FI60450C - SAETT ATT DETECTOR LED MATERIAL - Google Patents

Description

EÄSr ^ 1 [B1 (11) ANNOUNCEMENT ^ n / rn

Jfä & A l J <'UTLAGGNiNGSSKRIFT O U 4 0 0 C Patent granted 11 01 1932

Patent seddelat ^ (51) Kv.ik.3 / Int.ci.3 G 01 V 3/12, 3/38 SUOM I —Fl N LAN D (21) Putunttlhtkumu * - Pituntanrfknlni 3011/71 (22) Hakumlspllv · - An * eknlng * d «g 15.10.7l '(23) Primary Cloud — Glltighetsdag 15.10.7 ^ (41) Tullut | ulklMksl - Bllvlt offantllg 16.0U J6

National Board of Patents and Registration .... ......, __, ......

'(44) Date of leak in the Nihtivlktlpuion | -

Patent and Regulatory Authority AnsOkan utlagd och utl.skrifMn published 30.09.81 (32) (33) (31) Requested Muoikeu * —Bvjlrd prloritet (71) Barringer Research Limited, 30U Carlingview Drive, Rexdale, Ontario,

This invention relates to a method for the detection of conductive materials in a region, in particular in geophysical fields. to conduct mineral exploration and detect metals, mines and weapons by generating a primary electromagnetic field and directing said primary field towards that region, the primary field being determined by a time-varying waveform of known frequency and phase combination, the field containing a plurality of frequency components identification of the body, wherein the primary field causes induction of eddy currents in a conductive body present in said region which is intersected by the primary field and wherein the eddy currents cause the formation of secondary electromagnetic fields, and in which method the electromagnetic signals are received in the vicinity of a primary electromagnetic field, the first part of the received signals comprising signals from said regions corresponding in phase and amplitude to each frequency component of said secondary electromagnetic fields and the second part due to direct coupling to said primary electromagnetic field.

2 60450

Many classes of valuable mineral deposits contain a sufficient concentration of sulfide ores to become highly conductive compared to the rock type that encloses them. For many years, geophysical systems have used electromagnetic fields developed by alternating current conducting coils, which alternate currents induce eddy currents in underground conductive materials, to detect these occurrences. These eddy currents generate secondary electromagnetic fields that combine with the inducing primary field to produce electromagnetic resultant fields. These electromagnetic resultant fields can be expressed by suitable receiving coils and their phase and magnitude with respect to the primary field can be monitored during continuous override or from a group of predetermined positions.

The problem of detecting underground deposits increases substantially when they are at a depth of more than about 60 m and when they are covered by a conductive cover layer. Such a coating layer may be in the form of a glacial clay material, which is usually quite conductive due to the ability of the clay particles to carry charges. In addition, in dry areas, the topcoat can become very conductive due to the presence of salts that have been extracted on the surface due to the combined effects of varying rainfall and high evaporation rates. When electromagnetic research systems are used in the presence of a conductive cover layer, strong eddy currents are induced in the cover layer, thereby developing secondary fields that tend to mask the secondary fields originating from the mineral deposits below. In addition, the conductive cover layer tends to dampen the transmission of electromagnetic fields and reduce the effective penetration depth of electromagnetic geophysical survey systems.

Today, there are two main types of airborne electromagnetic research systems in use. One type uses a rigid mounting of the transmit and receive windings and the other type uses a non-rigid fit. In "rigid" systems, the transmit and receive coils are mounted on the tips, bow or stern of the wings of an aircraft or on opposite ends of a long rigid boom which is towed below the helicopter. In both systems, the windings are rarely more than 25 m apart and often much less. In such systems, for best results, it is necessary for the 60450 to support the coils with a very high degree of stiffness so that the intermediate and angular changes of the coils do not exceed more than a few parts per million. This is difficult to achieve in practice. As a result, it is generally desirable to use rigid systems in calm air conditions.

Another problem with many common rigid systems is that when the receiving coil is mounted on or behind the wingtips of an aircraft, small movements in the metal shell of the aircraft cause additional interference because such movements tend to change the nature of the eddy currents induced by the aircraft transmitter. In addition, stray currents from aircraft generators and other electrical equipment cause additional interference in the aircraft, which is sometimes difficult to eliminate.

