# Patent

## Title
Gas detection systems and methods with search directions

## Abstract

In some embodiments, at least one processor determines a range of search directions to a suspected gas leak source by receiving data representative of a plurality of wind direction measurements relative to a geo-referenced location of at least one gas concentration measurement point. The range of search directions extending from the location of the gas concentration measurement point is calculated according to a variability of the wind direction measurements.

## Background
CROSS REFERENCE TO RELATED APPLICATIONS
This application claims the benefit of U.S. provisional patent application 61/646,487 filed on May 14, 2012, titled “Gas Detection Systems and Methods”, which application is hereby incorporated by reference in its entirety.


BACKGROUND
The invention relates to systems and methods for detecting gas leaks such as a methane gas leaks.
A common means of distributing energy around the world is by the transmission of gas, usually natural gas. In some areas of the world manufactured gasses are also transmitted for use in homes and factories. Gas is typically transmitted through underground pipelines having branches that extend into homes and other buildings for use in providing energy for space and water heating. Many thousands of miles of gas pipeline exist in virtually every major populated area. Since gas is highly combustible, gas leakage is a serious safety concern. Recently, there have been reports of serious fires or explosions caused by leakage of gas in the United States as the pipeline infrastructure becomes older. For this reason, much effort has been made to provide instrumentation for detecting small amounts of gas so that leaks can be located to permit repairs.
Conventionally, search teams are equipped with gas detectors to locate a gas leak in the immediate proximity of the detector. When the plume of gas from a leak is detected, the engineers may walk to scan the area slowly and in all directions by trial and error to find the source of the gas leak. This process may be further complicated by wind that quickly disperses the gas plume. Such a search method is time consuming and often unreliable, because the engineer walks around with little or no guidance while trying to find the source of the gas leak.
Another approach to gas leak detection is to mount a gas leak detection instrument on a moving vehicle, e.g., as considered in U.S. Pat. No. 5,946,095. A natural gas detector apparatus is mounted to the vehicle so that the vehicle transports the detector apparatus over an area of interest at speeds of up to 20 miles per hour. The apparatus is arranged such that natural gas intercepts a beam path and absorbs representative wavelengths of a light beam. A receiver section receives a portion of the light beam onto an electro-optical etalon for detecting the gas. Although a moving vehicle may cover more ground than a surveyor on foot, there is still the problem of locating the gas leak source (e.g., a broken pipe) if a plume of gas is detected from the vehicle. Thus, there is still a need to provide a method and apparatus to locate the source of a gas leak quickly and reliably.
SUMMARY
According to one aspect, a method comprises employing at least one processor to determine a range of search directions to a suspected gas leak source by receiving data representative of a plurality of wind direction measurements relative to a geo-referenced location of at least one gas concentration measurement point. The range of search directions extending from the location of the gas concentration measurement point is calculated according to a variability of the wind direction measurements.
According to another aspect, an apparatus comprises at least one processor programmed to determine a range of search directions to a suspected gas leak source by receiving data representative of a plurality of wind direction measurements relative to a geo-referenced location of at least one gas concentration measurement point. The range of search directions extending from the location of the gas concentration measurement point is calculated according to a variability of the wind direction measurements.
According to another aspect, a non-transitory computer-readable medium encodes instructions which, when executed by a computer system, cause the computer system to determine a range of search directions to a suspected gas leak source by receiving data representative of a plurality of wind direction measurements relative to a geo-referenced location of at least one gas concentration measurement point. The range of search directions extending from the location of the gas concentration measurement point is calculated according to a variability of the wind direction measurements.
According to another aspect, a method comprises employing at least one processor to determine a probable direction and an estimated maximum distance to a suspected gas leak source by receiving data representative of wind direction and wind speed relative to a geo-referenced location of at least one gas concentration measurement point. The probable direction to the suspected gas leak source relative to the location of the gas concentration measurement point is determined according to the data representative of wind direction. The method also includes the step of determining a maximum detection distance value representative of the estimated maximum distance from the suspected gas leak source at which a leak can be detected. The maximum detection distance value is determined according to the data representative of wind speed.
According to another aspect, an apparatus comprises at least one processor programmed to determine a probable direction and an estimated maximum distance to a suspected gas leak source by receiving data representative of wind direction and wind speed relative to a geo-referenced location of at least one gas concentration measurement point. The probable direction to the suspected gas leak source relative to the location of the gas concentration measurement point is determined according to the data representative of wind direction. The processor is also programmed to determine a maximum detection distance value representative of the estimated maximum distance from the suspected gas leak source at which a leak can be detected. The maximum detection distance value is determined according to the data representative of wind speed.
According to another aspect, a non-transitory computer-readable medium encodes instructions which, when executed by a computer system, cause the computer system to determine a probable direction and an estimated maximum distance to a suspected gas leak source by receiving data representative of wind direction and wind speed relative to a geo-referenced location of at least one gas concentration measurement point. The probable direction to the suspected gas leak source relative to the location of the gas concentration measurement point is determined according to the data representative of wind direction. Also determined is a maximum detection distance value representative of the estimated maximum distance from the suspected gas leak source at which a gas leak can be detected. The maximum detection distance value is determined according to the data representative of wind speed.



BRIEF DESCRIPTION OF THE DRAWINGS
The foregoing aspects and advantages of the present invention will become better understood upon reading the following detailed description and upon reference to the drawings where:
FIG. 1 shows a gas leak detection apparatus according to some embodiments of the present invention.
FIG. 2 illustrates hardware components of a computer system according to some embodiments of the present invention.
FIG. 3 shows a number of application or modules running on a client computer system and a corresponding server computer system according to some embodiments of the present invention.
FIG. 4 is a schematic drawing of a screen shot on a graphical user interface displaying survey results on a street map according to some embodiments of the present invention.
FIG. 5 is a schematic drawing of a screen shot on a graphical user interface with GPS indicators according to some embodiments of the present invention.
FIG. 6 is a schematic drawing of a screen shot on a graphical user interface with weather station status indicators according to some embodiments of the present invention.
FIG. 7 is a schematic drawing of a screen shot on a graphical user interface with map controls according to some embodiments of the present invention.
FIG. 8 is a schematic diagram of three search area indicators according to some embodiments of the present invention.
FIG. 9 is a schematic diagram illustrating wind lines relative to the path of a mobile gas measurement device for detecting or not detecting a gas leak from a potential gas leak source according to some embodiments of the present invention.
FIG. 10 is a schematic diagram of wind direction and a path of a mobile gas measurement device used to estimate a probability of detection of a gas leak from a potential gas leak source at one or more measurement points along the path according to some embodiments of the present invention.
FIG. 11 is a graph of probability density vs. wind directions for estimating a probability of detection of a gas leak from a potential gas leak source according to some embodiments of the present invention.
FIG. 12 is a flow chart showing steps for performing a gas leak survey according to some embodiments of the present invention.
FIG. 13 is a flow chart showing steps for generating a search area indicator according to some embodiments of the present invention.
FIG. 14 is a flow chart showing steps for calculating a boundary of a survey area according to some embodiments of the present invention.
FIG. 15 is a flow chart showing steps for displaying layers overlaid or superimposed on a street map according to some embodiments of the present invention.
FIG. 16 is a graph of vertical dispersion coefficients of a gas plume as a function of downwind distance from a gas leak source according to some embodiments of the present invention.
FIG. 17 is a graph of crosswind dispersion coefficients of a gas plume as a function of downwind distance from a gas leak source according to some embodiments of the present invention.
FIG. 18 is a table of dispersion coefficients for various atmospheric conditions according to some embodiments of the present invention.



