Radar Seismograph Improvement

This invention is an improvement in my earlier original invention for which a patent application was filed Mar. 20, 1981, Ser. No. 245,945. The original invention makes use of the technologies of radar, exploration seismology and air transportation to provide a highly-mobile and economical system of geophysical exploration for petroleum and other minerals. Use is made of the Seismo-electric (SE) and Electro-seismic (ES) effects to momentarily alter the absorption in, and reflection or scattering from, the earth's surface of electromagnetic (radar) waves transmitted from, and received by, an aircraft, to thereby modulate seismic information on the reflected, scattered and received waves. The present invention applies the same basic apparatus as the original invention and, additionally, provides a great improvement in how the useful seismically-modulated radar wave signals are separated from the bulk reflected and scattered electromagnetic wave signals which might otherwise mask the smaller seismically modulated responses.

In the original invention, several methods and means of cancelling, or nulling, the bulk reflections were provided. In all of these a phase and amplitude balance was obtained as the radar scanned the terrain being explored, prior to the initiation of seismic responses; i.e., prior to dropping a weight or detonating an explosive charge, and the bulk reflections and scatterings thereby nulled in the receiving apparatus.

It is clear that the goodness of these methods and means depends upon the goodness of the phase and amplitude balance that can be obtained (the null). Since the balance is over at least one complete scan of the radar antenna (Practically, it is over many complete scans.), it is a broad average null covering a relatively large surface and, as such, cannot possibly be as good as might be desired. The most desirable arrangement would be one wherein the balance (the null) was automatically obtained for each and every small segment of the scan of the radar antenna. This kind of balance, or null, would then leave only the seismic response of each small segment of earth surface to be received, amplified, processed, and passed to form PPI pictures and recorded for additional processing at a later time. The object of this invention is to provide method and means whereby this ideal result is obtained.

FIG. 1, taken from Ser. No. 245,945, is a drawing outlining the preferred general system and method of the original invention and hence also of the present invention.

FIG. 2 is a sketch showing a typical scanning lobe of radiation as it is emitted from the rotating radar antenna of the present invention.

FIG. 3 is a sketch illustrating the composite scan of the present invention.

FIG. 4 is an electrical block diagram of the original system with the features of the present invention added.

FIG. 5 shows the construction of a typical delay line used in the present invention.

FIG. 6, similar to FIG. 4, is an electrical block diagram of the improved system with automatic features of time delay and radar scanning time added.

Referring to FIG. 1 in detail, 1 is the helicopter, or other aircraft, which transports most of the apparatus of the invention: the transmitting, receiving, nulling, displaying and recording apparatus of FIG. 4, as well as means of exciting elastic waves in the earth, comprising a tube, or gun, 4, shown in the cutaway 2, from which a projectile 5 is dropped or shot against the earth below. The cutaway 2 also symbolically shows the radar antenna 3 which alternately directs the transmitter radio waves against the earth and alternately receives the waves that are reflected or scattered back from the earth's surface. The remainder of the radar and recording apparatus is carried within the body of the aircraft but not shown.

As in the original invention, the projectile 5 is launched from 4 along a path 6 and strikes the earth 10, the impact of which deforms the earth at the point of contact and excites elastic waves, represented by the rays 7, some of which strike the subsurface structure 11 and are reflected back to the surface as rays 8. These, in turn, produce cyclical compressions and rarifactions of the soil at the surface and hence variations in the surface electrical conductivity as measured by Blau, Stathan (U.S. Pat. No. 2,054,067) and others. In all measurements, the cyclical variations in electrical conductivity, the SE-effect, were well above the noise level.

As practiced in the original invention, a broad nulling of reflections, prior to initiating a seismic response, would preferably first be obtained for an area being surveyed by one or more of the methods and means outlined in the original disclosure. The additional facilities of FIG. 4, items 34, 35, 36, and 37, would then be automatically utilized to obtain greatly improved results, as subsequently described.

