METHOD AND SYSTEM FOR ESTIMATING THE DRIFT OF A DATE BEAT OF SEISMIC DATA SAMPLES

Description

[0001] The present invention relates to a method and a system for estimating the drift over time of a physical operating parameter of a dating clock for seismic data samples.

[0002] The invention belongs to the field of seismic prospecting, whether marine or terrestrial, and applies in particular to the minimization of dating errors in the seismic data samples collected.

[0003] Every clock exhibits a certain time lag over time, which is expressed, for example, by an accumulated phase error, commonly called "drift." This drift is due to various factors, including the aging of the clock or the temperature of the environment in which it is located.

[0004] This drift affects both clocks of the type comprising a quartz oscillator (TCXO temperature-compensated oscillators, in English "Temperature Controlled Crystal Oscillators", or MCXO microcontroller-compensated oscillators, in English "Microcontroller Compensated Crystal Oscillators", or OCXO thermostat-controlled quartz oscillators, in English "Oven Controlled Crystal Oscillators") as well as atomic clocks (rubidium or caesium).

[0005] When the clock is used to date events such as receptions of seismic data samples, clock drift induces errors in the dating of these samples.

[0006] Current methods for correcting clock drift are generally based on the assumption that this drift is linear and therefore apply corrections that are also linear. This is the case, for example, in patent document US-A-9,417,359. Document EP 2,525,240 A2 discloses a method according to the preamble of claim 1.

[0007] This means that, if for example an accumulated clock phase error of 30 ms was measured after 30 days of operation, the drift is considered to be 1 ms per day and the correction is made accordingly.

[0008] However, this way of proceeding is not satisfactory because in practice we see that the clock drift is not linear.

[0009] The invention aims to remedy at least one of the aforementioned drawbacks of the prior art.

[0010] For this purpose, the present invention proposes a method for estimating the drift over time of a physical operating parameter of a dating clock for seismic data samples associated with a seismic data collection node, in particular at sea with a node adapted to remain on a seabed, in accordance with claim 1.

[0011] This results in a residual dating error by the clock that is lower than the residual error obtained when a linear correction is applied to the clock drift.

[0012] Furthermore, this method can be implemented either within the seismic data collection equipment itself or in a separate computer.

[0013] Furthermore, the clock drift can be estimated at any time following a seismic data collection campaign, or even during the campaign when it is particularly long.

[0014] Moreover, in case the clock is of the OCXO type, this method is robust with respect to the heating time of the clock oven.

[0015] Moreover, this process can be applied indifferently to any type of clock, including quartz clocks (including TCXO, MCXO and OCXO) and atomic clocks (including rubidium and cesium clocks).

[0016] The invention makes it possible to obtain a good quality estimate of the clock drift, which allows a relevant correction to be applied to the collected seismic data samples.

[0017] Particular embodiments are set forth in the dependent claims.

[0018] Still for the same purpose as that indicated above, the present invention also proposes a system for estimating the drift over time of a physical operating parameter of a dating clock for seismic data samples associated with a seismic data collection node, in accordance with claim 15.

[0019] The advantages and particular characteristics of the system being similar to those of the processes, they are not recalled here.

[0020] Other aspects and advantages of the invention will appear on reading the detailed description below of particular embodiments of the invention, given as non-limiting examples, with reference to the appended drawings, in which: there figure 1 is a flowchart generally illustrating steps in a method for estimating the time drift of a physical operating parameter of a seismic data sample dating clock; figure 2 is a flowchart illustrating steps in the process of the figure 1 in a first particular embodiment where the physical operating parameter is the instantaneous frequency of the clock; figure 3 is a flowchart illustrating steps in the process of the figure 1 in a particular embodiment not forming part of the invention, where the physical operating parameter is the phase of the clock; figure 4 is a flowchart illustrating steps in the process of the figure 1 in a particular embodiment not forming part of the invention, including a simulated mission; and the figure 5 is a graph representing the accumulated phase error as a function of time, in the case of a second-order correction and in the case of a third-order correction.

[0021] Throughout the following, a seismic data collection node is considered. These data are collected using various seismic sensors, including velocity sensors, accelerometers or even hydrophones and / or geophones. A given node may, for example, be associated with three geophones, or one hydrophone and three geophones, or one hydrophone and three accelerometers, any other combination of sensors being possible. In particular, although the data collection may be carried out on land or in a well, in a particular embodiment, we will focus on the collection of seismic data at sea, by a node which is in particular capable of being placed on the seabed for a seismic campaign before recovery.

