Patent Application
20110141850
Not applicable.
Not applicable.
1. Field of the Invention
The invention relates generally to the field of seismic signal acquisition and recording. More specifically, the invention relates to systems for signal recording synchronization and recorder position detection wherein the seismic recorders are autonomous.
2. Background Art
Geophysical data recording such as seismic signal recording includes the use of autonomous (“nodal”) signal recorders disposed at spaced apart positions above an area of the Earth's surface to be surveyed. A nodal signal recorder may be connected to one or more individual seismic sensors. Typically, a nodal recorder includes a clock or similar timing device and a data storage device. The clock or timing device enables indexing the signals that are recorded to a known time reference, such as the actuation time of a seismic energy source. When such surveys are conducted on the land surface, and there are few physical obstructions to detection of radio frequency signals by the nodal recorder, the nodal recorder may be configured to receive time reference signals from a global positioning system (GPS) satellite, and may receive such signals from a plurality of GPS satellites to determine the precise geodetic location of the nodal recorder. Such GPS timing and location techniques are known in the art. It is also known in the art to transmit timing reference signals from a master recording unit by a radio frequency communication system. The master recording unit may include a controller to actuate the seismic energy source, such that the nodal recorders are time synchronized to the actuation time of the source, even if not referenced to absolute time.
For conducting seismic surveys where obstructions to communication by radio frequency exist, for example in dense jungle, or in water covered areas, it is impracticable to synchronize nodal recording units or determine their geodetic locations using techniques known in the art. Direct cable connection could be used, but may be impracticable because of the terrain and/or the distances from the master recording unit to some of the nodal recorders.
There exists a need to synchronize nodal geophysical data recording devices and determine their geodetic positions without the need to use radio frequency communication or direct cable connection.
A synchronization system for a nodal geophysical data recorder according to one aspect of the invention includes an electromagnetic transmitter associated with a master recording unit. The master unit includes devices for determining time and geodetic position from an external reference. The transmitter includes a code generator to cause transmission of a time synchronization signal as a coded sequence. The transmitter is configured to induce an electromagnetic field in at least one of subsurface rock formations and a body of water. At least one nodal geophysical data recorder includes at least one geophysical data sensor. The at least one sensor has measurements therefrom stored in a data storage device associated with the recorder wherein the recorder includes a clock for time indexing the stored data measurements. The recorder includes a receiver for detecting and decoding the time synchronization signal in the coded sequence to synchronize the clock with the synchronization signal.
A method for geophysical surveying according to another aspect of the invention includes disposing at least one nodal data recorder proximate an area of the Earth's subsurface to be evaluated. The nodal recorder has associated therewith at least one geophysical sensor and a recorder. At selected times a geophysical energy source is actuated. Response of the at least one sensor in the respective nodal recorder is recorded. A time of recording of the sensor response is indexed. At selected times, a clock associated with the nodal recorder is synchronized by imparting a coded sequence electromagnetic field into the subsurface from a position proximate the surface and detecting the electromagnetic field at the nodal recorder. A time synchronization signal is obtained by cross correlating the detected electromagnetic field at the recorder with a reference copy of the coded sequence stored in each nodal recorder. The synchronization signal is used to synchronize the clock.
In one example, the coded sequence may be further encoded to include data. In one example a transmission time of the coded sequence is further encoded. Travel time of the coded sequence is determined by detecting the coded sequence time index and determining a difference therebetween and a time determined at the nodal recorder. Travel times may be used from transmitters located at a plurality of positions to determine the geodetic position of each nodal recorder.
Other aspects and advantages of the invention will be apparent from the following description and the appended claims.
An example geophysical data recording system using nodal data recorders is shown schematically in
A survey vessel 12 moves along the surface of the water 11. The survey vessel 12 includes equipment shown generally at 14 and for convenience referred to as a “master recording unit.” The master recording unit 14 may include components such as the following (none shown separately in
A plurality of nodal signal recorders 10 may be disposed on the water bottom 13 at spaced apart positions. The nodal recorders 10, an example of which will be explained in more detail with reference to
Each nodal signal recorder 10 may also have associated therewith an electromagnetic receiver antenna 22, for example, a pair of spaced apart electrodes as shown in
During some types of geophysical surveying, the energy source 16 is actuated at selected times, and the signals detected by the sensors 20 are recorded by the respective nodal signal recorders 10. The recordings are typically indexed to the actuation time of the energy source 16, so that travel time of the detected energy (e.g., seismic energy) may be used to infer certain properties of the subsurface formations 15 such as the structure thereof.
In order to ensure accurate time indexing, it is desirable to synchronize the nodal signal recorders 10 with the master recording unit 14 or other time reference. In the present invention, signals transmitted by the electromagnetic transmitter 18 may be used to transmit clock synchronizing (time reference) signals, and other information such as geodetic position of the transmitter antenna 18 and/or the absolute time of the transmission of the synchronizing signals, so that the geodetic position of each nodal signal recorder 10 may be determined.
An example of one of the nodal signal recorders 10 is shown in more detail in
The electromagnetic receiver antenna 22 (electrodes) is shown coupled to the input of a second preamplifier 24, the signals from which may be digitized in a second ADC 26. In the present example, the CPU 30 may store a version or copy of the coded sequence to be applied to the electromagnetic transmitter antenna (18 in
By switching the electromagnetic transmitter (18 in
The further encoding may convey information such as precise moments in time through encoded “time marks”, and other information encoded in such a fashion as to enhance signal to noise level and the retrieval of the encoded data in poor signal to noise ratio environments.
