System for temporal analysis of the spectral energy of seismic waves for detecting and viewing zones of fluid movements and alteration in a geological subsoil

Abstract

The invention relates to a system for temporal analysis of the spectral energy of seismic waves intended for detecting and viewing zones of fluid movements and alteration in a geological subsoil, the spectral energy being the distribution of the energy of a wave as a function of a plurality of frequencies, characterised in that the system includes a number M of sensors, M being an integer greater than or equal to 3, the sensors being configured to measure at least one wave propagating in the geological subsoil during an observation period in order to obtain at least one measurement signal, the sensors forming (see formula (1)) pairs of sensors, including a first sensor and a second sensor forming a pair of sensors and a controller for implementing a method for temporal analysis of the spectral energy of seismic waves intended for detecting and viewing zones of fluid movements and alteration in a geological subsoil.

Description

TEMPORAL ANALYSIS SYSTEM FOR THE SPECTRAL ENERGY OF SEISMIC WAVES INTENDED FOR THE DETECTION AND VISUALIZATION OF ZONES OF FLUID MOVEMENT AND ALTERATION IN A GEOLOGICAL SUBSURFACE

[0001] The invention relates to the field of detecting areas of fluid movement and alteration in a geological subsoil.

[0002] Prior to this, the scientific publication *SeismicAttenuationExtractionFromTrafficSignalsRecordedBy a SingleSeismicStation* (Yumin Zhao et al. 2022) describes a method for extracting seismic attenuation from road traffic signals recorded by a single seismic station. The seismic waves are analyzed from the ambient seismic noise generated by vehicle traffic. Specifically, the signals are recorded by a single seismic station located in an urban environment where ambient traffic-generated seismic noise is predominant. A spectral analysis is then performed to extract the wave characteristics. The frequency and amplitude of the waves are represented at specific time points. A linear relationship between the frequency and amplitude ratio of the matched instantaneous spectra is used to quantify the seismic attenuation, measuring how the waves dissipate as a function of their frequency.The collected data are analyzed daily to observe temporal variations in seismic attenuation. Recordings from different urban sites allow for spatial comparison to highlight local variations in the physical properties of the soil. However, some drawbacks remain. The method allows for the study of sensitivity to weather conditions. For example, variations in attenuation as a function of rainfall reveal a high sensitivity to changes in soil moisture, which impacts the propagation of seismic waves. The study is conducted by a single station, i.e., at a given point in space, which allows for a general study of the soil without precise spatial information. It is not possible, for instance, to map a region from a given seismic station. The method does not allow for the precise localization of soil areas where attenuation varies.

[0003] The objective of the present invention is to overcome these drawbacks and to be able to accurately visualize areas of interest on a map without directly measuring attenuation.

[0004] To achieve this objective, the invention proposes a time-domain analysis system for the spectral energy of seismic waves, intended for the detection and visualization of areas of fluid movement and weathering in a geological subsurface. Spectral energy is the distribution of a wave's energy as a function of a plurality of frequencies. The system is remarkable in that it comprises: an integer number M of sensors, said integer number M being greater than or equal to 3. These sensors are configured to measure at least one wave propagating in the geological subsurface during an observation period to obtain at least one measurement signal. The sensors form pairs of sensors, where a first sensor and a second sensor form a sensor pair; - a controller configured to: - reconstruct a series of waveforms by interferometry of said measurement signal for each pair of sensors at a regular time interval during said observation period to form a correlogram; - reconstruct a series of waveform spectra of said measurement signal for each pair of sensors over the regular time interval during said observation period to form a spectrogram; - calculate the spectral variance over time for each pair of sensors over said observation period for each frequency; - determine a frequency range from the non-zero spectral variance values, the frequency range being resolved into a finite number of frequencies and extending from a minimum frequency to a maximum frequency; - calculate the logarithm of the spectral ratio of each pair of sensors at a time by the following mathematical formula: with being a frequency value in the frequency range; being a spectral amplitude value at time for the frequency ;And being a reference spectral amplitude value for the frequency ;And being the finite number of frequencies for which the frequency range is resolved; the calculation of the logarithm of the spectral ratio from a plurality of sensor pairs determining time series of logarithm of the spectral ratio ; a logarithm of the spectral ratio negative corresponding to an attenuating zone, a logarithm of the spectral ratio positive corresponding to an amplifying zone; - spatially interpolate said plurality of time series of logarithm of the spectral ratio to determine a plurality of spectral ratio logarithm maps - Determine an average map from the logarithm maps of the spectral ratio representing the attenuating and / or amplifying areas over the observation period; - Determine a spectral variance map from the logarithm maps of the spectral ratio representing active and inactive areas, an active area being an area in which the logarithm of the spectral ratio presents a variance greater than or equal to a predetermined threshold value, an inactive zone being a zone in which the logarithm of the spectral ratio presents a variance lower than a predetermined threshold value.

