Method and system for estimating the drift of a datation clock of seismic data samples

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

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

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

Any clock exhibits a certain time lag over time, which is expressed for example by an accumulated phase error, commonly called a “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.

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 even quartz oscillators thermostatically controlled OCXO, in English "Oven Controlled Crystal Oscillators") than clocks of the atomic type (rubidium or cesium).

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

Current methods of correcting clock drift are generally based on the assumption that clock drift is linear and therefore also apply linear corrections. This is the case for example in the patent document US-A-9 417 359. This means that, if for example an accumulated clock phase error of 30 ms has been measured after 30 days of operation, we consider that the drift is 1 ms per day and we rectify accordingly.

However, this way of proceeding is not satisfactory because it is observed in practice that the drift of the clock is not linear.

The object of the invention is to remedy at least one of the aforementioned drawbacks of the prior art.

To this end, the present invention proposes a method for estimating the drift over time of a physical operating parameter of a clock for dating seismic data samples associated with a seismic data collection node, in particular by sea ​​with a knot adapted to stay on a seabed, remarkable in that:

at least one quantity associated with the clock is measured, at predetermined instants or during predetermined periods of time; and a non-linear predetermined law of variation of this quantity which depends on the values ​​collected during the measurement step is applied to this quantity, so as to obtain an estimate of the time drift of the physical parameter.

This results in a residual clock dating error that is less than the residual error obtained when applying a linear correction to the clock drift.

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

Furthermore, it is possible to estimate the drift of the clock at any time at the end of a seismic data collection campaign, or even during the campaign when the latter has a particularly long duration.

In addition, in the event that the clock is of the OCXO type, this method is robust with respect to the heating time of the clock oven.

In addition, this process can be applied equally to any type of clock, including quartz type clocks (including TCXO, MCXO and OCXO) and atomic type clocks (including rubidium and cesium clocks).

The invention makes it possible to obtain a good quality estimate of the drift of the clock, which makes it possible to apply a relevant correction to the samples of seismic data collected.

In a first particular embodiment, the method is remarkable in that during the measurement step:

before deployment of the node for a seismic data collection mission, the initial instantaneous frequency of the clock is measured and an internal time information signal of the node is synchronized with respect to a reference time information signal;

at a predetermined instant during the mission, the instantaneous frequency of the clock and the phase shift between the internal time information signal of the node and the reference time information signal are measured;

and during the step of applying the aforementioned law, the instantaneous frequency error of the clock is estimated on the basis of the initial instantaneous frequencies and at the aforementioned predetermined instant and of the phase shift.

The aforementioned instant can for example correspond to the last instant of the mission, that is to say at the end of the mission.

In this embodiment, according to a possible particular characteristic, according to the aforementioned predetermined law, the instantaneous frequency varies according to a polynomial equation of order 2, so that the phase of said clock varies according to a polynomial equation of order 3.

In this embodiment, according to a possible particular characteristic, during a preliminary step, the output frequency of the clock is adjusted so as to reduce the instantaneous frequency error of the clock. This preliminary step can be performed for example during a manufacturing phase or during a clock maintenance phase.

This further minimizes the residual clock dating error.

In this embodiment, according to a possible particular characteristic, the reference temporal information signal is provided by a GPS location system.

According to one possible particular characteristic, before deployment of the node for a seismic data collection mission, the change in the aforementioned quantity is also measured as a function of the temperature of the clock;

during the mission, the temperature of the clock is measured; and the aforementioned law takes into account the temperature.

Taking the temperature into account in estimating the drift of the clock makes it possible to improve the precision of this estimate.

In a second particular embodiment, the method is remarkable in that during the measurement step, the phase is measured for a first predetermined period of time before deployment of the node for a seismic data collection mission and for a second. predetermined period of time at the end of the mission, and during the step of applying the aforementioned law, an interpolation method is used, so as to obtain an estimate of the accumulated clock phase error.

This way of proceeding has the advantage of not requiring the instantaneous frequency of the clock to be measured and is therefore even simpler to implement than the first embodiment.

In a third particular embodiment, the method is remarkable in that during the measurement step, the accumulated phase error of the clock is measured during a period of time corresponding to a simulated seismic data collection mission. .