Transmission of an encoded sequence at a known moment in time enables the nodal recorder (10 in
The foregoing nodal recorder position determination may also be performed by deploying electromagnetic transmitters on vessels or vehicles that travel through the area where the nodal recorders are located. The transmitters each transmit a signal encoded with the precise time and location at the moment of the transmission. In some examples, local environment information such as water depth at the vessel location may be encoded in the transmitted electromagnetic signal. It will be appreciated that the vessel shown in
The transmitter, or each electromagnetic transmitter if a plurality of transmitters is used, may obtain information concerning its geodetic position and absolute time from external reference signals received, for example from global positioning system (GPS) satellites or other navigational radio aids deployed in the area.
Transmission encoding techniques may be used such as a scheme of predetermined precise transmission times and codes such that each transmitter is excited at a predetermined time, and each nodal recorder will have stored therein such times. Certain components of each nodal recorder can be switched on within a selected time range of the predetermined times and detect each such electromagnetic signal. At other times, the same components of the nodal recorder, for example the preamplifiers and ADCs may be, switched off to preserve battery life. Another purpose for detecting electromagnetic signal only within a selected time range of the predetermined times is to enable better results from stacking signal from a plurality of detection intervals under conditions of low signal to noise ratio. By limiting the detection time to specific time ranges about the predetermined times, it is more likely that signal to noise ratio will be increased by stacking a plurality of detected signals.
As explained above, and referring once again to
When required, the foregoing processes may used either together or individually A tradeoff is that repeating transmission of the coded sequence more frequently and summing the results in the mass data storage increases the capability to detect smaller and smaller coded signals, however, the length of time required to detect, decode and recognize the time synchronization signal and amount of data storage required increases correspondingly.
To obtain recognition of a synchronization time using repetitive transmissions, the transmissions of successive synchronization signals may be scheduled to occur at a series of predetermined times, with each transmission coded as “T” minus X milliseconds counting to the time mark “T” minus zero milliseconds. Each synchronization time coded sequence can also contain information (e.g., by polarity reversal as explained above) for the exact time at which “T” is predetermined to occur, encoded in each transmission. The occurrence of synchronization time T is compared to the predetermined time for each particular synchronization time to occur. Since it is expected that the detected synchronization time will be very near the predetermined time, or off by only one or more intervals, the CPU 30 may be configured, for example, so that when the synchronization time is determined to be within, for example, a half an interval the mark is considered to be valid. “Interval” in this context means the amount of time to transmit one bit of the encoding of the coded synchronization signal.
An example is to schedule to transmit the synchronization signal 10 times at 100 millisecond intervals beginning one second prior the desired sync 0 time “T−0.” Each transmission of the synchronization signal may be further encoded (explained above) to contain the information of “T minus XXX ms” followed by the unique synchronization code describing the moment of the “T” event. Each transmission “word” may be scheduled as T minus 900 ms, then T minus 800 ms, etc., to T minus 0 ms, with T being the moment of synchronization. The moment T may be represented by a unique digital code transmitted at a precise time, so the codes may be summed. For the codes to be summed they must occur at the expected moments in time within a small allowable drift in timing. Drift in time up to one half of one of the bits of the word (one half an interval) may be considered acceptable and the summation process will still work. Because the synchronization time code in this example includes a transmitted word of the predicted time of the occurrence, the CPU 30 compares the nodal recorder's expected time of occurrence (determined from the clock oscillator 32 value) to the time word in the detected synchronization signal, and if the timing between the two is within a selected tolerance, the next synchronization signal (T plus X milliseconds) will be used to adjust the clock oscillator 32. If the difference between the expected time of occurrence and the time word in the detected synchronization signal exceeds the selected tolerance, the detected time word will not be used to adjust the clock oscillator 32. For example, if the difference exceeds the selected tolerance, it may be that the synchronization signal became corrupted.
During survey conditions where electromagnetic signals have very good signal to noise ratios (e.g., on land), the foregoing stacking and further encoding techniques may not be used and a synchronization signal may be transmitted, for example with a single transmission pulse or encoded sequence transmitted at selectable time periods.
The clock oscillator 32, if of types known in the art, can be accurate enough such that between synchronization signal transmissions an accuracy of within 300 microseconds of absolute time may be maintained over a period of 12 hours. In such cases the electromagnetic synchronization technique described above may only need to occur as infrequently as once per several hours. The rate of synchronization may be determined by the amount of time error detected in each nodal data recorder 10 at each synchronization time detection. The rate of synchronization may vary from every few minutes to several hours between synchronization events depending on the accuracy of any individual clock oscillator 32. A record of the amount of required adjustment may be stored in the mass data storage unit 34 and reported as the nodal recorder 10 is retrieved and adjustments and or repairs may be made as is determined as necessary from the records.
It is within the scope of the present invention, as explained above, to further encode the coded synchronization time signal with other data. In one example, the other data may include the exact transmission time of the coded signal. Such time may be recovered at each nodal recording unit 10 during decoding. The recovered exact transmission time may be compared to the local time as measured by the CPU 30 from the clock oscillator 32. Differences between the exact transmission time and the local time at the nodal recorder 10 will represent the electromagnetic signal transmission time. In one example, the vessel 12 may be move to a plurality of locations, and the transmission time determination process repeated. Using transmission or travel times from signals transmitted from several different directions, a location of each nodal recorder 10 may be precisely determined. In other examples, the same function may be performed by operating a plurality of electromagnetic signal transmitters at known locations.
Using an electromagnetic synchronization system as described herein may enable more precise time indexing of geophysical data recordings made over substantial time periods without the need to physically access each data recorder. Such precise time indexing may be performed without the need for direct electrical and/or optical connection between a master recording unit and remote data recorders, and under conditions where transmission of a radio signal between the master recording unit and the remote recorders is impracticable.
While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims.
