Patent Publication Number: US-2023160789-A1

Title: Method for calculating a stream of at least one gas emitted by a source into the atmosphere, measurement method, and associated system and kit

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is a U.S. National Phase Application under 35 U.S.C. § 371 of International Patent Application No. PCT/EP2021/059173 filed Apr. 8, 2021, which claims priority of French Patent Application No. 20 03527 filed Apr. 8, 2020. The entire contents of which are hereby incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a method for computing a flow of at least one gas emitted by a source into the atmosphere, implemented by a computing system. 
     BACKGROUND 
     The gases to be measured are especially greenhouse gases such as methane or carbon dioxide. 
     Preoccupations regarding protection of the environment have contributed to reinforcement of legislation on polluting emissions, especially in Europe. 
     Thus, industrial units, such as present in the petroleum or chemical industry, must adapt to increasingly demanding environmental constraints. 
     In particular, greenhouse gases are emitted during operations of extracting, transporting, refining and storing hydrocarbons. These emissions are tracked by operators and are regularly subject to reduction measures. 
     It is in particular necessary to characterize the sources of these greenhouse gases and the amounts of greenhouse gases emitted by these sources, with a view to ensuring that they are controlled and to reporting progress made. 
     However, the techniques used to identify sources of greenhouse gases and quantify diffuse and short-lived emissions are still not entirely satisfactory. 
     Specifically, these emissions are very difficult to measure, because they are often unchannelled, and potentially located close to pools or lakes or inaccessible locations, for example at height or at the centre of the unit in question. 
     One major difficulty in evaluating the emissions of a point source within an installation is often the difficulty or even the inability to get as close as possible to the source in order to measure the flow of gas emitted by the source into the atmosphere. Furthermore, given wind, the flow of gas produced by the source disperses and propagates into the atmosphere in the form of a plume. Measuring the emissions emitted by a point source is therefore generally difficult and inaccurate when at a distance from the source. 
     SUMMARY 
     One aim of the invention is to provide a method for computing the flow of at least one gas emitted by a source into the atmosphere, in particular a greenhouse gas, the method not requiring any data taken as close as possible to the source, while still being accurate and easy to implement. 
     To this end, one subject of the invention is a method of the abovementioned type, comprising the following steps:
         retrieving data about amounts of at least one gas, said data being measured in the atmosphere at a distance from the source along a plurality of lines parallel to a first direction;   integrating the amounts read on each line in the first direction in order to obtain an integrated overall amount on each line;   integrating the product of the integrated overall amounts on each line and a wind speed present on the line in a second direction perpendicular to the first direction, in order to obtain a raw flow of gas;   determining the flow of gas emitted by the source based on the raw flow of gas.       

     The method according to the invention may comprise one or more of the following features, alone or in any technically possible combination:
         the method comprises, between the integration steps, a step of interpolating a curve of integrated overall amounts as a function of a coordinate in the second direction, based on the computed integrated overall amounts;   the interpolation is carried out using a cubic interpolation, in particular using a piecewise cubic interpolation;   the method comprises a preliminary step of retrieving data about wind speeds present on each line;   the method comprises a preliminary step of determining an average wind common to all of the lines;   the method comprises, after the step of retrieving the amount data, computing a continuous background of gas present in the atmosphere and processing the amount data in order to eliminate the continuous background;   determining the continuous background comprises, for each line, computing an average value of amounts, measured on the line, and then eliminating amounts above the average value, and repeating the previous steps until the difference between two successive average values is less than a convergence threshold;   the first direction is horizontal, the second direction being vertical;   the flows of at least two gases emitted by the source are computed.       

     Another subject of the invention is a method for measuring emissions of a source into the atmosphere, comprising the following steps:
         collecting amounts of at least one gas in the atmosphere at a distance from the source along a plurality of lines parallel to a first direction by flying a drone equipped with an assembly for measuring data representative of amounts of at least one gas;   transferring the collected representative data to a computing system;   using the computing system to implement the computing method as defined above.       

