Abstract:
A data transmission system, particularly for use in seismic data acquisition, transmits digital signals from remote units to a central control unit via a backbone network and root nodes. Data is transmitted wirelessly between the remote units and the root nodes by means of each remote unit having two transceivers, one of which acts as a client and one as an access point. The remote units transmit metrics which enable the adaptive formation of a mesh-like network.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates to wireless data transmission system and is particularly, but not exclusively, applicable to land seismic surveying systems. 
       BACKGROUND TO THE INVENTION 
       [0002]    In land seismic surveys, an array of geophones is used to detect reflections from subsurface earth formations of acoustic signals which are generated at, or near to, the surface of the earth. Geophysicists planning seismic surveys determine the positions where geophones are to be placed during a survey, normally on the earth&#39;s surface but commonly also in boreholes. These positions are known as stations, and one or more interconnected geophones may be placed at these stations. Such a collection of interconnected geophones is referred to as a geophone group, even if it consists of a single geophone. 
         [0003]    The output of a geophone group is an analogue signal which is required to be digitized by a high-precision 24-bit analogue-to-digital converter to facilitate the high fidelity recording of the signal. As the geophone groups are typically distributed over a wide geographical area, it has become a common technique to deploy digitizer units containing between one and eight analogue-to-digital converters across the survey area, and to interconnect these digitizing units using cable to create a data transport network to transfer the digitized geophone signals to the data recorder. 
         [0004]    Wireless systems have also come into use, as has the use of optical fibre cable to handle high data transfer rates. These developments, together with improved data processing, have allowed the use of larger seismic spreads and higher resolutions. 
         [0005]    In our U.S. Pat. No. 6,219,620 (=EP 0934538) there is described a seismic acquisition system in which the terrain is divided into cells, and digitizer units within each cell communicate with a cell controller by wireless techniques. The cell controllers then communicate with a central control unit by wireless or fibre optic cable. Such an arrangement greatly reduces the amount of work required to set up the seismic spread, and also allows a large amount of data to be processed virtually in real time. 
         [0006]    The present invention seeks to further enhance the deployment efficiency of the system by means of a self-configuring and self-adapting wireless data networking system. 
         [0007]    The invention provides a data transmission system for transmitting digital data between a multiplicity of remote stations and a central control unit. At the central control unit, the interface to the data transmission system is via one or more root nodes. Each of the remote stations comprises a first transceiver, a second transceiver and a control means; the first transceiver being operable as a wireless client, capable of communicating with an access point; and the second transceiver being operable as an access point to which the wireless client transceivers of other remote stations may connect. The controller within the remote station provides a means of routing data between the two transceivers. 
         [0008]    From another aspect the invention provides a seismic survey system comprising a data transmission system in accordance with the preceding paragraph and in which each of the remote stations is a remote acquisition unit connected to one or more geophones to form a geophone group. 
         [0009]    A further aspect of the invention provides a remote acquisition unit for use in seismic surveying comprising a remote station as defined above, input means for connection to one or more geophones, and means for storing and forwarding seismic data received from said geophone(s). 
         [0010]    Preferred features and advantages of the invention will be apparent from the claims and from the following description. 
     
    
     
       DESCRIPTION OF PREFERRED EMBODIMENT 
         [0011]    An embodiment of the invention will now be described, by way of example only, with reference to the drawings, in which: 
           [0012]      FIG. 1  is a schematic overview of a seismic surveying system; 
           [0013]      FIG. 2  is a block diagram illustrating one remote acquisition unit in the system of  FIG. 1 ; 
           [0014]      FIG. 3  is a flow chart of a process performed by the remote acquisition unit in establishing a communication route. 
           [0015]      FIG. 4  is a block diagram illustrating part of an example of a network embodying one aspect of the invention. 
       
    
    
