RU2076341C1 - Geophone - Google Patents

Abstract

FIELD: seismometry. SUBSTANCE: invention can be used in the capacity of borehole and bottom geophone in sea seismometry. Cylindrical body is completely filled with liquid inertial mass and houses N piezoelements in the form of symmetrical two-side bimorphous cells which planes are perpendicular to end faces of body placed in symmetry relative to major planes of body by its end faces. Piezoelements anchored close to different end faces of body are intercoupled electrically in counterphase. EFFECT: expanded application field, improved functional reliability. 4 cl, 3 dwg

Description

 The invention relates to the field of seismometry and can be used as a downhole seismic receiver, as well as as a bottom seismic receiver in marine seismometry.

In the field of seismometry and, in particular, marine seismometry, there is a need to measure low-frequency vibrations (frequencies below 10 ² Hz) of small amplitude: in the limit at the level of seismic noise (10 ^-8 m / s ² ), and there are problems of isolating small useful (measured ) signal against the background of electrical noise of the pre-amplifier (PU), as well as against the background of acoustic noise.

 In recent decades (starting from the 80s), piezoceramic seismic accelerometers have been widely used for these purposes, gradually displacing electrodynamic cycle meters and pendulum seismometers that are less convenient to use.

 Along with the positive qualities of piezoceramic seismic receivers (SP), such as: stability of parameters, mechanical strength, uniform frequency response of acceleration sensitivity, high constructivity of the active material of piezoceramics, which makes it possible to develop SPs for various purposes, piezoceramic SPs have a feature that substantially complicates their use in the low-frequency range , namely the capacitive nature of the internal resistance, which makes it difficult to match them with the preliminary amplifier low frequencies.

Известны сейсмоприемники, выполненные в виде одного или нескольких пакетов пьезоэлементов, работающих на продольных колебаниях, нагруженных инерционной массой.This seismic receiver should have a value of β sufficient to isolate seismic signals of approximately 10 ^-8 m / s ² against the background of acoustic noise of the sea, which at frequencies less than 10 ² Hz reach about 10 ^-3 Pa /

Known geophones made in the form of one or more packages of piezoelectric elements operating on longitudinal vibrations loaded with inertial mass.

могут обеспечить прием порогового сигнала не меньше 2,5 • 10^-5 м/с², при этом диапазон рабочих частот 0,1 100 Гц.APT-IM piezoelectric seismometers are known, for example, in the form of a symmetrical design with three pairs of piezoelectric elements and a central spherical inertial mass of 870 g, which, with a diameter of 12 cm and a height of 20 cm, have a sensitivity of 20 mV / m / s ² with a capacitance of 15900 pF and at the noise level of the pre-amplifier at a frequency of 1 Hz 0.5 μV /

can provide a threshold signal reception of at least 2.5 • 10 ^-5 m / s ² , while the range of operating frequencies is 0.1 to 100 Hz.

 An increase in sensitivity and, as a consequence, a decrease in the threshold signal is achieved by increasing the inertial mass and decreasing the capacitance, while, however, the operating range is reduced.

A known seismic receiver with an inertial mass of 3.6 kg and a capacitance of piezoelectric elements of 200 pF, while in the range of 0.01 to 20 Hz a sensitivity of 25 V / m / s ^{2 is} achieved and a threshold signal of approximately 10 ^-7 m / s ^{2 is} provided.

 It can be seen that, approaching a given value in terms of the minimum signal (still required an order of magnitude lower), this SP does not provide operation in the entire frequency range (only up to 20 Hz).

 Thus, it can be argued that, using the type of structures under consideration, it was not possible to completely satisfy the requirements of high sensitivity, high capacitance, and a wide range of frequencies in the low-frequency region.

This can be explained by the fact that in geophones, the mechanical system of which is the flexibility of a piezoelectric element loaded on an inertial mass, the sensitivity and resonant frequency f _{o are} directly related to each other through this flexibility, and an increase in f _o (widening the frequency range) leads to a decrease in sensitivity and vice versa.

