JPH0675055A - Control method for sweep oscillation source in ground investigation - Google Patents

Abstract

PURPOSE:To perform measurement of ground at high resolution upto a required depth by preliminarily oscillatins an electromagnetic vibrator with a reference signal in order to determine a transfer function and inverse filter thereof, driving a vibrator with a driving signal obtained based on the product of the reference signal and the inverse filter, thereby applying a vibration wave equivalent to the reference signal. CONSTITUTION:An electromagnetic vibrator 1 comprises a vibrator 1a to be excited through a power amplifier 2, and a base plate 1b vibrating relatively thereto and transmitting vibration to the ground. A preamplifier 3 takes in acceleration signals of the vibrator 1a and the base plate 1b and an IO box 4 produces a driving signal for the amplifier 2 based on a reference signal where the driving signal is corrected by the output from the amplifier 3. A controller 5 performs calculation processing and control. The vibrator 1a is driven with the reference signal to produce a vibration waveform which is subjected to Fourier transform in order to determine a transfer function and then an inverse filter is calculated. The inverse filter is multiplied by the reference signal to produce a drive signal for redriving the vibrator 1a which then produces a control signal matching with the reference signal. This constitution allows measurement of ground at high resolution upto a required depth.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for detecting a reflected wave of vibration applied to the ground and exploring the structure of the ground, and particularly to controlling a sweep source used in a continuous vibration method. It is about the method.

[0002]

2. Description of the Related Art In the past, many attempts have been made to devise an efficient method of generating vibrations used for reflection exploration as a means for knowing the geological structure of a shallow part. As a typical method of this, a method of hitting the ground by hitting with a hanging arrow or dropping a weight and using vibration generated at that time has been used as a simple method. In this striking method, the vibration generated is pulse-like, and the energy applied to the ground is the impact force obtained by the mass of the falling object and the acceleration at the time of falling. The vibration waveform generated by dynamite is a similar pulse wave. Such a pulsed source interferes with this noise because it is difficult to make the shape of the pulse wave uniform for each excitation, and because general vibrations generally propagate to the ground. There is a limit to obtaining higher resolution because the recording is unclear, and the frequency of the generated vibration wave that determines the exploration resolution cannot be arbitrarily selected, and the vibration energy that can be generated is also limited. was there. In order to solve such obstacles, large-scale exploration represented by oil field development,
A vibroseis method has been put to practical use as a vibroseis method in which a continuous wave is oscillated using a certain type of exciter and a pulse-shaped waveform is obtained by cross-correlation processing of an oscillation source waveform and a reflection waveform, which is a countermeasure against noise and increases oscillation energy ing. The principle of using this continuous wave is that the autocorrelation of a continuous wave whose frequency changes continuously within a certain time becomes a pulsed wave with an average frequency of the changed frequency, and vibration continues in terms of energy. It is known to be the sum total within a certain time.

[0003]

By applying this principle, in the vibroseis method, the vertical vibration is generated in the piston while continuously changing the frequency by hydraulic drive, and this vibration is input to the ground and observed as a reflected wave. The converted continuous signal and the input waveform are cross-correlated to be converted into a pulse wave. However, since this vibroseis method uses a hydraulic mechanism as a drive device for the purpose of generating a large amount of vibration energy, there is a limit in generating a high frequency, and a high resolution cannot be obtained.
In addition, a vibrator is used as a source for generating continuous waves instead of shock waves, and the received signal is compressed mainly by the cross-correlation with the waves of the epicenter, and the same record as the seismic record by the shock waves is obtained to search underground. In the method, due to the vibration mechanism of the vibrator, a difference in phase and amplitude changes is generated between the signal used for oscillation and the received signal that has passed through the stratum, and the compressed waveform is deformed, resulting in a particularly important travel time ( An error occurs in the measurement of the time difference between the oscillation time of the epicenter and the arrival time of the received signal. Further, resonance between the vibrator and the ground may occur, and if not sufficiently controlled, the compressed waveform may ring and may be significantly deformed. Therefore, if a signal with a frequency higher than a certain level is used, the influence of resonance can be avoided, but the penetrating force of the seismic wave in the formation becomes small, and the required exploration depth cannot be obtained.

The present invention solves the above-mentioned drawbacks of the prior art, reduces the measurement error, and enables stable ground exploration measurement with a high resolution to provide a sweep source in the ground exploration. A control method is provided.

[0005]

In order to achieve this object, a method for controlling a sweep vibration source in ground exploration according to the present invention is a method for pre-exciting an electromagnetic vibrator according to a desired sweep reference signal to transmit the electromagnetic vibrator. A step of obtaining a function and a step of obtaining an inverse filter of the transfer function,
Applying a vibration wave equivalent to the sweep reference signal from the electromagnetic vibrator to the measured ground by driving the electromagnetic vibrator with the process of obtaining a drive signal by the product of the reference signal and the inverse filter. And the process of doing.

