Method for monitoring a point of interest in a basement

Abstract

The present invention relates to a method for monitoring an area of ​​interest of a subsoil comprising a point of interest, comprising: an extraction of a first subset (1) of raw geophysical data associated with the point of interest, from a set (0) of raw geophysical data previously acquired to generate a first image (Im1) of the subsoil; a construction of a projection model (MOD1) from the first subset of data (1); a reduction of the first subset (1) of data to a second subset (2); the creation of a reference set (B) by applying the projection model (MOD1); an acquisition forming a third subset (3); the creation of a monitoring set (M) by applying the projection model (MOD1); a comparison of the first reference set (B) and the first monitoring set (M) to identify an evolution of the subsoil to be monitored. Abstract figure: Figure 6

Description

Title of the invention: Method for monitoring a point of interest in a basement Technical field

[0001] The present invention relates to the field of acquisition and processing of geophysical data.

[0002] More particularly, the application relates to methods for monitoring an area of ​​interest in a subsoil using geophysical data STATE OF THE ART

[0003] From the prior art, it is known to acquire geophysical data making it possible to generate an image of the geophysical structure of a terrestrial or marine subsoil. This image allows a person skilled in the art to determine with a high degree of certainty the presence or absence of the area of ​​interest, for example the presence or absence of natural resources.

[0004] Devices configured to generate images of a subsoil generally consist of a plurality of sources capable of producing seismic waves over different frequency ranges. These seismic waves are diffused in a subsoil and an area of ​​interest, which reflects a portion of said seismic waves towards a receiver. By studying the variations in propagation of the reflected waves in the subsoil, it is thus possible to produce an image of the subsoil studied.

[0005] These standard state-of-the-art techniques can be used at recurring intervals over time, to generate a succession of images of the subsoil, making it possible to determine the evolution of the geophysical profile of this subsoil and to carry out monitoring. This image is called 4D, that is to say an image of a volume of the subsoil evolving over time.

[0006] Currently, 4D study solutions are aimed at the complete update of a 3D acquisition of the subsoil for a considered area. In other words, the volume of seismic data in order to obtain a new complete 3D image. As a result, current techniques take a lot of time, on the order of several weeks to several months of computer processing of a significant volume of data, on the order of several gigabits to several terabits of data.

[0007] 4D studies remain costly in terms of resources, particularly in terms of software resources given the massive influx of data to be processed, which results in long processing times, because it is necessary to generate 4D images acquired at different calendar periods and to deduce from them an evolution of the geophysical structure of an area of ​​interest.

[0008] The applicant has in the past proposed various improvements to the techniques known from the state of the art, in particular to reduce the cost in financial, hardware and software resources of such devices, for example by proposing a method making it possible to identify from among a set of sources and a set of receivers, the receivers ideally positioned to measure an evolution of the geophysical structure in the area of ​​interest.

[0009] Despite these acquisition improvements, the raw recording of the acquired geophysical data may require a processing phase specific to the extraction of geophysical data useful for monitoring.

[0010] The invention aims to resolve the drawbacks of the state of the art by proposing a method allowing the extraction of geophysical data useful for a monitoring operation of a subsoil to be monitored. PRESENTATION OF THE INVENTION

[0011] More specifically, the invention relates to a method for monitoring at least one area of ​​interest of a subsoil to be monitored comprising a point of interest and a neighborhood of said point of interest, comprising: • an extraction of a first subset of raw geophysical data associated with the vicinity of the point of interest, from a set of raw geophysical data acquired previously, and processed previously to generate a first prior image of the subsoil to be monitored; • a construction of a first projection model from the first subset of data, the first projection model resulting from a geophysical data separation processing, to carry out the extraction of useful geophysical data used during the generation of the first image in the vicinity of the point of interest; and • a reduction of the first subset of data to a second subset comprising a predetermined minimum number of raw geophysical data; • the creation of a first reference set of useful geophysical data by applying the first projection model to the second subset. • an acquisition of raw geophysical data forming a third subset of raw geophysical data comprising the same minimum number of raw geophysical data as the second subset, each data item of the third subset corresponding to a data item of the second subset two; • the creation of a first set of useful geophysical monitoring data by applying the first projection model to the third subset; • a comparison of the first reference set and the first monitoring set to identify an evolution of the subsoil to be monitored.

