An apparatus for seismic surveying and a method of synchronizing systems for seismic surveying

Abstract: The synchronization of systems for seismic surveys, wherein the systems are composed of subsystems and may be land-based or marine, consists in the survey system being supplied with an absolute time standard and a time code based on the absolute time standard being generated in the survey system. There is further provided at least one programmable event generator in the survey system and this is synchronized with the absolute time standard. The time for a specific event is predetermined on the basis of given parameters and supplied with the time code which corresponds to this time. The time for a determined event is then supplied to the subsystems which are also provided with the absolute time standard. The event can thereby be executed in one or more of the subsystems on the basis of the supplied absolute time standard and synchronization is obtained with the desired accuracy. It is thus possible to synchronize shot times in a seismic signal generating system, and synchronize the shot recording time and sampling times in a seismic data recording system. It is also possible to determine the exact time of actually occurring events in a system for seismic surveys.

Description: The present invention relates to an apparatus for seismic surveying and a method of synchronizing systems for seismic surveying and of the systems' subsystems. The systems may be land-based or marine or combinations of these. The subsystems may comprise at least one navigation system, at least one seismic signal generating system and one or more recording systems. Such an apparatus is known from: NTC'83 IEEE 1983 NATIONAL TELESYSTEMS CONFERENCE, November 14-16, 1983, San Francisco, Ca, US; WARNKE ET AL: 'USE OF GPS FOR DETERMINING POSITIONS OF DRIFTING BOUYS', pages 209-213. When conducting seismic surveys, whether on land or at sea, it is vitally important to be able to determine the exact times of events and operations, and also to be able to establish the exact time relationship between the different operations. In seismic surveys, however, there are a number of potential sources of timing error and these errors will in turn affect the accuracy of the entire survey system. The use of modern computer technology in the processing of the seismic data has led to the need for an improvement of the accuracy in the time domain. It is a requirement that this timing accuracy should at least be substantially better than the accuracy and resolution which characterizes the software used, which normally is in the microsecond range. This will also make it possible to solve problems concerning the synchronization of the subsystems in a seismic survey system or the synchronization of several survey systems cooperating in a seismic survey. Synchronization in known seismic survey systems is based on the fact that the navigation system knows the time, vessel speed and position to an accuracy of approximately 100 ms in the time domain. The navigation system estimates the point in time when the ship arrives at the position for firing the next shot and, when this point in time is.reached, the navigation system generates an initiating pulse. This pulse is detected by a subsystem for the data recording which in turn generates a shot pulse to the source subsystem, which may be an airgun system. After a certain delay which corresponds to the source subsystem's response time, the shot is fired. At this point in time the source subsystem again generates a pulse which is detected by the recording subsystem, viz. the so-called "field-time-break" (FTB) pulse or time-mark signal. The recording subsystem then begins to sample the signals from the sensors, e.g., hydrophones. The sampling process is normally driven by a free-running oscillator. The signals are usually sampled every other microsecond in a period which normally lasts from 6 to 8 seconds. If more than one ship is used in a survey, the initiating pulse is transmitted via a radio modem line to the ship or ships participating in the survey. This radio modem normally uses an audio channel with the delays and inaccuracies inherent in this type of connection. If simultaneous surveys are conducted at widely scattered locations, a broadcast time signal is used to time-mark the data, i.e. the data are not initially synchronized, but marked in such a way that the lack of synchronization can be compensated for during the data processing. This known method therefore has a number of inherent weaknesses with regard to the synchronization of events. Operations and errors which occur at a point in time or in a subsystem will be transmitted through the system in the subsequent operations with the result that the system will be encumbered with errors in the time domain which may be difficult to quantify. A generally known technique has been to use standardised time code signals in order to synchronize processors in data recording systems. These time code signals may be based on known time code standards, such as IRIG (Inter-Range Instrumentation Group) A, B or G or NASA 36. There has, however, been no known method for satisfactorily implementing the use of time codes in seismic survey systems. Furthermore the time code has to be referred to an absolute time standard, e.g., Universal Time Coordinated (UTC). Such an absolute time standard can be obtained today via a satellite navigation system, such as the global positioning system (GPS) and, for example, be supplied to the navigation system on board a vessel. According to a first aspect of the present invention, there is provided a seismic surveying apparatus as claimed in appended Claim 1. According to a second aspect of the present invention, there is provided a method of synchronizing systems for seismic surveying as defined in appended Claim 15. Preferred embodiments of the invention are defined in the other appended claims. It is thus possible to provide a method of synchronizing systems for seismic surveying which eliminates the disadvantages associated with currently known methods for synchronizing in seismic survey systems. Such a method may distribute times