Patent Publication Number: US-10775262-B1

Title: Gas detection systems and methods using search area indicators

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 15/418,367, filed on Jan. 27, 2017 and issued on Jul. 2, 2019 as U.S. Pat. No. 10,337,946, which is a continuation of U.S. patent application Ser. No. 13/733,857, filed on Jan. 3, 2013, titled “Gas Detection Systems and Methods Using Search Area Indicators,” issued on Jan. 31, 2017 as U.S. Pat. No. 9,557,240, which claims the benefit of the filing date of U.S. provisional patent application 61/646,487 filed on May 14, 2012, titled “Gas Detection Systems and Methods,” which applications are hereby incorporated by reference in their entireties. 
    
    
     BACKGROUND 
     The invention relates to systems and methods for detecting gas leaks such as 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 generate content to be displayed. The content includes a street map and at least one search area indicator on the map that indicates a search area suspected to have a gas leak source. The search area indicator has an axis indicating a representative wind direction relative to a geo-referenced location of at least one gas concentration measurement point. The search area indicator also has a width relative to the axis. The width is indicative of a wind direction variability associated with a plurality of wind direction measurements in an area of the gas concentration measurement point. 
     According to another aspect, an apparatus comprises at least one processor programmed to generate content to be displayed. The content includes a street map and at least one search area indicator on the map that indicates a search area suspected to have a gas leak source. The search area indicator has an axis indicating a representative wind direction relative to a geo-referenced location of at least one gas concentration measurement point. The search area indicator also has a width relative to the axis. The width is indicative of a wind direction variability associated with a plurality of wind direction measurements in an area of the gas concentration measurement point. 
     According to another aspect, a non-transitory computer-readable medium encodes instructions which, when executed by a computer system, cause the computer system to generate content to be displayed. The content includes a street map and at least one search area indicator on the map that indicates a search area suspected to have a gas leak source. The search area indicator has an axis indicating a representative wind direction relative to a geo-referenced location of at least one gas concentration measurement point. The search area indicator also has a width relative to the axis. The width is indicative of a wind direction variability associated with a plurality of wind direction measurements in an area of the gas concentration measurement point. 
     According to another aspect, a method comprises employing at least one processor to generate content to be displayed. The content includes a street map and at least one search area indicator on the map that indicates a search area suspected to have a gas leak source. The search area indicator has an axis indicating a representative wind direction relative to a geo-referenced location of at least one gas concentration measurement point. The axis also has a length indicative of a maximum detection distance value representing an estimated maximum distance from the gas leak source at which a leak can be detected. The length is determined according to data representative of wind speed in the search area. 
     According to another aspect, an apparatus comprises at least one processor programmed to generate content to be displayed. The content includes a street map and at least one search area indicator on the map that indicates a search area suspected to have a gas leak source. The search area indicator has an axis indicating a representative wind direction relative to a geo-referenced location of at least one gas concentration measurement point. The axis also has a length indicative of a maximum detection distance value representing an estimated maximum distance from the gas leak source at which a leak can be detected. The length is determined according to data representative of wind speed in the search area. 
     According to another aspect, a non-transitory computer-readable medium encodes instructions which, when executed by a computer system, cause the computer system to generate content to be displayed. The content includes a street map and at least one search area indicator on the map that indicates a search area suspected to have a gas leak source. The search area indicator has an axis indicating a representative wind direction relative to a geo-referenced location of at least one gas concentration measurement point. The axis also has a length indicative of a maximum detection distance value representing an estimated maximum distance from the gas leak source at which a leak can be detected. The length is determined according to data representative of wind speed in the search area. 
    
    
     
       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 indictor 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  18   a - c  all connected through a wide area network  16  such as the Internet. Client computer systems  18   a - 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  18   c ) 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  18   c.    
       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  78   a ,  78   b , and  78   c  according to some embodiments of the present invention. Each of the search area indicators  78   a ,  78   b , and  78   c  has a respective axis  108   a ,  108   b , and  108   c  indicating a representative wind direction relative to a geo-referenced location of a corresponding gas concentration measurement point M 1 , M 2 , and M 3 . The gas concentration measurement points M 1 , M 2 , and M 3  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  78   c , preferably has a width W relative to its axis  108   c . The width W is indicative of a wind direction variability associated with wind direction measurements in the area of the gas concentration measurement point M 3 . 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  78   c  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 M 3 . 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 M 3 . 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 M 1 , M 2 , and M 3  on the map along with the search area indicators  78   a ,  78   b , and  78   c . 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  108   c  of the search area indicator  78   c  is preferably an axis of symmetry and points in a representative wind direction relative to the gas concentration measurement point M 3 . The representative wind direction is preferably a mean, median or mode of the wind direction measurements taken at or nearby the measurement point M 3 , 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  108   c  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  78   a ,  78   b , and  78   c  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 σ y  and σ z  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 σ y  and σ z , 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 X max , where this is the distance such that σ y σ z =Q/(πνc). If the wind is blowing gas directly from the gas leak source to the gas measurement device, the estimated maximum distance X max  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 Q min  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 C min  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 C min  is 30 ppb (parts-per-billion) above the background level (typically 1,800 ppb). Given these two values for Q min  and C min , and by predicting σ y  and σ z  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 X max  by determining the value for X max  that satisfies the following equality, Equation (3): 
     
       
         
           
             
               
                 
                   
                     
                       C 
                       min 
                     
                     = 
                     
                       
                         Q 
                         min 
                       
                       
                         π 
                         ⁢ 
                         v 
                         ⁢ 
                         
                           σ 
                           y 
                         
                         ⁢ 
                         
                           σ 
                           Z 
                         
                       
                     
                   
                   . 
                 
               
               
                 
                   [ 
                   3 
                   ] 
                 
               
             
           
         
       
     
     In some embodiments the relationship between σ y  and σ z  and X max  is provided by a functional relationship, a lookup table, or similar method. Because σ y  and σ z  are monotonically increasing functions of X max , 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: σ y =ax b ; σ z =cx d . 
     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 σ y (x) and σ z (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 σ y (x) and σ z (x) are documented in Section 1.1.5 of the User&#39;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 X max  beyond which a potential gas leak source may be missed by the gas measurement device.
 
       FIG. 16  is a graph of vertical σ z (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 σ y (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 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 X max  may depend upon an urban or rural environment for the gas concentration measurements and plume dispersion. For example, the estimated maximum distance X max  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 σ y /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&lt;t&lt;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&lt;t&lt;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 X max , 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 X max . 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 X max , 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 X max  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 X max , 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 apparatus 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 to 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.