Abstract:
Method and system for transferring synchronization data between a central control and seismic recording station and dependent stations in all the cases where the time of transmission through the connection channels between the elements may be liable to too great fluctuations.  
     In its principle, the solution retained essentially consists in providing the communicating stations: a central station (CCU) and intermediate local control and concentration stations (RRS), themselves connected to local acquisition units (RTU), with a common time reference or clock (H) (that can for example be provided by a satellite positioning network of a well-known type such as the GPS system, or based on the carrier frequency of a Hertzian transmitter) which is picked up by specialized receivers associated with the elements of the seismic device, and in using this common reference to calculate the real transmission time and to locally readjust seismic acquisition units (RTU).  
     Application: transmission of a TB concomitant with the triggering of a seismic source (S) for example.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to a transmission method and system using a standard transmission network for connecting elements of a seismic device, allowing to transmit with precision a signal indicative of a time break.  
           [0002]    The transmission system according to the invention can find applications in many fields where distant stations have to be synchronized without necessarily using dedicated communication means.  
         BACKGROUND OF THE INVENTION  
         [0003]    This is the case in the field of seismic prospecting where signals received by seismic receivers distributed on the ground surface for example and reflected by the discontinuities of the subsoil in response to the emission of seismic waves by a vibrational or impulsive source are recorded. Seismic data collection sets comprising a large number of acquisition units distributed on the site to be explored and suited to collect (amplify, filter, digitize and store) the signals are used. The stored data are transmitted to a central control and recording station from each acquisition unit at fixed time intervals (for example after each emission-reception cycle, after each daily session, etc.) or &lt;&lt;with the current&gt;&gt;, as soon as a transmission time interval is available, either directly or via intermediate stations or concentrators. Seismic acquisition systems are for example described in patents FR-2,511,772 (U.S. Pat. No. 4,583,206) or FR-2,538,194 (U.S. Pat. No. 4,628,494), FR-2,666,946 (U.S. Pat. No. 5,245,647), FR-2,692,384 (U.S. Pat. No. 5,550,787), FR-2,696,839 (U.S. Pat. No. 5,706,250), FR-2,710,757 (U.S. Pat. No. 5,563,847), FR-2,720,518 (U.S. Pat. No. 5,822,273), FR-2,766,580.  
           [0004]    It is important that all the acquisition units distributed on the site explored can be synchronized with a common time break, generally the time of triggering of the seismic source, whatever the distance from the central station that transmits the indicative signal thereto and whatever the transmission channel used therefore, failing which the centralized data combinations become very imprecise.  
           [0005]    There are well-known methods and devices allowing perfect readjustment in time of distant stations to a central station communicating by means of communication channels, provided that it has been possible to measure the time of propagation of the signals on these channels.  
           [0006]    Patent FR-2,538,194 (U.S. Pat. No. 4,628,494) filed by the applicant describes for example a method for synchronizing the acquisition of seismic signals by an acquisition unit in the field with a time break such as the time (TB) of triggering of a seismic source in the case where the time of propagation Δt thereto of the signal indicative of this time, via a transmission channel such as a cable or a radio channel, is known. The method essentially consists in sending a pre-signal initiating the acquisition of the signals coming from the seismic receivers and their storage in a local memory. When the synchronization signal subsequently emitted is received by the acquisition unit at a time t, all the samples stored from the time (t−Δt), i.e. the time break, are sought in the local memory. Patent application FR-98/15,792 describes a method and a device also allowing to produce, for each seismic signal, a series of samples of these signals, re-staggered from a time break, from a first series of digitized samples of this seismic signal produced from any time break prior to the time break, knowing the effective time interval between these two times. The method comprises determining coefficients of a digital filter likely to compensate for the fractional part of the effective time interval measured, and applying this digital compensation filter to the first series of samples, thus allowing to obtain a series of digitized samples re-staggered from the time break.  
           [0007]    However, these known locating and readjustment techniques are applicable only if the time of propagation of synchronization signals through the emitting and receiving stations is known with precision, transfers being managed by software means, notably within the scope of multitask management, especially if one considers that the uncertainty about the effective propagation time in a seismic transmission system as mentioned above for example should not exceed about 50 μs.  