In this third embodiment, it is possible to dispense with the application of a predetermined non-linear law of variation of the magnitude which depends on the values ​​collected during the measurement step.

For the same purpose as that indicated above, the present invention further provides a method for estimating the drift over time of a physical operating parameter of a clock for dating samples of seismic data associated with a node. of seismic data collection, remarkable in that:

at least one quantity associated with the clock is measured, at predetermined instants or during predetermined periods of time; and measuring the accumulated clock phase error for a period of time corresponding to a simulated seismic data collection mission.

According to a possible particular characteristic of this process:

before deployment of the node for a seismic data collection mission, the change in the aforementioned quantity is also measured as a function of the temperature of the clock;

during the mission, the temperature of the clock is measured.

In all of the aforementioned embodiments, the knot can be adapted for use on a seabed.

Still with the same aim as that indicated above, the present invention also proposes a system for estimating the drift over time of a physical operating parameter of a clock for dating samples of seismic data associated with a node for collecting seismic data, remarkable in that it comprises a module suitable for implementing the steps of a method as briefly described above.

The advantages and particular characteristics of the system being similar to those of the process, they are not recalled here.

Other aspects and advantages of the invention will become apparent on reading the detailed description below of particular embodiments of the invention, given by way of non-limiting examples, with reference to the appended drawings, in which:

FIG. 1 is a flowchart generally illustrating the steps of a method for estimating the drift over time of a physical operating parameter of a clock for dating samples of seismic data in accordance with the present invention ;

FIG. 2 is a flowchart illustrating steps of the method of FIG. 1 in a first particular embodiment where the physical operating parameter is the instantaneous frequency of the clock;

FIG. 3 is a flowchart illustrating steps of the method of FIG. 1 in a second particular embodiment where the physical operating parameter is the phase of the clock; and

FIG. 4 is a flowchart illustrating steps of the method of FIG. 1 in a third particular embodiment including a simulated mission.

In what follows, we consider a seismic data collection node. These data are collected by means of various seismic sensors, including velocity sensors, accelerometers or even hydrophones and / or geophones. Three geophones can for example be associated with a given node, or a hydrophone and three geophones, or a hydrophone and three accelerometers, any other combination of sensors being possible. In particular, although the data collection can be carried out on land or in a well, in a particular embodiment, we will be interested in a collection of seismic data at sea, by a node which is in particular able to be placed at sea. bottom of the water for a seismic campaign before recovery.

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

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 a high stability.

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 may for example be its phase with respect to a reference clock, or even its instantaneous frequency.

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

FIG. 1 illustrates the method for estimating this drift in accordance with the invention in general.

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

This measurement step 10 is followed by a step 12 during which a predetermined nonlinear law of variation is applied to this physical quantity. An estimate of the accumulated clock phase error is then obtained, this error being representative of the clock drift.

Two particular embodiments of this estimation method are described in detail below, depending on whether the physical quantity chosen is the instantaneous frequency and / or the phase of the clock.

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 in FIG. 2.

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, during a step 102, an internal time information signal of the node is synchronized with respect to a reference time information signal.

The internal time information 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 internal time information signal or internal PPS (pulse per second, in English "Puise Per Second").

This internal PPS allows a microcontroller to time stamp the samples of seismic data received by the seismic sensors and with the help of an analog-to-digital converter. The seismic data received are stored in a memory, which may for example be of the “flash” type.

During step 102, this internal PPS is set on a reference signal or external PPS, which can for example be supplied by a global positioning system or GPS (in English “Global Positioning System”), knowing that when the node is on a seismic vessel, it is usually connected to a GPS receiver.

This synchronization or timing operation can be performed by sending a reset signal by the microcontroller to the clock.

After synchronization, there is no longer a phase difference between the internal and external PPS.

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, the clock running continuously during the mission.

In the example of a collection of seismic data at sea, as the GPS signal is electromagnetic, it does not cross the water column. It therefore remains inaccessible for the clock. The GPS signal may also be inaccessible when collecting seismic data on land.

Over time, the instantaneous clock frequency error induces an accumulated phase error between the internal PPS and the external PPS. Indeed, the electronics time-stamps the seismic data by means of a signal 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 the aging of quartz oscillators (TCXO, MCXO or OCXO) as well as atomic oscillators (rubidium or cesium) are not negligible.