DETAILED DESCRIPTION
Apparatus and methods described herein may include or employ one or more interconnected computer systems such as servers, personal computers and/or mobile communication devices, each comprising one or more processors and associated memory, storage, input and display devices. Such computer systems may run software implementing methods described herein when executed on hardware. In the following description, it is understood that all recited connections between structures can be direct operative connections or indirect operative connections through intermediary structures. A set of elements includes one or more elements. Any recitation of an element is understood to refer to at least one element. A plurality of elements includes at least two elements. Unless otherwise required, any described method steps need not be necessarily performed in a particular illustrated order. A first element (e.g. data) derived from a second element encompasses a first element equal to the second element, as well as a first element generated by processing the second element and optionally other data. Making a determination or decision according to a parameter encompasses making the determination or decision according to the parameter and optionally according to other data. Unless otherwise specified, an indicator of some quantity/data may be the quantity/data itself, or an indicator different from the quantity/data itself. Computer programs described in some embodiments of the present invention may be stand-alone software entities or sub-entities (e.g., subroutines, code objects) of other computer programs. Computer readable media encompass storage (non-transitory) media such as magnetic, optic, and semiconductor media (e.g. hard drives, optical disks, flash memory, DRAM), as well as communications links such as conductive cables and fiber optic links. According to some embodiments, the present invention provides, inter alia, computer systems programmed to perform the methods described herein, as well as computer-readable media encoding instructions to perform the methods described herein.
FIG. 1 shows a gas leak detection system 10 according to some embodiments of the present invention. System 10 comprises a service provider server computer system 12 and a set of client computer systems 18a-c all connected through a wide area network 16 such as the Internet. Client computer systems 18a-c may be personal computers, laptops, smartphones, tablet computers and the like. A vehicle 24 such as an automobile may be used to carry at least some client computer systems (e.g. an exemplary client computer system 18c) and associated hardware including a mobile gas measurement device 26, a location/GPS measurement device 30, and a wind measurement device 32. In a preferred embodiment, the mobile gas measurement device 26 may be a Picarro analyzer using Wavelength-Scanned Cavity Ring Down Spectroscopy (CRDS), available from Picarro, Inc., Santa Clara, Calif. Such analyzers may be capable of detecting trace amounts of gases such as methane, acetylene, carbon monoxide, carbon dioxide, hydrogen sulfide, and/or water. In particular applications suited for detection of natural gas leaks, a Picarro G2203 analyzer capable of detecting methane concentration variations of 3 ppb may be used. Wind measurement device 32 may include a wind anemometer and a wind direction detector (e.g. wind vane). GPS measurement device 30 may be a stand-alone device or a device built into client computer system 18c. 
FIG. 2 schematically illustrates a plurality of hardware components that each computer system 18 may include. Such computer systems may be devices capable of web browsing and have access to remotely-hosted protected websites, such as desktop, laptop, tablet computer devices, or mobile phones such as smartphones. In some embodiments, computer system 18 comprises one or more processors 34, a memory unit 36, a set of input devices 40, a set of output devices 44, a set of storage devices 42, and a communication interface controller 46, all connected by a set of buses 38. In some embodiments, processor 34 comprises a physical device, such as a multi-core integrated circuit, configured to execute computational and/or logical operations with a set of signals and/or data. In some embodiments, such logical operations are delivered to processor 34 in the form of a sequence of processor instructions (e.g. machine code or other type of software). Memory unit 36 may comprise random-access memory (RAM) storing instructions and operands accessed and/or generated by processor 34. Input devices 40 may include touch-sensitive interfaces, computer keyboards and mice, among others, allowing a user to introduce data and/or instructions into system 18. Output devices 44 may include display devices such as monitors. In some embodiments, input devices 40 and output devices 44 may share a common piece of hardware, as in the case of touch-screen devices. Storage devices 42 include computer-readable media enabling the storage, reading, and writing of software instructions and/or data. Exemplary storage devices 42 include magnetic and optical disks and flash memory devices, as well as removable media such as CD and/or DVD disks and drives. Communication interface controller 46 enables system 18 to connect to a computer network and/or to other machines/computer systems. Typical communication interface controllers 46 include network adapters. Buses 38 collectively represent the plurality of system, peripheral, and chipset buses, and/or all other circuitry enabling the inter-communication of devices 34-46 of computer system 18.
FIG. 3 shows a number of applications or modules running on an exemplary client computer system 18 and corresponding server computer system 12. Authentication applications 54, 62 are used to establish secure communications between computer systems 12, 18, allowing client computer system 18 selective access to the data of a particular customer or user account. A client data collection module 56 collects real-time gas concentration, location data such as global positioning system (GPS) data, as well as wind speed and wind direction data. A graphical user interface (GUI) module 58 is used to receive user input and display survey results and other GUI displays to system users. A client-side real-time data processing module 60 may be used to perform at least some of the data processing described herein to generate survey results from input data. Data processing may also be performed by a server-side data processing module 68. Server computer system 12 also maintains one or more application modules and/or associated data structures storing current and past survey results 64, as well as application modules and/or data structures storing reference data 66 such as plats indicating the geographic locations of natural gas pipelines.
FIG. 4 is a schematic drawing of a screen shot on a graphical user interface, displaying survey results on a street map 70 according to some embodiments of the present invention. The GUI screenshots are most preferably displayed on a client device in the vehicle, which may be connected to a server as described above. The illustrated screenshots show both exemplary user input, which may be used to control system operation, and exemplary real-time displays of collected/processed data. In the example, it includes the geo-referenced street map 70 showing plat lines 72. The plat lines 72 are preferably derived from gas company records. An active pipeline plat boundary 71 may also be displayed on the map 70. A user-selectable button 96 may be selected to overlay a selected pipeline plat on the map 70. Superimposed on the map 70 are one or more lines (preferably in a distinguishing color not shown in patent drawings) indicating the path 74 driven by the vehicle with the mobile gas measurement device on one or more gas survey routes. In this example, the path 74 shows the vehicle U-turned at the Y-shaped intersection. Optionally, a current location icon 75 may be overlaid on the map 70 to indicate the current surveyor location, e.g., the position of the vehicle with a gas measurement device and wind measurement device. A user-selectable button 94 may be selected to center the map 70 by current surveyor location. Also provided is a user-selectable start button 102 and stop button 100 to start/stop capturing gas for analysis. An analyzer control button 104 is user-selectable to control analyzer operations (e.g., shut down, start new trace, run isotopic analysis, etc.).
Peak markers 76 show the locations along the path 74 where peaks in the gas concentration measurements, which satisfy the conditions for being likely gas leak indications, were identified. The colors of the peak markers 76 may be used to distinguish data collected on different runs. The annotations within the peak markers 76 show the peak concentration of methane at the locations of those measurement points (e.g., 3.0, 2.6, and 2.0 parts per million). An isotopic ratio marker 77 may be overlaid on the map 70 to indicate isotopic ratio analysis output and tolerance (e.g., −34.3+/−2.2). Also displayed on the map 70 are search area indicators 78, preferably shown as a sector of a circle having a distinguishing color. Each of the search area indicators 78 indicates a search area suspected to have a gas leak source. The opening angle of the search area indicator 78 depicts the variability in the wind direction. The axis of the search area indicator 78 (preferably an axis of symmetry) indicates the likely direction to the potential gas leak source. Also displayed on the map 70 are one or more survey area indicators 80 (shown as hatched regions in FIG. 4) that indicate a survey area for a potential gas leak source. The survey area indicator 80 adjoins the path 74 and extends in a substantially upwind direction from the path. The survey area marked by each indicator 80 is preferably displayed as a colored swath overlaid or superimposed on the map 70. For example, the colored swaths may be displayed in orange and green for two runs. In preferred embodiments, the parameters of the search area indicators 78 and the survey area indicators 80 (described in greater detail with reference to FIGS. 8-11 below) are derived from measurements of the wind, the velocity of the vehicle, and optionally the prevailing atmospheric stability conditions.
Referring still to FIG. 4, the surveyor user interface also preferably includes a real-time CH4 concentration reading 82. A wind indicator symbol 84 preferably displays real-time wind information, which may be corrected for the velocity vector of the vehicle to represent the true wind rather than the apparent wind when the vehicle is in motion. Average wind direction is preferably indicated by the direction of the arrow of the wind indicator symbol 84, while wind direction variability is indicated by the degree of open angle of the wedge extending from the bottom of the arrow. Wind speed is preferably indicated by the length of the arrow in the wind indicator symbol 84. An internet connection indicator 98 blinks when the internet connection is good. A data transfer status button 86 is user-selectable to display data transfer status (e.g., data transfer successful, intermittent data transfer, or data transfer failed). An analyzer status button 88 is user-selectable to display current analyzer status such as cavity pressure, cavity temperature, and warm box temperature. A map control button 106 is user-selectable to open a map controls window with user-selectable layer options, discussed below with reference to FIG. 7.
FIG. 5 is a schematic drawing of a screen shot on the graphical user interface, displaying a GPS status window 91, according to some embodiments of the present invention. A user-selectable GPS status button 90 may be selected to open the GPS status window 91. The GPS status window 91 preferably includes indicators of the current GPS status, such as “GPS OK”, “Unreliable GPS signal”, “GPS Failed”, or “GPS Not Detectable”.
FIG. 6 is a schematic drawing of a screen shot on the graphical user interface, displaying a weather station status window 93, according to some embodiments of the present invention. A user-selectable weather station status button 92 may be selected to open the weather station status window 93. The weather station status window 93 preferably includes indicators of the current weather station status, such as “Weather Station OK”, “Weather Station Failed”, or “Weather Station Not Detectable”. Weather station data are preferably received in real-time and may include wind data and atmospheric stability conditions data relevant to the area being surveyed.
FIG. 7 is a schematic drawing of a screen shot on the graphical user interface, displaying a map control window 95. Various elements displayed on the map 70 are regarded as layers which may be turned on or off. In this example, map controls window 95 includes six user-selectable buttons named “Hide Peak Markers”, “Hide Search Area Indicators”, “Minimum Amplitude”, “Hide Isotopic Analysis”, “Hide Field of View”, and “Hide Plat Outline”. The “Hide Peak Markers” button may be selected so that the markers indicating peak gas concentration measurements are not displayed on the map 70. The “Hide Search Area Indicators” button may be selected so that the search area indicators are not displayed on the map 70. The “Minimum Amplitude” button may be selected so that gas concentration peaks not meeting a minimum amplitude requirement are not displayed on the map 70. The “Hide Isotopic Analysis” button may be selected so that isotopic ratio analysis information is not displayed on the map 70 next to the peak markers. The “Hide Field of View” button may be selected so that the survey area indicator(s) are not displayed on the map 70. The “Hide Plat Outline” button may be selected so that the plat lines are not displayed on the map 70.
FIG. 8 is a schematic diagram of three search area indicators 78a, 78b, and 78c according to some embodiments of the present invention. Each of the search area indicators 78a, 78b, and 78c has a respective axis 108a, 108b, and 108c indicating a representative wind direction relative to a geo-referenced location of a corresponding gas concentration measurement point M1, M2, and M3. The gas concentration measurement points M1, M2, and M3 are positioned along the path 74 traveled by the vehicle 24 that carries a GPS device, a mobile gas measurement device, and wind measurement device for taking wind direction measurements and wind speed measurements. Each of the search area indicators, such as the search area indicator 78c, preferably has a width W relative to its axis 108c. The width W is indicative of a wind direction variability associated with wind direction measurements in the area of the gas concentration measurement point M3. In preferred embodiments, the width W is indicative of a variance or standard deviation of the wind direction measurements. Also in preferred embodiments, the search area indicator 78c has the shape of a sector of a circle, with the center of the circle positioned on the map at the location of the gas concentration measurement point M3. Most preferably, the angle A subtended by the sector of the circle is proportional to a standard deviation of the wind direction measurements taken at or nearby the measurement point M3. For example, the angle A may be set to a value that is twice the angular standard deviation of the wind direction measurements. It is not necessary to display the gas concentration measurement points M1, M2, and M3 on the map along with the search area indicators 78a, 78b, and 78c. As previously shown in FIGS. 4 and 7, the measurement points and associated gas concentration measurements are preferably map layer options for an end-user that may be turned on or off.
Referring again to FIG. 8, the axis 108c of the search area indicator 78c is preferably an axis of symmetry and points in a representative wind direction relative to the gas concentration measurement point M3. The representative wind direction is preferably a mean, median or mode of the wind direction measurements taken at or nearby the measurement point M3, and indicates the likely direction to a potential gas leak source. The wind direction measurements may be taken from the vehicle 24 as it moves and converted to wind direction values relative to the ground (e.g., by subtracting or correcting for the velocity vector of the vehicle). In some embodiments, the axis 108c has a length L indicative of a maximum detection distance value representative of an estimated maximum distance from a potential gas leak source at which a gas leak from the source can be detected. For example, the length may be proportional to the maximum detection distance value, or proportional to a monotonically increasing function of the maximum detection distance value, such that longer maximum detection distance values are represented by longer axis lengths. In preferred embodiments, the maximum detection distance value and corresponding length L are determined according to data representative of wind speed in the search area. In some embodiments, the maximum detection distance value and the corresponding length L are determined according to data representative of atmospheric stability conditions in the search area. Each of the search area indicators 78a, 78b, and 78c may thus provide a visual indication of a likely direction and estimated distance to a potential gas leak source. Although a sector of a circle is the presently preferred shape for a search area indicator, alternative shapes for a search area indicator include, but are not limited to, a triangle, a trapezoid, or a wedge.
FIG. 9 is a schematic diagram illustrating an example of detecting or not detecting a gas leak from a potential gas leak source, according to some embodiments of the present invention. An indicator of a survey area (also sometimes referred to as a “field of view”) is intended as an indication of how well the measurement process surveys the area around the path 74 traveled by the vehicle 24 that carries a GPS device, a mobile gas measurement device, and wind measurement device. The survey area indicator is designed such that if a potential gas leak source is located in the survey area and has a rate of leakage meeting a minimum leak rate condition, then an estimated probability of detection of a gas leak from the potential gas leak source at one or more measurement points P along the path 74 satisfies a probability condition.
Whether or not a potential gas leak source of a given strength is detectable by a gas measurement device of a given sensitivity depends on the separation distance of the source from the gas measurement device and on whether the wind is sufficient to transport gas from the gas leak source to the gas measurement device at some point along the path 74. In some embodiments, a physical model is employed that relates the measured gas concentration peak at the location of the vehicle 24 (in ppm, for example) to the emission rate of the potential gas leak source (in g/sec, for example) and the distance between the source and the detection point.
There are multiple possible models that describe the propagation of a gas leak as a plume through the atmosphere. One well-validated physical model for a plume (Gifford, F. A., 1959. “Statistical properties of a fluctuating plume dispersion model”. Adv. Geophys, 6, 117-137) is to model the plume as a Gaussian distribution in the spatial dimensions transverse to the wind direction, or (for a ground level source), the concentration c (x, y, z) at a distance x downwind, y crosswind, and at a height z from a gas leak source of strength Q located on the ground is given by Equation (1):




