Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01V—GEOPHYSICS; GRAVITATIONAL MEASUREMENTS; DETECTING MASSES OR OBJECTS; TAGS
  • G01V1/00—Seismology; Seismic or acoustic prospecting or detecting
  • G01V1/16—Receiving elements for seismic signals; Arrangements or adaptations of receiving elements
  • G01V1/18—Receiving elements, e.g. seismometer, geophone or torque detectors, for localised single point measurements
  • G01V1/181—Geophones
  • G01V1/183—Geophones with moving magnet
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01H—MEASUREMENT OF MECHANICAL VIBRATIONS OR ULTRASONIC, SONIC OR INFRASONIC WAVES
  • G01H11/00—Measuring mechanical vibrations or ultrasonic, sonic or infrasonic waves by detecting changes in electric or magnetic properties
  • G01H11/02—Measuring mechanical vibrations or ultrasonic, sonic or infrasonic waves by detecting changes in electric or magnetic properties by magnetic means, e.g. reluctance
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01V—GEOPHYSICS; GRAVITATIONAL MEASUREMENTS; DETECTING MASSES OR OBJECTS; TAGS
  • G01V1/00—Seismology; Seismic or acoustic prospecting or detecting
  • G01V1/16—Receiving elements for seismic signals; Arrangements or adaptations of receiving elements
  • G01V1/162—Details
  • G01V1/164—Circuits therefore

Definitions

• the invention relates to a sensor for measuring mechani ⁇ cal vibrations, in particular seismic waves, comprising a mechanical transducer with an electronic processing circuit.
• the sensor transduces the acceleration into a digital output signal.
• seis ⁇ mometers or geophones For measuring a seismic signal, use is made of seis ⁇ mometers or geophones.' These geophones are, generally, passive analog sensors connected in series in groups and are con ⁇ nected with a measuring station. By a movement of the geo- phone, a voltage is induced in a coil which is movably sus ⁇ pended in the magnetic field of a permanent magnet. To achieve a high sensitivity, the mass of the magnet is large, which unfavourably influences the coupling at high frequencies between the geophone and the ground in which it is implanted.
• the analog connection between the geophones and the measuring station is sensible for disturbances by external electro-magnetic fields.
• the analog output signal of the geophones is amplified, sampled and digitized. Because of the high demands put on the resolution, the analog/digital con ⁇ verter and the anti-alias filter required to that end are so sensitive to component tolerances that manufacturing in IC- technology is almost impossible.
• an accelerometer producing a digital output signal.
• the sensor assembly consists of a sensor element determining the position of an inertial mass and a drive coil exerting a repositioning force on the inertial mass.
• a current pulse is sent through the drive coil.
• a Lorentz force is exerted thereby on the inertial mass, which is opposite to the force caused by the acceleration to be measured. The movement of the inertial mass will be reduced to substantial zero by the repositioning force.
• the acceleration can be computed.
• the velocity of the mechanical input signal to be measured is proportional to the frequency of the output signal. In fact a sensor having a frequency output is obtained in this manner.
• This accelerometer has some disadvantages, and is, therefore, not suitable for seismic measurements.
• the closed-loop gain should be chosen very large, so that the damping of the sensor assembly will be strongly reduced, and instability can occur. This can be prevented by including a differentiating network in the feed-back loop.
• the combination of a position sensing element arid a differentiator forms, then, a velocity sensing element.
• the geophone can be used in a horizontal and a vertical orientation.
• the measurement range should be large enough for compensating the gravitational acceleration of the inertial mass.
• the measurement range should, then, be unnecessarily large, since, for using seismic measurements, a measurement range of 1 m/s 2 i•s suffi•cx•ent.
• This known accelerometer can, furthermore, only be tested by means of a mechanical input signal, which, in view of the large number of implanted geophones, will be objectionable.
• This geophone is described in claim 1 in more detail, and has the following properties: because of the digital communication, the in ⁇ fluence of disturbances on the cable will be small, and a large distance between the geophone and the measuring station is possible; the analog/digital converter and the anti-alias filter are so insensitive for component tolerances that realization thereof with IC-technology is possible, so that the analog/digital converter and the anti-alias filter can be included in the geophone; the band-width of the geophone is large, and.