The accumulated phase error can be measured when the external PPS is available, that is to say, in the example of a mission at sea, when the node is repatriated on board the seismic survey vessel and reconnected to the GPS receiver.

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 external PPS.

As a variant, this second phase can take place at a predetermined instant during the mission, in which case the instantaneous frequency of the clock is measured at this instant as well as the phase shift at this instant between the internal PPS and the external PPS.

During step 12 of applying the predetermined nonlinear law of variation of the instantaneous frequency of the clock, the final instantaneous frequency error of the clock is estimated from the initial instantaneous frequencies fi and final ff and of the final phase shift.

To do this, in accordance with the invention, the hypothesis is formulated 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.

Let e ​​(t) be the instantaneous frequency error of the clock as a function of time t. The second-order polynomial equation is written:

ε (ί) = 8 [+ a. t + β.ί² where = ε (0) denote the initial frequency error and a and β denote predetermined coefficients.

The accumulated phase error is given by:

(p (t) = J 8 (t) dt

The accumulated phase error is therefore given by the following third-order polynomial equation:

t² t³ <p (t) = Ei.t + a.— + β.—

Let T be the moment of the end of the mission. The final instantaneous frequency error and the final phase error measured at time T are noted:

ε (Τ) = £_f <p (T) = <Pf

We can then calculate the coefficients a and β, given that we have two equations with two unknowns:

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£_f- £ j- β. T²

These coefficients can be calculated either in the node, or in postprocessing, in a separate computer.

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

Before deployment of the node, the instantaneous frequency error of the clock can be reduced by performing a preliminary step of adjusting the output frequency of the clock.

This step can be carried out either during the manufacture of the clock, or during maintenance of the clock. Commercial clocks have an input allowing this setting.

To take into account the temperature changes which may occur in particular at the start or at the end of the mission, due to the deployment or repatriation of the node and the associated electronics in cold water, for example in the case of a mission at sea, we can improve the estimate of the drift of the clock by introducing a temperature parameter.

To this end, before deployment of the node for a seismic data collection mission, the change in the instantaneous frequency of the clock is measured as a function of the temperature of the clock. Then during the mission, the temperature of the clock is measured.

We then introduce into the law of nonlinear variation of the instantaneous frequency of the clock a parameter £_Temp representative of the instantaneous clock frequency error due to the change in temperature.

The third order polynomial equation giving the accumulated phase error of the clock then becomes:

t³ &lt;p (t) = (/ i + ÏTemp) · t + a. - + β. ~ Hu with

T β^— 7Ï ÏÂ

I --- IT³\ 3 2J '¹ β 2. £ f_empa = —---------------- Figure 3 illustrates the flow of the method according to the invention in a second embodiment, where the physical operating parameter of the clock used is not the instantaneous frequency, but the phase of the clock.

In this embodiment, the measurement step 10 takes place in two stages.

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 ΔΤ1 so as to know the evolution of the accumulated phase error during this period ΔΤ1.

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 ΔΤ2, so as to know the evolution of the accumulated error phase during this period ΔΤ2.

Then, during a step 204, the accumulated phase error during the mission, that is to say between the two periods ΔΤ1 and ΔΤ2, is estimated, using an interpolation method such as for example the method of splines, a spline being a function defined in pieces by polynomials. This method of interpolation is known per se. This example is not limiting: one could use another interpolation method.

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

In a third embodiment, illustrated in FIG. 4, upstream of the mission, the accumulated phase error during a simulated mission, in production or in maintenance, at different temperatures is measured during a step 300. . 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.

The system for estimating the drift over time of the clock in accordance with the invention may comprise an electronic or computer module, either on board the node, or deported in a computer or computer or separate electronic circuit, capable of measuring the time. '' clock frequency and / or phase error at the start and at the end of the mission, which makes it possible to estimate the phase error accumulated according to a polynomial of order 3, while 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.

More particularly, the aforementioned module is configured to carry out the steps described above in connection with FIGS. 1 to 4. When the module is embedded in the node, it can consist of a separate unit from the functional units already present in the node, such as as the microcontroller of the node, or it may be functionally integrated into such units. By way of example, the microcontroller of the node is configured to further perform at least some of the steps described above in connection with FIGS. 1 to 4.