A seismic receiver is also known, which contains a spherical piezoelectric element in contact with a liquid inertial mass, which is placed in its internal volume. The seismic receiver has an attachment element in the form of a spherical segment, on which the piezoelectric element is supported through a flexible layer [1] In the range of 0.1 to 120 Hz, the seismic receiver provides a sensitivity of γ of approximately 60 mV / m / s ² and the reception of a threshold signal of 5 • 10 ^-6 m / s ² . The seismic receiver is designed to receive the vertical component of vibration.

 The use of liquid inertial mass allows for the transfer of force over the entire surface of the piezoelectric element without the use of rigid special structural means. This leads to an increase in the reliability of the seismic receiver and an increase in its strength to the blast wave, which is especially important in seismometry.

 However, the sensitivity of such a geophone is insufficient, and its increase is associated not only with a decrease in the resonant frequency, but also with the need to increase the size of the piezoceramic sphere, which is associated with technological difficulties and leads to a decrease in the strength of the geophone.

 Closest to the proposed is a seismic receiver [2] containing a cylindrical body, completely filled with a liquid inertial mass. On the ends of the cylindrical body from their inner side there are round one-sided bimorph elements electrically connected antiphase. In such a seismic receiver, it is possible to significantly increase the sensitivity to vibration, as indicated in [2] by an order of magnitude, but this is not enough in the conditions considered above. In addition, the seismic receiver considered in [2] does not possess sufficient field stability and is not selective enough with respect to the unmeasured transverse component of vibration, since no special measures have been taken for this.

 The objective of the invention is to provide a seismic receiver with high sensitivity, capable of operating in a wide range of low frequencies, including in a liquid medium, i.e., as a marine bottom seismic receiver, insensitive to the action of pressure signals and transverse vibrations not subject to reception.

To solve this problem, in a seismic receiver containing a cylindrical body, completely filled with a liquid inertial mass, at the ends of which bimorph piezoelectric elements are connected, connected in antiphase, new features are introduced:
at each end of the body symmetrically with respect to the main planes of the body, N piezoelectric elements are fixed;
each piezoelectric element is made in the form of a sealed bilateral bimorph cell, which is a pressure receiver, and
the plane of the cells is perpendicular to the ends of the cylindrical body.

 Pressure receivers located symmetrically relative to the main planes of the housing have the same sensitivity to pressure.

 To increase the sensitivity and noise immunity to the pressure of the geophone by increasing the symmetry of the system, the fastening element is located on the outer perimeter of the rigid body in its central section. The seismic receiver can be equipped with a durable external sealed housing, separated from the cylindrical body by an air gap to reduce the sensitivity of the seismic receiver to acoustic pressure.

 The proposed technical solution combines the idea of using an inertial liquid mass with the use of a large number of receivers measuring the pressure that arises in this mass in the place where it has maximum value. In this case, there is a fundamental possibility of increasing both the sensitivity and the expansion of the frequency band, since the sensitivity of each of the 4N active elements can be the same as the piezoelectric element of the prototype receiver, and the resonant frequency increases with decreasing receiver sizes. In addition, a high degree of symmetry of the seismic receiver as a whole and of each individual pressure receiver provides deep compensation for vibrational (transverse component of vibration) and acoustic (sea noise pressure) interference.

 Figure 1 shows an example of the design of a device that measures vibration in the direction of the Z axis; figure 2 plot of the pressure distribution in the liquid inertial mass along the height of the housing; figure 3 is a section of a plate piezoelectric ceramic pressure receiver used in the design of the geophone.

 As an example of a specific implementation of the present invention, consider the seismic receiver shown in figure 1, having dimensions commensurate with the dimensions of modern downhole geophones, namely, a vertical size of 420 mm and a diameter of 120 mm The body 1 of the seismic receiver is made in the form of a cylinder of steel 5 mm thick with two end caps 8 mm thick. It is desirable that the height of the liquid column is less than 1 / 8λ, which corresponds to the upper frequency of the 500 Hz range. The internal volume of the housing is filled with liquid 2 (water). A flange 3 is provided in the central part of the casing, which serves to tightly connect the two halves of the casing through a rubber seal, as well as for fastening to a vibrating object or to an external strong casing (not shown in Fig. 1).

 Two groups of pressure receivers with 12 receivers in each group are located in two end parts of the housing. The receivers 4 are rigidly fixed by the mount 5 relative to the housing.

 Figure 3 shows a section of the receiver 4.