           [0008]    Transmission without more or less random time lag is possible and easier to implement when one owns the network that is used and if one is in control of the form of the signals emitted and of the coding mode. When the TB signal indicating the effective time of emission by the source reaches the central station, it is possible, via suitable circuits, to inject this signal directly into the communication channel that connects it to the receiving station, and thus to prevent possible non-constant delays due to the station management information system.  
           [0009]    Standard communication networks are advantageous. They allow high-rate transmissions, they are relatively simple to use and open-ended. They however work according to a particular communication protocol with specialized pilot and control circuits which one has to adapt to in order to transmit signals representative of time breaks with precision.  
           [0010]    This is the case in the system described in patent application FR-99/12,113 filed by the applicant, which uses a standard communication network of Ethernet type for example in order to connect a central control and seismic recording station to dependent stations: local intermediate control and concentration stations, local acquisition units, and to transmit a synchronization signal (TB) thereto. The time of transit of the data on the transmission channels between the central station and the acquisition units being measured beforehand, acquisition of the seismic data by the acquisition units is preinitiated, the retention time interval in relation to the time of reception of the synchronization signal being fixed and greater than the transit time. An adjusted compensation delay is applied to the synchronization signal in the intermediate station in order to take into account the fluctuations of the effective transfer time linked with its passage through multitask control means of the intermediate stations and the transmission network so as to respect the retention time interval. All the acquisition units can thus be perfectly synchronized.  
           [0011]    This method is quite suitable when the time of transit via the transmission channel is constant and can be measured beforehand, which is the case with a standard communication network comprising material links (low-rate wired Ethernet network for example).  
           [0012]    On the other hand, in any other case where the time of transit on the available channel of the standard network (radio link for example) is likely to fluctuate within a time interval incompatible with the precision required for transmission of a synchronization signal such as a TB, the compensation means of the prior system are inadequate.  
         SUMMARY OF THE INVENTION  
         [0013]    The transmission method according to the invention allows to use a standard communication network for transferring synchronization data between a central control and seismic recording station and dependent stations (local seismic data acquisition, processing and transmission units depending on the central station either directly or via one or more intermediate local control and concentration stations, itself connected to local acquisition units), in all the cases where the transmission time on the connection channels between the elements may be liable to too great fluctuations.  
           [0014]    In its principle, the solution retained essentially consists in providing the communicating stations with a common time reference that can be delivered for example by signals picked up by receivers and transmitted by a satellite positioning network of a well-known type such as the GPS system or the carrier frequency of a Hertzian emitter, and in using this common reference to calculate the real transmission time and to locally readjust the seismic acquisition time break.  
           [0015]    The transmission method according to the invention allows to transfer, via a standard communication network (LAN), a synchronization signal indicating a time break (TB) between a central control and seismic recording station (CCU) and seismic signal acquisition and storage units (RTU) distributed in the field, when the time of transit of the synchronization signal through at least part of the connection channels of the standard network between the central station and at least one dependent station (which can be an intermediate station (RRS), itself connected by cables or fibers to acquisition units in the field (RTU) or possibly each acquisition unit in configurations where they directly controlled by the central station) may be liable to fluctuations within a determined fluctuation margin.  
           [0016]    The method comprises:  
           [0017]    a learning stage including formation of a specific frame and storage thereof by each station, and detection in each said dependent station of signals delivered by a clock exterior to the device,  
           [0018]    pre-initiation of the acquisition of seismic data by acquisition units (RTU) with a retention time interval in relation to the time of reception of synchronization signal (TB) which is fixed and greater than the maximum transit time, considering the fluctuation margin,  
           [0019]    first precise dating of time break (TB) from the external clock and transmission of the data obtained with this first dating in form of the specific frame to the dependent stations,  
           [0020]    detection of the time of arrival, in each dependent station, of the specific frame and second dating of this time of arrival from the external clock, and  
           [0021]    measurement of the effective transit time equal to the time interval between the data associated with the first dating and those associated with the second dating, and consequently readjustment of the time break to the seismic data stored in acquisition units (RTU).  
