Abstract:
A method and apparatus for detecting relative movement of two system (S, S 2 ) by using electromagnets induction (B, v, i) on a sensor with a defined magnetic field (B m ) and compensating for the influence of an external interfering magnetic field (B s ) The magnetic field (B m ) is generated in at least two areas (M, M 2 ) with reversed polarity and a conductor arrangement is designed so that an addition induction effect (i, i 2 ) occurs on the conductor and the external interfering field (B 2 ) is corrected by the additive induction effect.

Description:
[0001]     This application is a continuation of application Ser. No. 10/454,595, filed Jun. 5, 2003, which is a continuation of Ser. No. 09/715,059, filed Nov. 20, 2000. 
     
    
     BACKGROUND AND SUMMARY OF THE INVENTION  
       [0002]     The present invention relates to a method of detecting a relative movement of two systems.  
         [0003]     The present invention is based on problems which occur when detecting seismic processes by means of sensors which utilize electromagnetic induction: It cannot be distinguished whether a recorded signal is generated on the basis of the seismic process and thus of the movement of an electric conductor with respect to an established magnetic measuring field or on the basis of a changing interfering magnetic field.  
         [0004]     Seismic processes may also be accompanied by seismically caused electromagnetic fields. When sensors of the above-mentioned type are used, because of magnetic fields possibly accompanying seismic processes, measuring signals can be interpreted only with great uncertainty exclusively as being caused by movement, not taking into account additional non-seismically caused interfering magnetic fields. In addition, sensors for detecting seismically caused movement must be able to also detect extremely low-frequency signals, far below 10 Hz, even below 1 Hz, with a high sensitivity.  
         [0005]     This results in the principal object on which the present invention is based, specifically to be able to detect the movement of seismic processes by means of utilizing induction and, in the process at least reducing the influence of interfering electromagnetic fields, whether they are caused seismically or otherwise. The following described solution on which the present invention is based while addressed to problems when detecting movements on the basis of seismic processes can easily be used for other movement sensor applications, particularly if similarly high demands are made on a suppression of interferences and sensitivity is demanded into the lowest frequency ranges.  
         [0006]     The problem of interfering magnetic fields in the case of seismic sensors had not been considered to be very important.  
         [0007]     The present invention achieves the above-mentioned object in that the influence of an external interfering magnetic field is counteracted by compensation.  
         [0008]     Herein the term interfering magnetic field applies to any type of magnetic field except for the one which is established in a targeted manner for measuring purposes and which we want to call measuring magnetic field.  
         [0009]     According to the invention, the influence of interfering magnetic fields which affects the measurements can be suppressed at the lowest frequency interference signals and with a sensitivity required during seismic measurements.  
         [0010]     In a preferred embodiment of the method according to the invention, the defined measuring magnetic field, in at least two locally separated adjacent areas, is additively coupled with a conductor arrangement, but the interfering magnetic field is subtractively coupled. In this case, two magnetic fields with reversed polarity can be coupled to a given conductor arrangement, but are nevertheless added to one another in their induction effect because the conductor arrangement is correspondingly guided geometrically in the measuring fields. In contrast, an interfering magnetic field is essentially homogeneous in the area of the sensor, whereby, when the above-mentioned measures are taken, its induction effect is subtracted on the conductor arrangement.  
         [0011]     Preferably, the measuring magnetic field is applied to the same extent in the two above-mentioned areas. However, this is not absolutely required.  
         [0012]     In a preferred embodiment, the described conditions are implemented in that the defined measuring magnetic field, bound to one of the systems, is generated with at least two partial fields of different polarity which are radial with respect to an axis, and the conductor arrangement is constructed as an arrangement of at least two coils which are wound in opposite directions, are bound to the second system and whose coil axes are at least approximately coaxial with respect to the axis of the radial partial fields.  
         [0013]     In addition to the compensation measures according to the invention, it is, however, easily possible to also provide shielding measures. If these are to be effective also at lowest-frequency signals, preferably at least the area of the sensor on which the induction is established is surrounded by means of an electrostatic shield. The shield is implemented, for example, by an electrically highly conductive coating or foil. The wall of a shield carrier housing, which is significantly thicker than the shield, is produced from electrically non-conducting, non-magnetic material, for example, of a plastic material.  