     The measuring method according to the invention may comprise one or more of the following features, alone or in any technically possible combination:
         the method comprises a preliminary step of determining a wind direction and/or a configuration of an emission plume downstream of the source, the drone being flown based on the predetermined plume configuration.   the method comprises a step of measuring the wind speed.       

     Another subject of the invention is a system for computing a flow of at least one gas emitted by a source into the atmosphere, comprising:
         a module for obtaining data about amounts of at least one gas, said data being measured in the atmosphere at a distance from the source along a plurality of lines parallel to a first direction;   a module for integrating the amounts read on each line in the first direction in order to obtain an integrated overall amount on each line;   a module for integrating the product of the integrated overall amounts on each line and a wind speed present on the line in a second direction perpendicular to the first direction, in order to obtain a raw flow of gas;   a module for determining the flow of gas emitted by the source based on the raw flow of gas.       

     Another subject of the invention is a kit for measuring the emissions of at least one gas by a source into the atmosphere, comprising:
         a drone, able to fly in the atmosphere at a distance from the source along a plurality of lines parallel to a first direction;   the drone being able to measure data representative of amounts of at least one gas along each line parallel to the first direction;   a computing system as defined above, able to receive the representative data measured by the drone.       

     The kit according to the invention may comprise one or more of the following features, alone or in any technically possible combination:
         the drone is able to continuously measure data representative of the amounts of at least two gases present in the atmosphere.       

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be better understood on reading the following description, which is given merely by way of example, and with reference to the appended drawings, in which: 
         FIG.  1    is a schematic view of a first emissions measurement kit according to the invention; 
         FIG.  2    is a view of a gas source within an installation, and of the plume emitted by the gas source; 
         FIG.  3    is a detailed view of the plume resulting from the source emitting in established wind conditions; 
         FIG.  4    is a view of the flight plan implemented by the drone of the kit from  FIG.  1   ; 
         FIG.  5    is a view of the measurements carried out on a horizontal line when implementing the flight plan from  FIG.  4   ; 
         FIG.  6    is a view of a curve linking integrated amounts obtained from multiple lines as a function of altitude, and the interpolation performed between these amounts; 
         FIG.  7    is a view of an estimate of wind speed as a function of altitude, able to be used in the implementation of the method according to the invention; 
         FIG.  8    is a view similar to  FIG.  3   , in the case of an emission in low-wind conditions. 
     
    
    