[0022] A clock is associated with this node to date the seismic data samples received by the node.

[0023] Knowing that, for example during a seismic data collection campaign at sea, the node can remain at the bottom of the sea for several months, the timestamping of the seismic data samples is ensured by a high-end clock, presenting high stability.

[0024] This clock can be of the quartz oscillator type (for example TCXO, MCXO or OCXO) or of the atomic type (for example rubidium or cesium). It is defined by a physical operating parameter, which can be for example its phase relative to a reference clock, or its instantaneous frequency.

[0025] As explained in the introduction, over time and after an initial synchronization, the clock undergoes a drift which results after a predetermined duration in an accumulated phase error or an accumulated instantaneous frequency error.

[0026] There figure 1 illustrates the method of estimating this drift in accordance with the invention in its generality.

[0027] It includes a step 10 of measuring a physical quantity associated with the clock. This physical quantity is measured either at predetermined times or during predetermined periods of time.

[0028] This measurement step 10 is followed by a step 12 during which a predetermined non-linear variation law is applied to this physical quantity, in particular a polynomial law of order greater than or equal to 2. An estimate of the accumulated phase error of the clock is then obtained, this error being representative of the clock drift.

[0029] Two particular methods of implementing this estimation method are detailed below, depending on whether the physical quantity chosen is the instantaneous frequency and / or the phase of the clock.

[0030] In a first embodiment where the instantaneous frequency and the phase of the clock are used as physical quantities to be measured, the detail of the measurement step 10 is illustrated by the figure 2 .

[0031] This measurement step is carried out in two phases: during a first phase, which takes place before deployment of the node for a seismic data collection mission, the initial instantaneous frequency fi of the clock is measured during a step 100 and an internal time information signal of the node is synchronized in a step 102 with respect to a reference time information signal.

[0032] The internal time signal of the node is produced by the clock. It usually operates at a frequency of several MHz, for example 10 MHz, and through a clock frequency divider, the clock also provides a signal at another frequency, for example 1 Hz, which is used as the internal time signal or PPS (pulse per second).

[0033] This internal PPS allows a microcontroller to time-stamp the seismic data samples received by the seismic sensors and with the help of an analog-to-digital converter. The received seismic data is stored in a memory, which can for example be of the "flash" type.

[0034] In step 102, this internal PPS is aligned with a reference signal or external PPS, which may for example be provided by a global positioning system or GPS (in English “Global Positioning System”), knowing that when the node is on a seismic prospecting vessel, it is generally connected to a GPS receiver.

[0035] This synchronization or calibration operation can be carried out by sending a reset signal by the microcontroller to the clock.

[0036] After synchronization, there is no longer any phase difference between the internal and external PPS.

[0037] At the end of this first phase, the node is deployed and the seismic data collection mission takes place for a certain number of days, weeks or even months, with the clock running continuously during the mission.

[0038] In the example of seismic data collection at sea, because the GPS signal is electromagnetic, it does not pass through the water column. It therefore remains inaccessible to the clock. The GPS signal may also be inaccessible during seismic data collection on land.

[0039] Over time, the instantaneous clock frequency error induces an accumulated phase error between the internal PPS and the external PPS. Indeed, the electronics timestamp the seismic data using a signal that is supposed to have a constant frequency of 1 Hz in the example described here, but in practice, this is not the case, because the temperature instability and aging of quartz oscillators (TCXO, MCXO or OCXO) as well as atomic oscillators (rubidium or cesium) are not negligible.

[0040] The accumulated phase error can be measured when the external PPS is available, i.e., in the example of an offshore mission, when the node is brought back on board the seismic survey vessel and reconnected to the GPS receiver.

[0041] Thus, during a second phase, which takes place at the end of the seismic data collection mission, during a step 104, the final instantaneous frequency ff of the clock is measured, as well as the final phase shift between the internal PPS and the external PPS.

[0042] Alternatively, this second phase can take place at a predetermined time during the mission, in which case the instantaneous frequency of the clock at this time is measured as well as the phase shift at this time between the internal PPS and the external PPS.