           [0022]    The pre-initiation time is for example the time of transmission to a seismic source (S) of a fire order.  
           [0023]    In the case where the (or each) dependent station is a local station (RRS) connected to at least one acquisition unit (RTU) by a material link with a fixed transit time, and to central station (CCU) by a fluctuating transit time link, a readjustment to the seismic data stored in acquisition units (RTU) is applied by taking into account the fixed transit time.  
           [0024]    The method comprises for example:  
           [0025]    direct detection, in central station (CCU), of synchronization signal (TB),  
           [0026]    formation, from the synchronization signal detected, of the specific frame and direct application thereof to an interface module (TCI) of central station (CCU),  
           [0027]    detection, in an interface module (TCI) of each dependent station, of a specific frame of synchronization signal (TB), and  
           [0028]    application, to synchronization signal (TB) of the dependent station, of a time lag equal to the measured transit time.  
           [0029]    The method is particularly flexible since it allows, by means of an adjustable transmission delay time compensation, to easily take into account the various data transfer rates of the transmission channels available in a complex transmission system as used for example in modern seismic prospecting devices.  
           [0030]    The transmission system according to the invention allows to transfer, by means of a standard communication network (LAN), a synchronization signal indicating a time break (TB) between a central control and seismic recording station (CCU) and seismic data acquisition and storage units (RTU) distributed in the field, when the time of transit of the synchronization signal through at least part of the connection channels of the standard network between the central station and at least one dependent station may be liable to fluctuations within a determined fluctuation margin. The system comprises:  
           [0031]    means for forming a specific frame, storage means allowing each element of the seismic device to store this specific frame, and a local clock (RXGPS) controlled by synchronization signals provided by said external clock (H) in order to generate a dating time scale, this local clock being associated with the elements of the seismic device,  
           [0032]    means for pre-initiating, in acquisition units (RTU), acquisition of the seismic data with a retention time interval in relation to the time of reception of synchronization signal (TB) that is fixed and greater than the maximum transit time, considering the fluctuation margin,  
           [0033]    counting means associated with each local clock so as to perform a first precise dating of time break (TB) in accordance with the external clock and transmission means for transmitting the data obtained with this first dating in form of the specific frame to the dependent stations,  
           [0034]    means for detecting the time of arrival, at each dependent station, of the specific frame and counting means associated with the local clock for performing a second dating of the time of arrival in accordance with external clock (H), and  
           [0035]    counting means for determining the effective transit time equal to the time interval between the data associated with the first dating and those associated with the second dating, and means for accordingly readjusting the time break to the seismic data stored in acquisition units (RTU).  
           [0036]    The system comprises for example an interface set (TBG/I) in central station (CCU) for directly generating on a transmission channel, to each dependent station, a frame carrying the data from the first dating, an interface set (TBD/I) for decoding said specific frame, counting means (D-CPT) for performing the second dating in accordance with external clock (H) and for determining the effective transit time of the synchronization signals.  
           [0037]    In the case where the (or each) terminal station is a local station (RRS), it comprises a generator (SYNCTB-G) for generating a signal (SYNCTB) synchronous with the delayed signal to acquisition units (RTU).  
           [0038]    The common external clock consists for example of synchronization signals emitted by a satellite positioning system, each element of the seismic device comprising a suitable detection module.  
           [0039]    The common external clock can also consist for example of synchronization signals emitted by a Hertzian transmitter, each element of the seismic device comprising a suitable detection module.  
           [0040]    This combination of a fixed retention time applied by all the acquisition units and of intermediate delay adjustment means suited to complete the propagation times measured on the transmission channels by reference to an external clock allows all the acquisition units to be adjusted together to the same time break.  