     DETAILED DESCRIPTION 
     A kit  10  for measuring emissions of at least one gas emitted by a source into the atmosphere is illustrated schematically in  FIG.  1   . The kit  10  is intended to implement a method for measuring emissions of an industrial installation  12 , shown schematically in  FIG.  2   . 
     Preferably, the emission of at least two gases present in the atmosphere is measured by the method according to the invention. The gases are preferably methane and carbon dioxide. 
     In some variants, other gases may be measured, such as aromatic gases, especially benzene or even 1,3-butadiene, carbon monoxide, ethane and more generally volatile organic compounds. 
     The industrial installation  12  is in particular a petroleum installation, in particular a hydrocarbon extraction, transportation, refining, processing or storage installation located at sea or on land. The installation  12  comprises at least one source  14  emitting gases the amount of which is measured. 
     In the example shown in  FIG.  3   , the source  14  is a flare. It emits gases in a plume  16  that is released from the source  14  and that propagates under the effect of the wind V. 
     The plume  16  is entrained by the wind V blowing in the atmosphere close to the source  14 . It advantageously has an area  18  in which the plume  16  rises, which is substantially vertical, and an area  20  in which the plume propagates, which is substantially horizontal in this example. 
     In the example of  FIG.  8   , if the wind V is lower, the rise area  18  is higher and the propagation area  20  extends in a manner inclined with respect to the horizontal. 
     With reference to  FIG.  1   , in order to implement the measuring method, the measurement kit  10  comprises a drone  22  for collecting data representative of amounts of at least one gas, preferably of at least two gases, at a plurality of positions in the atmosphere, at a distance from the source  14 . 
     The kit  10  furthermore comprises a computing system  24 , able to implement a method for computing a flow of the or of each gas emitted by the source  14  into the atmosphere, based on data representative of amounts of each gas in the atmosphere as measured by the drone  22 . 
     The drone  22  is able to carry out the measurements needed to collect data representative of the amounts of at least one gas present in the plume  16 , at a distance from the source  14 . It comprises a chassis  30 , and a propelling assembly  32 , which is able to allow the chassis  30  to take off away from the ground and it to move by flying through the atmosphere above the ground. 
     The drone  22  furthermore comprises a measuring assembly  34 , a control assembly  36  for controlling the measuring assembly  34 , and preferably a remote transmission system  38 . 
     With reference to  FIG.  1   , the propelling assembly  32  comprises a plurality of propelling members  32 A, which here are propellers driven to rotate by a motor. 
     The propelling assembly  32  furthermore comprises a power source  32 B, formed here by a battery, and a system  33  for locating and for controlling the movement of the drone  22  through the atmosphere. 
     In this example, the drone  22  is a multi-rotor rotary-wing drone. It does not have any fixed wings, its lift being generated by the propelling assembly  32 . 
     The drone  22  is for example a rotary-wing quadcopter drone, and especially a DJI M200 drone as sold by DJI. 
     The propelling assembly  32  comprises a plurality of propellers that rotate about substantially vertical axes. “Substantially vertical” is generally understood to mean that the axes of rotation of the propellers are inclined by less than 30° with respect to the vertical. 
     When the motors of the propellers are supplied with electric power by the battery, the propellers are driven to rotate about their axis, driving a downward flow of air. 
     The locating and control system  33  comprises a position sensor, especially a GPS and/or an inertial measurement unit. It furthermore comprises a control unit, which is able to control the movement of the drone  22  along a path pre-recorded before the flight and loaded into the system  33 , or remotely and manually via a remote control. 
     The drone  22  is thus able to automatically follow a predefined path, or, alternatively, to be controlled manually by an operator, in order to implement a flight plan. 
     Preferably, in order to implement the measuring method, the drone  22  is able to take a path following a ladder-shaped movement, as illustrated by  FIG.  4   . 
     The drone  22  moves along a plurality of lines  50  parallel to a first direction D 1 , with a connecting segment  52  between each pair of adjacent parallel lines  50 . The connecting segment  52  follows a second direction D 2  transverse to the first direction D 1 . 
     Here, the first direction D 1  is a horizontal direction and the second direction D 2  is a vertical direction. 
     In this example, all of the parallel lines  50  scanned by the drone  22  extend substantially in one and the same vertical measuring plane Pm. 
     The extent E 1  of the lines  50  in the first direction D 1  is chosen based on the width of the plume  16 , in order to scan the entire plume  16 . This extent E 1  is generally greater than 20 m and is between 20 m and 500 m. 
     The distance between the lines  50  is defined by an extent E 2  of the connecting segments  52  in the second direction. This extent E 2  is for example greater than 1 m and in particular between 1 m and 50 m. 
     The measuring assembly  34  comprises at least one sensor able to carry out measurements of data representative of amounts of at least one gas present in the atmosphere, at a plurality of points along each line  50 . 
     Preferably, the data representative of the amounts of at least two gases are collected by the measuring assembly  34  along each line  50 . 
     The measurements are carried out continuously along the line  50 . The measurement frequency of data representative of each gas amount is for example greater than 1 Hz and in particular between 1 Hz and 100 Hz. 
     One example of a measuring assembly  34  is described in application no. 20 03027 from the Applicant, filed at the Institut National de la Propriété Industrielle in France, entitled “Drone for measuring data representative of amounts of at least two gases present in the atmosphere away from the ground and associated measuring method”. 