• the inertial mass is small, so that the geophone has a small mass as well as a small volume, ensuring a good ground coupling; on the basis of the small deviation of the inertial mass, use can be made of springs with a large transversal stiffness; the geophone is usable in any position, and is only sensible for an axial vibration; by means of a digital test signal, the transfer and distortion of the geophone can be measured.
• Fig. 1 a circuit diagram of the digital geophone according to the invention
• Fig. 2 a circuit diagram of the digital geophone, in which, by means of a second feed-back loop, an improved noise suppression is obtained;
• Fig. 3 a* circuit diagram of the digital geophone in which a higher sampling rate is possible;
• Fig. 4 the frequency characteristic of the input signal and the test signal ; also the influence of the quantization noise is visible;
• Fig. 5 the mechanical transducer used in the geophone;
• Fig. 6 a mechanical transducer which may be used for the geophone, in which the centering forces on the inertial mass are absorbed by a rigid construction;
• Fig. 7 a circuit element of the circuit, by means of which the geophone can be tested; this circuit measures, to that end, the harmonic distortion.
• Fig. 1 shows a circuit diagram of the digital geophone according to the invention.
• the mechanical signals are shown by dotted lines, and the electrical signals by drawn lines.
• the geophone undergoes an input acceleration X(s), causing a force Fa to act on the inertial mass 2. If the resulting force of this force Fa and the Lorentz force Fl to be elucidated below is not equal to zero, this will has as a consequence a movement of the inertial mass 2 of the mass-spring system 3, - A - which is detected by the velocity sensor 4.
• the operation of the velocity sensor 4 will be elucidated in more detail by Figs. 5 and 6.
• the sensor element 4. has an output voltage, but in view of the realization thereof with IC-technology, this voltage is con ⁇ verted into an output current by the analog input amplifying stage.
• the amplifying stage of the sensor element 4 is amplified by the amplifier 5.
• the signal is sampled by the sampling element 6 after the command "HOLD" of a clock 7.
• the clock 7 is controlled by an external synchronisation signal "SYNC”.
• the sampled signal is converted by the analog/digital converter 8 into a digital signal after the command "START" of the clock 7.
• the sampling frequency fs is high, and, at any rate, much higher than the resonance frequency of the mass- spring system 3. Because of the limited resolution of the analog/digital converter 8, the latter can be realized in an IC-process.
• the digital output signal Y(z) is inverted by an inverter 9 , so that the signal in the feed-back circuit is in phase opposition to the input signal.
• a digital adder 10 adds a test signal T(z) to the inverted output signal. During measurement the signal T(z) is equal to zero. Testing the sensor assembly 1 will be elucidated in more detail by reference to Fig. 7.
• the sum signal of the adder 10 is converted by the digital/analog converter 11 into a current i2.
• a force transducer 12 exerts a Lorentz force Fl on the inertial mass 2 which is proportional to the current i2. Because of the Lorentz force, the movement of the inertial mass 2 will be substantially reduced to zero.
• the transfer function of the digital geophone can be tested by means of the test signal T(z).
• T(z) By means of the digital adder 10, the test signal is added with the inverted output signal, and, by the digital/analog converter 11, a current i2 is sent through the force transducer 12.
• the inertial mass 2 is excited in the same manner as by the input acceleration signal.
• the transfer function Y(z)/T(z) is, in the seismic band width, almost equal to the transfer function Y(s)/X(s), apart from a frequency in ⁇ dependent factor.
• the output signal and the test signal are being syn ⁇ chronized. Therefore, the "START" command will be generated by the clock by means of the external synchronization signal.
• Fig. 2 shows an alternative circuit diagram of the digital geophone.
• the output current of the velocity sensing element 4 is, now, added with a current il to be discussed below, and is amplified by the amplifier 5'.
• the amplifier 5' has a very high gain factor and a low-pass transfer character- istic. The cut-off point of the frequency characteristic is very low, so that the input signal is integrated.
• the output signal of the low-pass filter 5' is sampled by the sampling element 6 after the command "HOLD".
• the analog/digital con ⁇ verter 8 converts the voltage into a digital output signal Y(z), and after inversion by the inverter 9 the output signal Y(z) is added to the test signal T.z).