In non-rigid airborne electromagnetic systems, the receiving windings are generally towed behind the aircraft in a non-metallic streamlined aerodynamic tank or "bird" typically connected to the aircraft by a tow cable at least about 50 m long. The advantage of this arrangement is that it removes the bird from the immediate vicinity of the aircraft and thus eliminates some of the above-mentioned sources of interference. However, when the towed receiving coil or coils continuously change their connection with the primary field, it is necessary to eliminate the primary field, for example, by expressing only those secondary fields that are exactly one-quarter phase difference, i.e. 90 ° out of phase with the primary field. However, one disadvantage of such quadrant systems is that a large number of secondary fields are out of order because it is in phase with the primary field. In addition, the relatively limited amount of information received from the Quarter System does not allow for a complex conductivity analysis of the terrain below.

Another technique to eliminate the effects of variable coupling with respect to the primary field is to use a series of high-power pulses and express a short-term secondary field that follows each pulse. This achieves a time difference between the primary and secondary fields. This arrangement is known as the short-term induced pulse technique. The main point of this technique is that a very large part of the secondary field 60450 is obsolete because it was developed during the primary pulse period. As a result, it is necessary to use a very much more powerful transmitter than is typically used in other systems to compensate for the fact that most of the secondary field is out of order. A system of this type is disclosed in U.S. Patent 3,105,934.

According to the present invention, the advantages of both rigid and non-rigid airborne electromagnetic systems have been combined to achieve high signal-to-interference ratios and a significant difference from the conductive coating under complex conductivity conditions. The invention is applicable to both rigid and non-rigid airborne electromagnetic shapes.

The invention utilizes an inductive primary electromagnetic field having a complex waveform, i.e., the waveform comprises multiple frequencies, to induce eddy currents in conductive materials in the vicinity of the field, which in turn will repeatedly radiate electromagnetic secondary fields. These electromagnetic secondary fields will have a waveform that is distorted relative to the corresponding primary field waveform by an amount that depends on the size, shape, conductivity, polarizability, and permeability of the ore or conductive material. Such a distortion is due to the fact that the complex waveform contains several frequency components, all of which have different repetitions (as a result of eddy currents) with different relative amplitudes and phase shifts with respect to the primary waveform. The amplitude and phase shift of the secondary field for each frequency component is determined by the characteristics of the conductive substance. In addition, the overlay layer and mineral deposits generally have certain separately identifiable groups of responses that can be broadly classified and distinguished from each other.

The detection of a conductive substance in the region of the present invention is characterized by storing a plurality of reference or reference signals characterized by (i) predetermined components of said primary field and (ii) predetermined components of secondary fields emanating from a plurality of predetermined of the type of conductive bodies when the primary field 60450 intersects these bodies, wherein the first and second portions of the received electromagnetic signals are compared to the stored reference waveforms and the reference waveforms that provide optimal correlation to one or more selected components of said received signals are detected, respectively.

Computer-aided signal processing techniques are best used to obtain a comparison between the waveforms of the received signals and the stored waveforms of the reference corresponding to known geological structures or conditions. One advantage of the present invention over systems, e.g., the short-term induced pulse technique disclosed in U.S. Patent No. 3,105,934, is that the entire secondary field can be analyzed if desired, i.e., the field received while the primary field is operating. In fact, the present invention also provides narrowband filtering of the desired waveforms and the elimination of spherical interference caused by near and far thunderstorms, etc., by interference signals such as thematic interference in the receiving coil, microwaves in the receiver.

In the drawings:

Figure 1 schematically shows a research aircraft carrying a transmitting and receiving device.

Figures 2-10 and 12-15 show graphical illustrations, illustrating the waveforms referred to below.

Figure 11 shows a block diagram of the main parts of a preferred embodiment of the invention.

The invention will be explained with reference to the use of a primary field, the primary field of which is determined by a fast-scanning periodic frequency modulated signal, for example a signal whose frequency varies continuously between 165 Hz and 5280 Hz, with a repetition rate of about 80 Hz. Such a signal can be generated by a series of capacitors that are sequentially coupled to the oscillation circuit to provide a fast sweep frequency modulated signal. Silicon resistors are very suitable for connecting capacitors at zero current consumption points and are capable of handling very high powers.

Referring to the drawings, a primary electromagnetic field, the waveform of which is shown in Figure 2, is generated by conducting a current generated by the transmitter 14 through a multilayer circumferential antenna 10 mounted on an aircraft 11 or other vehicle. The power supplied by the transmitter 14 should be at least about one kilowatt. The primary field and the secondary fields reflected from the conductive materials in the terrain below are received in one or more receiving coils 15 towed by a ribbed torpedo-shaped flying vehicle, hereinafter referred to as a bird, indicated at 12 in Figure 1. If desired, the bird may be provided with small wings. ascent to a normal towing position at the rear of the airplane, where it does not catch on trees when flying at low altitudes. It is common to place the receiving windings at right angles to each other, with one winding in maximum coupling with the primary field generated by the circumferential antenna 10. The receiving windings are connected to preamplifiers, the output power of which is transferred to the signal handling device in the aircraft 11 via the electrical wires in the tow cable 13.