           [0041]    Whether the time of transmission by the network is known with precision or not, it is possible to combine such a network with conventional dedicated transmission channels using optical fibers, radio or cable links, while maintaining a perfect synchronism between these various channels.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0042]    Other features and advantages of the method and of the system according to the invention will be clear from reading the description hereafter of a non limitative example, with reference to the accompanying drawings wherein:  
         [0043]    [0043]FIG. 1 diagrammatically shows a seismic device positioned in the field with various transmission channel possibilities between the elements,  
         [0044]    [0044]FIG. 2 diagrammatically shows a local concentration station RRS,  
         [0045]    [0045]FIG. 3 diagrammatically shows general central station CCU,  
         [0046]    [0046]FIG. 4 diagrammatically shows a concentration module DCU inside a local station RRS,  
         [0047]    [0047]FIG. 5 is a block diagram showing the progress, in central station CCU, of the operations of emission of synchronization signal TB,  
         [0048]    [0048]FIG. 6 is a block diagram showing the progress, in a local station RRS, of the operations of reception and delay adjustment of synchronization signal TB,  
         [0049]    [0049]FIG. 7 diagrammatically shows an interface circuit TBI adjustable, as the case may be, to the detection or the generation of a frame characterizing the synchronization signal, and  
         [0050]    [0050]FIG. 8 is a general block diagram of the software management of the functions fulfilled by a local station.  
     
    
     DETAILED DESCRIPTION  
       [0051]    The method and the system according to the invention will be described within the particular scope of an application to a seismic prospecting device already described notably in patents FR-2,692,384, FR-2,696,239 and FR-2,720,518 already mentioned above.  
       I) Seismic Device  
       [0052]    The seismic device comprises (FIG. 1) an often considerable series (several hundreds to several thousands) of seismic receivers R producing each a &lt;&lt;seismic trace&gt;&gt;in response to the transmission in the ground of seismic waves produced by a source S coupled with the ground and reflected by discontinuities of the subsoil. Receivers R are subdivided into n groups GR 1 , GR 2 , . . . , GRn, comprising each a certain number q of receivers R. Local acquisition and transmission units RTU referenced BA 11 , . . . , BApk, . . . , BApn, similar for example to those described in patent FR-2,720,518 mentioned above, are placed in the field, each one for digitizing and temporarily storing the seismic data collected by one or more receivers R of each group. Any group GRk of order k for example comprises a certain number q of receivers respectively connected to p local acquisition units RTU referenced BA 1 k, BApk for example. Numbers p and q can be different if at least part of the local units RTU (BAp 2  for example in FIG. 1) is intended to collect the seismic data coming from more than one seismic receiver R. The seismic device can thus comprise for example several hundred local units RTU.  
         [0053]    The various groups of acquisition units RTU are respectively controlled by local control and concentration stations RRS referenced RRS 1 , RRS 2 , . . . , RRSi, . . . , RRSk, . . . , RRSn. These local stations are equipped to fulfil extensive functions:  
         [0054]    control the acquisition units RTU of their respective groups,  
         [0055]    collect the seismic data (seismic traces) of the various units RTU, store them in a mass memory (one or more disks for example),  
         [0056]    transfer on request to central station CCU at least part of the data stored: either parameters indicative of the proper operation of the acquisition units or of the local stations (RRS), or at least a part, possibly compressed, of each seismic trace, in order to carry out a quality control,  
         [0057]    manage test and initialization operations on the connection means (Hertzian channels or lines) and on the field equipment (seismic receivers, electronic devices in each acquisition unit RTU), check the progress of seismic operations, and inform on request of the results and possible anomalies.  
         [0058]    The various concentrators RRS, like source S, are controlled by a central control and recording station CCU where all the seismic data are finally grouped together. Local stations RRS communicate with central station CCU via a local network LAN of a well-known type (Ethernet™ for example) comprising material links (cable or optical fiber L) or radio links RF 1 , RF 2 , . . . RF n , this network working according to a specific transmission protocol.  
       II) Local Control and Concentration Stations (RRS)  
       [0059]    Each local station RRS comprises (FIG. 2) a central unit CPU with an extended RAM memory (32 Mo for example), communicating by means of an internal bus BUS with a mass memory MM of several Go, and a concentration unit DCU (see FIG. 4) for managing the communications of each local station RRS with local acquisition units RTU, either by means of Hertzian channels F kj  and/or by transmission cables or lines Ci.  