  
    
      
        
          C
          ⁡
          
            (
            
              x
              ,
              y
              ,
              z
            
            )
          
        
        =
        
          
            Q
            
              π
              ⁢
              
                
              
              ⁢
              v
              ⁢
              
                
              
              ⁢
              
                σ
                y
              
              ⁢
              
                σ
                z
              
            
          
          ⁢
          
            e
            
              
                
                  
                    -
                    
                      y
                      2
                    
                  
                  /
                  2
                
                ⁢
                
                  
                
                ⁢
                
                  σ
                  y
                  2
                
              
              -
              
                
                  
                    z
                    2
                  
                  /
                  2
                
                ⁢
                
                  σ
                  z
                  2
                
              
            
          
        
      
    
    
      
        [
        1
        ]
      
    
  




where ν is the speed of the wind, and the plume dispersion half-widths σy and σz depend on x via functions that are empirically determined for various atmospheric stability conditions.

If we consider the plume center, where y=z=0, the concentration at the center is given by Equation (2):




  
    
      
        
          C
          peak
        
        =
        
          Q
          
            π
            ⁢
            
              
            
            ⁢
            v
            ⁢
            
              
            
            ⁢
            
              σ
              y
            
            ⁢
            
              σ
              z
            
          
        
      
    
    
      
        [
        2
        ]
      
    
  




The dimensions of the Gaussian distribution horizontally and vertically, half-widths σy and σz, increase with increasing distance from the source. The amount they increase can be estimated from measurements of wind speed, solar irradiation, ground albedo, humidity, and terrain and obstacles, all of which influence the turbulent mixing of the atmosphere. However, if one is willing to tolerate somewhat more uncertainty in the distance estimation, the turbulent mixing of the atmosphere can be estimated simply from the wind speed, the time of day, and the degree of cloudiness, all of which are parameters that are available either on the vehicle 24 or from public weather databases in real time. Using these available data, estimates of the Gaussian width parameters can be estimated using the Pasquill-Gifford-Turner turbulence typing scheme (Turner, D. B. (1970). “Workbook of atmospheric dispersion estimates”. US Department of Health, Education, and Welfare, National Center for Air Pollution Control), or modified versions of this scheme.
For a given sensitivity of the gas measurement device, there is a minimum concentration which may be detected. Given a gas leak source of strength greater than or equal to the minimum concentration, the source will be detected if it is closer than an estimated maximum distance Xmax, where this is the distance such that σyσz=Q/(π νc). If the wind is blowing gas directly from the gas leak source to the gas measurement device, the estimated maximum distance Xmax is the distance beyond which the source may be missed. This estimated maximum detection distance may depend upon atmospheric stability conditions as well as wind speed. The formula diverges to infinity when the wind speed is very small, so it is advisable to set a lower limit (e.g., 0.5 m/s) for this quantity.
The minimum leak rate Qmin is determined by the requirements of the application. For natural gas distribution systems, a minimum leak rate of 0.5 scfh (standard cubic feet per hour) may be used; below this level, the leak may be considered unimportant. Other minimum leaks rates (e.g. 0.1 scfh, 1 scfh, or other values within or outside this range) may be used for natural gas or other leak detection applications. The minimum detection limit of the plume Cmin is given either by the gas detection instrument technology itself, or by the spatial variability of methane in the atmosphere when leaks are not present. A typical value for Cmin is 30 ppb (parts-per-billion) above the background level (typically 1,800 ppb). Given these two values for Qmin and Cmin, and by predicting σy and σz given atmospheric measurements (or with specific assumptions about the state of the atmosphere, such as the stability class), one may then determine the estimated maximum detection distance Xmax by determining the value for Xmax that satisfies the following equality, Equation (3):




  
    
      
        
          C
          
            m
            ⁢
            
              
            
            ⁢
            i
            ⁢
            
              
            
            ⁢
            n
          
        
        =
        
          
            
              Q
              
                m
                ⁢
                
                  
                
                ⁢
                i
                ⁢
                
                  
                
                ⁢
                n
              
            
            
              π
              ⁢
              
                
              
              ⁢
              v
              ⁢
              
                
              
              ⁢
              
                σ
                y
              
              ⁢
              
                σ
                z
              
            
          
          .
        