The primary field is expressed by the receiving coil 15 in the form of its derivative, because the coil 15 is sensitive to the degree of change in the magnetic flux. The received waveform of the primary field is shown in Figure 3. In the presence of a conductive substance, for example an underground ore, the received waveform of the secondary field generated by such a conductive substance is distorted as shown in Figure 4. The positions of the waveform zero points have shifted and the relative amplitudes of the low and high frequency components of the waveform have changed in the received signal compared to the primary field.

One suitable way to describe the response of a typical conductive substance is to express its conductivity thickness product. Thus, the answer is a function of the absolute conductivity of the substance in which the eddy current flows multiplied by the thickness of the conductive layer containing the eddy current. This means the fact that a large number of naturally occurring conductive substances or formations in the ground have a plate-like shape.

7 60450

Secondary field responses excited by a scanning frequency waveform in a series of conductive layers with conductivity-thickness inputs of 1.6,3.2,6.4, 12.8, 25.6, and 51.2 The seeds are shown in Figures 4-9, respectively. It should be noted in Figure 4, where the conductivity-thickness input is the lowest, that there is a considerable attenuation of the low frequency component compared to the high frequency component. There are also zero phase shifts at all frequencies. According to Figures 8 and 9, at the end of the high conductivity-thickness input, the distortion of the secondary field amplitude is very small with respect to the differences between the high and low frequency components, and the zero phase shifts are limited to the lowest frequency components.

For a material with a relatively high conductivity-thickness input, such as an ore having a conductivity-thickness input of, for example, 12.8 Seeds and below a conductive cover layer with a lower conductivity-thickness input (e.g., 1.6 sianens), the combined secondary field response is shown in Figure 10.

It should be noted that the waveforms of each conductivity-thickness input have distinctly different characteristics, and the image of the combined waveform showing the ore below the cover layer is also different from the response of the conductive cover layer alone. The signal generation system of the invention has a plurality of output channels, respectively, indicating the existence of a correspondence between the received signal and a plurality of stored signals, which signals are typical of a variety of coating and ore conductivity thickness inputs. When the ore is below the conductive cover layer, the output is obtained from two channels, one corresponding to the conductivity characteristics of the cover layer and the other to the conductivity characteristics of the ore.

A block diagram of a preferred embodiment of the signal generation system of the invention is shown in Figure 11. The receiving coil 15 in the bird 12 is designed to have sufficient bandwidth to faithfully follow the waveform of the primary field and is connected to a preamplifier 16. The output power of the preamplifier 16 is 17 along the electrical wires in the cable 13.

8 60450

The analog-to-digital converter 17 is connected to a partition memory store 19 which synchronously reproduces the waveform and stores it in a digital buffer memory unit 20. The buffer memory unit 20 is discharged at periodic intervals, e.g. twice in scKun and digitally connected to a small computer 21. which are characteristic of typical cladding and ore responses, for the group.

The method used in computer 21 provides an optimal matching of the reference waveforms, together with the primary waveform, for the buffered 20 stored, received waveforms. Consistently, the average of the received waveforms is calculated for each of the receiving windings from about 32 cycles to improve their message / interference ratio. Each time-averaged waveform obtained is then represented mathematically as the sum of the primary waveform and the plurality of reference waveforms, with the amplitudes of the primary and reference waveforms being considered as optimizable parameters. A suitable optimization method is the least squares method, in which the sum of the squares of the differences between the received waveform and its mathematical representation is made as small as possible for all the ordinates of the waveform.

During flight, the continuous oscillation of the bird around its center position causes a time-dependent variation of the primary field amplitude in each receiving coil. Compensation for the effect of bird motion is achieved by keeping the amplitude of the primary field as a linear function of time while adding one additional parameter to be determined during the least squares optimization.

In some forms of the transmission and reception windings, the response from the ground is always positively connected to one of the reception windings, in which case the amplitudes of the reference waveforms are forced to be positive. When constraint irregularities occur, the sum of squares has several local minima that are judged according to the goodness of the fit criterion to provide the optimal solution.

Other factors included in the desired optimization scale are the correction of the magnetic field induced from the eddy currents in the aircraft wings to the receiving coils and the frequency dependent phase shift in the reference waveforms for deep conductive materials due to the effect of the conductive coating.

The invention has been described as being applied to a scanning frequency type waveform but there is generally a wide variety of types of primary waveforms that can be used. It is required that the waveform of the primary field be known and vary over time and include a sufficient number and range of frequency components to enable ore identification.