First, refer to FIGS. 2 and 3 for the principles being applied. There, 21 is the radar antenna, 22 is a representative lobe of radiation emitted by 21, and 23 is an arrow indicating the rotation of the antenna, and hence also, the movement of the scanning lobe 22. Whereas FIG. 3 shows two lobes, the radar antenna produces only one lobe, as in FIG. 3; what is shown in FIG. 3 is a sequence of lobe positions wherein the broken-line lobe 24 is the position of the incident radiation at time T; whereas, the solid-line lobe 22 is the position of the lobe at time T-t; in other words, the two lobe positions are separated by a time interval, or period, t. If during this time, the seismic wave advances a distance, d, producing SE- and ES-changes in conductivity and dielectric responses, respectively, the reflection of the radar wave will change slightly during the time, t; and if we somehow bring the responses into opposition what essentially will be left are the reflection and scattering differences produced by the seismic disturbances in the small shaded area, 25, where the lobe of radiation intercepts the earth surface.

To accomplish this objective, it is obvious that the reflection of lobe 24 must be delayed by time, t, in the receiving and processing apparatus, so that the reflection of lobe 22 can be put in opposition thereto, and this is the essence of what has been added to the original invention.

Referring again to FIG. 4, 34 is the Delay Mechanism by means of which the time delay, t, is obtained; 35 represents the Direct Path of the signal into Subtractor 36 where it is combined in opposition to the delayed signal via 34. Thus, what is passed on to Rectifier 37, as the Radar Antenna 3 rotates, is the video representation of the differences in radar reflections occuring during the interval, t. After rectification in 37 what then is passed along to the PPI and Recorder is the electrical representation of the difference in seismic wave compressions and rarefications occuring during the time, t; which is to say: the seismic responses by means of which geological structure is manifest. The requirements are similar to those of Dopler Radar which is used to observe small moving objects in space, except we aren't observing directly the microscopic movement of the earth surface; rather, we are observing the variations in the conductive and dielectric properties of the earth's surface produced by the microscopic motions of the ions and particles responsible for the these electrical properties.

It would be most desirable to delay the reflection of lobe 24 by electromagnetic means but this turns out to be highly impractical. For example, if the seismic disturbance is travelling at a velocity of 5,000 ft./sec., and the radar reflection is confined to the first foot of earth surface, the optimum time, t, will be of the order of 200 micro-seconds. An electromagnetic wave in free space (essentially the same along a conductor in air) will travel at the rate of 982.times.10.sup.6 ft./sec., approximately 10.sup.9 ft./sec. Thus, an electromagnetic delay line would have to be 200,000 ft. long, and such a line, considering both bulk and weight, would be highly impractical to transport in a helicopter or other small aircraft; moreover, the attenuation of such a line would be of the order of several hundred Db.

Fortunately, as has been done in Dopler Radar and similar devices, an acoustic delay line can be substituted for an electromagnetic delay. Since the velocity of propagation of acoustic waves is much less than the velocity of propagation of electromagnetic waves, a delay line of reasonable proportions becomes available. This adaptation of the present invention is shown in FIG. 4. The reflections are picked up by Antenna 3, which in a pulsed radar can be used in common with for the functions of receiving and transmitting; or a separate antenna for receiving can be provided. In any event, the reflection signals in the receiving mode are transformed into electric currents by the antenna and are passed to Receiver 33 where they are selected and amplified, and the receiver output divided into a Direct Path 35 and into a path through Delay Mechanism 34. The two out-phased signals are then joined in Subtractor 36 and the difference in signals, that part representative of the seismic disturbance, passed to Rectifier 37 and thence to the PPI Display Unit 38 and Recorder 39. Thus, the discrete reflections (what is left of them considering prior cancellation in Receiver 33 of bulk reflections, with the help of Phase & Amplitude Network 32) from each small segment of terrain are process together, masking reflections further removed and the seismically-produced variations preserved.