      
    
    
      
        [
        3
        ]
      
    
  




In some embodiments the relationship between σy and σz and Xmax is provided by a functional relationship, a lookup table, or similar method. Because σy and σz are monotonically increasing functions of Xmax, a unique value can be determined from this process. For example, one useful functional form is a simple power law, where the coefficients a, b, c, and d depend on atmospheric conditions: σy=axb; σz=cxd.
In some embodiments, the concentration C measured close to the ground of a Gaussian plume due to a gas leak source on the ground depends on the rate of emission Q of the source, the distance x between the source and the gas measurement device, and the speed of the wind blowing from the source to the gas measurement device, in accordance with an expression of the form (Equation 4):




  
    
      
        C
        =
        
          Q
          
            π
            ⁢
            
              
            
            ⁢
            v
            ⁢
            
              
            
            ⁢
            
              
                σ
                y
              
              ⁡
              
                (
                x
                )
              
            
            ⁢
            
              
                σ
                z
              
              ⁡
              
                (
                x
                )
              
            
          
        
      
    
    
      
        [
        4
        ]
      
    
  




The expressions for σy(x) and σz(x) depend on the stability class of the atmosphere at the time of measurement. In some embodiments, the stability class of the atmosphere is inferred from the answers to a set of questions given to the operator, or from instruments of the vehicle, or from data received from public weather databases. As shown in the table of FIG. 18, coefficients A, B, C, D, E and F may depend on surface wind speed and atmospheric conditions such as day or night, incoming solar radiation, and cloud cover. Mathematical forms for σy(x) and σz(x) are documented in Section 1.1.5 of the User's Guide for Industrial Source Complex (ISC3), Dispersion Models Vol. 2 (US Environmental Protection Agency document EPA-454/B955-003b September 1995). Given the sensitivity of the gas measurement device and the rate of emission of the smallest potential gas leak source of interest, equation (4) may be solved to find the estimated maximum distance Xmax beyond which a potential gas leak source may be missed by the gas measurement device.