 The receiver is made in the form of a two-sided bimorph cell. The two bimorph elements included in it consist of each of two piezoceramic disks 6, with a diameter of 30 mm and a thickness of 1 mm, glued on both sides to a metal (titanium) substrate 7 with a thickness of 0.5 mm. The substrates 7 are made integrally with the ring supports, hermetically glued to each other.

 Inside the cell is air. Piezoelectric elements are connected in parallel-series. The receiver has a capacitance of 13 tpF and a sensitivity of 1.5 to 2 mV / Pa in the operating frequency range.

 The operation of the seismic receiver is as follows: the measured vertical component of vibration W is transmitted through the flange 3 to the housing 1 and the liquid 2 enclosed in it.

Due to the interaction of inertial forces of the liquid and the walls of the housing, dynamic pressures P arise, which change in time with the frequency of the active vibration and are distributed along the height of the cylinder as shown in FIG. The pressure P reaches a maximum value at points furthest from the central plane and is determined by the formula:
P = Wρh (2)
where W is acceleration;
ρ is the density of the fluid used;
h the size of the housing in the direction of the measured vibration component, and the pressure phase at points symmetrical with respect to the central plane differs by 180 ^o .

,
где P определяется выражением (2), h' (см.фиг.1). Таким образом, если h'

340 мм 0,34 м, то при W 1 м/м² величина P будет: P' 1•10^-3 • 0,34 340 Па.The pressure receivers 4 are affected by the pressure P 'averaged over their working surface, the amplitude of which is determined by the formula

,
where P is determined by the expression (2), h '(see Fig. 1). So if h '

340 mm 0.34 m, then at W 1 m / m ^{2 the} value of P will be: P '1 • 10 ^-3 • 0.34 340 Pa.

The pressure receivers located at points h 'receive the pressure P', and the electric signal at their output will be proportional to their sensitivity γ _p and pressure P ', i.e.

При соединении 24 приемников, вводя их в состав сейсмоприемника последовательно (с учетом необходимости изменения полярности у приемников одной группы, относительно другой) получаем общую чувствительность двух групп к давлению:
γ $\genfrac{}{}{0ex}{}{\text{Σ}}{\text{p}}$ =(1,5-2)•24=36-48 мВ/Па=мВ/Па
Чувствительность сейсмоприемника к вибрации определится, как произведение чувствительности γ $\genfrac{}{}{0ex}{}{\text{Σ}}{\text{p}}$ на величину действующего давления при вибрации 1 м/с² т.е. на 340 Па:
γ $\genfrac{}{}{0ex}{}{\text{Σ}}{\text{n}}$ =42,340=14380 мВ/мс^-2=14 В/м.с^-2
при суммарной емкости С- 650 пФ.v = γ _p P ′ = γ _p Wρh ′
and sensitivity to the vibration component in the direction of the axis of the cylindrical body

When connecting 24 receivers, introducing them into the seismic receiver in series (taking into account the need to change the polarity of the receivers of one group, relative to the other), we obtain the total sensitivity of the two groups to pressure:
γ $\genfrac{}{}{0ex}{}{\text{Σ}}{\text{p}}$ = (1.5-2) • 24 = 36-48 mV / Pa = mV / Pa
Sensitivity of the geophone to vibration is defined as the product of sensitivity γ $\genfrac{}{}{0ex}{}{\text{Σ}}{\text{p}}$ the value of the effective pressure during vibration 1 m / s ² i.e. at 340 Pa:
γ $\genfrac{}{}{0ex}{}{\text{Σ}}{\text{n}}$ = 42.340 = 14380 mV / ms ^-2 = 14 V / ms ^-2
with a total capacitance of C - 650 pF.