[0043] In step 12 of applying the predetermined non-linear law of variation of the instantaneous frequency of the clock, the final error of the instantaneous frequency of the clock is estimated from the initial instantaneous frequencies fi and final ff and the final phase shift.

[0044] To do this, in accordance with the invention, we formulate the hypothesis that the instantaneous frequency of the clock varies according to a polynomial equation of order 2, from which it follows that the phase of the clock varies according to a polynomial equation of order 3; we could take a higher order of the polynomial.

[0045] Let ε(t) be the instantaneous frequency error of the clock as a function of time t. The second-order polynomial equation is written: ε t = ε i + α . t + β . t 2 Or ε i = ε (0) denotes the initial frequency error and α And β designate predetermined coefficients.

[0046] The accumulated phase error is given by: φ t = ∫ ε t dt

[0047] The accumulated phase error is therefore given by the following third-order polynomial equation: φ t = ε i . t + α . t 2 2 + β . t 3 3

[0048] Let T be the end time of the mission. The final instantaneous frequency error and the final phase error measured at time T are noted: ε T = ε f φ T = φ f

[0049] We can then calculate the coefficients α And β , given that we have two equations with two unknowns: β = φ f − T 2 ε f + ε i 1 3 − 1 2 . T 3 α = ε f − ε i − β . T 2 T

[0050] These coefficients can be calculated either in the node or in post-processing, in a separate calculator.

[0051] This embodiment, for which the time drift of the clock is described according to an order 3 equation, is only possible if the node has a means to precisely measure the instantaneous frequency of its clock.

[0052] Estimating the time drift of the clock according to a cubic equation allows to reduce the maximum dating error of seismic samples compared to a second-order or parabolic equation.

[0053] Indeed, using a second-order equation is based on the assumption that the frequency evolves linearly over time. However, the phenomenon of clock aging causes the frequency to vary according to a non-linear law.

[0054] Thus, as shown in the figure 5 , which represents the accumulated phase error (in seconds) as a function of time (in days), the residual dating error of the seismic samples has a bell shape when the applied correction is of order 2, with the maximum error in the middle of the mission (top curve on the figure 5 ).

[0055] This phenomenon is eliminated when using a cubic equation (bottom curve on the figure 5 ), which minimizes the maximum dating error.

[0056] Alternatively, instead of a frequency variation law based on a polynomial equation, one could use other forms of evolution, such as a logarithmic evolution for example.

[0057] Before node deployment, the instantaneous clock frequency error can be reduced by performing a preliminary step of adjusting the clock output frequency so as to reduce the instantaneous clock frequency error.

[0058] This step can be performed either at the time of clock manufacture or during clock maintenance. Commercial clocks have an input for this adjustment. This further minimizes the residual dating error of the clock.

[0059] To take into account temperature changes that may occur, particularly at the beginning or end of the mission, due to the deployment or repatriation of the node and associated electronics in cold water, for example in the case of a mission at sea, the estimation of the clock drift can be improved by introducing a temperature parameter. Indeed, taking temperature into account in the estimation of the clock drift improves the accuracy of this estimation.

[0060] To this end, before deploying the node for a seismic data collection mission, the evolution of the instantaneous frequency of the clock as a function of the clock temperature is measured. Then, during the mission, the temperature of the clock is measured.

[0061] We then introduce into the law of non-linear variation of the instantaneous frequency of the clock a parameter ε Temp representative of the instantaneous clock frequency error due to temperature change. The third-order polynomial equation giving the accumulated clock phase error then becomes: φ t = ε i + ε Temp . t + α . t 2 2 + β . t 3 3 with β = φ f − T 2 ε f + ε i 1 3 − 1 2 . T 3 α = ε f − ε i − β . T 2 − 2 . ε Temp T

[0062] There figure 3 illustrates the progress of the method according to the invention in an exemplary embodiment not forming part of the invention, where the physical operating parameter of the clock used is not the instantaneous frequency, but the phase of the clock.

[0063] In this embodiment, measurement step 10 takes place in two stages.

[0064] First of all, before deployment of the node for a seismic data collection mission, the phase of the clock is measured during a step 200 continuously for a predetermined period of time ΔT1 so as to know the evolution of the accumulated phase error during this period ΔT1.