FIG. 16 is a graph of vertical σz(x) dispersion coefficients of a gas plume as a function of downwind distance from a gas leak source according to some embodiments of the present invention. FIG. 17 is a graph of crosswind σy(x) dispersion coefficients of a gas plume as a function of downwind distance from a gas leak source according to some embodiments of the present invention. The graphs are from de Nevers, 2000, Air Pollution Control Engineering, The McGraw-Hill Companies, Inc. The dispersion coefficients are functions of downwind distance x. In this example, dispersion coefficients are calculated based on atmospheric stability. The table of FIG. 18 gives the atmospheric stability class as a function of wind speed, day or night, cloud cover, and solar radiation. In some embodiments, the dispersion coefficients and/or the estimated maximum distance Xmax may depend upon an urban or rural environment for the gas concentration measurements and plume dispersion. For example, the estimated maximum distance Xmax may be less in an urban environment with buildings or other structures than in a rural environment.
The actual distance at which a gas leak source may be detected is reduced if there is some variability or uncertainty in the direction of the wind. This is because there is a probability that the wind blows gas in a direction such that it does not intercept the path 74 of the vehicle 24 (FIG. 9). In practice this uncertainty is usually larger than the intrinsic angular uncertainty σy/x implied by the Gaussian plume model. In order to determine the effective survey area of the mobile gas measurement device, assume for this example that the wind speed remains approximately constant within a time interval −T<t<T bounding the time t=0 at which the vehicle 24 passes through a particular point P on the path 74, but that the wind direction (angle) is distributed as a Gaussian with a known mean and standard deviation.
As shown in FIG. 9, we consider the line 110 through the measurement point P pointing toward the direction of the mean wind, and whether a candidate point Q on this line qualifies to be within the boundary of the survey area (i.e., within the field of view of the mobile gas measurement device of the vehicle 24). We also consider drawing a sample from the distribution of wind directions and drawing a line through the candidate point Q in this direction. If this line intersects the path 74 of the vehicle 24 within the time interval −T<t<T, and the distance from the candidate point Q to the point of intersection with the path 74 is less than or equal to the estimated maximum distance Xmax, then this is regarded as detectable by the mobile gas measurement device since the potential gas leak source at the candidate point Q would have been detected along the path 74. The quantity T sets the time interval during which it is expected to detect the gas coming from the candidate point Q at measurement point P. Theoretically, the time interval can be large, but it may not be reasonable to assume that the wind statistics remain unchanged for an extended period of time. In some embodiments, the wind direction measurements are taken during a time interval less than or equal to about 2 minutes, during which time interval a gas concentration is measured at the gas concentration measurement point P. More preferably, the time interval is in the range of 10 to 20 seconds.
FIG. 10 is a schematic diagram showing the estimation of a probability of detection at the measurement point P of a gas leak from a potential gas leak source at the candidate point Q, according to some embodiments of the present invention. The probability of detection at measurement point P is estimated according to an angle θ subtended by a segment 79 of the path 74 relative to the candidate point Q for the potential gas leak source. The path segment 79 is positioned within a distance of the candidate point Q that is less than or equal to the estimated maximum distance Xmax. The probability of detection is preferably estimated according to a cumulative probability of wind directions with respect to the subtended angle θ. The cumulative probability of wind directions may be determined according to a representative wind direction (e.g., a mean, median, or mode of the wind direction measurements) and a wind direction variability (e.g., variance or standard deviation) calculated from the wind direction measurements.
The candidate point Q is deemed to be within the boundary of the survey area if the probability of successful detection of a potential gas leak source at the candidate point Q, over the distribution of wind directions, satisfies a probability condition. In some embodiments, the probability condition to be satisfied is an estimated probability of successful detection greater than or equal to a threshold value, typically set at 70%. In general, as the candidate point Q is moved a farther distance from the gas concentration measurement point P, the range of successful angles becomes smaller and the probability of success decreases, reaching a probability threshold at the boundary of the territory deemed to be within the survey area.
FIG. 11 is a graph of probability density vs. wind directions for estimating a probability of detection of a gas leak from a potential gas leak source, according to some embodiments of the present invention. The area under the curve spans a range of possible angles θ for the successful detection of a potential gas leak from a candidate point. The probability density is preferably generated as a Gaussian or similar distribution from the calculated mean and standard deviation of the wind direction measurements in the area of the gas concentration measurement point P, FIG. 10. If the angle θ subtended by the path segment 79 relative to the candidate point Q encompasses a percentage of possible wind vectors that is greater than equal to a threshold percentage (e.g., 70%, although the percentage may be adjusted to other values such as 50%, 60%, 67%, 75%, 80%, or 90% in some embodiments), and if the distance from the candidate point Q to the measurement point P is less than the estimated maximum distance Xmax, then the candidate point Q is deemed to be within the survey area.
The above process is repeated as different measurement points along the path 74 are chosen and different candidate points are evaluated for the probability of successful detection of a potential gas leak source. The cumulative distribution of the wind direction function together with a root finding algorithm are useful for efficiently determining the boundary of the survey area. For example, referring again to FIG. 10, the root finding algorithm may consider candidate points along the line of mean wind direction starting at the estimated maximum distance Xmax from measurement point P, and iteratively (e.g. using a bisection or other method) moving closer to the measurement point P along the mean wind direction line until the angle θ subtended by the path segment 79 is sufficient to meet the probability threshold, as determined from the cumulative probability of wind directions over the subtended angle θ, FIG. 11. Referring again to FIG. 4, the survey area indicator 80 may be displayed on the map 70 as a colored “swath” adjoining the path 74 and extending in a substantially upwind direction from the path.
FIG. 12 is a flow chart showing a sequence of steps to perform a gas leak survey according to some embodiments of the present invention. In step 200, the survey program is started, preferably by an operator in the vehicle using a graphical user interface (GUI). The operator begins to drive the vehicle on a survey route while the GUI displays a street map (FIG. 4). Gas concentration measurements are preferably performed rapidly along the survey route (e.g., at a rate of 0.2 Hz or greater, more preferably 1 Hz or greater). This enables the practice of driving the vehicle at normal surface street speeds (e.g., 35 miles per hour) while accumulating useful data. The gas concentration is measured initially as a function of time, and is combined with the output of the GPS receiver in order to obtain the gas concentration as a function of distance or location. Interpolation can be used to sample the data on a regularly spaced collection of measurement points. The concentration of methane typically varies smoothly with position, for the most part being equal to the worldwide background level of 1.8 parts per million together with enhancements from large and relatively distant sources such as landfills and marshes.
In step 210, at least one processor (e.g. of a client device, server device, or a combination) receives data representative of measured gas concentrations, wind direction measurements, wind speed measurements, and GPS data. In decision block 220, it is determined if a peak in gas concentration is identified. A peak may be identified from a gas concentration measurement above a certain threshold (or within a certain range), or exceeding background levels by a certain amount, which may be predetermined or user-selected. In some embodiments, the gas concentration and GPS data are analyzed using a peak-location method, and then each identified peak is subsequently fit (using linear or nonlinear optimization) for center and width. The functional form used for this fitting step may be a Gaussian pulse, since a Gaussian is commonly the expected functional form taken by gas plumes propagating through the atmosphere.
If a peak in gas concentration is not identified, then the program proceeds to step 250. If a peak in gas concentration is identified, then a peak marker is generated in step 230. The peak marker may be displayed on the map as a user-selectable layer, as previously discussed with reference to FIG. 4. In step 240, a search area indicator is generated to indicate the likely location of a gas leak source corresponding to the identified peak in gas concentration. The search area indicator may be displayed on the map as a user-selectable layer, as shown in FIG. 4. In step 250, the survey area boundary is calculated, and a survey area indicator may be displayed on the map as a user-selectable layer (hatched region in FIG. 4). In decision step 260, it is determined if the operator wishes to continue surveying (e.g., by determining if the “Stop Survey” button has been selected). If yes, the survey program returns to step 210. If not, the survey results are stored in memory in step 270 (e.g., in the survey results 64 of FIG. 3), and the survey program ends.