         [0014]     Seismic movement sensors are known, in which case electromagnetic induction is utilized for recording movements. Reference is made, for example, to the sensors of the SM3-KV or SM3-KVE Types of the Russian Academy of Sciences, Design Bureau for Geophysical Instruments, or the sensor S-13 of Geotech Instruments Company, LLC.  
         [0015]     These known seismic movement sensors are extremely suitable to be equipped by means of the measures according to the invention. In this case, a measuring magnetic field is bound to a journal bearing of a lever on the known sensor. In the above-mentioned bearing, the swivel pin of a lever is disposed, on which first a seismic mass and then a conductor arrangement is mounted which can be moved in the measuring magnetic field together with the lever. A spring arrangement also acts upon the lever, against the torque caused by the seismic mass. However, for the interfering magnetic field compensation according to the invention, the measuring magnetic field and the conductor arrangement are now constructed according to the invention.  
         [0016]     The method according to the invention is particularly suitable for the detection of seismic processes and their analysis; in this case, particularly for finding hydrocarbon occurrences for obtaining information on hydrocarbon occurrences and/or on the extent of hydrocarbon occurrences.  
         [0017]     This provides the possibility of separately recording movements caused by seismic processes and magnetic fields caused by seismic processes and of analyzing them as separate components of one and the same process.  
         [0018]     Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]      FIG. 1  is a schematic and perspective view of the induction effect of a magnetic field on a movable conductor, as a basis for understanding the present invention;  
         [0020]      FIG. 2  is a representation analogous to  FIG. 1  of the basic implementation according to the invention of a measuring magnetic field and a conductor arrangement moving therein;  
         [0021]      FIG. 3  is a longitudinal sectional view of a preferred embodiment of a sensor operating according to the method of the invention;  
         [0022]      FIG. 4  is a schematic view of a seismic sensor of the invention constructed on the basis of a known seismic sensor;  
         [0023]      FIG. 5   a  is a view of the signal in the 50 Hz interfering field environment received by means of a conventional, not interfering-field-compensated inductive measuring head, in the time range;  
         [0024]      FIG. 5   b  is a view of the signal according to  FIG. 5   a  in the frequency range;  
         [0025]      FIG. 6   a  is a view, analogous to the view of  FIG. 5   a,  of the measuring signal in the same environment with an interfering-field-compensated sensor constructed according to the invention;  
         [0026]      FIG. 6   b  is a view of the signal according to  FIG. 6   a  in the frequency range; and  
         [0027]      FIG. 7  is a view of the amplitude characteristics and the phase frequency characteristics of the sensor of the invention constructed according to  FIG. 4  with an installed operating point control and with phase frequency characteristics compensation. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0028]     The relative movement of a system S 1  with respect to a system S 2  is to be detected. For the respective utilization of the induction, a measuring magnetic field B m  is generated on the one system S 1 . When, in this induction field B m , a conductor  3 , which extends perpendicular to the lines of flux of the measuring magnetic field B m , is moved at the speed v in the illustrated direction, a voltage is induced in the conductor  3 .  
         [0029]     Thus, it now becomes possible to measure the speed v in the indicated direction by measuring the induced voltage on the conductor  3  of the conductor arrangement. In contrast, a position determination of the conductor  3  in the static field B m , which is assumed to be uniform, is not possible by means of induction.  
         [0030]     When a time-variable interfering field B s (t) exists in the field-filled space area M, because of its time variation, independently of whether or not system S 2  is moved with the conductor  3 , this interfering field B s (t) causes a voltage in the conductor  3 . For this reason, it cannot be discriminated on an electric signal tapped at the conductor  3  whether it is generated only because of a relative movement of the systems S 1 , S 2 , only because of the time variation of the interfering field B s  or because of a combination of both causes.  