[0012] By the notion of "neighborhood" of the point of interest, we mean an area which corresponds to the resolution of the image generated by the geophysical method used at the point of interest. For example, if the geophysical method is a seismic method, the resolution is commonly evaluated at a quarter of the wavelength, the wavelength being a function of the propagation speed of the wave in the geological formations and the dominant frequency of the recorded signal.

[0013] Thanks to such a combination of characteristics, it is thus possible to note, using the monitoring method, an evolution of the geophysical response of the area of ​​interest by means of a limited acquisition of data, which would for example be insufficient for the construction of a 4D image but sufficient for the detection of possible modifications of geophysical property. Thus, the software resources and the time required for processing the useful geophysical data are significantly reduced. In addition, the constructed projection model is invariant over time, that is to say that once created, it can be reused over calendar time.

[0014] Advantageously, the monitoring method comprises, after the creation of the first set of geophysical monitoring data: • extracting a second image from the first image, the second image corresponding to the vicinity of the point of interest; • constructing a second projection model from the first set of useful reference geophysical data and the second image, such that applying the second projection model to the set of reference geophysical data substantially matches the second image to create a second set of useful reference geophysical data; • applying the second projection model to the first set of useful geophysical monitoring data to create a second set of useful geophysical monitoring data; • a comparison of the second set of useful reference geophysical data and the second set of monitoring geophysical data to identify a subsoil evolution to be monitored.

[0015] In such a configuration, the quality of the analyses produced by the application of such projection models is significantly improved.

[0016] Advantageously, at least one new iteration of the acquisition of raw geophysical data forming the third subset and the following steps are carried out over time. Thus, it is possible to note an evolution of the geophysical structure during regular or regular calendar intervals, for example of the order of several weeks to several months.

[0017] Advantageously, the minimum number of useful geophysical data is equal to one recording. In such a configuration, the volume of data to be processed is significantly reduced, which further reduces the resources required for monitoring an area of ​​interest in a subsoil. In such a configuration, the recording is for example selected by means of an inverse ray tracing method from the point of interest to identify the source and receiver position representative of the point of interest and which generated the recording of the first subset.

[0018] Advantageously, the raw geophysical data are seismic wave recordings and the useful geophysical data are reflected, surface or refracted seismic waves.

[0019] Advantageously, the acquisition of prior seismic wave data is a 2D, 3D or 4D acquisition and in that the acquisition of the raw geophysical data used to form the third subset is another 4D acquisition.

[0020] Advantageously, the projection onto the projection model comprises at least: • deconvolution and / or frequency filtering of geophysical data; • a separation of geophysical data by applying a filter of slope defined in the time-distance plane and / or in the frequency-wavenumber plane; • static and dynamic correction of geophysical data.

[0021] In such a configuration, any predefined set of usual physical treatments, single-channel or multi-channel, can be part of the projection model. PRESENTATION OF THE FIGURES

[0022] The invention will be better understood on reading the following description, given solely by way of example, and referring to the appended drawings given by way of non-limiting examples, in which identical references are given to similar objects and in which:

[0023] [Fig.l] is a flowchart describing the steps of a method for monitoring a subsoil to be monitored according to a first embodiment of the invention;

[0024] [Fig.2] is a flowchart describing the steps of a method for monitoring a subsoil to be monitored according to another embodiment of the invention;

[0025] [Fig.3] is a diagram illustrating a subset of raw geophysical data;

[0026] [Fig.4] is a diagram illustrating the projection model constructed using the raw geophysical data set of [Fig.3];

[0027] [Fig.5] is a diagram illustrating a recording obtained by reducing the subset of the diagram of [Fig.3] to which signal processing operations are successively applied;