FIG. 13 is a flow chart showing a sequence of steps performed to generate a search area indicator according to some embodiments of the present invention. When a local enhancement in the gas concentration is detected, the likely direction and estimated distance to the potential gas leak source is preferably calculated from data representative of wind direction and wind speed measured during a time interval just prior to or during which the gas concentration was measured. The time interval is preferably fewer than 2 minutes, and more preferably in the range of 5 to 20 seconds. Calculating statistics from wind measurements may require some conversion if the measurements are made using sensors on a moving vehicle. A sonic anemometer is preferably used to measure wind along two perpendicular axes. Once the anemometer has been mounted to the vehicle, these axes are preferably fixed with respect to the vehicle. In step 241, wind speed and wind direction values that were measured relative to the vehicle are converted to wind speed and wind direction values relative to the ground by subtracting the velocity vector of the vehicle, as obtained from the GPS data. When the vehicle is stationary, GPS velocity may be ineffective for determining the orientation of the vehicle and wind direction, so it is preferable to use a compass (calibrated for true north vs. magnetic north) in addition to the anemometer.
In step 242, wind statistics are calculated from the converted wind values to provide the parameters for the search area indicator. The statistics include a representative wind direction that is preferably a mean, median, or mode of the wind direction measurements. The statistics also include a wind direction variability, such as a standard deviation or variance of the wind direction measurements. In step 243, an angular range of search directions, extending from the location of the gas concentration measurement point where the local enhancement was detected, is calculated according to the variability of the wind direction measurements. In optional step 244, atmospheric conditions data are received. Step 245 is determining a maximum detection distance value representative of the estimated maximum distance from the suspected gas leak source at which a leak can be detected. In some embodiments, the maximum detection distance value is determined according to Equation (3) or Equation (4), and the data representative of wind speed and/or atmospheric stability conditions. Alternatively, the maximum detection distance value may be a predetermined number, a user-defined value, empirically determined from experiments, or a value obtained from a look-up table. In step 246, the search area indicator is generated with the determined parameters, previously discussed with reference to FIG. 8.
FIG. 14 is a flow chart showing a sequence of step performed to calculate a boundary of a survey area according to some embodiments of the present invention. In step 251, wind speed and wind direction values that were measured relative to the vehicle are converted to wind speed and wind direction values relative to the ground by subtracting the velocity vector of the vehicle, as previously described in step 241 above. In optional step 252, atmospheric conditions data are received. Step 253 is determining a maximum detection distance value representative of the estimated maximum distance from a suspected gas leak source at which a leak can be detected. In some embodiments, the maximum detection distance value is determined according to Equation (3) or Equation (4), and the data representative of wind speed and/or atmospheric stability conditions. Alternatively, the maximum detection distance value may be a predetermined number, a user-defined value, empirically determined from experiments, or a value obtained from a look-up table. In step 254, it is determined what angle θ is subtended by a segment of the path of the vehicle relative to the candidate point Q for the potential gas leak source. The path segment is positioned within a distance of the candidate point Q that is less than or equal to the estimated maximum distance.
In step 255, a representative wind direction (e.g., a mean, median, or mode of the wind direction measurements) and a wind direction variability (e.g., variance or standard deviation) are calculated from the wind direction measurements. In step 256, the probability of detection is estimated according to a cumulative probability of wind directions with respect to the subtended angle θ. In step 257, the survey area boundary is calculated with a probability threshold. For example, if the angle θ subtended by the path segment relative to the candidate point encompasses a percentage of possible wind vectors that is greater than equal to a threshold percentage (e.g., 70%), and if the distance from the candidate point Q to the measurement point P is less than the estimated maximum distance Xmax, then the candidate point Q is deemed to be within the survey area. In decision step 258, it is determined if the survey area boundary function is to continue with the next measurement point. If yes, steps 251-257 are repeated as different measurement points along the path are chosen and different candidate points are evaluated for the probability of successful detection of a potential gas leak source. If not, then the boundary function ends.
FIG. 15 is a flow chart showing steps for displaying layers overlaid or superimposed on a street map according to some embodiments of the present invention. In step 310, a street map is displayed, preferably on a GUI visible to the operator in the vehicle. In step 320, the path of the vehicle with the mobile gas measurement device is displayed on the map. Various elements displayed on the map are regarded as layers which may be turned on or off. In this example, the map controls window (FIG. 7) includes six user-selectable buttons named “Hide Peak Markers”, “Hide Search Area Indicators”, “Minimum Amplitude”, “Hide Isotopic Analysis”, “Hide Field of View”, and “Hide Plat Outline”. In decision step 330, it is determined if one or more of these layers is selected. If yes, the selected layer is displayed overlaid or superimposed on the street map in step 340. If not, it is determined if the survey is to continue. If yes, display steps 310-350 are repeated. If not, the display options may end.
The exemplary systems and methods described above allow a surveyor to locate potential gas leak sources efficiently and effectively in highly populated areas. The search area indicators provide likely direction and estimated maximum distance to the source of detected gas leaks, while the survey area indicators provide an estimated statistical measure of confidence that an area was successfully surveyed for potential gas leaks. These aspects provide significant improvement in finding gas leak sources over conventional methods where engineers scan the area very slowly and in all directions by trial and error to find the source of a gas leak. These aspects also account for wind that may quickly disperse a gas plume.
It will be clear to one skilled in the art that the above embodiments may be altered in many ways without departing from the scope of the invention. For example, gas leaks may include, but are not limited to: leaks from gas pipes or transportation systems (e.g., natural gas leaks), leaks from gas processing or handling facilities, and emissions from gas sources into the environment (e.g., pollution, gas emission from landfills, etc.). Gas concentration measurements are preferably performed rapidly (e.g., at a rate of 0.2 Hz or greater, more preferably 1 Hz or greater). This enables the concept of driving a vehicle at normal surface street speeds (e.g., 35 miles per hour) while accumulating useful gas concentration and wind measurement data. However, embodiments of the invention do not depend critically on the gas detection technology employed. Any gas concentration measurement technique capable of providing gas concentration measurements can be employed in some embodiments.
Although the gas concentration measurements are preferably performed while the gas measurement device is moving, at least some gas concentration measurements can be performed while the gas concentration measurement device is stationary. Such stationary gas concentration measurements may be useful for checking background gas concentrations, for example. While real-time measurements are preferred, post analysis of more sparsely sampled data, e.g., via vacuum flask sampling and later analysis via gas chromatography or other methods, may be used in some embodiments. Optionally, measurements can be made on different sides of the road or in different lanes to provide more precise localization of the leak source. Optionally, the present approaches can be used in conjunction with other conventional methods, such as visual inspection and/or measurements with handheld meters to detect emitted constituents, to further refine the results. Optionally, measurements can be made at reduced speed, or with the vehicle parked near the source, to provide additional information on location and/or source attribution.
Optionally, the system can include a source of atmospheric meteorological information, especially wind direction, but also wind speed or atmospheric stability conditions, either on-board the vehicle or at a nearby location. The stability of the atmospheric conditions can be estimated simply from the wind speed, the time of day, and the degree of cloudiness, all of which are parameters that are available either on the vehicle or from public weather databases. Optionally, the computer system can include an on-board video camera and logging system that can be used to reject potential sources on the basis of the local imagery collected along with the gas concentration and wind data. For example, a measured emissions spike could be discounted if a vehicle powered by natural gas passed nearby during the measurements. Optionally, repeated measurements of a single location can be made to provide further confirmation (or rejection) of potential leaks. Accordingly, the scope of the invention should be determined by the following claims and their legal equivalents.

