Hydrocarbon exploration with display of re-radiated and reflected microwave energy

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.

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 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.degree. , 60.degree. 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.degree. arc, a synchro 
27 produces a signal representing the position of the antenna. 
Specifically, synchro 27 produces a signal representing the angle .theta. 
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 
Sin.theta. and Cos.theta.. The Sin.theta. and Cos.theta. 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.degree. 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.