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

METHOD AND SYSTEM FOR ESTIMATING THE DERIVATIVE OF A SEAT CLOCK OF SEISMIC DATA SAMPLES

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 in the samples of seismic data collected.

Every clock has a certain time difference 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.

This drift affects both clocks of the type comprising a quartz oscillator (temperature compensated oscillators TCXO, in English “Temperature Controlled Crystal Oscillators”, or compensated oscillators with MCXO microcontroller, in English “Microcontroller Compensated Crystal Oscillators, or even quartz oscillators OCXO thermostats, in English "Oven Controlled Crystal Oscillators") than atomic type clocks (rubidium or cesium).

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

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

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

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

To this end, the present invention 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 for collecting seismic data, 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 times 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 drift in time of the physical parameter.

This results in a residual clocking error lower than the residual error obtained when applying a linear correction to clock drift.

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

In addition, the clock drift can be estimated at any time after a campaign to collect seismic data, or even during the campaign when it has a particularly long duration.

In addition, if 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 to any type of clock, including clocks of the quartz type (including TCXO, MCXO and OCXO) and clocks of the atomic type (including those of rubidium and those of cesium).

The invention makes it possible to obtain a good quality estimate of the clock drift, 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 a reference time information signal;

at a predetermined instant during the mission, the instantaneous clock frequency 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 from the initial instantaneous frequencies and at the aforementioned predetermined instant and of the phase shift.

The aforementioned instant may 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 particular characteristic possible, 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 clock output frequency is adjusted so as to reduce the instantaneous clock frequency error. This preliminary step can be carried out 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 time information signal is supplied by a GPS location system.

According to a possible particular characteristic, before deployment of the node for a seismic data collection mission, the evolution of the aforementioned quantity is further 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 temperature into account.

Taking temperature into account in estimating clock drift improves the accuracy 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 mission to collect seismic data and for a second predetermined period of time at the end of the mission, and during the stage of application of the above-mentioned law, an interpolation method is used, so as to obtain an estimate of the accumulated error of phase of the clock.

This procedure has the advantage of not having to measure the instantaneous frequency of the clock 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 error of phase of the clock is measured for a period of time corresponding to a simulated mission of collecting seismic data. .

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

For the same purpose as that indicated above, the present invention also proposes 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 times 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 evolution of the aforementioned quantity is further measured as a function of the temperature of the clock;

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

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

Still for the same purpose as that indicated above, the present invention also provides 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 includes a module adapted to implementing steps of a method as succinctly described above.

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

Other aspects and advantages of the invention will appear 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:

- Figure 1 is a flowchart generally illustrating steps of a method for estimating the drift over time of a physical operating parameter of a clock for dating seismic data samples according to the present invention ;

- Figure 2 is a flowchart illustrating steps of the method of 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 of the method of Figure 1 in a second particular embodiment where the physical operating parameter is the phase of the clock; and

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

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

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

Knowing that, for example during a campaign for collecting seismic data at sea, the node can remain at the bottom of the sea for several months, the time stamping of the samples of seismic data 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 can for example be its phase relative to a reference clock, or 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 duration, in an accumulated phase error or an accumulated error of instantaneous frequency.

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

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

This measurement step 10 is followed by a step 12 during which a non-linear predetermined 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 detailed 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 measure, 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 in step 100 and synchronizing, during a step 102, an internal time information signal of the node with respect to a reference time information signal.

The internal time information signal of the node is produced by the clock. It generally operates at a frequency of several MHz, for example 10 MHz and, by means of a clock frequency divider, the clock also provides a signal at another frequency, of 1 Hz for example, which is used as an internal time information or PPS signal (pulse per second, in English "Puise Per Second") internal.

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-digital converter. The received seismic data are stored in a memory, which can for example be of the “flash” type.

During step 102, this internal PPS is wedged on a reference signal or external PPS, which can 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.

This synchronization or setting operation can be carried out by sending a reset signal by the microcontroller to the clock.

After synchronization, there is no longer any 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 collecting seismic data at sea, since 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 earthquake data on land.

Over time, the instantaneous clock frequency error induces an accumulated phase error between the internal PPS and the external PPS. Indeed, electronics time stamp 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, i.e. in the example of a sea mission, when the node is repatriated on board the seismic prospecting 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 difference 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 at this instant as well as the phase shift at this instant between the internal PPS and the external PPS are measured.

During step 12 of applying the predetermined nonlinear law of variation of the instantaneous frequency of the clock, the final error of instantaneous frequency 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, whence it follows that the phase of the clock varies according to a polynomial equation of order 3.

Let e ​​(t) be the instantaneous clock frequency error as a function of time t. The polynomial equation of order 2 is written:

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

The accumulated phase error is given by:

(p (t) = J s (t) dt

The accumulated phase error is therefore given by the following polynomial equation of order 3:

t² t² <p (t) = s_The.t + a. - + β.—

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 (Τ) = £_f <p (T) = <Pf

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

<img file="FR3083326A1_D0001.tif" />

£_f- £ j- β. T²

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

As a variant, 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 deploying the node, the instantaneous clock frequency error can be reduced by carrying out a preliminary step of adjusting the clock output frequency.

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

To take into account the temperature changes which can occur in particular at the beginning 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 estimation of clock drift by introducing a temperature parameter.

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

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

The polynomial equation of order 3 giving the accumulated phase error of the clock then becomes:

&lt;P (O = (and + s_Temp). t + a. - + β. y with

<img file="FR3083326A1_D0002.tif" />

Ff β. T 2. £_Pemp a = ------------------ Figure 3 illustrates the progress of the process 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 continuously measured during a step 200 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 again measured, in 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 phase error accumulated during the mission is estimated, that is to say between the two periods ΔΤ1 and ΔΤ2, using an interpolation method such as for example the method of splines, a spline being a function defined in pieces by polynomials. This interpolation method is known per se. This example is not limitative: we 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 even simpler electronics than in the first embodiment.

In a third embodiment, illustrated in FIG. 4, upstream of the mission, during a step 300, the phase accumulated error during a simulated mission, in production or in maintenance, is measured at different temperatures. . This is 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 in time of the clock in accordance with the invention may include an electronic or computer module, either on board the node, or remote in a computer or separate 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 phase error accumulated according to a polynomial of order 3, while conventional equipment is only capable of measuring phase drift and therefore of estimating the phase error accumulated 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 unit distinct from the functional units already present in the node, such than the node microcontroller, or it can 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.