Abstract:
A seismic signal reception device on land comprising a large number of sensors each associated with an adjusting means which may be remote-controlled from a central control and recording station, by means of seismic data acquisition boxes distributed in the field. It is possible to rapidly change at any time, from the central station, the number of sensors forming each seismic trace, the weighting coefficients of the various elementary signals constituting a trace and to control pre-processing of the signals picked-up before their transmission to the recording laboratory. By modeling the features of the reception device, interpretation of the recordings may be highly simplified.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a seismic exploration device whose configuration and/or features may be adapted or modelled according to particular conditions, and to a method for the implementing thereof. 
     2. Description of the Prior Art 
     Seismic exploration devices used for seismic prospecting generally comprise a reception device consisting of a very large number of seismic receivers which are placed in various locations of the explored zone for receiving the seismic waves reflected or refracted by the subsoil discontinuities, in response to vibrations transmitted into the ground by a seismic source. The receivers generally consist each of a combination of several elementary sensors (geophones, hydrophones) interconnected electrically and arranged in the explored zone so as to filter for example surface noises within the scope of seismic reflection prospecting runs. Each of these receivers provides a seismic “trace” which is an electric average of the signals produced by all the elementary sensors. The receivers may be distributed in the field or along a well, or arranged along a seismic streamer towed through the water by a ship. In modern reception devices, the groups of sensors are associated with signal acquisition boxes. Each box collects the signals delivered by one or several groups of sensors, to digitize, store and average several traces recorded successively. By order of a central control and recording station, each acquisition box transmits sequentially the stored traces to a recording device through one or several common communication channels (radio cables or channels). The lay-out of the various sensors of a single “seismic trace” has a considerable effect on the rejection of the surface noises. The sensors may be arranged at regular or unequal intervals in some cases to filter surface noises better without degrading useful signals. In onshore seismic prospecting, it is well-known to carry out “noise shootings” whose analysis allows the best relative arrangement of the sensors of a single trace to be defined. 
     These sensors may be associated with means for modifying the characteristics of the resultant signals, passive networks or local amplification modules to weight the sensors respective sensitivities or gains, active low-pass filters to attenuate undesirable frequency bands, etc. Various active or passive weighting processes are for example described in U.S. Pat. Nos. 2,698,927, 2,747,172, 3,400,783, 3,863,200, 3,863,201, etc. 
     An analog processor for modelling the frequency gain or spectrum of the various sensors allows a substantial attenuation of the noise level before the digitization of the useful signals in the acquisition boxes or in the central recording laboratory, and therefore make it possible to reserve the total dynamic range of the digitization circuit for processing these signals. 
     The major drawback of all analog pre-processing equipments lies in their rigid structure. The filtering parameters must be known in advance and, assuming that the reception device that is effectively set in the field allows these parameters to be changed, appropriate adaptations have to be carried out on the location of these local processors, which delays the recording operations. Moreover, the possible corrections which may be achieved before a “shooting” cannot be changed during recording. 
     Now, there are numerous cases, which will be shown in the description below, where trace adjustments and re-arrangements would be very useful adapting to the subsoil being explored and to obtain more readable seismic profiles. By extending the possibilities of existing recording laboratories, a solution could be considered to model a seismic reception device more easily. For example, if a 20- to 24-bit dynamic laboratory, capable of acquiring 500 to 4000 different channels (instead of the 12 to 15 bits currently obtained for the acquisition of 50 to 400 traces) was available, the analog filtering could be eliminated since it would be possible to acquire the signals of each of the elementary sensors distributed in the field with sufficient dynamic range. The optimum filtering of noise would be obtained from the recorded signals. However, the cost of such a laboratory with high dynamic range and a large number of traces, together with the cost of the processing of the large volume of data being obtained would be prohibitive. 
     SUMMARY OF THE INVENTION 
     The seismic exploration method according to the invention comprises installating, in a zone to be explored, an emission-reception device consisting of an array of receivers producing a set of seismic traces, a source of seismic signals and of at least one control and recording station. The invention is characterized in that each trace of at least part of the set of seismic traces is obtained through the combination of several seismic signals coming from several elementary sensors associated with control elements, and this combination may be modified at any time from the control and recording station by changing the combined elementary signals and/or by modifying the configuration of the elementary sensors going into the combination. 
     The method may comprise applying remote controls allowing for example: 
     differentiated control of the amplitude of the various signals constituting at least one of the traces, or 
     application of selective phase shifts to the signals received by the sensors constituting at least one of the traces, or 
     selection of the number of signals picked up constituting at least one of the traces, or 
     a selected combination of signals received by the sensors of at least one of said traces, such as convolutional or recursive filterings for example, to be obtained, or 
     inclusion of particular sensors in several adjacent seismic traces to be obtained. 
     The previous adjustments may be combined and also modified in time during the recording of a shooting for example. 