     The control system  33  comprises a data collection unit that comprises at least one memory able to store the data representative of each amount of each gas, in association with the geographical position along each line  50 . 
     The data collection unit is connected to the remote transmission system  38  in order to allow the data to be exported to the computing system  24  when the drone is flying or after the drone has flown. 
     The computing system  24  is located on the ground here. It comprises at least a computer  60  and a human-machine interface comprising a control member  62  such as a keyboard, a mouse and/or a touchscreen, the human-machine interface also comprising a display  64 , in particular a screen. 
     The computer  60  comprises, as is known, at least a processor  66  and a memory  68  comprising software modules able to be executed by the processor  66  in order to carry out functions. As a variant, the computer  60  comprises programmable logic components or dedicated integrated circuits intended to carry out the functions of the modules that will be described below. 
     With reference to  FIG.  1   , the memory  68  contains a module  70  for obtaining and initially processing data representative of amounts of at least one gas in order to compute, on each parallel line  50 , successive amounts of at least one gas along the parallel line  50 , one example of which may be seen in  FIG.  5   . 
     The memory  68  furthermore contains a module  72  for integrating the amounts on each line  50  in the first direction D 1  in order to obtain an integrated overall amount TGI on each line  50 . 
     The memory  68  furthermore contains a module  74  for interpolating a curve  75  of integrated overall amounts TGI along the second direction D 2  transverse to the first direction D 1  (see  FIG.  6   ), based on the integrated overall amounts TGI computed on each line  50 , and a module  76  for integrating the product of the integrated overall amount TGI in the first direction and a wind speed V, the integration being carried out in the second direction D 2 , in order to maintain a raw flow Qb of gas flowing in the plume  16 . 
     The memory  68  also contains a module  78  for determining a flow Qg of gas emitted by the source  14  by correcting the raw flow of gas Qb as a function of the structure of the plume to obtain. 
     The obtaining and processing module  70  is able to receive the data representative of the measured amounts of at least one gas, preferably of at least two gases, along each line  50 , as measured by the drone  22  at each measurement point, in association with the geographical position X of the measurement point along the line  50 . 
     It is able to transform the measured representative data into amounts of each of the gases at each measurement point X on each line  50  on the basis of a calibration curve associated with each gas. 
     A curve  71  of amounts T of each gas as a function of a first coordinate X along the line  50  in the direction D 1  is thus obtained, as illustrated in  FIG.  5   . 
     The obtaining and processing module  70  is furthermore possibly able to filter the obtained amounts. 
     According to a first method, the obtaining and processing module  70  is able to detect amount peaks  71 A on each curve  71 , on the basis of a predetermined threshold S for the occurrence of a peak, and then to eliminate the observed peaks  71 A from the obtained curve in order to obtain a curve of background values as a function of the first coordinate X. 
     In one variant, the obtaining and processing module  70  is able to implement an iterative algorithm in which the average value of the amounts along the line  50  is computed, and then in which all of the amounts above the average value are eliminated from the curve  71 , and then to repeat the steps of computing the average value and of subtracting amounts above the average value until a convergence criterion is met. 
     The convergence criterion is for example that the difference between the successive average values between two iterations is less than a predetermined value, for example less than 10%. 
     A continuous background is thus determined and is subtracted from the curve  71  representing the amounts T as a function of the position X on each line  50 . 
     The integration module  72  is able to integrate the curve  71  representing the amounts of each gas along each line  50 , in the first direction D 1 , over the entire width of the line  50  in order to obtain an integrated overall amount TGI on each line  50 , using the following equation: 
         TGI=∫   Xmin   Xmax   T ( X ) dX            where X min and X max are the geographical coordinates characterizing the limits of the plume as defined along the extent E 1  of the plume parallel to the first direction D 1 .       
     According to the first data processing method performed by the module  70 , the integral of the curve of the background values is also computed and is subtracted from the previous integral. 
     According to the second method, the curve of background values is subtracted from the curve  70  of the amounts before integration. 
     Thus, for each line  50  in which a measurement has taken place, corresponding to a coordinate Z in the second direction D 2 , an integrated overall amount TGI(Z) is obtained. 
     The interpolation module  74  is able to interpolate, based on the integrated overall amounts TGI(Z) on each line  50 , in combination with their coordinates Z in the second direction, a continuous curve  75  of integrated overall amounts TGI as a function of the coordinate Z in the second direction D 2 , as illustrated by  FIG.  6   . The interpolated curve  75  is for example obtained using a cubic interpolation, in particular a piecewise cubic interpolation until convergence. 
     The integration module  76  is able to integrate the product of the wind speed V(Z) measured or obtained at each coordinate Z along the second direction D 2  with the integrated overall amount TGI(Z) corresponding to this coordinate, obtained from the interpolated curve  75 , in order to obtain a raw flow Qb passing through the measuring plane Pm using the following formula: 
         Qb=∫   Zmin   Zmax   V ( Z )× TGI ( Z ) dZ  
         where Zmin and Z max are the minimum and maximum coordinates along the second direction D 2  for which a line  50  of measurements was obtained.       