         [0031]     Thus when, by means of induction, the movement of a seismic process is to be monitored, the measuring result does not indicate with certainty whether a time-variable interfering field and/or a movement was detected.  
         [0032]     However, specifically when examining seismic processes, it is very important to separately record movements and seismically caused electromagnetic fields, apart from the fact that magnetic fields which are not seismically caused and which also enter in B s , are never of interest here.  
         [0033]     Based on the problems indicated in conjunction with  FIG. 1 ,  FIG. 2  shows the principle of the solution according to the invention. Accordingly, the measuring magnetic field B m  is applied in at least two mutually separated areas of system S 1 ; according to  FIG. 2 , in area M 1  as partial magnetic field B m1  and in area M 2  as partial magnetic field B m2 . With respect to the movement direction of system S 2 , corresponding to {overscore (v)}, and the alignment of the respective conductors  3   1  and  3   2  in the two areas M 1  and M 2 , the partial magnetic fields B m1  and B m2  are reversely polarized. As a result of the corresponding wiring of the conductors  3   1  and  3   2 , as schematically illustrated at reference number  4 , they are connected in series such that the induction voltages occurring during a joint system movement {overscore (v)} are added in the conductors  3   1  and  3   2 . A measuring instrument is schematically illustrated by means of reference number  5 .  
         [0034]     When, analogous to  FIG. 1 , the time-variable interfering magnetic field B s (t) is considered,  FIG. 2  illustrates that the field uniformly fills the two adjacent areas M 1  and M 2  and, as a result, the resulting induction voltages are also compensated in the conductors  3   1  and  3   2 .  
         [0035]     It is not absolutely necessary that the induction effect is doubled in spaces M 1  and M 2  of the measuring magnetic field B m , but it is important that the induction effect in both above-mentioned spaces, if possible, is identical with respect to the interfering magnetic field B s , so that an interfering signal compensation takes place which is as complete as possible.  
         [0036]      FIG. 3  is a schematic view of a preferred embodiment of a sensor according to the invention. A symmetrical annular-gap magnet  10  comprises a pole section  12  which is cylindrical with respect to an axis A, optionally with two inward jutting pole flanges  14  coaxial with respect to the axis A. Pole section  12  is constructed of a magnetically soft material, for example, made of iron or steel.  
         [0037]     On the base side, pole section  12  is closed off by a base plate  16  made of a non-magnetic material, for example, of a plastic material or of stainless steel, which coaxially to the axis A in the pole section  12  carries a core  18 , with magnetically soft pole parts  20  and  21  as well as a magnet arrangement  23  disposed in-between, preferably constructed as a strong permanent magnet, such as a neodymium magnet. By means of this arrangement, a radial measuring magnetic field B m  is formed. Naturally, it is also easily possible to use, instead of or in addition to the permanent magnet  23 , for example, also in or on the wall of the pole section  12 , additional magnet arrangements and always permanent and/or electromagnets.  
         [0038]     On a support  19  of the system S 2  illustrated in  FIG. 3  by a broken line, coils  25   2  and  25   1 , which form conductor arrangements according to  FIG. 2 , are mounted such that, together with the system S 2 , the coils can be moved in a non contact manner over the core  18  and in the radial magnetic partial fields in areas M 1  and M 2  respectively.  
         [0039]     Analogous to the representation of  FIG. 2 , the coils  25   1  and  25   2  are series-connected such that the induction voltages resulting from the system movement v are added up. Between the magnetically soft core parts  20  and  21 , on the one hand, and the pole section  12 , on the other hand, two cylindrical air gaps, which are coaxial with respect to the axis A, are formed corresponding to the areas M 1  and M 2  of  FIG. 2 . These are filled with the radial measuring magnetic fields of reversed polarity which act upon the coil arrangements  25  displaceable in the air gaps. Interfering magnetic fields fill both air gaps in a uniform manner, whereby, corresponding to the explanations with respect to  FIG. 2 , their induction effect on the coils  25   1  and  25   2  is compensated.  