• the obtained sum signal is converted, by the digital/analog converter 11, into two output currents il and i2.
• the current il is added to the out ⁇ put current of the velocity sensing element 4, and is inte- grated by the low-passfilter 5 1 .
• the force transducer 12 exerts a Lorentz force Fl on the inertial mass 2 which is proportional to the current i2.
• the gain factors of the low-pass filter 5' should be high.
• the feed-back in the sensor assembly 1 for a signal with a frequency zero is interrupted, since the velocity sensor 4 can only detect a movement of the inertial mass 2.
• the offset voltage of the low-pass filter 5' appears with a very high gain factor at the output, and, thus, limits the dynamic range of the sensor. In IC-technology, an offset compensation can be difficultly realized, and is, therefore, expensive. By providing a second feed-back circuit with the current il, this disadvantage is restricted.
• the offset of the low-pass filter 5' is reset back by il, and the gain factor of the low-pass filter 5 can, now, be chosen very high without the offset restricting the dynamic range.
• the digital/analog converter 11 with two output currents can be realized in IC-technology by means of a current mirror with a multiple output.
• Fig. 3 shows an alternative circuit diagram.
• the currents il and i2 are generated by means of two separate digital/analog converters 11 and 11'.
• the sampling rate by means of which the digital word is converted into a current il, can be chosen much higher than the sampling frequency generating the current i2.
• the advantage thereof is that the sampling rate is not limited by the maximum frequency of the current i2, the voltage across the drive coil 12 then being lower than the supply voltage.
• An arithmetical unit 10" adds, now, the inverted output signal Y(z) to the test signal T.z), integrates the sum signal, and sends, after a clock pulse of the circuit 7, the signal towards the digital/analog converter 11.
• the integration of the sum signal obtained by the low-pass characteristic of the arithmetical circuit 10 ' improves the resolution of the signal fed back.
• Fig. 4 shows the frequency characteristic of the sensor assembly.
• the mass-spring system 3 has a resoncance frequency lying within the seismic band-width, but because of the high open-loop gain, the transfer of the sensor assembly 1 within the seismic band-width will be hardly influenced by the frequency characteric of the mass-spring system 3.
• the open-loop gain is small because of the low transfer function of the mass-spring system 3 and the low-pass filter 5'.
• the transfer function Y(s)/X(s) of the sensor assembly 1 will then decrease with an increase of the frequency. Since the velocity sensing element 4 will only detect the movement of the inertial mass 2, the transfer function of the sensor assembly 1 will be low at frequencies lower than the seismic band-width.
• the resolution of the analog/digital converter 8 is low, and quantization noise is added to the output signal.
• This quantization noise is uniformly distributed over the frequency band, but, because of the high open-loop gain at low frequencies, the contribution of the quantization noise to the output signal will be small. At higher frequencies the open- loop gain will decrease, and the quantization noise is no longer suppressed.
• Fig. 4 the contribution of the quantiza- tion noise in the output signal is represented by a hedged area. From this Figure it appears that the contribution of the quantization noise in the seismic band-width (up to the fre ⁇ quency fc) will be strongly suppressed. By selecting a very high sampling rate or by using a multiple feed-back loop, a maximum suppression of the quantization noise is obtained.
• Fig. 5 shows the mechanical transducer 13.
• This trans ⁇ ducer 13 consists of an inertial mass 2 comprising a magnet 14, a distance piece 15 and inner pole pieces 16, as well as a housing 17 with a sensing coil 18, a driving coil 19, a com ⁇ pensation coil 20 and a tubular pole shoe 21.
• the inertial mass 2 is suspended in the housing 17 by means of springs 22. A movement of the inertial mass 2 changes the magnetic flux within the sensing coil 18, and, then, induces a voltage.
• the sensing coil 18 is in series with the compensation coil 20, and is connected with the input of the electronic processing, device.
• the drive coil 19 is connected to the electronic processing device, and is situated in the magnetic field of the magnet 14. If a current is sent through the drive coil 19, the Lorentz force Fl — B.i.l. will exert a force on the inertial mass 2. Since the magnetic flux density B and the coil length 1 are constant, the Lorentz force Fl is proportional to the current i.