     In the example illustrated by  FIG.  2   , the wind speed V(Z) is taken as a constant average wind speed along the second coordinate Z. 
     As a variant, a curve of wind V(Z) as a function of the second coordinate along the second direction Z is established, as illustrated by  FIG.  7   , and the wind speed V(Z) at each second coordinate Z is used to compute the product with the integrated overall amount TGI(Z) at the second coordinate Z and carry out the integration. 
     The integration module  76  is thus able to obtain a total raw flow Qb of each measured gas passing through the measuring plane Pm, which may be seen in  FIG.  3   . 
     Next, the determination module  78  is able to correct the value of the measured total raw flow Qb in order to take into account the structure of the plume  16 . 
     For example, if the measuring plane Pm is vertical, an angle of incline a of the direction of the flow in the plume  16  in the measuring plane Pm is computed, as a function of a value of the height of the rise area  18 , and of an assumed plume shape in the transport area  20 , computed as a function of the wind. 
     A total gas flow Qt passing through a plane Pp perpendicular to the flow direction is then computed on the basis of the raw flow Qb computed by the integration module  76  and of the determined angle of incline a, for example assuming that the cross section of the plume is circular perpendicular to the flow. 
     According to the principle of the conservation of mass, the flow of gas Qg emitted by the source  14  is then equal to the flow passing through the plane Pp. 
     A measuring method will now be described. Initially, the drone  22  is put into flight in order to take a path following a ladder-shaped movement in a measuring plane Pm, as illustrated by  FIG.  4   . 
     As indicated above, the drone  22  moves along a plurality of lines  50  parallel to a first direction D 1  with a connecting segment  52  between each pair of adjacent parallel lines  50 , the connecting segment  52  following a second direction D 2  transverse to the first direction D 1 . 
     The data representative of the amounts of at least one gas, preferably of at least two gases, are collected by the measuring assembly  34  along each line  50 . 
     The measurements are carried out continuously along the line  50 . 
     The memory of the data collection unit stores the data representative of each amount of each gas, in association with the geographical position X along each line  50 . 
     Next, while the drone  22  is flying or after the drone  22  has flown, the remote transmission system  38  exports data to the computing system  24  on the ground. 
     The obtaining and processing module  70  receives the data representative of the measured amounts of at least one gas, preferably of at least two gases, along each line  50 , as measured by the drone  22  at each measurement point, in association with the geographical position X of the measurement point along the line  50 . 
     It transforms the representative data into amounts of each of the gases at each measurement point X on each line  50  on the basis of a calibration curve associated with each gas. For each line  50 , a curve  71  of amounts of each gas as a function of a first coordinate X along the line  50  in the direction D 1  is thus obtained, as may be seen in  FIG.  4   . 
     The obtaining and processing module  70  possibly filters the obtained amounts, for example using the first method or the second method described above. 
     Next, the integration module  72  integrates the curve  71  representing the amounts of each gas along each line  50 , in the first direction T 1 , over the entire width of the line  50  in order to obtain an integrated overall amount TGI on each line  50 , using the equation given above: 
         TGI=∫   Xmin   Xmax   T ( X ) dX    
     According to the first data processing method performed by the module  70 , the integral of the curve of the background values is also computed and is subtracted from the previous integral. 
     According to the second method, the curve of background values is subtracted from the curve  70  of the amounts before integration. 
     Thus, for each line  50  in which a measurement has taken place, corresponding to a coordinate Z in the second direction D 2 , an integrated overall amount TGI(Z) is obtained. 
     The interpolation module  74  then interpolates, based on the integrated overall amounts TGI on each line  50 , in combination with their coordinates Z in the second direction, a continuous curve  75  of integrated overall amounts TGI as a function of the coordinate Z in the second direction D 2 , as illustrated by  FIG.  5   . The interpolated curve  75  is for example obtained using a cubic interpolation, in particular a piecewise cubic interpolation until convergence. 
     The integration module  76  then integrates the product of the wind speed V(Z) measured or obtained at each coordinate Z along the second direction D 2  with an integrated overall amount TGI(Z) corresponding to this coordinate (Z), obtained from the interpolated curve  75 , in order to obtain a raw flow Qb passing through the measuring plane using the following formula: 
         Qb=∫   Zmin   Zmax   V ( Z )× TGI ( Z ) dZ  
 