for events to all parts of a data recording system, thereby avoiding the use of trigger pulses, while at the same time each subsystem is supplied with the time at the desired high level of accuracy. These advantages may be achieved by providing a survey system with an absolute time standard such as, for example, Universal Time Coordinated (UTC), generating via a time code generator in the survey system a time code based on the absolute time standard, providing at least one programmable event generator in the survey system, the event generator being connected with the time code generator, synchronizing the event generator with the absolute time standard, predetermining the time for a defined event on the basis of given parameters, specifying the time for a defined event, the time code corresponding to the point in time, supplying the time for the defined event to the relevant subsystems, similarly continuously providing the subsystems with the absolute time standard, the event thereby being executed in one or more relevant subsystems on the basis of the continuously supplied absolute time standard. The invention will further be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 illustrates schematically the principle of synchronization in a prior art seismic survey system; Figure 2 illustrates schematically synchronization in a seismic survey system constituting an embodiment of the present invention; Figure 3 illustrates synchronization between two systems for seismic surveys, wherein the present invention is implemented in a decentralized manner; Figure 4 illustrates synchronization between two systems for seismic surveys, with centralized and decentralized implementation of the present invention, respectively; Figure 5 illustrates synchronization between two systems for seismic surveys wherein one survey system is land-based and the other survey system is ship-based. Figure 1 illustrates synchronization in a known seismic survey system. All events are actuated by trigger pulses based on an initiating pulse from a navigation subsystem. The various subsystems such as the source subsystem and the recording subsystem are not supplied with the current or local time. Therefore the accuracy of the timing of the trigger pulses is dependent on synchronization with some kind of time standard and the initiating pulses. It is obvious that the transfer and triggering of the pulses in themselves can cause delays and inaccuracies which will be transmitted throughout the entire survey. An apparatus employing the method of the present invention is schematically illustrated in Figure 2. A receiver receives signals from a satellite navigation system, in this case the so-called Global Positioning System or GPS, which supplies a time standard to a navigation subsystem which forms part of the seismic survey system. This time standard can, e.g., be Universal Time Coordinated. This time standard is supplied to various subsystems such as the source subsystem and the recording subsystem, while at the same time these systems are supplied with shot times. For providing synchronization with another survey system, the time for events (for example, the shot time) is also provided to a radio modem for transfer to the other survey system. The synchronization is based on so-called deterministic event generation. This implies that an event will take place simultaneously, i.e. within a period of 10 µs in two or more survey systems. The time for the event or the local time (Time of Day, TOD) is predicted and broadcast from the main system, e.g. a survey vessel, a few seconds before the time when the event is to take place. The broadcast is in the form of a simple computer message. Each system involved in the execution of the event will generate the event internally based on the supplied local time (TOD) and its internal, extremely accurate clock, which, e.g., can indicate the time to an accuracy of 1 µs. In practice this means that a shot cycle is initiated by the navigation system in the main survey system, the navigation system being used to predict the correct firing position for the source, e.g. an airgun, approximately 2 s before firing takes place. The reference time t=0 or the so-called field-time-break (FTB) is broadcast as local time (TOD) to all survey systems participating in the survey, e.g. all survey vessels. The source generating system must have a signal prior to the field-time-break, viz. the firing signal, in order to be able to fire the individual sources or guns correctly in order to produce peak pressure at t=0. This is generated internally by loading secondary local time for this event to the register of a so-called event generator. The recording is initiated at t=0 in a similar manner. If the recording is performed during sampling, resynchronization of the sampling process can be performed, e.g., some time before t=0. The above-mentioned programmable event generator is a device whereby a software module can control the time for an event relative to an absolute time standard, e.g. UTC, with an accuracy and resolution which is far better than the time which is available to the actual software. Thus a programmable event generator can be programmed so that it generates pulses at specific points in time when these are defined, e.g. in UTC or Universal Time Co-ordinated with an accuracy of 1 µs. A programmable event generator is therefore essentially a sophisticated time control unit and can, for example, be implemented as a central unit which distributes start pulses or initiating pulses to the various subsystems in a seismic survey system, while at the same time keeping absolute time internally. However, it can also be designed as a decentralized system in which absolute time is distributed to time control units, i.e. event generators, in the various subsystems. In both cases the programmable event generator can be synchronized with a broadcast time signal such as GPS. The navigation subsystem in the survey system knows, for example, the time, vessel speed and vessel position to an accuracy of 100 µs. It estimates the time for the next shot some time in advance and distributes the shot time to the other'subsystems. Every