While it is generally most desirable to use a primary field having a periodic and repetitive waveform, it is also possible to use a primary field having a waveform formed of "pseudo-random" noise having a suitable range of frequency components. In this case, the periodic time samples of the pseudorandom noise field transmitted by the circumferential antenna 10 are modified in the computer 21 to obtain a series of reference waveforms representing the response of the conductive region. These are compared in computer 21 by a pseudo-random signal detected by the receiver;

Iin waveform for analyzing this signal into primary and secondary field components, as shown in conjunction with the scanning frequency waveform.

In a system designed for the study of conductive ore deposits in air, the practical limits of the frequencies in the primary field are 100 to 10,000 Hz. However, the system can be applied to other special geological applications, such as surface geology conductivity mapping, where it may be desirable to raise the upper frequency as high as 100 kilohertz. In a ground system for ore exploration, the frequency limits may be advantageously reduced at the lower end and in some cases to frequencies as low as 10 Hz.

The invention has been shown to be applied to a received full waveform. However, when using a pulsating primary field, it is possible to use only the transient portion of the received signal immediately following the primary pulse. This eliminates a very broad primary waveform and makes the task of achieving a comparison of the received waveform and the reference waveform somewhat less critical. However, this means reduces the signal-to-noise ratio because the part of the secondary field covered by the primary waveform is no longer in use. Pulse waveforms that can be used with the short-term portion of the received waveform include half-blue pulses, sawtooth and rectangular pulses, and all other pulses in which the main signal is suddenly followed by a period in which it has either constant rate of change. In either case, the differentiated waveform expressed in the receiving winding has a zero rate of change during the period used to detect the transient portions of the secondary field.

Although the receiving means used in the invention are shown as windings, it is clear that a magnetometer with a sufficiently high sensitivity can be used as an alternative. Most magnetometers are not sensitive enough for this purpose, but as the state of the art evolves, new magnetometers will be developed to

- Q

the sensitivity in the vicinity of 10 Gauss 3a is better.

The use of a single horizontal axis receiving coil is described above. However, there are some advantages to using two or three rectangular receiving coils, as disclosed in U.S. Patent 3,105,934 for additional information on the geometry of the expressed conductive material. Thus, the vertical conductive layer can be more easily separated from the inclined layer by using receiving coils with vertical and horizontal axes and comparing the two outputs on the computer to pairs of signal signals stored.

With respect to the circumferential antenna 10, it is also possible to use vertical axis transmission coils instead of the horizontal axis winding shown in Figure 1. Such a coil may surround the aircraft and be attached to the bow, wingtips and stern.

The invention has been presented for the purpose of comparing a waveform in a receiver comprising a digital computer, and it is clear that this computer can be placed either on an aircraft to perform the calculation of signals immediately after they are received or it can be placed on the ground. In the latter case, the received signals can either be remotely measured by radio link to a ground station where processing can be performed or they can be recorded on a tape in an aircraft for later analysis. A suitable way to record on tape is to use a partitioned and synchronously integrated 11,60450 buffer pool that collects and averages the return signals over periods of about 1/4 second and then decompresses the stored signals into a digital magnetic recorder. This reduces the amount of tape required and provides a dynamic wide area output that is ready to be processed on the ground using the system shown. However, it is possible to recover the raw signal using the band frequency modulation technique as opposed to the digital integration method mentioned above. In the above description, reference has been made to the use of an aircraft, but it is clear that the invention is also applicable to ground-based systems in which the orbital antenna is supported by a land vehicle or is mounted on the ground. The received! the signal analysis can again be performed immediately or the signal can be remotely measured or recorded on tape for processing at a central station.

In the normal air application according to the invention, parallel overflights are performed, usually at an altitude in the vicinity of 60 m, with a flight distance in the vicinity of 300 m. Profile maps are produced from the amplitudes of different output powers drawn in graphical form along flight paths. Data drawing methods in the field of airborne geophysical research are well known.

Although the invention has been described with reference to a digital computer, it will be appreciated that analog processing methods may be used instead. However, digital processing is more flexible compared to the reference waveform types of the recorders and the mathematical analysis technique used.

With respect to the evolution of the reference waveforms for comparison purposes, it is possible to correlate the received signal with the sets of waveforms representing the responses of the conductive layers. Small conductivity-thickness inputs representing the top layer can be compared with horizontal conductive layers, while higher conductivity-thickness inputs representing conductive sulfides can be formed as responses to vertical or steeply curved conductive layers.