## Claims

1. A method comprising:
performing a gas leak detection survey by
measuring a plurality of gas concentration values at a corresponding plurality of measurement points using a gas concentration measurement device carried by a moving vehicle along a survey path,
determining a plurality of geospatially-referenced locations of the plurality of measurement points using a positioning device carried by the moving vehicle, and
measuring a plurality of wind direction values using a wind direction determination device carried by the moving vehicle, wherein the plurality of wind direction values are measured within a time interval during which the plurality of gas concentration values are measured, the time interval being less than or equal to 2 minutes;

determining, using at least one hardware processor, a range of search directions to a suspected gas leak source extending from a location of a gas concentration measurement point according to a variability of the wind direction values, wherein the range of search directions defines a width of a graphical indicator of the variability of the wind direction values; and
employing the range of search directions to identify a location of the suspected gas leak source, to facilitate a repair of the gas leak source.

2. The method of claim 1, wherein the variability comprises a variance or standard deviation of the wind direction values.

3. The method of claim 2, wherein the range of search directions is proportional to the standard deviation of the wind direction values.

4. The method of claim 1, further comprising employing the at least one processor to determine a representative wind direction relative to the gas concentration measurement point.

5. The method of claim 4, wherein the representative wind direction comprises a mean, median or mode of the wind direction values.

6. The method of claim 1, further comprising employing the at least one processor to determine, according to wind speed data relative to the gas concentration measurement point, a maximum detection distance value representative of an estimated maximum distance from the suspected gas leak source at which a gas leak is detectable by the survey.

7. The method of claim 6, wherein the maximum detection distance value is further determined according to data representative of atmospheric stability conditions along the survey path.