         [0060]    An interface set NCI is also connected to internal bus BUS. Network LAN comprising one or more lines L and/or radio transmission channels RFB, used for communication with central unit CCU, is connected to interface set NCI by means of a channel switch SW. An interface TBI intended for detection, on network LAN, of the signals indicative of the time break when seismic source S is triggered, is connected to interface set NCI on the one hand and to internal bus BUS on the other. The functionalities of elements NCI and TBI are described below in connection with FIGS. 5 and 6.  
       III) Central Control Station CCU  
       [0061]    Central station CCU also comprises a central-unit CPU provided with an extended RAM memory communicating by means of an internal bus BUS with a mass memory MM of high capacity, sufficient for storage of the seismic traces transmitted by the acquisition units via control and concentration units RRS. It also comprises a local dialogue terminal UI for the operator; a printer PR allowing high-definition printing of seismic sections, maps, etc., is connected to internal bus BUS by an interface card DI. A high-capacity mass memory DB for storing a database consisting of seismic data, geographic data, etc., is also connected to internal bus BUS. An image scanner ISC used to enter into the database of memory DB possibly a geographic map of the zone where seismic operations are carried out is also connected to this bus by means of an interface element. Such an image of the zone of operations can be used to match the points of installation of the field pickups with precise geographic coordinates.  
         [0062]    Seismic source S is controlled by a control box SC forming, when triggered, an indicative signal TB that is applied to central unit CCU by means of an interface card SCI.  
         [0063]    An interface set NCI specifically suited to control network LAN is also connected to internal bus BUS. Local network LAN (lines L and/or radio transmission channels Rfi) used for connection with each local station RRS is connected to interface set NCI by means of a channel switch SW. An interface circuit TBI is interposed in parallel between interface elements NCI and SCI. The functionalities of elements NCI and TBI are described below in connection with FIGS. 5, 6 and  7 .  
         [0064]    Concentration module DCU of each local station RRS (FIG. 4) is intended to relay the transmission of the commands of control unit CPU to local stations RRS and, conversely, the reception of the seismic data. It comprises two electronic cards. A first card carries a set CiV whose functions will be described in connection with FIG. 5, a synchronous line detection circuit LSD that communicates through input/output ports P with one or more transmission lines Ci for communication with acquisition units RTU in the field (FIG. 1). A second card carries memory modules FM with DMA, accessible by means of an internal bus DMAB. Interface circuit FOI allows exchanges between exchange bus DMAB and central unit CPU (FIG. 2), by means of internal bus BUS. The first three cards CPU, LSD, FM communicate by means of an internal bus PB.  
         [0065]    Each concentration unit DCU comprises a radio emission-reception unit CRTU similar to the unit described in patent FR-2,720,518 mentioned above, which is suited, when this mode of connection is established, to establish communications by Hertzian channel with local units RTU. This unit CRTU comprises (FIG. 4) an interface circuit RI connected to exchange bus DMAB, to internal bus BUS and to a radio transmitter Tx emitting for example in the TFM (Tamed Frequency Modulation) mode well-known to specialists, and a synchronous Hertzian modulation detection circuit RSD connected to a radio receiver Rx.  
       IV) Programming Activities by Tasks  
       [0066]    As also described in the aforementioned patent FR-2,720,518, the various activities allowing proper progress of the processes are divided into catalogued tasks, each dedicated to a specific process and each in the form of programmes integrated in the computers in central station CCU  1 , in local stations RRS and local units RTU.  
         [0067]    The tasks can be carried out sequentially or concurrently via switches. A real-time distribution programme manages the start or the interruption of tasks by taking into account their respective priority degrees or their resumption when they have been interrupted a) when all the required data were no available at a time of their execution, or b) upon reception of an interrupt message from another task, or c) as a result of an exterior event.  
