Abstract:
In the exploration for hydrocarbon gas, microwave energy is radiated from an antenna transported along a traverse above the surface of the earth. Microwave energy re-radiated from gas seeps is detected. Microwave energy reflected from hard targets along the traverse is also detected. Video monitors simultaneously display the detected re-radiated and reflected microwave energy. In this way, hydrocarbon gas seeps are displayed in relation to topographical features along the traverse.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to the exploration for hydrocarbon gas by radiating it with microwave energy and detecting the re-radiated energy and more particularly, the invention relates to the simultaneous display of re-radiated and reflected microwave energy. 
     Airborne exploration for hydrocarbons is described, for example, in U.S. Pat. No. 3,651,395 -- Owen et al. In such exploration, a microwave transmitter and antenna radiate the surface of the earth with microwave energy. 
     Hydrocarbon gas often seeps from the surface of the earth above important hydrocarbon reservoirs. For example, propane has commonly been observed seeping from underground petroleum or natural gas reservoirs. Techniques for quickly and accurately locating these gas seeps are needed. 
     My copending application Ser. No. 628,689, filed Nov. 3, 1975, describes a particularly useful microwave radiation and detection technique for use in the exploration for hydrocarbon gases. A radar transmitter emits a pulse of microwave energy which excites molecules of certain species of gas to new molecular rotation states from which they emit, or re-radiate, energy at characteristic frequencies. This re-radiated energy is detected by the radar receiver and the resultant signal is displayed as an indicator of hydrocarbon gas. 
     The frequency of the re-radiated energy is substantially independent of the frequency of the radiating energy. Because of this, the transmitter frequency can be different from the expected re-radiation frequency of the gases undergoing exploration. Because the frequency of the expected re-radiation is different from the transmitter frequency, detection is possible, even at the low energy levels likely to exist at aircraft flight altitudes. 
     My copending application identified above describes a system which separates the detected re-radiation energy from background energy. This background is primarily due to excitation of other naturally occurring constituents of the atmosphere. 
     Also energy at the transmitter frequency is reflected from radiated hard targets. This reflected microwave energy contains valuable information identifying the topographical features along the line of exploration. 
     SUMMARY OF THE INVENTION 
     In accordance with this invention, detected re-radiated and reflected microwave energy are simultaneously displayed to identify detected hydrocarbon gas seeps in relation to topographic features along a line of exploration. 
     In carrying out the invention, an antenna is transported along a traverse above the surface of the earth while radiating microwave energy at a transmitter frequency from the antenna. Microwave energy re-radiated from gas seeps at a different frequency is detected. The detected signal from the antenna includes background, energy at the re-radiated frequency, and reflected energy at the transmitter frequency. The background component is cancelled from the received signal to produce a signal representing only re-radiated energy. This signal is applied to a plan position indicator to produce a display representing hydrocarbon gas seeps. The reflected energy signal is applied to another plan position indicator to produce a display of topographical features along the line of traverse. Television cameras convert these displays into video signals which can be recorded. 
     In accordance with a further aspect of this invention, different color phosphors are used on the two different displays. Color cameras convert these two different colored displays into video signals which are mixed to form a composite video signal which, when displayed, shows the topographic features along the traverse in one color and the hydrocarbon gas seeps superimposed in another color thereon. 
     In accordance with another aspect of the invention, navigational equipment produces a digital display of the location along the traverse. This digital display is converted to a video signal which is mixed with the video signal representing the topographic features and the gas seeps. The navigational information appears as an integral part of the final display. 
     The foregoing and other objects, features and advantages of the invention will be better understood from the following more detailed description and appended claims. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows the display system of this invention; 
     FIG. 2 depicts side-by-side displays of topographic features and hydrocarbon gas seeps; and 
     FIG. 3 depicts a display of hydrocarbon gas seeps superimposed upon a display of topographic features. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The antenna 11 is of the sector scanning type mounted in the nose of commercial aircraft for weather radar. In an actual embodiment of the invention the radar rotates through an arc of 120° , 60° on either side of the path of the aircraft. A cam arrangement rotates the antenna through its complete arc in two seconds and then reverses the movement so that the antenna is returned over the same arc in 2 seconds. Alternatively, a phased array antenna could be used. 
     The duplexor 12 and transmitter 13 are conventional radar components which produce a microwave pulse having a width between 0.25 and 2.5 microseconds, a pulse repetition rate about 1,000 per second and a peak power of 80 KW. The transmitter is triggered by oscillator 14. The transmitter frequency is approximately 20 MHZ above or below the characteristic frequency of molecular resonance of the gas undergoing exploration. In one embodiment of the invention the transmitter includes a magnetron mechanically tunable from 8600 to 9600 MHZ and which is nominally tuned to 9375 MHZ. This is 20 MHZ above the observed re-radiated signal from propane which has a frequency of 9355 MHZ. 
     The duplexor 12 alternately applies transmitter power to the antenna 11 and then applies the signal detected by the antenna to the receiver. The receiver includes a mixer 15. 
     The received microwave signal is applied from the antenna to the mixer 15 as is the signal from local oscillator 16. The local oscillator is tuned several megaherz away from one of the resonance frequencies of the gas to be identified. For example, the local oscillator is tuned to 9317 MHZ, 38 MHZ away from the re-radiation frequency of propane. Preferably mixer 15 is a non-linear balanced mixer which reduces local oscillator noise. Mixer 15 beats the local oscillator signal with the received signal to produce sum and difference signals. If the particular species of gas undergoing exploration is present in the atmosphere, a difference signal having a 38 MHZ component is produced at the output of the mixer. 