[0006]

Principle of the Invention As shown in FIG. 2 (a), the electromagnetic bi-plater used in the present invention is a reaction mass (mass M) that vibrates by a signal (reference signal) used for oscillation.
_r , displacement X _r ) and a base plate (mass M _b , displacement X _b ) that oscillates relatively to transmit the vibration to the ground. The force that the vibrator pushes on the ground is the sum of the reaction mass and base plate acceleration weighted by each mass.

It is widely accepted that (M _r d ^z X _r / dt ^z ) + (M _b d ^z X _b / dt ^z ). A signal for oscillation (reference signal) using this amount (exciting force) as a control signal
The vibration of the vibrator is controlled so that the phase of the vibrator and the phase of the vibrator match. F _t is a static force that crimps the base plate to the ground. In order to perform this control completely for all waveforms, the sweep source uses the concept of transfer function. If the vibrator is excited by a reference signal having a spectrum and a control signal having a spectrum F (ω) is observed, the transfer function H (ω) is

## EQU00002 ## H (.omega.) = F (.omega.) / S (.omega.) (1). Therefore, the spectrum P (ω) equal to the reference signal
To get the control signal with

[Equation 3] A (ω) = P (ω) H (ω) ^-1 ……………………………………………………………………………………………………………… (2) (Drive signal)
Drive the vibrator with. Here, H (ω) ⁻¹ is an inverse filter, which is given by a coefficient sequence in the time domain.

That is, a vibrator is oscillated with a reference signal to obtain a transfer function, and an inverse filter is calculated. This is combined with the reference signal to calculate the drive signal, and if the vibrator is vibrated by this, the reaction force of the ground that causes deformation vibration to the desired sweep reference signal to be applied to the ground A control signal equal to the reference signal is obtained by eliminating all obstacles in the system such as the airbag of the above.

[0008]

The present invention will be described in detail below. FIG. 1 is a connection system diagram of an apparatus for carrying out the method of the present invention. Here, 1 is an electromagnetic vibrator, and the vibrator 1 is excited by the drive signal amplified by the power amplifier 2.
a and a base plate 1b that vibrates relatively to the reaction mass due to the vibration of the vibrator 1a that is pressed against the ground and transmits the vibration to the ground. 3 is a preamplifier that extracts and amplifies the acceleration signal of the base plate 1b and the acceleration signal of the reaction mass Mr, and 4 creates a drive signal to the power amplifier 2 based on the reference signal and corrects the drive signal by the acceleration signal from the preamplifier. It is an expansion IO box for doing. A control device 5 includes a personal computer and a transputer and performs high-speed calculation processing and control necessary for the present invention.

The present invention seeks to obtain high resolution by using an electromagnetic vibrator capable of generating a high frequency for reflection exploration.
It is possible to generate vibration up to Hz. Usually, a commercially available electromagnetic vibrator is composed of an electromagnet at the center and an electromagnet forming an outer shell, and by applying a signal current thereto, a desired vibration is generated by the same principle as an electromagnetic speaker. At this time, since the mass of the inner core is several times smaller than that of the outer shell, the inner core mainly vibrates and the outer shell hardly vibrates. Therefore, since the vibration is hardly transmitted to the ground as it is, it is necessary to establish a method for transmitting the vibration to the ground mainly by the vibrating inner core portion. In the present invention, it is possible to use an electromagnetic vibrator configured as shown in FIG. 2B so that the vibrating inner core vibrator directly transmits the vibration to the ground. At this time, a mechanism for movably holding the outer shell in a predetermined range position with respect to the inner core is required, and the airbag 1c is arranged between the inner core oscillator and the outer shell. Further, when the upward acceleration exceeds 1 G, the device is lifted due to the contact with the ground, and the signal waveform transmitted to the ground becomes a deformed waveform in which a part of the negative (pull) peak is missing. In order to prevent this, the vibration acceleration may be driven to be smaller than 1 G, but in order to vibrate with a larger vibration energy, a holddown mass F _t may be attached to the base plate 1 b as a separate preload.

FIG. 3A is an example of the sweep reference signal used in the present invention, and FIG. 3B is an example of the ground model.
In this case, the vibration waveform of the base plate becomes a vibration waveform significantly different from the reference waveform because all the characteristics of these systems against vibration and the vibration response characteristics of the ground itself as an elastic body are combined. Such an oscillating waveform in which complicated characteristics of various systems are combined becomes a complicated post-correlation waveform even if the cross-correlation of the reference waveform is taken, and the expected pulse-like waveform cannot be obtained.

In a vibrator using electromagnetic vibration, there is a large phase difference between the reference signal used for vibration and the vibration of each part of the vibrator, and it is difficult to align the phase with the reference signal by simple feedback. Fig. 5 shows the correlation recording between the signal received by a nearby well and the reference signal when the ground is vertically excited by an electromagnetic vibrator with a base plate attached to a small electric vibrator that does not perform any control. . It can be seen that the P and S waves of the direct wave proceed, but the waveform shows significant ringing, and due to the resonance between the vibrator and the ground,
This indicates that a significant phase difference has occurred.