Greater sophistication can be obtained in the system by expressing the responses of 12,60450 more conductive substances with the response of spherical or flattened oval spheres. The choice of models can be varied in a given study area depending on the type of object assumed. The more precise the waveforms used to represent the different conditions that are expected to be encountered, the more complete the separation and discrimination in computer comparison and optimal filtering.

In the case where the conductive ore deposits are below the conductive cover layer, the modifications in the response characteristics of the conductive deposit are caused by the interaction with the conductive cover layer.

Delays in the propagation of the signal passing through the frequency-dependent overburden in both directions can be allowed in computer programs so that a better and more accurate fit is obtained with respect to the response of the underground ore deposit. This type of treatment is a feature of the invention which makes it possible to separate a difficult-to-detect mineral deposit thickly covered with a conductive top layer.

The development of suitable reference waveforms representing separate conductive conditions for use in computer comparison can be accomplished by mathematical design or field measurements. In many cases, batches of mathematical models can be stored and applied to special situations with answers stored above known objects in the field. Thus, pending the re-occurrence of an ore deposit of a certain class in the area, the response of a known ore deposit can first be recorded and used as a reference value against which the signals obtained are compared during the research program.

Although the invention has been described for implementation primarily in connection with geophysical surveys of mineral deposits and the like, it also has use as a metal detector, a mine detector and a weapons detector. The transmit and receive windings can be mounted in a rigid parallel station and the frequency sweep signal used covers the range of 1,000 Hz to 20 kilohertz. The characteristic features of the response can be analyzed in the same way, but the frequency requirements are much higher due to the small size of the objects to be detected. The main difference is between the responses of ferrous and non-ferrous metal objects, and the response marks are also very much a function of size. A suitable computer that compares the change in characters can provide better separation than previous single-frequency and dual-frequency metal detection systems. Examples of waveforms are shown in Figures 12-15.

Fig. 12 shows the shape of the current wave in the transmitter and Fig. 13 shows the shape of the voltage wave coming from the primary field at the receiver. Fig. 14 shows the waveform of the secondary field from a small non-ferrous metal object and Fig. 15 shows the waveform from a small steel object.

Claims (8)

14 60450 A method for detecting conductive materials in an area, in particular for geophysical mineral exploration and for detecting metals, mines and weapons, generating a primary electromagnetic field and directing said primary field towards said area, the primary field being determined by a known frequency and a combination of phases. , which allow the identification of a conductive body in the region and wherein the primary field causes the induction of eddy currents in the conductive body present in said region, which is intersected by the primary field and the eddy currents cause secondary electromagnetic fields to form, and in which electromagnetic signals are received in the vicinity of the primary the part comprises signals from said areas which phase their frequency and amplitude correspond to each frequency component of said secondary electromagnetic fields and the second part is due to direct coupling to said primary electromagnetic field, characterized by storing a plurality of reference or reference signals characterized by (i) predetermined components of said primary field and (ii) predetermined components of said primary field; emanating from a plurality of conductive bodies of a predetermined type when the primary field intersects these bodies, the first and second portions of the received electromagnetic signals being compared with the stored reference waveforms and expressing those reference waveforms that provide an optimal correlation to one or more of said received signals. A geosyphalic survey method according to claim 1, characterized in that said comparison step comprises successive steps of subtracting the shape of each said stored reference wave from said received signals to provide an optimal comparison by the least squares method. 3.  A geophysical survey method according to claim 1, characterized in that said primary field is provided by a fast-scanning, periodically frequency-modulated waveform.  15 60450 A geophysical survey method according to claim 3, characterized in that the primary field contains frequency components in the range of about 165 to 5,300 Hz. A geophysical survey method according to claim 4, characterized in that the primary field is provided by a periodic waveform having a repetition frequency of about 80 Hz. A geophysical survey method according to claim 1, characterized in that the locations where said electromagnetic signals were received are transported over the land area to be surveyed and in said land area. 1. The provisions of this Regulation apply to the presence of faces, in particular the use of geophysical casting and the detection of metallic, minor and vapor, and to the presence of primary electromagnetic fields. with the frequency and the frequency of the connection, the colors of the internal frequency of the source, the brightness of the identification of the power supply and the color of the primary power supply of the power supply, the power supply of the power supply the second electromagnetic signal is used to indicate the electromagnetic signal strength of the electromagnetic signal, the signal to the electromagnetic signal, the signal to the electromagnetic signal to the electromagnetic signal is limited to the second frequency and, in the case of direct direct copying of these primary electromagnetic fields, the reference voltage, the reference reference or high-voltage signal set, for which the specific specification is (i) the secondary component of the primary component (s) vilka utgär