In the radar science of the 1960s, at least 40 Db additional selectivity could be obtained by the method and means of comparing delayed and direct-path signals. Since the delay required by the present invention is about ten times less than in the usual Dopler Radar; since 30 to 40 Db selectivity can be obtained by the nulling methods of the initial invention; i.e., the application for example, of 32, FIG. 4, to the selection processes of Receiver 33; and since many improvements have been made since 1960 in selecting and amplifying the weak signals of space probes, it appears that at least 80 Db selectivity can be obtained by applying the advanced art of electronics to the present invention; i.e., selecting and processing differences of the order of one part in ten thousand. The big advantage here, however, is that we can make nulling balances for small terrain segments, the exact size used under conditions of seismic disturbances, rather than balances obtained during complete scans of an entire area.

The construction of a typical acoustic delay line is shown in FIG. 5. The transmitting and receiving crystals, 52 and 55, respectively, are piezoelectric devices which transform electric currents into mechanical motions, and mechanical motions into electric currents. They are cemented to Delay Line 54 by means of a suitable adhesive, as indicated by 53, and Delay Line 54 may be a rod of quartz, aluminum, etc., or a more complicated multiple-reflecting body well known to the art. The end pieces, 51 and 56, are reflecting and/or absorbing bodies used to make better impedance matches of the crystals 52 and 55 to Delay Line 54, as well as help attenuate unwanted side effects.

To assist in making the two transmission paths of FIG. 4 as identical as possible, and thereby perfecting good balances, I propose putting a delay line in each path, one having a 200 micro-second, or thereabouts, greater delay than the other. This can be accomplished by making one delay rod longer than the other, or in multiple-reflecting bodies, by providing fewer reflection paths in one than in the other. In this way, the transmission paths are made identical except for effective lengths, and hence better balances more readily obtained.

The delay line, or lines, are operated at some I-F or video frequency out of the receiver, in the range 5-60 Mhz., for example.

Considering the radar scanning frequency that might be required by the proposed system, suppose that a sector width of 6.degree. is used; i.e., the scanning beam, or lobe, moves 6.degree. in 200 micro-seconds, or 360.degree. in 12,000 micro-seconds. This results in a repetition rate of 83.33 r.p.s.=5,000 r.p.m., and this is a rate consistent with the observation of seismic wave travel in the earth's surface.

To be most effective, the time delay, t, of one signal path must be such that when the signals of this path are combined with the signals of another path, the reflections from identical, or nearly identical, earth surfaces are nulled, as already pointed out. In view of the fact that the observing platform (helicopter, airplane or space ship) is moving it is of course literally impossible to present absolutely identical earth surfaces for nulling on successive scans of the radar antenna. The best we can do is provide identical sectors for nulling; however, an aircraft travelling at 200 miles/hr. would move only 0.704 inches in 200 micro-seconds, and this is quite insignificant. Other speeds and distance would be proporational.

If the time delay, t, is 200 micro-seconds, as an example, and identical sectors are scanned each revolution, the radar antenna must scan 360.degree. of azimuth in each 200 micro-second period, or rotate at a frequency of 5,000 r.p.m. Thus, an electronic antenna and related scanning mechanism would be required.

With the time delay, t, fixed, the simplest mode of operation, the frequency of the energy applied to the azimuth and elevation controls of an electronic antenna would need to be adjustable, set at f=1/t, and locked in. I propose accomplishing this by incorporating the Delay Mechanism 34, FIG. 4, into the feedback mechanism of the sweeping Azimuth Control Oscillator, as disclosed in FIG. 6, so that the Delay Mechanism comprises a half-wave-length of the azimuth sweeping frequency, for example. Considering the Delay Mechanism 34 to be Delay Line 70, FIG. 6; and in view of the fact that the I-F or video signal frequencies of the radar system, and the frequency of the azimuth sweep, are far apart, as 5-60 Mhz compared with 5 Khz, Delay Line 70 can be used in common by merely providing two sets of piezoelectric crystals to activate the delay line and receive corresponding motions at the terminal end.