subsystem knows its own parameters and calculates internal times for events. These times are loaded to the locally provided event generator which in turn generates the correct pulses at the pre-programmed times. If several vessels are involved in the seismic survey, the shot time is transmitted via radio to the other ship or ships, which are equipped with similar systems and can synchronize these in the same way. The source generating subsystem receives the time for the next shot and, on the basis of inherent knowledge of the delays in different guns, times are calculated for the firing pulses for the individual guns. These times are supplied to the event generator which generates the firing pulses at the correct times. The recording subsystem also receives the time for the next shot and calculates all sampling times for this shot. Accuracy and resolution in the time domain are now more than adequate. The seismic signals are monitored by receivers, such as hydrophones. The hydrophone signals are then sampled at exact times relative to an absolute time standard such as UTC during the entire acquisition period which may, e.g., be 6-8 s. There is therefore no need for a free-running oscillator. If there is more than one survey system, for example, if several ships are involved in the seismic survey, the same operations are performed on all ships. This guarantees that no significant difference occurs in the shot time and sampling times in the survey systems involved. If the survey is conducted at widely scattered locations, there is also the guarantee that the sampling will be synchronous in all the locations, since the absolute time standard used is, of course, the same and supplied in the same way. It should be noted that the required accuracy in time before the shot is fired is dependent on the speed of the vessel and approximately 100 µs is usually sufficient, while the required accuracy after the shot has been fired, i.e. during the data acquisition, is dependent on the sound velocity in the subsurface and thus an accuracy and resolution of 10 µs between the individual survey systems, e.g. survey vessels, is necessary. Figure 3 illustrates the synchronization of seismic survey systems constituting a decentralized implementation of the present invention. Two survey systems are used, which may be first and second vessels. Each survey system comprises a navigation subsystem which is connected to a GPS receiver. In the navigation subsystem there is provided a time code generator which generates a time code based on an absolute time standard which is received via the GPS receiver, and which, for example, can be UTC. The time code used can, for example, be NASA 36 or an IRIG time code such as IRIG-A, IRIG-B or IRIG-G. The three last-mentioned time codes operate with basic periods of 0.1 s, 1 s and 0.01 s. A programmable event generator is provided in each subsystem and each event generator receives the absolute time standard in the form of a time code from the time code generator. The subsystems communicate via a local area network (LAN) which is linked to the rest of the vessel's communication system. Communication between the individual survey systems takes place via a radio modem. Since synchronization in every survey system is based on an absolute time standard transmitted via GPS, it is only the time for the event which has to be transmitted via radio and synchronization takes place in the same way and with the same accuracy in all the survey systems. Figure 4 illustrates an example of a surveying system comprising two vessels. Vessel 2 has a decentralized implementation which was described in the systems in Figure 3, while vessel 1 is equipped with a centralized system. The event generators are now provided together with the time code generator which is synchronized with the absolute time standard. The time for the event is supplied from each of the event generators to the respectively provided subsystem. Once again, all subsystems are linked to a local area network (LAN) and communication between the survey systems takes place, as before, by radio. The only difference therefore is that the synchronization of the survey system indicated as vessel 1 takes place centrally and not in each of the subsystems. Figure 5 illustrates a system similar to that shown in Figure 4, for synchronizing a land-based survey system and a marine survey system. The land-based survey system corresponds to the implementation on vessel 1 in Figure 4, the navigation system being replaced by a positioning system. Centralized implementation may be preferable in land-based systems, where equipment and survey methods differ from those used in marine surveys. Moreover, in a centralized implementation, it is conceivable that all synchronization means such as the time code generator and the event generator could be integrated with the navigation system, which may also include the GPS receivers. As mentioned in the introduction, the method according to the invention is primarily concerned with the synchronization of shot times, shot recording times and sampling times in seismic data recording systems. However, the method can also be used for quality control in a system for seismic surveys. The invention can be used to test the accuracy in the time domain, the real times for events via the event generator and expressed in an absolute time standard being supplied to a central data processing system. A person skilled in the art will realize that in principle this represents almost a reversal of the actual synchronization method. A quality control of this kind, however, is extremely relevant in modern seismic data processing, where the use of three-dimensional seismic techniques places a completely different set of demands on accuracy and data processing capacity than was the case in two-dimensional seismic technology. The invention can be implemented in other ways than those described here and may also be used in data recording and measuring systems outside the seismic field. The invention is not limited to use with those time codes and time standards described here, nor does it require to be based on the Global Positioning System.