     In the example illustrated by  FIG.  2   , the wind speed V(Z) is taken as a constant average wind speed along the second coordinate Z. 
     As a variant, a curve of wind V(Z) as a function of the second coordinate along the second direction Z is established, as illustrated by  FIG.  7   , and the wind speed V(Z) at each second coordinate Z is used to compute the product with the integrated overall amount TGI(Z) at the second coordinate Z and carry out the integration. 
     The integration module  76  thus obtains a total raw flow Qb of each measured gas passing through the measuring plane Pm. 
     Next, the correction module  78  corrects the value of the measured total raw flow Qb in order to take into account the structure of the plume  16 , as described above. 
     For example, if the measuring plane Pm is vertical, an angle of incline a of the direction of the flow in the plume  16  in the measuring plane Pm, along with a speed of change of the plume  16  in the measuring plane Pm, are computed as a function of a value of the height of the rise area  18 , and of an assumed plume shape in the transport area  20 . 
     A total gas flow Qg passing through a plane Pp perpendicular to the flow direction is then computed on the basis of the raw flow Qb computed by the integration module  76  and of the determined angle of incline a and of the computed speed of change of the plume  16 , for example assuming that the cross section of the plume is circular perpendicular to the flow. The speed of change is computed along the axis of the plume and corresponds to the norm of the speed vector of the plume, which is perpendicular to the plane Pp. 
     According to the principle of the conservation of mass, the flow Qg of gas emitted by the source  14  is then equal to the flow passing through this plane. 
     The measuring method according to the invention is therefore particularly easy to implement, since it requires a simple measuring campaign using a drone  22  flying directly into the plume  16 , at a distance from the source  14 . 
     Following this measuring campaign, the computation is simple and effective in order to obtain an accurate determination of the flow emitted by the source  14 . 
     This method is able to be implemented close to various industrial installations  12 , even if these installations are inaccessible or/and require safety precautions. The measurements may be performed at low cost and frequently, thereby making it possible especially to track the evolution of emissions brought about by the source  14 , and to ensure that they are under control or that they are reduced. 
     In one variant, shown in  FIG.  8   , for low-wind conditions, the plume  16  has a vertical configuration and the measuring plane Pm is a horizontal plane. The first direction D 1  is then a horizontal direction, and the second direction D 2  is a horizontal direction perpendicular to the first direction D 1 . The measuring process, including the measuring method, remain similar to those described above. 
     Advantageously, the measuring method according to the invention comprises an initial step of determining structural characteristics of the plume  16 , for example by measuring the wind rose applicable to the source  14  at the time of the measuring campaign. 
     In another variant, the wind speed is measured directly right at the installation  12 , for example by a lidar wind measuring device  100 , as illustrated by  FIG.  2   . 
     As a variant, the wind speed is measured by a sensor carried by the drone  22 , when the drone  22  is present on each line  50 , or even advantageously at each point of measurement of an amount of gas on a line  50  by the drone  22 . 
     Advantageously, the wind speed is measured at a plurality of successive times, preferably corresponding to the times of measurement of an amount of gas on each line  50 . 
     In these variants, the integration module  72  is able to integrate the curve representing the products T(X, Z) x V(X, Z) of the amounts T(X, Z) of each gas read at each measurement point along each line  50 , in the first direction D 1 , over the entire width of the line and the wind speed V(X, Z) at the measurement point, in order to obtain an integrated overall product PGI(Z) on each line  50 , using the following equation: 
         PGI ( Z )=∫ Xmin   Xmax   T ( X,Z )× V ( X,Z ) dX  
         where X min and X max are the geographical coordinates characterizing the limits of the plume as defined along the extent E 1  of the plume parallel to the first direction D 1 .       