         [0068]    Definition of a task requires taking into account its function, all the data required for its execution, the required control programs (drivers), all the interruptions imposed by the task and the pre-established priority degree thereof.  
         [0069]    The tasks can have access to a database consisting of parameters entered by the operator, of the acquired seismic data and of the seismic system control parameters.  
         [0070]    Interdependence relations of the various tasks fulfilled by each local control and concentration station RRS for example are shown in the diagram of FIG. 8.  
       V) Transmission of Synchronization Signal TB by Local Network LAN  
       [0071]    Repetitive transmission, without delay, to the dependent stations (local intermediate stations RRS or directly to acquisition units RTU in the case where they are directly connected to central station CCU), of signal TB indicative of the precise time of triggering of source S (time of firing) is performed by respecting the following procedures:  
         [0072]    No traffic takes place on all of network LAN when firing is initiated.  
         [0073]    If the effective time of transit of the signals on each transmission channel of network LAN between central station CCU and the dependent stations is constant and reproducible: cable link, optical fiber link, etc., it is measured with precision once and for all when connecting each dependent station to network LAN, as described in the aforementioned patent application FR-99/12,113.  
         [0074]    If the effective transit time of these signals is fluctuating but below a known limit value: case of wireless connections for example, one uses clock signals provided by an external time reference accessible to all the elements of the seismic device (dating system), accessible where the seismic operations are carried out, as described hereafter.  
         [0075]    These signals can be, for example, clock signals emitted at regular intervals (every second for example) by a positioning system such as the GPS (Global Positioning System) system or synchronization signals emitted by a radio station and adjusted to an atomic clock, these synchronization signals being picked up by specialized receivers associated with the elements of the seismic device.  
         [0076]    Emission of signal TB of local stations RRS on the cables connecting them to acquisition units RTU in the configuration shown in FIG. 1 is retarded by a known fixed delay. This delay, which takes into account all the transit times in the transmission system and the allowable fluctuation margins, is known by acquisition units RTU.  
         [0077]    When signal TB is received by acquisition boxes RTU, acquisition of the seismic signal has already started before with a starting presignal (pre-TB). As already described in patent FR-2,666,946, the acquired seismic samples are stored in a buffer memory of acquisition units RTU whose size is sufficient to contain all the samples acquired during a time interval between the presignal and the first sample to be held, acquired after the reference time break.  
         [0078]    This time interval is conventionally selected greater than the maximum time of propagation of the signals on the physical transmission channels used while remaining compatible with the size of the buffer memories. Since the real transmission time of the synchronization signal is likely to fluctuate when multitask microcomputers or various transmission means: optical fibers, Hertzian channels or cables, are used, the method will comprise, if necessary, applying intermediate adjustable delays (by using retarding counters) intended to complete the intangible delays (propagation times measured once and for all when the seismic device is installed in the field) or the delays measured by reference to a common time scale, so that all the acquisition units RTU are synchronous with the time break after this time interval.  
       V-1) In Central Station CCU  
       [0079]    Dating or timing of events is based on the recognition of a particular frame emitted, which requires a prior learning stage. Reference frame FRAME-TB is emitted by the central station and stored in set TBG/I of interface TBI. It is this frame that will afterwards be emitted upon reception of synchronization signal TB indicating the start of the seismic acquisition.  
         [0080]    The prior learning stage of the reference frame, whatever the form thereof in the specific transmission protocol, allows to adapt automatically to any possible change in the stantard network LAN used.  
         [0081]    The progress of the firing sequence operations in the station is as follows:  
         [0082]    Operator O initiates the seismic shooting (FIG. 5) and the corresponding signal F is sent to a task TB of central station CCU.  
         [0083]    A pre-initiation signal is transmitted through network LAN to local stations RRS which transmit it to acquisition units RTU. Upon reception of this signal, the acquisition units start acquiring the signals coming from the receivers in the field and store them in a buffer memory.  
         [0084]    Task TB emits a signal F that is relayed to source S through its control box SC, thus causing its initiation.  
         [0085]    This initiation time is signalled to central station CCU by a signal TB.  