     Implementation of the method according to the invention allows the reception device to be configured at any time before each triggering of the seismic source or even during the reception of the seismic signals after the triggering of the source. The previous configuration, which are usual with reception systems of fixed configuration, are thus avoided or shortened. Various modifications or processings may be applied to the traces in real time during the recording sessions in the field without requiring a laboratory with extended processing capacity. 
     The seismic exploration device according to the invention comprises an array of seismic receivers producing a set of seismic traces, a source of seismic signals, and at least one control and recording station for collecting the seismic traces coming from the various receivers. It is characterized in that each receiver of at least one group of the array of receivers comprises a plurality of elementary seismic sensors associated with remotely controllable control elements, and means for combining the seismic signals resulting from the control elements, a remote control for remotely controlling the control elements so as to modify at will and at any time the configuration of the elementary sensors contituting each of the receivers and/or for modifying at least part of the elementary signals combined to produce each seismic trace coming from the group. 
     The device may comprise a assembly of acquisition apparatus distributed in the zone to be explored to collect each the signals of at least one seismic receiver, with at least one communication channel between the acquisition apparatus and the control and recording station for the transmission of control data and signals, and the remote control being connected to the remotely controllable control elements by means of the at least one communication channel and of conducting means arranged between them and the acquisition apparatus. 
     The control elements may for example comprise opto-electronic elements. 
     The conducting means reach for example the remotely controllable elements associated with arrays of sensors, the remote control means comprise means for coding to associate address words with control signals and the remotely controllable elements may also comprise address decoders. The conducting means comprise for example adjusting means. 
     The connections going from several sensors may also be split to associate each one with several adjacent traces. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other features and advantages of the invention will be clear from reading the description hereafter of embodiments given by way of non limitative example, with reference to the accompanying drawings in which: 
     FIG. 1 shows a seismic exploration device arranged in a zone to be explored, 
     FIG. 2 diagrammatically shows a first embodiment of the invention with an individual link between each adjusting means and the associated acquisition apparatus; 
     FIG. 3 diagrammatically shows a second embodiment of the invention with an addressable link between the adjusting means and the associated acquisition apparatus; 
     FIG. 4 diagrammatically shows a first embodiment of an adjusting element of opto-electronic type associated with a seismic sensor; and 
     FIG. 5 shows a variant of the previous embodiment with addressable adjusting elements. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference to FIG. 1 onshore seismic exploration system generally comprises a source of acoustic waves S and a seismic reception and acquisition apparatus comprising a large number of seismic receivers R 1 , R 2 , . . . , Rn which are distributed in the field, and a central control and recording station  1  installed in a vehicle  2  to control the triggering of the source S and to centralize the seismic data picked up by the various sensors R 1  to Rn. Each seismic receiver Ri is connected individually to the central station  1  or preferably to a box B 1 , B 2 , . . . , Bk containing a data acquisition apparatus for collecting the signals produced by one or several receivers and, by order of central station  1 , to transmit them thereto through a communication channel V such as a linking cable or a radio link. Various exploration devices utilizing acquisition apparatus communicating with a central station by radio and/or by cable are described in U.S. Pat. Nos. 4,583,206; 4,815,044 and 4,908,803 assigned to the same assignee. 
     In prior seismic reception devices, each seismic receiver generally comprises several elementary sensors C 1 , C 2 , . . . , Cp interconnected electrically so that the resulting signal or seismic trace is the average of several elementary signals having possibly predetermined weightings which are difficult to modify. 
     According to the embodiments of FIGS. 2 to  5 , the device in accordance with the invention comprises seismic traces that may be configured at will, for example, from the central station. 
     To that end, the elementary sensors C 1  to Cp of a single seismic receiver R 1  . . . Rn are associated (FIGS. 2,  3 ) with local processing circuits arranged to produce defined combinations of the signals produced by several elementary sensors, several examples of which will be given below. Several lay-outs are possible. Each elementary sensor C 1   i , C 2   i , C 3   i  . . . Cpi is associated with an adjusting element  3  for processing the individual signals picked up. The outputs of the various adjusting elements  3  are connected to inputs of a combining element  4 , such as a stacking device for example. The output of combination element  4  is connected to an input of an acquisition apparatus Ai in a local box B 1 , B 2 , etc, through a common line  5 . According to the embodiment of FIG. 2, each adjusting element  3  is controlled individually through a line lc 1 ,  1 c 2  . . . lcp connecting it to the associated acquisition apparatus Ai. According to the variant of FIG. 3, the inputs controlling the various adjusting elements  3  are all connected to a common line linked to acquisition apparatus Ai. In this case, the adjusting elements  3  are all addressable. The commands addressed to one of the interconnected elements are emitted on the common line  1 cc (FIG. 3) associated with a destination address, and each one of them is provided with an address decoder (not shown). 