     Advantageously, as described above, a curve of wind V(Z, t) as a function of the second coordinate in the second direction Z is established at a plurality of successive times t. The wind speed V(X, Z) used to compute the product with the measured amount T(X, Z) is chosen to be equal to the wind speed V(Z, t) measured at the second coordinate Z of the line  50  on which the amount T(X, Z) is measured, at the time closest to the time of measurement of the amount T(X, Z). 
     When the wind speed V(X, Z) is measured by a sensor carried by the drone  22 , the wind speed V(X, Z) is preferably measured upon each measurement of the amount T(X, Z). 
     The product T(X, Z) x V(X, Z) is thus obtained at each measurement point on each line  50  using measured values of T(X) and V(X, Z). 
     According to the first data processing method performed by the module  70 , the integral of the curve of the background values is also computed and is subtracted from the previous integral. 
     According to the second method, the curve of background values is subtracted from the curve  70  of the amounts before integration. 
     Thus, for each line  50  in which a measurement has taken place, corresponding to a coordinate Z in the second direction D 2 , an integrated overall product PGI(Z) is obtained. 
     The interpolation module  74  is able to interpolate, based on the integrated overall products PGI(Z) on each line  50 , in combination with their coordinates Z in the second direction, a continuous curve of integrated overall products PGI as a function of the coordinate Z in the second direction D 2 . The interpolated curve is for example obtained using a cubic interpolation, in particular a piecewise cubic interpolation until convergence. 
     The integration module  76  is able to integrate the integrated overall products obtained from the interpolated curve  75  in order to obtain a raw flow Qb passing through the measuring plane Pm using the following formula: 
         Qb=∫   Zmin   Zmax   PGI ( Z ) dZ            where Zmin and Z max are the minimum and maximum coordinates along the second direction D 2  for which a line  50  of measurements was obtained.       
     The integration module  76  is thus able to obtain a total raw flow Qb of each measured gas passing through the measuring plane Pm, which may be seen in  FIG.  3   . 
     The methods that are described as a variant greatly improve the accuracy of the measurement of the total raw flow Qb, while retaining simple integration in comparison with Kriging methods. 
     Moreover, the total raw measurement Qb is more accurate if the wind speed changes rapidly or if an optical present between the source  14  and a line  50  affects the wind speed on the line  50 .