на частоте 20 Гц (и порядке 60 нВ/

на частоте 1 Гц). При этом частота собственного резонанса сейсмоприемника, определенная резонансом столба жидкости высотой h, будет лежать значительно выше верхней границы рабочего диапазона 10² Гц. Действительно условие резонанса будет

При этом экспериментально определено, что размещение 24 приемников описанной конструкции приводит к понижению резонансной частоты не более, чем в 1,6 раза.With these parameters of the SP, it is possible to ensure reliable reception of the threshold signal of 10 ^-8 m / s ² (when exceeding by 10 dB over the interference), using a pre-amplifier with a domestic field-effect transistor 2P303A at the input, capable of providing an interference level of approximately 3 nV /

at a frequency of 20 Hz (and about 60 nV /

at a frequency of 1 Hz). In this case, the frequency of the intrinsic resonance of the seismic receiver, determined by the resonance of a liquid column of height h, will lie much higher than the upper limit of the operating range of 10 ² Hz. Indeed, the resonance condition will be

Moreover, it was experimentally determined that the placement of 24 receivers of the described design leads to a decrease in the resonant frequency by no more than 1.6 times.

0,34 м, d'=0,05 м, h'/d'=7), а, во-вторых, за счет вычитания сигналов близкой чувствительности, а также за счет виброустойчивости конструкции самой биморфной ячейки. В результате чувствительность к поперечной вибрации составит не более 1,4% от чувствительности к измеряемой вибрации, что меньше, чем у измерительных виброприемников фирмы Брюгло и Кьер.The signal from acoustic noise by subtracting the signals of the receivers of different groups having close sensitivity (with a difference in γ _{p of} less than 5%) is reduced by at least 10 times. Reducing the sensitivity spread of pressure receivers is achieved by pre-calibrating the pressure receivers in the small-volume chamber, which gives high accuracy in relative calibration, and by completing the pressure receivers taking into account their sensitivity. The presence of an external durable housing allows you to weaken the pressure of acoustic noise by at least 10 ³ times. As a result, the acoustic noise signal of 10 ^-3 Pa decreases to 10 ^-7 Pa and amounts to only 3% compared to the signal from vibration at 10 ^-8 m / s
The sensitivity to the immeasurable component of the transverse vibration is weakened by at least 10 times, firstly, due to the difference in the longitudinal and transverse sizes of the vibration receiver, more precisely in the ratio h '/ d', where d 'is the distance between the centers of the bimorphic cells in the group (see .ig. 2), (in the example under consideration for h '

0.34 m, d '= 0.05 m, h' / d '= 7), and, secondly, due to the subtraction of signals of close sensitivity, as well as due to the vibration resistance of the design of the bimorph cell itself. As a result, the sensitivity to transverse vibration will be no more than 1.4% of the sensitivity to the measured vibration, which is less than that of measuring vibration receivers of Bruhlo and Kier.

 The advantage of the proposed seismic receiver is also that its design does not contain complex parts that require technological testing, given that pressure receivers are used, the design of which has been mastered in serial production.

 The manufacture of parts such as a cylindrical body, including fittings for filling it with liquid and sealed cable leads, are also mastered and do not present technological difficulties.

 The considered design has increased impact and explosion resistance, as It does not contain brittle structural elements, and the impact on pressure receivers occurs through the liquid evenly over the entire surface.

 Due to this, you can count on a design that has good reproducibility in manufacture and reliability in operation.

 It should be noted that the advantage of the proposed design is its efficiency. The weight of the piezoceramics included in one receiver does not exceed 20 g, i.e., 24 will have less than 0.5 kg. Taking into account the weight of the receiver housings, the weight of the active elements will not exceed 1 kg, while in seismic receivers of a similar purpose, but having lower sensitivity at a lower value of intrinsic resonance, i.e. with a narrower frequency range, the weight of the active element is much larger, and therefore the cost of its manufacture is greater and less reliable in operation, especially when exposed to shock (explosive) loads.

Claims (4)

 1. A seismic receiver containing a cylindrical body, completely filled with a liquid inertial mass, at the ends of which piezoelectric elements are fixed on the inside, connected electrically antiphase, characterized in that each body end is symmetrically relative to its main planes fixed on N piezoelectric elements made in the form of sealed bilateral bimorphic cells whose planes are perpendicular to the ends of the cylindrical body.  2. The seismic receiver according to claim 1, characterized in that the housing is equipped with a fastening element located in its central section along the outer perimeter.  3. The seismic receiver according to claim 1, characterized in that it is equipped with an additional strong sealed housing, separated from the cylindrical body by an air gap.  4. The seismic receiver according to claims 1 to 3, characterized in that the piezoelectric elements symmetrical with respect to the main planes of the cylindrical body have the same pressure sensitivity.

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