[0005] Thanks to the invention, it is possible to visualize more precisely the areas likely to exhibit fluid movements or alterations of the geological subsoil.

[0006] Advantageously, the said integer M is equal to 3, the said sensors being aligned.

[0007] Thus, with only 3 sensors, it is possible to study a given area.

[0008] Preferably, these sensors are geophones or hydrophones.

[0009] Thus, it is possible to detect and visualize areas of fluid movement and alteration in a terrestrial or marine geological subsoil.

[0010] Advantageously, the reference spectral amplitude value is a spectral amplitude value given at a predetermined time over the time interval.

[0011] In this way, it is possible to compare a situation with a known state in order to track its evolution.

[0012] Alternatively, the reference spectral amplitude value is the average of all spectral amplitude values ​​over the observation period.

[0013] Alternatively, the reference spectral amplitude value is the median of all spectral amplitude values ​​over the observation period.

[0014] This proves useful in situations where the operator has no knowledge of the system to which they would like to compare themselves.

[0015] Furthermore, the invention relates to a time-domain analysis method of the spectral energy of seismic waves intended for the detection and visualization of zones of fluid movement and alteration in a geological subsoil, spectral energy being the distribution of the energy of a wave in a plurality of frequencies, said method implementing a time-domain analysis system of the spectral energy of seismic waves intended for the detection and visualization of zones of fluid movement and alteration in a geological subsoil previously described, remarkable in that said method comprises the following steps: - a step of acquiring at least one measurement signal during an observation period; - a step of reconstructing a series of waveforms by interferometry of the measurement signal for each pair of sensors at a first regular time interval during an observation period to form a correlogram;- a step of reconstructing a series of waveform spectra for each pair of sensors over a second regular time interval during said observation period to form a spectrogram, the first regular time interval being identical to the first regular time interval; - a step of calculating the spectral variance over time for each pair of sensors during said observation period for each frequency; - a step of determining a frequency range resolved into a finite number of frequencies; and extending from a minimum frequency to a maximum frequency for each pair of sensors based on the non-zero spectral variances calculated during the spectral variance calculation step over time; - a step for calculating the logarithm of the spectral ratio of each pair of sensors by the following mathematical formula: with being a frequency value in the frequency range; being a spectral amplitude value at time for the frequency ; being a reference spectral amplitude value for the frequency ;And being the finite number of frequencies for which the frequency range is resolved; the calculation of the logarithm of the spectral ratio from a plurality of sensor pairs determining time series of logarithm of the spectral ratio ; a logarithm of the spectral ratio negative corresponding to an attenuating zone, a logarithm of the spectral ratio positive corresponding to an amplifying zone; - a spatial interpolation step of said plurality of time series of logarithm of the spectral ratio to determine a plurality of spectral ratio logarithm maps each logarithm map of the spectral ratio being determined from one of the time series at the same time - a step of determining an average map from the logarithm maps of the spectral ratio - the average map representing the attenuating and / or amplifying areas over the observation period; - a step of determining a spectral variance map from the logarithm maps of the spectral ratio The variance map represents active and / or inactive areas, an active area being an area in which the logarithm of the spectral ratio presents a variance greater than a predetermined threshold value.