       [0016]    Referring to  FIG. 1 , a seismic survey system comprises a number of remote acquisition units (RAUs)  10  distributed across a survey terrain. Each RAU  10  is connected to one or more geophones  11  forming a geophone group. It will be appreciated that  FIG. 1  is schematic and that in practice several thousand RAUs may be used. 
         [0017]    Seismic data from the geophones is ultimately transferred to a central control unit (CCU)  12 . In the present embodiment, data is transferred from each RAU  10  by a wireless system to be described to a root node  14 , and the root nodes  14  communicate with the CCU  12 . Each root node  14  takes the form of one or more wireless access points which are connected to the CCU  12  by a high speed data network  16  which will typically be fast Ethernet or Gigabit Ethernet which may use copper, fibre optic or wireless as transmission medium. 
         [0018]    Turning to  FIG. 2 , each RAU  10  has an input  20  for receiving geophone signals, an analog-to-digital converter  22  (not required if the geophone signals are digital), and a memory  24  for temporarily storing the digital signals. The RAU  10  also comprises two radio transceivers, namely a first transceiver  26  referred to herein as an “upstream” transceiver, and a second transceiver  28  referred to herein as a “downstream” transceiver, and a control circuit  27 . 
         [0019]    The upstream transceiver  26  operates as a wireless client while the downstream transceiver  28  operates as an access point, as will be described. Each of the root nodes  14  includes a wireless transceiver operating as an access point. 
         [0020]    Each of the downstream wireless transceivers  28  and root node  14  wireless transceivers operating as an access point may be configured to broadcast a beacon signal. This beacon signal contains a parameter indicating the logical distance of the node from the CCU. The root node  14  wireless transceiver shall have this parameter set to 0. 
         [0021]    When a seismic array is deployed, as in  FIG. 1 , on being powered up, a RAU  10  enables its upstream transceiver  26  and seeks to establish communication with a root node  14  by searching for a beacon signal with a logical distance parameter of 0. On detecting the beacon, the transceiver associates with the root node and is enabled as a wireless client of the network. The downstream transceiver  28  of the same RAU  10  is enabled as an access point using a different wireless frequency, and broadcasts a beacon with the logical distance parameter set to 1, identifying the RAU as a relay node. 
         [0022]    If the upstream transceiver  26  cannot establish communications with a root node  14 , it then searches for a beacon signal broadcast by a relay node. If multiple beacons are detected, the upstream transceiver  26  will preferentially connect to an access point broadcasting a beacon containing the lowest logical distance. If the lowest logical distance is detected from multiple beacons, preference is give to the one which is evaluated to have the best communications path based on a set of metrics, including, but not limited to, received signal strength, packet error rate and link data rate. On detecting an appropriate beacon, the transceiver associates with the access point transmitting the beacon and is enabled as a wireless client of the network. The downstream transceiver  28  of the same RAU  10  is then enabled as an access point using a different wireless frequency, and broadcasts a beacon with the logical distance parameter set to a value of 1 greater than that contained in the beacon detected by the upstream transceiver  26 . 
         [0023]    It will be appreciated that as the system is brought into use, the RAUs will adaptively form a network with optimum efficiency. It is preferred that the evaluation carried out by the RAUs is repeated at intervals during use of the system to take account of changes in signal propagation and environmental factors. 
         [0024]      FIG. 3  illustrates in flow chart form the process of evaluating and association carried out within an RAU. 
         [0025]      FIG. 4  shows a very small part of the network to illustrate connections which may be made. RAU  10 A is able to communicate directly with root node  14 . RAU  10 B cannot communicate directly with root node  14 , and establishes communication via RAU  10 A. RAU  10 C communicates via  10 A and  10 B. RAU  10 D can communicate with either of  10 B and  10 C and will select the route which is most efficient on the basis of the metrics received. This would most likely be via  10 B as requiring the fewest hops, but could be via  10 C if the channel from  10 D directly to  10 B is of poor quality. 
         [0026]    In the network formed in this way the operation of each RAU is analogous to that of an Ethernet switch on a copper Ethernet network. Each RAU has an associated IP address and the central control unit  12  maintains a routing table. Once the network has been established, the routing table is relatively static. 
         [0027]    The system is similar in topography to a wireless mesh network, but is significantly different in operation. In a conventional wireless mesh network there is a single transmitter/receiver in each unit. While one unit is transmitting other units on the same route are limited to receiving. The effect is that as the mesh grows there is increasing latency and the effective bandwidth is greatly reduced. In the present system, by using two transceivers per unit there is a small degree of latency and effectively zero (or very small) reduction in bandwidth as the system grows. 
         [0028]    US 2005/0143133 A1 describes a wireless communication system based on nodes. Each node contains two transceivers. This might appear at first sight to be similar to the present invention. However, in this prior art documents the two transceivers are provided for specific purposes, namely one for handling wireless communication between nodes and the other acting as a wireless LAN station for working with wireless devices outside the communication mesh, and thus would suffer as discussed above from increasing latency as the mesh grows. US 2005/0143133 A1 does not suggest one transceiver acting as a wireless client communicating with an access point and the other acting as an access point for other similar devices. The arrangement of the present invention effectively provides full duplex communication between RAUs or nodes. 
         [0029]    Although described with particular reference to land seismic surveying, the invention is equally applicable to other uses where large quantities of data must be collated from a large number of dispersed locations.