Method for monitoring fugitive gas emissions from the soil, via vertical concentration measurements
Method for the quantification of the fugitive gas flow from a dispersed source (A) by monitoring with a remote detection optical instrument mounted on an aircraft (UAV) which moves at a determined height along a plane (S) perpendicular to the direction of the wind field (u), such wind field being known through suitable positioning of meteorological stations within and in areas neighboring the site to be monitored according to known techniques and use of commercially available diagnostic meteorological models. By this instrument discrete vertical measurements are carried out of the fugitive gas concentration averaged over said height along the whole width (W) of the plane (S) to yield corresponding mean vertical concentration values and, according to mean wind speed values detected at said discrete vertical measurements, a value of the fugitive gas flow (Q) is obtained by integrating the product of the mean vertical concentration values and of the corresponding mean wind speed values with respect to the surface of the sampling plane (S). The obtained value of the fugitive gas flow (Q) is corrected by a corrective factor (a) obtainable by comparing concentration values obtained by direct measurements and values calculated by dispersion models.
1. A method for the quantification of the flow from the soil of a fugitive gas from a dispersed source (A) of said gas, comprising the following steps:
providing an aircraft (UAV) equipped with a remote detection optical instrument;
defining a downwind sampling plane (S) perpendicular to the wind direction, having a width (W) sufficient to contain the projection of the site to be monitored and placed at such a distance from the site as to contain, at the aircraft sampling height, the whole gaseous effluent released by the site; said method being characterized in that it further comprises the steps of
flying over said sampling plane (S) for the whole width (W) at a height (H) with said aircraft (UAV) so that said whole fugitive gas remains below said height,
carrying out discrete vertical measurements of fugitive gas concentration averaged over said height along the whole width (W) of said plane (S) to yield corresponding mean vertical concentration values,
detecting mean wind speed values at said discrete vertical measurements of concentration;
integrating the product of said mean vertical concentration values and said corresponding mean wind speed values with respect to the surface of the sampling plane (S) to obtain a value of the fugitive gas flow (Q); and
correcting the obtained value of the fugitive gas flow (Q) by a corrective factor (α) dependent from the vertical gradient of speed and concentration and obtainable by comparing concentration values obtained by direct measurements and values calculated by dispersion models in order to take into account the speed variation with the height thereby improving the accuracy of the obtained value of the fugitive gas flow (Q) from the dispersed source (A).
2. The method according to claim 1 , wherein said remote detection optical instrument is a spectroscopic absorption diode laser (TDLAS).
3. The method according to claim 1 , wherein said aircraft (UAV) is an autopilot aircraft.
4. The method according to claim 1 , wherein the mean wind speed values are obtained from diagnostic meteorological models that process data from meteorological stations placed in strategic points of the field to be monitored.
5. The method according to claim 1 , wherein the dispersion models for calculating said correction coefficient (α) are Gaussian dispersion models or commercially available dispersion models (CALPUFF).
6. The method according to claim 1 , further comprising:
carrying out vertical concentration measurements along sampling planes S 1 , . . . , S i , . . . , S i+n+1 parallel to said sampling plane (S), in the direction opposite to that of the wind and also intersecting the surface of said dispersed source (A), in the planes S 1 , . . . , S i not intersecting said surface, the value of the flow being constant,
defining the sampling planes S i+1 , . . . , S i+n+1 intersecting the surface of said dispersed source (A) at which the value of the total fugitive gas flow decreases until zero, and obtaining the coordinates of said planes to localize said dispersed source.