Claims: An apparatus for seismic surveying, comprising at least one navigation subsystem, at least one seismic signal generating subsystem and at least one recording subsystem, characterized by a time code generator for generating a time code in response to an absolute time standard, determining means for determining the time for events of the seismic surveying on the basis of given parameters, and at least one programmable event generator responsive to the time code generator and the determining means for enabling each event to be executed in a relevant subsystem on the basis of the absolute time standard. An apparatus as claimed in Claim 1, characterized in that the time code generator is arranged to receive the absolute time signal from a satellite navigation system. An apparatus as claimed in Claim 1 or 2, characterized in that the time code generator is arranged to generate an "Inter-Range Instrumentation Group" time code (IRIG time code). An apparatus as claimed in any one of the preceding claims, characterized in that the time code generator is connected with at least one programmable event generator in each subsystem. An apparatus as claimed in any one of the preceding claims, characterized in that the at least one programmable event generator is synchronized on the basis of Universal Time Coordinated (UTC), the synchronization taking place by means of the time code supplied by the time code generator. An apparatus as claimed in any one of Claims 1 to 4, characterized in that the at least one programmable event generator is synchronised on the basis of Universal Coordinated Time received from a satellite navigation system. An apparatus as claimed in any one of the preceding claims, characterized in that the time for at least one predetermined event is supplied to a programmable event generator in the relevant subsystem. An apparatus as claimed in Claim 7, characterized in that the programmable event generator in each subsystem is supplied with the time code for the absolute time standard. An apparatus as claimed in Claim 8, characterized in that the at least one event is initiated by the programmable event generator of the relevant subsystem on the basis of the predetermined time and in synchronization with the real time as indicated by the continuously supplied time code. An apparatus as claimed in any one of the preceding claims, characterized in that the parameter which is given for predetermining the time for the event is generated on the basis of continuous position finding in the navigation subsystem. An apparatus as claimed in any one of the preceding claims, characterized in that the signal generating subsystem controls an at least one shot time on the basis of the absolute time standard. An apparatus as claimed in any one of the preceding claims, characterized in that the recording subsystem controls the recording time on the basis of the absolute time standard. An apparatus as claimed in any one of the preceding claims, characterized in that the recording subsystem controls the sampling times of the seismic data on the basis of the absolute time standard. An apparatus as claimed in any one of the preceding claims, characterized in that the apparatus is arranged to determine the precise point of time in absolute time for an actually occurring event during a seismic survey. A method of synchronizing systems for seismic surveying, the systems comprising at least one navigation subsystem, at least one signal generating subsystem and at least one recording subsystem, characterized by the steps of generating from an absolute time standard via a time code generator a time code based on the absolute time standard, synchronizing at least one event generator with the absolute time standard, determining the time for at least one event of the seismic surveying on the basis of given parameters, and supplying the time for the at least one event to the at least one event generator such that the at least one event generator causes the at least one event to be executed in at least one of the subsystems on the basis of the absolute time standard.

Inventors: BRAGSTAD HELGE, HELGERUD PER, BRAGSTAD, HELGE, HELGERUD, PER
Applicants: GECO AS, GECO A.S.
Application year: 1993
Main classification: G01S1
IPC codes: G01V1, G01S1