Referring to FIG. 6 for appropriate details, numerals 3, 30, 31, 33, 36 and 40 refer to units previously described in connection with FIG. 4. 70 is the multiply-used Delay Line as set forth above, and 71 is a Phase Shift Oscillator such as that of FIG. 24 (c), page 505, of Terman's Radio Engineer's Handbook, typical of the vacuum-tube era and often employed in audio and low-frequency generators. In the present application, Delay Line 70 replaces the tripple-section RC-network of Terman's oscillator and provides 180.degree. phase shift in the feedback path from plate to grid. Alternatively, more modern solid-state forms of generators may be used, and any required additional gain to overcome losses in the delay line also provided. The cardinal principle is that Delay Line 70 must provide 180.degree. of phase shift (or some integral number of degrees which divide evenly into a full cycle of 360.degree.) so that a one-on-one correspondence can be maintained for the Sweep Control Device 40 of Radar Antenna 3, using harmonic amplifiers as may be required.

When the phase shift is 180.degree., the output of Phase Shift Oscillator 71 is fed into Harmonic Amplifier 72 where the second harmonic of the oscillator fundamental frequency is selected, suitably amplified and passed to the Antenna Azimuth Controls of Sweep Control Device 40. In this way, the sweep of the antenna is sychronized with the video delay time, t, so that regardless of how much delay is chosen the period of the antenna sweep, and the video delay time, t, are one and the same thing. Obviously, the sweep of the antenna elevation controls can be synchronized in the same way. Thus, identical sectors of the earth's surface and, in so far as the aircraft's movements during time t do not contribute significantly to the motion, identical intercept surfaces 25, FIG. 3, will be progressively compared, the unwanted reflections and scatterings cancelled in Subtractor 36, leaving only the differences in reflections and scatterings that are produced by the seismic signals as they move to the surface.

The actual delay time, t, can be chosen to best reveal seismic information and modulate it upon the radar signal; as, for example, making it equal to the period of a half-cycle of the predominate component of the seismic wave. If this component is 100 Hz, say, the delay time, under this assumption, would want to be 5,000 micro-seconds rather than 200 micro-seconds. Such might possibly provide the greatest contrast; however, delays of this magnitude are difficult to obtain. Multiple reflection delay lines, such as those of FIGS. 4.9 and 4.10, Skolnik, "Introduction to Radar Systems," McGraw-Hill Book Co., N.Y., 1962, would probably need to be used. It might be better to base the delay time on the anticipated depth of penetration of the r dar wave into the earth, as proposed in the foregoing. Fundamentally, it is only essential that the time delay be long enough that significant changes in earth conductivity and/or dielectric properties, are produced by the seismic wave during the time of an antenna sweep. It matters not how many sweeps are required to develop the greatest contrast as long as significant information is provided by each sweep. This information can all be stored on a tape and developed and analyzed later.

The output of the Phase Shift Oscillator 71 can further be applied to the TR/ATR Switch 31, if desired, to synchronize the radar pulse periods with the delay time, although such is by no means necessary. In this instance, the oscillator output might be applied through either a harmonic amplifier, as for the sweep, or through a multi-vibrator, 73, or like device, depending upon the delay time, t, that is used, and whether the TR/ATR is operated on a harmonic or sub-harmonic of the oscillator frequency.

With the foregoing sensitive method and means of comparing successive reflections and/or scatterings of radar waves from specific segments of the earth's surface, due to the variations in surface conductivity and dielectric properties caused by the SE- and ES-effects, the source of the seismic disturbance producing the seismic waves is immaterial. A weight can be dropped; an explosive charge detonated, or use can be made of naturally occuring phenomena such as earthquakes (particularly the after shocks), volcanoes, or even ocean waves beating on the shore. Too, use can be made of large man-made disturbances such as those produced nuclear explosions or the launching of rockets and spacecraft, providing the efforts are appropriately coordinated.