[0028] [Fig.6] is a collection of diagrams comparing a single record of the projection model of [Fig.4] and the record of [Fig.5] post-processing;

[0029] [Fig.7] is a diagram illustrating a subset of raw geophysical data as well as a plurality of acquisitions each forming a subset of raw geophysical data, each acquisition being carried out at a different calendar period;

[0030] [Fig.8] is a diagram illustrating a set of useful reference geophysical data and a plurality of sets of useful monitoring geophysical data resulting from the application of the first projection model to the data of [Fig.7];

[0031] [Fig.9] is a diagram illustrating the detections of the differences in arrival of the waves by comparing the data of [Fig.8] at the point of interest;

[0032] [Fig. 10] is a diagram illustrating a third set of useful geophysical data resulting from the application of the second projection model to the data of [Fig.8].

[0033] It should be noted that the figures set out the invention in detail to enable the invention to be implemented; although not limiting, said figures serve in particular to better define the invention where appropriate. DETAILED DESCRIPTION OF THE INVENTION

[0034] The invention relates to a method for monitoring at least one area of ​​interest of a subsoil to be monitored comprising a point of interest and a neighborhood of said point of interest by geophysical method. In the embodiment illustrated in [Fig.l], the monitoring method is in particular implemented by seismic method, any other geophysical method being applicable.

[0035] By the notion of "neighborhood" of the point of interest, we mean an area which corresponds to the resolution of the image generated by the geophysical method used at the point of interest. For example, if the geophysical method is a seismic method, the resolution is commonly perhaps a quarter of the wavelength, the wavelength being a function of the propagation speed of the wave in the geological formations and the dominant frequency of the recorded signal.

[0036] The present continuation of the description seeks to describe step by step the monitoring method according to the first embodiment of [Fig.l]. The method comprises a first step of extracting a first subset 1 of raw geophysical data associated with the vicinity of the point of interest, from a set 0 of raw geophysical data acquired previously, and processed previously to generate a first previous Iml image of the subsoil to be monitored.

[0037] The term "previously" means that the set 0 of raw geophysical data has already been used to generate a first image Iml of the subsoil during an imaging process, for example 2D or 3D. The term "2D or 3D" means imaging produced in two or three spatial dimensions. Thus, although the set 0 of raw geophysical data exists in a version that has already been processed, by signal processing operations, said set 0 is used in the context of the process in an unprocessed version, for example stored on a storage medium.

[0038] The subset 1 of raw geophysical data illustrated in [Fig. 3] comprises at least the raw geophysical data which, when previously processed, contributed to a portion of the first image Iml comprising the point of interest and its vicinity. In the described embodiment, the subset 1 is a grouping of raw 2D seismic recordings. By the term "2D" is meant a recording according to two spatial coordinates.

[0039] The method further comprises a step of constructing a first projection model M0D1 from the first subset of data 1, the first projection model M0D1 resulting from a geophysical data separation processing, to carry out the extraction of useful geophysical data used during the generation of the first image Iml in the vicinity of the point of interest. A projection model is a geophysical model, in this case a seismic model making it possible to improve the quality of a subset of raw geophysical data to a level at least sufficient to allow monitoring of an area of ​​interest by favoring the useful geophysical data.

[0040] To create the first projection model M0D1, a first single-channel processing is applied, for example a suppression of direct arrivals and / or a deconvolution and / or a frequency filtering. Optionally, a multi-channel processing is applied after the single-channel processing(s). The multi-channel processing may for example comprise a normal shift correction, better known to the person skilled in the art under the English term “normal moveout correction”, a filtering in the frequency plane, a filtering in the frequency-wavenumber plane, a filtering in the frequency-distance plane or a singular value decomposition or any multi-channel signal processing method known from the state of the art. In the case of a seismic method, the constructed projection model comprises useful geophysical data, i.e. useful for the generation of the first Iml image, for example seismic recordings of reflected waves, as shown in [Fig.4]. Alternatively, the geophysical data can be refracted seismic waves or surface seismic waves. The first projection model M0D1 is stored, for example on a storage medium for subsequent implementations of the method.