[0065] Then at the end of the mission, the phase of the clock is measured again, during a step 202, continuously for a predetermined period of time ΔT2, so as to know the evolution of the accumulated phase error during this period ΔT2.

[0066] Then, during a step 204, the accumulated phase error during the mission is estimated, i.e. between the two periods ΔT1 and ΔT2, using an interpolation method such as, for example, the spline method, a spline being a function defined piecewise by polynomials. This interpolation method is known per se. This example is not limiting: another interpolation method could be used.

[0067] This embodiment is advantageous in that it does not require measuring the instantaneous frequency of the clock. It can therefore be implemented by even simpler electronics than in the first embodiment.

[0068] In another exemplary embodiment not forming part of the invention, illustrated by the figure 4 , upstream of the mission, the accumulated phase error is measured during a step 300 during a simulated seismic data collection mission, in production or maintenance, at different temperatures. This constitutes a form of calibration. The law described above is then applied to estimate the accumulated phase error during a real mission, during a step 302.

[0069] The system for estimating the clock time drift according to the invention may comprise an electronic or computer module, either embedded in the node or remoted in a separate calculator or computer or electronic circuit, capable of measuring the frequency and / or phase error of the clock at the start and end of the mission, which makes it possible to estimate the accumulated phase error according to a polynomial of order 3, whereas conventional equipment is only capable of measuring the phase drift and therefore of estimating the accumulated phase error according to a polynomial of order 1.

[0070] More specifically, the aforementioned module is configured to carry out the steps described above in connection with the figures 1 à 4 . When the module is embedded in the node, it may consist of a unit separate from the functional units already present in the node, such as the node microcontroller, or it may be functionally integrated with such units. For example, the node microcontroller is configured to further perform at least some of the steps described above in connection with the figures 1 à 4 .

Claims

1. Method for estimating the drift over time of a physical operating parameter of a clock for dating samples of seismic data, associated with a seismic data collection node, wherein: at least one variable associated with said clock is measured (10) at predetermined instants in time or for predetermined periods of time; and a predetermined non-linear law of variation of said variable, which depends on the values collected in said measurement step (10), is applied (12) to said variable so as to obtain an estimate of the drift over time of said physical parameter, said method being characterized in that, in said measurement step (10): before deploying said node for a seismic data collection mission, the initial instantaneous frequency of said clock is measured (100) and an internal time information signal of said node is synchronized (102) with respect to a reference time information signal; at a predetermined instant in time during said mission, the final instantaneous frequency of said clock and the phase offset between said internal time information signal of said node and said reference time information signal are measured (104); and, in said step (12) of applying said law, the instantaneous frequency error of said clock is estimated based on said initial instantaneous frequencies and at said predetermined instant in time and said phase offset, and in that, according to said predetermined law, said instantaneous frequency varies according to a polynomial equation of order greater than or equal to 2, such that the phase of said clock varies according to a polynomial equation of order greater than or equal to 3.

2. Method according to Claim 1, characterized in that, in a preliminary step, the output frequency of said clock is adjusted so as to reduce the instantaneous frequency error of said clock.

3. Method according to Claim 2, characterized in that said preliminary step is performed during a phase of manufacturing said clock.

4. Method according to Claim 2, characterized in that said preliminary step is performed during a phase of maintaining said clock.

5. Method according to any one of the preceding claims, characterized in that said reference time information signal is provided by a GPS location system.

6. Method according to any one of the preceding claims, <b>characterized in that: before deploying said node for a seismic data collection mission, the evolution of said variable is furthermore measured as a function of the temperature of said clock; during said mission, the temperature of said clock is measured; and said predetermined law takes into account said temperature.

7. Method according to any one of the preceding claims, characterized in that said predetermined instant in time during the mission corresponds to the end of the mission.

8. Method according to any one of the preceding claims, characterized in that, according to said predetermined law, said instantaneous frequency varies according to a 2nd-order polynomial equation, such that the phase of said clock varies according to a 3rd-order polynomial equation.

9. Method according to any one of the preceding claims, characterized in that said node is designed for use on a seabed.

10. System for estimating the drift over time of a physical operating parameter of a clock for dating samples of seismic data, associated with a seismic data collection node, characterized in that it comprises a module designed to implement steps of a method according to any one of the preceding claims.