[0016] Preferably, the process includes a step for determining the slope of the logarithm of the spectral ratio between said step of calculating the logarithm of the spectral ratio and said spatial interpolation step.

[0017] Thus, it is possible to obtain additional information, including the speed of extension of an attenuating zone or the speed of retraction of an amplifying zone.

[0018] Advantageously, said spatial interpolation step is carried out by a tomographic inversion process.

[0019] Spatial interpolation allows the correction of geometric effects resulting from all the sensors present in the studied territory.

[0020] The invention also relates to a computer program comprising program code instructions for executing the steps of the method for temporal analysis of the spectral energy of seismic waves intended for the detection and visualization of areas of fluid movement and alteration in a previously described geological subsoil, when said program is running on a controller.

[0021] The invention will be further detailed by the description of non-limiting embodiments, and on the basis of the attached figures illustrating variants of the invention, in which: a time-domain analysis system of the spectral energy of seismic waves intended for the detection and visualization of zones of fluid movement and alteration in a geological subsoil according to an embodiment of the invention; a time-domain analysis method of the spectral energy of seismic waves intended for the detection and visualization of zones of fluid movement and alteration in a geological subsoil according to an embodiment of the invention.

[0022] A time-domain analysis system for the spectral energy of seismic waves, designed for the detection and visualization of fluid movement and weathering zones in the geological subsurface, is schematically illustrated in Figure 1. Figure 1 represents a surface 1 delimiting the geological subsurface to be studied. Four sensors are arranged on this surface 1. The sensors are geophones, or hydrophones when located on the seabed. The geological subsurface contains a plurality of waves 3 originating from continuous seismic noise. Continuous seismic noise corresponds to all the waves 3 naturally present in the subsurface, without any human intervention. The sensors 2 measure the frequency and amplitude of the mechanical wave 3 generated by the fluid present in the geological subsurface. This information is then transmitted to a controller (not shown in Figure 1) for processing.