     Each adjusting element  3  may comprise for example (FIGS. 4,  5 ) a resistive bridge  6  one branch of which comprises a variable resistance  7  consisting of a photoresistance associated with a photo-emitter connected to the local acquisition apparatus through particular conductors lci. The application, to the conductors lci, of a variable electric current allows the voltage delivered by sensor Ci to be varied and possibly cancelled for some applications. The photo-emitter of each adjusting element may also be controlled (FIG. 5) by means of an address decoder  8 . All the address decoders  8  associated with the sensors C 1   i -Cpi of a single receiver Ri are interconnected on a control line lc c  linked with the local acquisition apparatus. Application of a particular command intended for one of the adjusting elements  3  is transmitted by associating the address of the decoder  8  concerned therewith. 
     In the embodiments described, control of the adjusting elements  3  is achieved, for example, from the central. In order to act upon any adjusting element  3 , address words designating specifically the local acquisition box and the associated adjusting element are associated with the control. The message is transmitted through the transmission channel V used (cable or channel) to the acquisition apparatus. The message is directed towards the particular control conductors lci of the adjusting element  3  (FIG. 4) or towards the common control line lcc (FIG. 5) where it is intercepted by the local decoder  8  at the specified address. 
     The central control and recording station  1  principally comprises a recorder  16 , a control set  17  programmed to manage the seismic recording cycles and to work out the controls of the various adjusting elements  3  associated with the various receivers Ri in the field, and a communication set  18  for the communication channel linking the central station  1  to the various acquisition boxes. 
     Numerous examples of use of the invention will be given hereafter to show the large variety of seismic data processings which may be achieved from the central station by operators. 
     With the invention as described, many complex operations, well-known to geophysicists, may be carried out. 
     Within the scope of a multi-channel filtering, it is for example possible to: 
     select through all or nothing the sensors whose elementary signals are combined to form the various traces, 
     choose at will the particular attenuation coefficients to be applied to the signals of each sensor, 
     apply selected delays to the various signals before the combining thereof, 
     mark out at will each trace through particular gain and delay values, 
     vary the gain or attenuation values, as well as the delays applied during each recording time, a 
     subtract a noise model, etc. 
     Within the scope of a single-channel filtering, a frequency filtering known as convolutional or other more complex filterings, such as recursive filtering, may also be performed. 
     Thus, the filtering mode known as trace filtering may be optimized and applied in a particular way at any point of the space of co-ordinates (x,t) in order to better save the characteristics of the signals. 
     The method according to the invention allows development procedures to be highly simplified. In surface seismic prospecting, a plurality of noise “shootings” may be performed by selecting each time a single sensor per trace, so as to determine, by means of a seismic laboratory computer, the trace filtering parameters and then, with the values found for these filtering parameters, to achieve a development shooting. Noise shootings and development shootings may be performed with the same sensors. This is not the case usually, insofar as the sensors used for noise measurement and those used for development are generally not the same. The sensors must therefore be changed while trying to keep the previous positioning of the noise sensor. As it is well-known, this sensor change is the cause of mistakes in the noise parameters. 
     The simplification provided by the remotely controllable means for adjusting the weightings makes it however possible to perform a noise filtering without any previous noise shootings. In the case of vectorial sensors with several reception axes, allowing the polarization of the signals received to be determined, is available, and trace filtering may also be made more efficient by utilizing the polarization data in order to characterize better the useful signals and the noise to be suppressed. 
     The possibility, given to field operators, of configuring the receivers is also very useful in the field of well seismic prospecting where the guided waves or Stoneley waves, whose amplitude is much higher than that of the useful signals, have to be minimized. Once the characteristics of the noise to be suppressed has been measured, it is possible to obtain, through an action on the adjusting means, a very efficient filtering before recording, and thus to keep all the digitization dynamics for the useful signals. When 3D seismic prospecting operations are carried out with sensors distributed regularly over a surface and a source which is displaced successively in many points of the surface, it is well-known that the noise propagation directions vary with the displacement of the source. With the remotely controllable adjusting being used, the optimum filtering direction may be easily adjusted for each trace so as to take account of the changes in the noise propagation. For some applications, the traces are stretched by increasing the distance between the sensors so that adjacent traces may overlap slightly. In this case, one is led to position sensors substantially in the same place on the ground, each one of them being linked to a different trace. With the signal modelling means described, the signals of a single sensor may be combined with those of two different traces, on condition that connections allowing these signals to be transferred from one to the other are provided. It is possible, in this case, to decrease the total number of necessary sensors without any filtering efficiency loss. 
     The examples given above are in no way limitative, 
     Without departing from the scope of the invention, adjusting elements remote controllable from the central station and combined with a conventional reception device where the receivers are all connected directly to the control and recording station, and not by means of local acquisition apparatuses, may be used.