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.