[0041] The method further comprises a step of reducing the first subset 1 of data to a second subset 2 comprising a predetermined minimum number of raw geophysical data. In the described embodiment, the minimum number of geophysical data is a single record, commonly referred to as a "trace" by those skilled in the art, as illustrated in [Fig. 5]. Here, the record is for example selected by means of an inverse ray tracing method from the point of interest to identify the position of the source and receiver set representative of the point of interest and which generated the record of the first subset 1.

[0042] The method further comprises a step of creating a first reference set of useful geophysical data B by applying the first projection model M0D1 to the second subset 2. An application of a projection model. This operation is referred to as "projection" of the second subset 2 onto the first projection model M0D1. The projection may for example be carried out using a projection operator, to improve the quality of the recording obtained during the previous inverse ray tracing step. [Fig.6] successively illustrates, from left to right, the first projection model M0D1, the first reference set of useful geophysical data B, and a comparison of the first projection model M0D1 and the reference geophysical data set B.Here, a correlation coefficient between the first projection model M0D1 and the reference geophysical dataset B is calculated and considered to be very high, i.e., greater than 0.99. Advantageously, the correlation coefficient is greater than or equal to 0.7.

[0043] The method further comprises a step of acquiring raw geophysical data forming a third subset of raw geophysical data 3 comprising the same minimum number of raw geophysical data as the second subset 2, each data item of the third subset 3 being similar to a data item of the second subset 2. By the term "similar", it is meant that the third subset of raw geophysical data 3 comes from an acquisition allowing the monitoring of the same point of interest or its vicinity as the point of interest as the second subset of raw geophysical data 2.

[0044] Advantageously, at least one new iteration of the acquisition of raw geophysical data forming the third subset 3 is carried out in calendar time. [Fig.7] illustrates a grouping on the same diagram of the second subset 2, located at the zero position, designated the reference calendar date, as well as five subsets 3 acquired on different calendar dates 31, 32, 33, 34 35, an approach referred to in English as “Monitoring spot gathers” or “MSG”. The acquisitions can be made at regular or irregular intervals in calendar time.

[0045] The method further comprises creating a first set of useful geophysical monitoring data M by applying the first projection model M0D1 to the third subset 3. In the case where the acquisition of the third subset 3 is iterated in time, each third subset 3 is projected onto the first projection model M0D1 in the same way as the raw geophysical data subset 2 was previously projected onto the first projection model M0D1. [Fig.8] illustrates a MSG of the reference useful geophysical data set B located at the zero position, as well as five sets of useful geophysical monitoring data M acquired at different calendar dates M1, M2, M3, M4, M5. The acquisitions can be made at regular or irregular intervals in calendar time.

[0046] The method further comprises a comparison of the first reference set B and the first monitoring set M to identify an evolution of the subsoil to be monitored. In the case where there are a plurality of useful geophysical monitoring data sets M, each monitoring set is compared one by one with the base reference set. In such a configuration, it is thus possible to note, using the monitoring method, an evolution of the geophysical response of the area of ​​interest by means of a limited acquisition of data, which would for example be insufficient for the construction of a 4D image but sufficient for the detection of possible modifications of geophysical property. Thus, the software resources and the time required for processing the useful geophysical data are significantly reduced.

[0047] Advantageously, several comparisons using different theoretical models can be carried out. [Fig.9] thus illustrates a measurement of a temporal drift between the reference useful geophysical data set B and a plurality of monitoring useful geophysical data sets M acquired on different calendar dates. For each date, a difference in the calculation of the arrival time dDT of the seismic echo of the point of interest between the reference useful geophysical data set BASE and each of the useful geophysical data sets of monitoring M is calculated with an associated uncertainty dependent on the signal-to-noise ratio and bandwidth known to the person skilled in the art.