• the current through the drive coil 19 has, moreover, as a consequence that by mutual in- duction between the drive coil 19 and the sensing coil 18 an induction voltage is induced.
• the compensation coil 20 is provided. Since the relative permeability value of the distance piece 15 is equal to that of the magnet 14, i.e. that of air, the induced voltage in the compensation coil 20 will be equal to the induction voltage in the sensing coil 18. The winding sense of both coils is, however, opposite, so that by a series connection of both coils the induction voltage as a consequence of a current through the drive coil 19 will be null.
• the inner pole shoes 16 are connected, by means of a core 23, with one another and with the electric mass of the electronic processing device.
• the material of the core 23 is electrically conductive but magnetically non- conductive, and is, for instance, made of copper.
• the core 23 also serves for mechanically mounting the pole shoes 16, the magnet 14 and the distance piece 15 to the springs 22. A mechanical connection by means of glueing is possible, but, in view of the shock resistance, not attractive.
• the mechanical construction _.ccording to Fig. 5 is simple, but has the disadvantage that the outer pole shoe 21 made of a material conducting the magnetic field is attracted by the inner pole shoes 16 and the magnet 14.
• the springs should, therefore, be sufficiently rigid in the radial direction for creating a centering force which is sufficient for compensating the attraction force of the magnet 14.
• a mechanical transducer 13' is shown which does not have this disadvantage.
• the inertial mass 2' is, now, formed by the magnet 14, a distance piece 15, inner pole shoes 16 and an outer pole shoe 21 ' .
• the housing 17 comprises the sensing coil 18, the drive coil 19 and the compensation coil 20.
• the outer pole shoe 21 ' is rigidly fixed to the outer side of the inner pole shoes 16 by means of spokes 23'.
• the magnetic attraction force between the inner pole shoes 16, the magnet 14 and the outer pole shoe 21" is now absorbed by the rigid mechanical connection.
• the inertial mass 2' is suspended in the housing 17 by means of the springs 22.
• Fig. 7 elucidates a method for measuring the distortion level of the sensor 1.
• a read-only memory 24 contains a digitized sign signal.
• a switch 25 reads the read-only memory, and produces the test signal T(z).
• the period of the test signal T(z) is determined by the clock frequency generated by the clock generator 16 and a counter 27.
• the frequency of the test signal can be varied by adjusting nl .
• the test signal T(z) excites the inertial mass 2, and simulates an acceleration input signal X.s).
• the output signal Y(z) contains the response of the test signal T(z) and the acceleration signal X.s) .
• a second switch 28 reads the read-only memory 26 with a period determined by the clock generator 26 and a counter 29.
• This test signal is generated with a fre- quency which is n2/nl times higher than the frequency of the first test signal.
• This signal is multiplied with the output signal of the sensor 1 by a multiplier 30.
• the product formed by the multiplier 30 comprises sum and difference frequencies of the output signal Y(z) and the test signal having a fre- quency with an n2/nl times higher frequency.
• the harmonic distortion in the output signal having a frequency which is n2/n2 times higher can be determined by measuring the difference in respect of a zero frequency at the output of the multiplier 30.
• the output signal V(z) is the harmonic distortion having a frequency which is n2/nl times the fre ⁇ quency of the test signal T(z).
• a low-pass filter 31 filters this frequency from the signal of the multiplier 30.
• the cut ⁇ off frequency of the low-pass filter 31 determines the width of the filter in the spectrum of the seismic signal. As the cut-off frequency is selected lower, the contribution of the environmental noise as a consequence of the input acceleration will decrease.

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• 1988
  • 1988-08-11 NL NL8802000A patent/NL8802000A/nl not_active Application Discontinuation
• 1989
  • 1989-08-10 DE DE68913550T patent/DE68913550T2/de not_active Expired - Lifetime
  • 1989-08-10 WO PCT/NL1989/000063 patent/WO1990001712A1/en active IP Right Grant
  • 1989-08-10 EP EP89909443A patent/EP0434702B1/en not_active Expired - Lifetime
  • 1989-08-10 US US07/613,825 patent/US5172345A/en not_active Expired - Lifetime
  • 1989-08-10 JP JP1508738A patent/JP2717231B2/ja not_active Expired - Lifetime

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