         [0086]    Signal TB is sent to an interruption controller IT-C in interface module SCI, which leads to the interruption of the task in progress and an interruption signal IT-TB is taken into account nearly immediately. Task TB takes signal IT-TB into account in order to control the proper progress of the operations. Control box SC simultaneously produces a signal GEN-FRAME-TB. Interface circuit TBI comprises a set TBG/I connected to network LAN between network control interface NCI and switch SW and suited to generate a frame TB-FRAME therein (see FIG. 7).  
         [0087]    The receiver of the GPS positioning system producing signals at intervals of the order of one second, a dating or timing counter supplied by a 1-MHz internal clock for example, intended to provide an intermediate time scale (1 μs period), is associated therewith, which allows to date any event occurring in each interval. The emission of each GPS signal initializes the dating counter. The value displayed by counter TBDATE is stored upon reception of signal GEN-FRAME-TB.  
         [0088]    Upon reception of signal GEN-FRAME-TB, value TBDATE of the dating counter is inserted into reference frame FRAME-TB and transmitted without delay to the dependent stations: acquisition units or intermediate stations.  
       V-2) In local stations RRS  
       [0089]    Interface TBD/I (of the type described in FIG. 7) is connected to network LAN between switch SW and network control interface NCI to detect the specific frame of the TB. Set DCU in each local station RRS comprises, in set CiV (FIG. 4), a delay time counter D-CPT and a generator SYNCTB-G producing a frame signal synchronous with signal TB.  
         [0090]    Task RXTB (FIG. 6) initializes counter D-CPT with the predetermined fixed delay greater than the propagation time through all the connection channels used.  
         [0091]    The learning stage is similar to that described for the central station.  
         [0092]    From the reception of frame Pre-TB, frame detection interface TB monitors the traffic on the physical link in order to detect frame TB.  
         [0093]    As soon as frame TB is detected, interface TBD/I takes the value from dating counter TBDATE 1  in station RRS, extracts value TBDATE from the frame and subtracts it from TBDATE 1 . The value obtained is applied to counter D-CPT in order to adjust the emission delay of signal SYNCTB emitted towards acquisition units RTU. Signal FRAME-TB-REC then starts counter D-CPT.  
         [0094]    The end of counting of counter D-CPT causes the emission of a signal TX-SYNC to a circuit SYNCTB-G in the DCU, which generates a signal SYNCTB that is then effectively transmitted to acquisition boxes. When counter D-CPT is stopped, a signal IT-TB-EM is emitted towards task RXTB to indicate the end of processing of TB.  
         [0095]    The DCU also comprises a counter (not shown) allowing to delimit a time slot (signal T-LATE) after which, if no TB signal is detected, the waiting procedure in progress is cancelled.  
         [0096]    The organization of the tasks in each local station RRS is shown in FIG. 8. The various abbreviations used to designate the elements and the tasks respectively mean:  
         [0097]    DRVETH: local network driver;  
         [0098]    DRVCRT: console driver;  
         [0099]    TSKTB: TB task;  
         [0100]    TSKSEQ : sequencing task;  
         [0101]    TSKTRACE : seismic trace storage task in a mass memory;  
         [0102]    TSKFORM: field equipment installation control task;  
         [0103]    TSKTEST : test task;  
         [0104]    TSKREAD: data reading task;  
         [0105]    TSKCMD: command emission task;  
         [0106]    DRVTFM: radio reception driver;  
         [0107]    DRVHDB3: reception on lines driver;  
         [0108]    DRVCMD: command emission driver;  
         [0109]    DRSSTATUS: status driver, and  
         [0110]    DRVRADIO: radio link driver.  
         [0111]    Embodiments where standard network LAN is used for connecting the central station to the intermediate stations have been described. The same technique could of course be used without departing from the scope of the invention in cases where the dependent stations on network LAN are acquisition units RTU.  
         [0112]    It is also clear that the technique of measuring the delay with which synchronization signal TB is taken into account and the adjustable transmission delay compensation can be applied if a network referred to as owner network is used instead of a standard LAN type network.