     A frequency monitor 17 provides a display of local oscillator frequency accurate to plus or minus 1 MHZ. It is important that the local oscillator 16 be tuned accurately. If the local oscillator drifts in frequency, then the system will not detect the gas under exploration, for example, the re-radiation of propane near 9355 MHZ. 
     The difference signal from the mixer 15 is amplified by the wide band pass preamplifier 18 having, for example, a band width of 100 MHZ centered about 60 MHZ. The output of preamplifier 18 is applied to three I.F. amplifiers 19, 20 and 21. I.F. amplifiers 20 and 21 are tuned, for example, to 38 MHZ and 58 MHZ respectively. That is, I.F. amplifier 20 amplifies signals 38 MHZ below and above the tuned frequency of local oscillator 16. I.F. amplifier 20 produces an output having both the re-radiated component, if it is present, and the background component of the received signal. I.F. amplifier 21 passes a signal having a component with frequencies 58 MHZ above or below the tuned frequency of oscillator 16. This component represents background energy. 
     The outputs of video amplifiers 20 and 21 are respectively applied to video amplifiers 22b and 22c, the outputs of which are applied to the cancellation circuit 23. The I.F. amplifiers 20 and 21, video amplifiers 22b and 22c and cancellation circuit 23 are more fully described in my copending application Ser. No. 628,689, filed Nov. 3, 1975, the disclosure of which is incorporated herein by reference. Briefly, the cancellation circuit 23 substracts the 58 MHZ background energy component from the 38 MHZ re-radiated energy component. The output of the cancellation circuit 23 represents only the re-radiated energy component. This signal is applied to modulate the intensity of the sweep of the plan position indicator 24. In order to generate a signal representing hard targets, the I.F. amplifier 19 is tuned to the transmitter frequency. The output of I.F. amplifier 19 is applied to video amplifier 22a. The component representing energy reflected from hard targets is applied to the plan position indicator 25. 
     Sweep signals for plan position indicators 24 and 25 are produced by oscillator 14 and sweep circuit 26. Each time the transmitter 13 is fired, a pulse from oscillator 14 starts the sweep of plan position indicators 24 and 25. Each sweep on the indicator originates at the bottom center of the screen and proceeds upward and outward in a direction coinciding with the direction of the antenna. Radial distance from the bottom center represents time and/or range and the sweep is intensity modulated to represent signals received along this range. 
     As the antenna 11 mechanically moves through its 120° arc, a synchro 27 produces a signal representing the position of the antenna. Specifically, synchro 27 produces a signal representing the angle θ between the direction of the antenna and the nominal aircraft heading denoted by the dashed line 28. This signal from the synchro 27 is applied to sweep circuit 26 which generates two signals representing the Sinθ and Cosθ. The Sinθ and Cosθ signals are respectively applied to the X and Y sweep inputs of the indicators 24 and 25. 
     As a result, the indicators produce a pie-shaped display on the screen representing topographic features and gas seeps in 60° sectors on either side of the heading of the aircraft. FIG. 2 depicts such displays with the left hand display depicting topographic features as detected by reflections from hard targets while the right hand display depicts re-radiated energy produced for example, by propane seeps. 
     Conventional television cameras 28 and 29 are used to record the displays produced by indicators 24 and 25. The output of each camera is recorded on a video tape recorder, or recorders, 30. The output can be viewed in real time on the monitors 31 and 32 or can be replayed later on these monitors. Monitors 31 and 32 produce displays of the type shown in FIG. 2 wherein the topographic features and the gas seeps are displayed side-by-side. An interpreter can easily locate the gas seeps with respect to known topographic features. In FIG. 2, two gas seeps are indicated by the displays 33 and 34 of re-radiated signals. The display on the left hand side indicates topographic features, for example, a coastline 35. 
     As an alternative or as an additional refinement, the display of the hydrocarbon seeps may be superimposed on the display of the topographic features by using color television cameras and different color phosphors in the displays 24 and 25. For example, the display 24 can have a red emitting phosphor and the display 25 can have a green emitting phosphor. Color television cameras 28 and 29 produce video signals representing these two displays and the signals are mixed in the video mixer 36. Video mixer 36 can be a common commercial unit for superimposing two or more video pictures. On example of such a mixer is the Sony special effects video recorder. Alternatively, the primary display can be in black and white and false color can be employed in color cameras, instead of different phosphors. 
     When the superimposed displays are reproduced on the color monitor 27 a display of the type depicted in FIG. 3 is produced. In this case the gas seeps are represented in red as indicated at 38 and 39 whereas the topographic features such as the coastline 40 are displayed in green. 
     Video mixer 36 can also be usefully employed to display navigational data concurrently with the gas seep display. Conventional aircraft navigational equipment 41 controls a digital display 42 which provides a read-out of latitude, longitude and time. Television camera 43 records this display. Mixer 36 superimposeds the navigational data on the bottom of the display as is shown in FIGS. 2 and 3. 
     While a particular embodiment of the invention has been shown and described, other modifications are within the true spirit and scope of the invention. The appended claims are, therefore, intended to cover all such modifications.