         [0040]     Although the approach discussed here is particularly suitable for the interfering-field-compensated recording of movements which originate in seismic processes, correspondingly constructed movement sensors can also be used for the detection of relative movements between systems which have different causes.  
         [0041]      FIG. 4  schematically shows a particularly preferred embodiment of a seismic movement sensor constructed, on the one hand, of the basic mechanism of the known sensor SM3-KV or SM3-KVE of the initially mentioned firm and, on the other hand, supplemented by the measures according to the invention, as explained by means of  FIG. 3 . From a physical point of view, this is a pendulum arrangement.  
         [0042]     The known seismic movement sensor comprises a lever  29  with a large seismic mass  31  disposed in a swivel bearing  27  on a support system S 1 . At the end of the lever  29 , the support  19  of the inductively acting measuring head  33 , which is now constructed according to the invention, is mounted, as explained according to  FIG. 3 . A spring arrangement  30  acts against the torque of the mass  31  on the lever  29 .  
         [0043]     The calibrating and adjusting measures provided on the known sensor are not illustrated in  FIG. 4 . As shown in  FIG. 3 , it is also important in the case of the approach according to the invention that, for utilizing a linear movement/signal transmission range which is as large as possible, in the inoperative condition, system S 2  is returned with respect to system S 1  into a defined operating point position which is preferably symmetrical with respect to areas M 1  and M 2 . For this reason, an operating point control is provided on the movement sensor in a known manner (not shown).  
         [0044]     In order to minimize the influence of electric interfering fields in all embodiments of the method of the invention, as schematically illustrated in  FIG. 4 , the measuring head, with the induction-effective space areas, is installed in a housing  41  whose wall material has poor conduction. A plastic material or stainless steel can, for example, be used as the wall material. The interior wall of the housing  41  is covered or coated with a thin shield  43  of an electrically well conducting material, such as copper, or with a conducting lacquer. The shield  43  is preferably applied to a measuring reference potential. This prevents a capacitive interference of electric fields.  
         [0045]     A seismic movement sensor of the SM3-KV Type of the initially mentioned firm, as schematically illustrated in  FIG. 4 , but with a conventional, inductively acting measuring head, was set up in a 50 Hz-interfering-field-contaminated environment and was mechanically blocked and, on the single provided induction coil, the resulting signal was recorded in an intensified manner. In the time range, the signal illustrated in  FIG. 5   a  is obtained; in the frequency range, the  FIG. 5   b  signal is obtained. The 50 Hz interfering field as well as another interfering field at approximately 16⅔ Hz are clearly visible.  
         [0046]     When a retooling took place to the measuring head of the invention according to  FIG. 4  or  FIG. 3 , while the conditions were otherwise identical, in the time range, the signal according to  FIG. 6   a  was obtained, and in the frequency range, the signal according to  FIG. 6   b  was obtained.  
         [0047]     The influence of the compensation according to the invention is extremely clear with an interfering signal reduction by a factor of approximately 25.  FIG. 7  shows additional amplitude and phase frequency characteristics of the movement sensor according to  FIG. 4  with an operating point control. In this case, the phase frequency characteristics were optimized by a correction filter on the amplifier circuit connected behind the measuring head  33  according to  FIG. 4 .  
         [0048]     By means of the method according to the invention, it now becomes possible seismic processes.  
         [0049]     Additionally, because magnetic field sensors are known, it now becomes possible to separately record both components of seismic processes, specifically the resulting movements and the resulting magnetic fields, while these magnetic fields do not interfere with the recording of the movements. This therefore results in a new dimension of the analysis of seismic processes, and thus for finding underground hydrocarbon occurrences and/or for determining the extent of such occurrences.  
         [0050]     In general, the suggested approach provides the possibility of inductively detecting movements between systems to the lowest frequencies, while an interfering field which may be present at the measuring site will not interfere with the measurement.  
         [0051]     The foregoing disclosure has been set forth merely to illustrate the invention and is not intended to be limiting. Since modifications of the disclosed embodiments incorporating the spirit and substance of the invention may occur to persons skilled in the art, the invention should be construed to include everything within the scope of the appended claims and equivalents thereof.