In order to solve this problem, the present invention is carried out as follows. The vibrator is driven by the reference signal, the control signal waveform in this case is taken out, the noise is removed by the filter, the transfer function is obtained by using the Fourier transform, and the inverse filter is calculated. An inverse filter is integrated with the reference signal to calculate a signal for driving the vibrator. When the vibrator is re-driven using this drive signal, the control signal obtained at this time matches the reference signal.

Since all waveforms are controlled by this operation, the amplitude of the reference signal is also controlled to be constant. That is, this method is waveform control in which harmonic suppression, amplitude control, and phase control are simultaneously performed. As a result, the reflected wave becomes as shown in FIG. 3C unlike the case of FIG.
Due to the principle that the waveform of FIG. 4B is obtained by autocorrelation as shown in FIG. 4A, an extremely accurate and stable reflected waveform can be obtained as shown in FIG. 3D. However, the excitation for obtaining the transfer function and the excitation for obtaining the earthquake record
It is necessary to excite once. Also, the control operation using this transfer function requires three FFT calculations, but since the sweep time needs to be lengthened to about 10 seconds, this operation should be performed within a practical time on the current personal computer. Is impossible.

Therefore, as shown in FIG. 1, a transputer (parallel arithmetic processing board), which has recently been put on the market, is attached to a personal computer, and a control device is prototyped to achieve a calculation speed close to that of a workstation. In the prototype device, the upper limit frequency of the reference sweep signal is about 200 Hz,
A sweep time of 15 seconds, an observation time of about 1 second, and a sampling rate of 1 ms were used, and a fast Fourier transform (FFT) of 16,384 points was used. 1 by this device
A processing time of a few seconds was achieved. At a processing speed of this level, one shot is completed in about 1 minute by adding the time required for two vibrations.
It is possible to work with sufficient efficiency as long as it is not multi-source work and multi-channel work such as dimensional exploration.

In the prototype control device of FIG. 1, the acceleration of the base plate, the acceleration of the reaction mass, and the exciting force are used as control signals, as in the case of the large hydraulic vibrator. 6, FIG. 7 and FIG. 8 show the amplitude ratio and the phase difference of the control signal with respect to the reference signal when the vibrator is directly driven by the reference signal, the transfer function is obtained from each control signal, and the vibrator is driven by the drive signal derived from the transfer function. It is a figure which compared the amplitude ratio and phase difference with respect to the reference signal of the control signal at the time of vibrating. In the case of not using the control device (a), a remarkable change in amplitude and phase due to resonance can be seen. However, when control is performed by each transfer function, the amplitude spectrum is as shown in each figure (b) in each case. It becomes flat and the phase difference becomes zero over the entire frequency range, which shows that the control by this transfer function is extremely effective. It should be noted that although the amplitude ratio is slightly lowered at both ends of the frequency range, this is due to the effect of the bandpass filter for removing harmonics.

[0016]

As described in detail above, according to the present invention, the desired vibration having a uniform phase difference from the reference signal can be transmitted from the electromagnetic vibrator to the ground to be measured.
It is possible to carry out ground measurement with high resolution and stability to the required exploration depth with a small measurement error, and it is a great contribution to the improvement of ground exploration technology.

[Brief description of drawings]

FIG. 1 is a system diagram showing an apparatus arrangement for carrying out the method of the present invention.

FIG. 2 is an equivalent circuit diagram (a) and a cross-sectional structure diagram (b) showing an electromagnetic vibrator used in the present invention.

FIG. 3 is a waveform diagram for explaining the operation of the present invention.

FIG. 4 is a waveform diagram illustrating autocorrelation used in the present invention.

FIG. 5 is a diagram showing a measurement waveform of a conventional method.

FIG. 6 is a characteristic comparison diagram between the case of the conventional method and the case of the method of the present invention.

FIG. 7 is a characteristic comparison diagram between the case of the conventional method and the case of the method of the present invention.

FIG. 8 is a characteristic comparison diagram between the case of the conventional method and the case of the method of the present invention.

[Explanation of symbols]

 1 Electromagnetic Vibrator 1a Vibrator (Reaction Mass) 1b Base Plate 2 Power Amplifier 3 Preamplifier 4 Expansion IO Box 5 Controller

Claims (2)

[Claims] 1. A process of pre-exciting an electromagnetic vibrator with a desired sweep reference signal to obtain a transfer function of the electromagnetic vibrator, a process of obtaining an inverse filter of the transfer function, and a process of obtaining the reference signal and the inverse filter. Sweep in ground exploration including a step of obtaining a drive signal by integration, and a step of driving the electromagnetic vibrator by the drive signal to apply a vibration wave equivalent to the sweep reference signal from the electromagnetic vibrator to the measured ground Source control method. 2. The electromagnetic vibrator includes an outer shell portion, an inner core portion that is disposed inside the outer shell portion via an air bag and mainly vibrates, and an outer shell portion at one end of the inner core portion at a position outside the outer shell portion. The method for controlling a sweep source in ground exploration according to claim 1, further comprising a base plate for applying vibration to the measured ground.