[0023] Figure 2 schematically illustrates, in the form of a flowchart, a time-domain analysis method for the spectral energy of seismic waves, designed for the detection and visualization of fluid movement and weathering zones in the geological subsurface. Spectral energy is the energy distribution of a wave as a function of a plurality of frequencies. During an acquisition step E1, a seismic noise measurement signal is recorded over an observation period. The seismic noise data is obtained via sensors, M being an integer greater than or equal to 3, and these sensors being implemented in pairs. Among the M sensors, there are Pairs of sensors, including at least one first sensor and one second sensor forming a pair. When the number M is equal to 3, the sensors are aligned so that the various trajectories connecting each sensor to the other overlap. The sensors are connected to a controller. Data concerning the frequency and amplitude of the waves detected by the pair of sensors are transmitted electrically to the controller for processing. Preferably, the controller is a computer. During a waveform reconstruction step E2, a series of waveforms are reconstructed by interferometry from the measurement signal of each pair of sensors at regular time intervals during the observation period. Seismic interferometry is a technique for investigating the subsurface by studying superimposed waves, thus creating a phenomenon of constructive or destructive interference.The waves initially collected during the acquisition step E1 are combined and reconstructed according to whether the interference is constructive or destructive. This reconstruction is performed at regular intervals during the previously defined observation period. For example, the regular time interval is between 10 and 50 minutes, preferably 30 minutes. Thus, the waves are reconstructed every 30 minutes. The waveform reconstruction step E2 results in the formation of a correlogram. During the waveform spectra reconstruction step E3, a series of waveform spectra of the measurement signal is reconstructed for each pair of sensors at regular intervals. The waveform spectra reconstruction step E3 results in the formation of a spectrogram.During the spectral variance calculation step E4, the spectral variance of each sensor pair is calculated for each pair of sensors over the observation period. The spectral variance calculation is performed for each frequency. During the frequency range determination step E5, the frequency range is determined from the non-zero spectral variances obtained during the spectral variance calculation step E4. Indeed, if the spectral variance is non-zero, a zone of fluid movement and / or a zone of alteration is considered to have been detected. The frequency range is then resolved into a finite number of frequencies. and extends from a minimum frequency to a maximum frequency. During a step in calculating the logarithm of the spectral ratio E6, the logarithm of the spectral ratio is calculated for each pair of sensors using the following mathematical formula: with being a frequency value within the frequency range, being a spectral amplitude value at time for the frequency , being a reference spectral amplitude value for the frequency And being the finite number of frequencies for which the frequency range is resolved. Calculating the logarithm of the spectral ratio The value of each pair of sensors allows us to determine a set of time series of the logarithm of the spectral ratio When the logarithm of the spectral ratio is negative, this means that the area studied by the given pair of sensors is an attenuating area. Conversely, when the logarithm of the spectral ratio If the value is positive, it means that the area studied by the given sensor pair is an amplifying area. Preferably, the reference spectral amplitude value is a spectral amplitude value given at a predetermined time within the time interval. Alternatively, said reference spectral amplitude value is the mean or median of all spectral amplitude values ​​over the observation period. It can also be any other statistical tool that allows the data to be synthesized to enable reliable comparison with a given spectral amplitude value. During a spatial interpolation step E7, the plurality of time series of the logarithm of the spectral ratio The data from each pair of sensors is processed by spatial interpolation in order to form a plurality of logarithm maps of the spectral ratio Indeed, a logarithm map of the spectral ratio is issued for each time series at the same time Preferably, said spatial interpolation step E7 is performed by tomographic inversion. Optionally, said controller is capable of determining the slope of the logarithm of the spectral ratio and, consequently, the process includes a step of determining the slope of the logarithm of the spectral ratio Thus, it is possible to determine the expansion rate of the attenuating area or the contraction rate of the amplifying area for each pair of sensors. The next step involves determining the slope of the logarithm of the spectral ratio. is performed after the aforementioned step of calculating the logarithm of the spectral ratio During a step in determining an average E8 map, the average map is created from the logarithm maps of the spectral ratio The average map indicates attenuating and / or amplifying areas over the observation period. During the determination of a spectral variance map (E9), the spectral variance map is determined from the logarithm maps of the spectral ratio The spectral variance map allows visualization of active and / or inactive areas over the observation period. An active area is an area in which the logarithm of the spectral ratio exhibits a spectral variance greater than or equal to a predetermined threshold value. An inactive zone is a zone in which the logarithm of the spectral ratio presents a spectral variance lower than said predetermined threshold value.

[0024] Finally, the invention also relates to a computer program configured to implement said method of analyzing the spectral energy of seismic waves intended for the detection and visualization of areas of fluid movement and alteration of the geological subsoil.

