Data processing: measuring – calibrating – or testing – Measurement system in a specific environment – Earth science
Reexamination Certificate
2001-04-27
2003-04-22
Lefkowitz, Edward (Department: 2862)
Data processing: measuring, calibrating, or testing
Measurement system in a specific environment
Earth science
C367S049000, C181S112000
Reexamination Certificate
active
06553316
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
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.
Description of the Prior Art
In the field of seismic prospecting signals received by seismic receivers distributed on the ground surface, which are reflected by 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, as soon as a transmission time interval is available. The transmission of stored data is either direct, or via intermediate stations or concentrators.
Seismic acquisition systems are for example described in U.S. Pat. Nos. 4,583,206, 4,628,494, 5,245,647, 5,550,787, 5,706,250, 5,563,847 and 5,822,273.
It is important that all the acquisition units distributed on the explored site 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, collection of centralized data can be very imprecise.
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.
U.S. Pat. No. 4,628,494, filed by the assignee describes, 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 &Dgr;t 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 sends 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-&Dgr;t), i.e. the time break, are sought in the local memory.
French Patent Application 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 the seismic signal produced from any time break prior to the time break, based upon knowledge of 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 obtaining of a series of digitized samples re-staggered from the time break.
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 are, managed by software using multitask management with uncertainty about the effective propagation time in a seismic transmission system as mentioned above not exceeding about 50 &mgr;s.
Transmission without a random time lag is possible and easier to implement when one owns the network that is used is owned by the party performing the test and in control of the emitted signals a coding mode is maintained. 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.
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 have to be adapted to in order to transmit signals representative of time breaks with precision.
This is the case in the system described in French Patent Application 99/12,113 filed by assignee which uses a standard communication network of Ethernet type 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 is measured beforehand, acquisition of the seismic data by the acquisition units is pre-initiated; and the retention time interval in relation to the time of reception of the synchronization signal is fixed and greater than the transit time. An adjusted compensation delay is applied to the synchronization signal in the intermediate station in order to account for the fluctuations of the effective transfer time linked with its passage through a multitask control 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.
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).
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 schemes of the prior system are inadequate.
SUMMARY OF THE INVENTION
The transmission method according to the invention uses 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 the case when the transmission time on the connection channels between the elements is subject to great fluctuations.
The transmission system according to the invention has applications in many fields where distant stations have to be synchronized without necessarily using dedicated communication means.
In principle, the invention provides 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 radio transmitter, and in using this common reference to calculate the real transmission time and to locally readjust the seismic acquisition time break.
The transmission method according to the invention transfers, 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 de
Bary Renate
Guion Michel
Thierry Gerard
Institut Francais du Pe'trole
Lefkowitz Edward
Taylor Victor J.
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