuttles are not properly anchored to the borehole wall and any data for that period is of suspect quality. Anchoring arm force in one of a number of auxiliary measurements and operations that can be made at each shuttle. These include temperature measurement, anchoring arm clutch position measurement, arm position measurement, anchoring motor operation and shaker operation. Since it is not necessary to have all of these auxiliary functions available at all times, a smaller number of channels are made available for the signals, typically three channels although other numbers of channels may be used depending on availability. Operation of these functions is on a multiplexed basis according to received command signals. Consequently, while seismic data acquisition is on a continuous basis, auxiliary functions are performed on a periodic basis. 
     When it is desired to move the array to another location in the borehole, a signal is sent from the surface to the cartridge which then passes commands to the shuttles to stop acquiring data and release the anchoring arm for each shuttle. The auxiliary sensors in each shuttle allow confirmation that it has released and the array can be moved to another location where the shuttles can be locked in place again using the anchoring arms. Again the auxiliary sensors allow confirmation of proper deployment of each shuttle before new data acquisition begins. 
     On startup, each sensor in the shuttles  160  begins acquiring data at a predetermined sampling rate (e.g. 0.5 ms, 1 ms, 2 ms, 4 ms, etc.), which are transmitted to the tool cartridge  150  and stored in the buffer memory MEM. At the beginning of the session, the initial clock value TO is latched and transmitted to the surface unit  100 . At every second FTB frame following this, the clock value is latched and transmitted to the surface unit together with the corresponding value from a clock in the DTC (not shown) which is synchronised with a clock in the surface unit  100 . Thus, for an FTB frame length of 16 ms, every 32 ms the surface unit  100  receives a pair of values comprising the VSI clock t(n) and the corresponding DTC time stamp DTS Time Stamp t(n) (which relates to the clock value in the surface unit  100 ). The sequence is as follows: 
     1. Startup 
     2. Latch VSI clock and transmit t(O) to surface. (Begin data acquisition from shuttles an store in buffer with corresponding VSI clock value t(n)) 
     3. Miss one FTB frame. 
     4. Latch VSI clock and transmit value VSI clock t(n) to surface together with DTC slave clock time stamp, DTS Time Stamp t(n). 
     5. Miss one FTB frame. 
     6. Latch VSI clock and transmit value VSI clock t(n) to surface together with DTC slave clock time stamp, DTS Time Stamp t(n). 
     7. Miss one FTB frame. 
     8. etc. 
     In the surface system  100 , the latest  256  pairs of VSI clock t(n) and DTS Time Stamp t(n) are accumulated in memory. 
     When it is desired to retrieve samples of the acquired signals, the clock in the surface system  100  is latched according to the time Te of some event. This can be set internally in the surface system  100  or can be triggered by an external event such as the firing command of a source at the surface or detection of source firing. The surface system translates Te from surface clock time (DTS Time) into VSI clock time using the stored  256  values of VSI clock t(n) and DTS Time Stamp t(n) and simple extrapolation to Te. The time Ts to commence sampling of the data is then computed in terms of VSI clock value which is in phase with the VSI data/time stamp pairs in the buffer MEM. The DTS Time Ts is computed from the extrapolation and used to generate a command signal in the surface system which is transmitted to a surface sensor (if present) and downhole over the telemetry system. This command provides the VSI Ts value and the number of samples to be transmitted uphole. The cartridge uses this command to determine which data are to be retrieved from the buffer MEM and passed to the telemetry cartridge for communication to the surface system  100  over the cable. The sequence is as follows: 
     1. Latch surface clock to obtain Te 
     2. Translate Te from DTS time to VSI clock time 
     3. Compute Ts in VSI clock time from Te 
     4. Translate Ts from VSI clock time to DTS time and generate command signal 
     5. Transmit command signal downhole 
     6. Receive command signal at telemetry cartridge DTC downhole and pass to tool cartridge over FTB 
     7. Receive FTB command signal in tool cartridge and determine VSI clock time value Ts to start data to be retrieval from buffer and the number of samples to be retrieved 
     8. Retrieve data and transmit to DTC for communication to surface over cable 
     Using the system described above, it is possible to separate the acquisition of data from the transmission of data to the surface (by the use of the VSI clock) and to only transmit to the surface the data required (by correlating the VSI clock with the surface clock). This optimises use of the telemetry bandwidth by avoiding transmitting unwanted data. While the sampling rate is typically predetermined for the shuttles, it can be adjusted by providing the necessary command signals from the surface. 
     Because the VSI clock runs independently of the surface clock, it is necessary when determining Te to round its value to the nearest VSI clock value. This rounding varies from case to case by up to one sampling interval (typically 1ms). Since this amount is measurable in the surface system, it can be applied later when the data is analysed. While the invention has been described above in relation to an array seismic tool, it will be apparent that the concept can be applied to other tools either in the form of arrays of similar sensors or strings of different sensors and tools.