Claims

A time-domain analysis system for the spectral energy of seismic waves, intended for the detection and visualization of fluid movement and weathering zones in a geological subsurface, spectral energy being the energy distribution of a wave as a function of a plurality of frequencies, characterized in that said system comprises: - an integer number M of sensors, said integer number M being greater than or equal to 3, said sensors being configured to measure at least one wave propagating in said geological subsurface during an observation period in order to obtain at least one measurement signal, said sensors forming pairs of sensors, where a first sensor and a second sensor form a sensor pair; - a controller configured to: - reconstruct a series of waveforms by interferometry of said measurement signal for each pair of sensors at a regular time interval during said observation period to form a correlogram; - reconstruct a series of waveform spectra of said measurement signal for each pair of sensors over the regular time interval during said observation period to form a spectrogram; - calculate the spectral variance over time for each pair of sensors over said observation period for each frequency; - determine a frequency range from the non-zero spectral variance values, the frequency range being resolved into a finite number of frequencies and extending from a minimum frequency to a maximum frequency; - calculate the logarithm of the spectral ratio of each pair of sensors at a time by the following mathematical formula: with being a frequency value in the frequency range; being a spectral amplitude value at time for the frequency ;And being a reference spectral amplitude value for the frequency ;And being the finite number of frequencies for which the frequency range is resolved; the calculation of the logarithm of the spectral ratio from a plurality of sensor pairs determining time series of logarithm of the spectral ratio ; a logarithm of the spectral ratio negative corresponding to an attenuating zone, a logarithm of the spectral ratio positive corresponding to an amplifying zone; - spatially interpolate said plurality of time series of logarithm of the spectral ratio to determine a plurality of spectral ratio logarithm maps - Determine an average map from the logarithm maps of the spectral ratio representing the attenuating and / or amplifying areas over the observation period; - Determine a spectral variance map from the logarithm maps of the spectral ratio representing active and inactive areas, an active area being an area in which the logarithm of the spectral ratio presents a variance greater than or equal to a predetermined threshold value, an inactive zone being a zone in which the logarithm of the spectral ratio presents a variance lower than a predetermined threshold value. System according to claim 1 characterized in that said integer M is equal to 3, said sensors being aligned. System according to claim 1 or 2 characterized in that said sensors are geophones or hydrophones. System according to any one of claims 1 to 3 characterized in that the reference spectral amplitude value is a spectral amplitude value given at a predetermined time over the time interval. System according to any one of claims 1 to 3 characterized in that the reference spectral amplitude value is the average of all spectral amplitude values ​​over the observation period. System according to any one of claims 1 to 3 characterized in that the reference spectral amplitude value is the median of the set of spectral amplitude values ​​over the observation period. A method for temporal analysis of the spectral energy of seismic waves intended for the detection and visualization of zones of fluid movement and alteration in a geological subsoil, spectral energy being the distribution of the energy of a wave in a plurality of frequencies, said method implementing a temporal analysis system of the spectral energy of seismic waves intended for the detection and visualization of zones of fluid movement and alteration in a geological subsoil according to any one of claims 1 to 6, characterized in that said method comprises the following steps: - an acquisition step (E1) of at least one measurement signal during an observation period; - a reconstruction step of a series of waveforms (E2) by interferometry of the measurement signal for each pair of sensors at a first regular time interval during an observation period to form a correlogram;- a step of reconstructing a series of waveform spectra (E3) for each pair of sensors over a second regular time interval during said observation period to form a spectrogram, the first regular time interval being identical to the first regular time interval; - a step of calculating the spectral variance over time (E4) over time for each pair of sensors during said observation period for each frequency; - a step of determining a frequency range (E5) resolved into a finite number of frequencies; and extending from a minimum frequency to a maximum frequency for each pair of sensors based on the non-zero spectral variances calculated during the spectral variance calculation step over time (E4); - a step for calculating the logarithm of the spectral ratio (E6) of each pair of sensors by the following mathematical formula: with being a frequency value in the frequency range; being a spectral amplitude value at time for the frequency ; being a reference spectral amplitude value for the frequency ;And being the finite number of frequencies for which the frequency range is resolved; the calculation of the logarithm of the spectral ratio from a plurality of sensor pairs determining time series of logarithm of the spectral ratio ; a logarithm of the spectral ratio negative corresponding to an attenuating zone, a logarithm of the spectral ratio positive corresponding to an amplifying zone; - a spatial interpolation step (E7) of said plurality of time series of logarithm of the spectral ratio to determine a plurality of spectral ratio logarithm maps each logarithm map of the spectral ratio being determined from one of the time series at the same time t; - a step of determining an average map (E8) from the logarithm maps of the spectral ratio - the average map representing the attenuating and / or amplifying areas over the observation period; - a step of determining a spectral variance map (E9) from the logarithm maps of the spectral ratio The variance map represents active and inactive areas, an active area being an area in which the logarithm of the spectral ratio presents a variance greater than a predetermined threshold value. A method according to claim 7, characterized in that the method comprises a step of determining the slope of the logarithm of the spectral ratio between said step of calculating the logarithm of the spectral ratio (E6) and said spatial interpolation step (E7). Method according to claim 7 or 8 characterized in that said spatial interpolation step (E7) is carried out by a tomographic inversion process. Computer program comprising program code instructions for executing the steps of the method for temporal analysis of the spectral energy of seismic waves intended for the detection and visualization of areas of fluid movement and alteration in a geological subsoil according to any one of claims 7 to 9, when said program is running on a controller.

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