[0048] The remainder of the description focuses on describing the invention according to another embodiment, illustrated in [Fig.2]. In this embodiment, the monitoring method further comprises, after the creation of the first set of geophysical monitoring data M: • extracting a second image Im2 from the first hnl image, the second image Im2 corresponding to the vicinity of the point of interest; • constructing a second projection model M0D2 from the first set of useful reference geophysical data B and the second image Im2, such that applying the second projection model M0D2 to the set of reference geophysical data B substantially matches the second image IMAGE2 to create a second set of useful reference geophysical data Bi; • applying the second projection model M0D2 to the first set of useful geophysical monitoring data M to create a second set of useful geophysical monitoring data Mi; • a comparison of the second set of useful reference geophysical data Bi and the second set of monitoring geophysical data Mi to identify an evolution of the subsoil to be monitored.

[0049] [Fig. 10] illustrates an MSG of the second set of useful reference geophysical data Bi located at the reference calendar position, as well as five second sets of useful monitoring geophysical data Mi acquired on different calendar dates Mil, Mi2, Mi3, Mi4, Mi5. From such an MSG, it is then possible to carry out analyses making it possible to conclude on the evolution of the geophysical structure of the area of ​​interest over the course of calendar time, in particular measurements of temporal drifts similar to those carried out for the embodiment described in [Fig.9].

[0050] It will also be noted that the invention is not limited to the embodiments described above. It will indeed appear to the person skilled in the art that various modifications can be made to the embodiment described above, in light of the teaching which has just been disclosed to him.

[0051] In the detailed presentation of the invention which is made above, the terms used must not be interpreted as limiting the invention to the embodiment set out in the present description, but must be interpreted to include all equivalents whose prediction is within the reach of the person skilled in the art by applying his general knowledge to the implementation of the teaching which has just been disclosed to him.

Claims

1. Claims Method for monitoring at least one area of ​​interest of a subsoil to be monitored comprising a point of interest and a neighborhood of said point of interest, comprising: • an extraction of a first subset (1) of raw geophysical data associated with the vicinity of the point of interest, from a set (0) of raw geophysical data acquired previously, and processed previously to generate a first prior image (Iml) of the subsoil to be monitored; • a construction of a first projection model (M0D1) from the first subset of data (1), the first projection model (M0D1) resulting from a geophysical data separation processing, to carry out the extraction of useful geophysical data used during the generation of the first image (Iml) in the vicinity of the point of interest; and • a reduction of the first subset (1) of data to a second subset (2) comprising a predetermined minimum number of raw geophysical data; • the creation of a first reference set of useful geophysical data (B) by applying a projection operator to the second subset (2), the projection operator being configured so that a correlation coefficient between the first reference set of useful geophysical data (B) obtained and the first projection model (M0D1) is greater than 0.7; • an acquisition of raw geophysical data forming a third subset of raw geophysical data (3) comprising the same minimum number of raw geophysical data as the second subset (2), each data item of the third subset (3) corresponding to a data item of the second subset (2); • the creation of a first set of useful geophysical monitoring data (M), by application of the same projection operator to the third subset (3); • a comparison of the first reference set (B) and the first monitoring set (M) to identify an evolution of the subsoil to be monitored.

2. Monitoring method according to any one of the preceding claims, characterized in that at least one new iteration of the acquisition of raw geophysical data forming the third subset (3) and the following steps are carried out over time.

3. Monitoring method according to any one of the preceding claims, characterized in that the minimum number of useful geophysical data is equal to one recording.

4. A monitoring method according to any one of the preceding claims, characterized in that the raw geophysical data are seismic wave recordings and the useful geophysical data are reflected, surface or refracted seismic waves.

5. Monitoring method according to claim 4, characterized in that the acquisition of previous seismic wave data is a 2D, 3D or 4D acquisition and in that the acquisition of the raw geophysical data used to form the third subset (3) is another 4D acquisition.

6. Monitoring method according to any one of the preceding claims, characterized in that the application of the projection operator comprises at least: • a deconvolution or frequency filtering of the geophysical data; • a separation of the geophysical data by application of a slope filter defined in the time-distance plane and / or in the frequency-wavenumber plane; • a static and dynamic correction of the geophysical data.

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