8. The method of claim 1, wherein the location of the gas concentration measurement point corresponds to a peak in gas concentration identified from the plurality of gas concentration values.

9. An apparatus comprising:
a gas concentration measurement device configured to measure a plurality of gas concentration values at a corresponding plurality of measurement points along a survey path travelled by a moving vehicle conducting a gas leak detection survey, the vehicle carrying the gas concentration measurement device;
a positioning device configured to determine a plurality of geospatially-referenced locations of the plurality of measurement points;
a wind direction determination device configured to measure a plurality of wind direction values within a time interval during which the plurality of gas concentration values are measured, the time interval being less than or equal to 2 minutes; and
at least one hardware processor coupled to the gas concentration measurement device, the positioning device, and the wind direction measurement device, and programmed to determine a range of search directions to a suspected gas leak source extending from a location of a gas concentration measurement point according to a variability of the wind direction values, wherein the range of search directions defines a width of a graphical indicator of the variability of the wind direction values.

10. The apparatus of claim 9, wherein the variability comprises a variance or standard deviation of the wind direction values.

11. The apparatus of claim 10, wherein the range of search directions is proportional to the standard deviation of the wind direction values.

12. The apparatus of claim 9, wherein the at least one processor is configured to determine a representative wind direction relative to the gas concentration measurement point.

13. The apparatus of claim 12, wherein the representative wind direction comprises a mean, median or mode of the wind direction values.

14. The apparatus of claim 9, wherein the gas concentration measurement point corresponds to a peak in gas concentration identified from the plurality of gas concentration measurements.

15. The apparatus of claim 9, wherein the at least one processor is further programmed to determine, according to wind speed data relative to the gas concentration measurement point, a maximum detection distance value representative of an estimated maximum distance from the suspected gas leak source at which a gas leak is detectable by the survey.

16. The apparatus of claim 15, wherein the maximum detection distance value is further determined according to data representative of atmospheric stability conditions along the survey path.

17. A method comprising:
performing a gas leak detection survey by
measuring a plurality of gas concentration values at a corresponding plurality of measurement points using a gas concentration measurement device carried by a moving vehicle along a survey path,
determining a plurality of geospatially-referenced locations of the plurality of measurement points using a positioning device carried by the moving vehicle, and
measuring a plurality of wind direction values and a plurality of wind speed values using at least one wind measurement device carried by the moving vehicle, wherein the plurality of wind direction values and the plurality of wind speed values are measured within a time interval during which the plurality of gas concentration values are measured, the time interval being less than or equal to 2 minutes; and

determining, using at least one hardware processor, a probable direction and an estimated maximum distance to a suspected gas leak source relative to a geo-referenced location of a gas concentration measurement point, wherein the maximum distance represents an estimated maximum distance from the suspected gas leak source at which a gas leak is detectable by the survey, wherein the probable direction is determined according to the plurality of wind direction values, wherein the maximum distance is determined according to the plurality of wind speed values, and wherein the maximum distance defines a length of a graphical indicator of the maximum distance.

18. The method of claim 17, wherein the maximum distance is further determined according to data representative of atmospheric stability conditions in an area of the gas concentration measurement point.

19. The method of claim 17, wherein the location of the gas concentration measurement point corresponds to a peak in gas concentration identified from the plurality of gas concentration values.

20. An apparatus comprising:
a gas concentration measurement device configured to measure a plurality of gas concentration values at a corresponding plurality of measurement points along a survey path travelled by a moving vehicle conducting a gas leak detection survey, the vehicle carrying the gas concentration measurement device;
a positioning device configured to determine a plurality of geospatially-referenced locations of the plurality of measurement points;
at least one wind measurement device configured to measure a plurality of wind direction values and a plurality of wind speed values within a time interval during which the plurality of gas concentration values are measured, the time interval being less than or equal to 2 minutes; and
at least one hardware processor coupled to the gas concentration measurement device, the positioning device, and the at least one wind measurement device, and programmed to determine a probable direction and an estimated maximum distance to a suspected gas leak source relative to a geo-referenced location of a gas concentration measurement point, wherein the maximum distance represents an estimated maximum distance from the suspected gas leak source at which a gas leak is detectable by the survey, wherein the probable direction is determined according to the plurality of wind direction values, wherein the maximum distance is determined according to the plurality of wind speed values, and wherein the maximum di stance defines a length of a graphical indicator of the maximum distance.

21. The apparatus of claim 20, wherein the maximum distance is further determined according to data representative of atmospheric stability conditions in an area of the gas concentration measurement point.

22. The apparatus of claim 20, wherein the location of the gas concentration measurement point corresponds to a peak in gas concentration identified from the plurality of gas concentration values.

23. A non-transitory computer-readable medium encoding instructions which, when executed by a computer system, cause the computer system to:
receive a plurality of gas concentration values measured at a corresponding plurality of measurement points using a gas concentration measurement device carried by a moving vehicle along a survey path,
receive a plurality of geospatially-referenced locations of the plurality of measurement points determined using a positioning device carried by the moving vehicle, and
receive a plurality of wind direction values and a plurality of wind speed values measured using at least one wind measurement device carried by the moving vehicle, wherein the plurality of wind direction values and the plurality of wind speed values are measured within a time interval during which the plurality of gas concentration values are measured, the time interval being less than or equal to 2 minutes; and
determine a probable direction and an estimated maximum distance to a suspected gas leak source relative to a geo-referenced location of a gas concentration measurement point, wherein the maximum distance represents an estimated maximum distance from the suspected gas leak source at which a gas leak is detectable by the survey, wherein the probable direction is determined according to the plurality of wind direction values, wherein the maximum distance is determined according to the plurality of wind speed values, and wherein the distance defines a length of a graphical indicator of the maximum distance.

24. The medium of claim 23, wherein the maximum distance is further determined according to data representative of atmospheric stability conditions.

25. A non-transitory computer-readable medium encoding instructions which, when executed by a computer system, cause the computer system to:
receive a plurality of gas concentration values measured at a corresponding plurality of measurement points using a gas concentration measurement device carried by a moving vehicle along a survey path;
receive a plurality of geospatially-referenced locations of the plurality of measurement points determined using a positioning device carried by the moving vehicle, and
receive a plurality of wind direction values measured using a wind direction determination device carried by the moving vehicle, wherein the plurality of wind direction values are measured within a time interval during which the plurality of gas concentration values are measured, the time interval being less than or equal to 2 minutes; and
determine a range of search directions to a suspected gas leak source extending from a location of a gas concentration measurement point according to a variability of the wind direction values, wherein the range of search directions defines a width of a graphical indicator of the variability of the wind direction values.