JP3136000B2 - Control Method of Sweep Source in Ground Exploration - Google Patents

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 a vibration applied to a ground and exploring the structure of the ground, and more particularly to a control of a sweep vibration source used in a continuous vibration method. It is about the method.

[0002]

2. Description of the Related Art Hitherto, many attempts have been made on a method for efficiently generating vibrations used for reflection survey as a means for knowing the geological structure of a shallow portion. As a typical method, 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 generated vibration is pulse-like, and the energy applied to the ground is an impact force obtained from the mass of the falling object and the acceleration at the time of the falling. The vibration waveform generated by dynamite is a similar pulse wave. In such a pulsed vibration source, it is difficult to equalize the shape of the pulse wave for each excitation, and since noise generally propagates universally on the ground, it interferes with this noise. There is a limit to obtaining higher resolution because the recording is obscured, and the frequency of the generated vibration wave that determines the exploration resolution is limited, and the generated vibration energy is also limited. was there. To solve such obstacles, large-scale exploration, represented by oil field development,
A continuous wave was oscillated using a certain type of vibrator, and a pulse-like waveform was obtained by cross-correlation between the source waveform and the reflected waveform. 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 pulse-like wave with the average frequency of the changed frequency, and that the vibration continues in terms of energy It is known that the sum is within a certain time.

[0003]

By applying this principle, in the vibroseis method, 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 cross-correlation between the continuous signal and the input waveform is converted into a pulse-like wave. However, since the vibroseis method uses a hydraulic mechanism as a driving device for the purpose of generating large vibration energy, there is a limit in generating a high frequency, and high resolution cannot be obtained.
In addition, a vibrator is used as a source to generate a continuous wave instead of a shock wave, and the received signal is compressed mainly by cross-correlation with the wave of the source, and a record similar to the earthquake record by the shock wave is obtained, and the underground is explored. The method uses a vibration mechanism of a vibrator to generate a difference in phase or amplitude between the signal used for oscillation and the received signal that has passed through the stratum, and the compressed waveform is deformed. An error occurs in measuring the time difference between the oscillation time of the epicenter and the arrival time of the received signal. Furthermore, if the resonance between the vibrator and the ground occurs and the control is not sufficiently performed, the compression waveform may ring and cause a significant deformation. Therefore, if a signal having a frequency higher than a certain level is used, the influence of resonance can be avoided, but the penetration force of the seismic wave in the stratum becomes small, and the required exploration depth cannot be obtained.

SUMMARY OF THE INVENTION The present invention solves the above-mentioned drawbacks of the prior art, and reduces the measurement error so that a stable ground exploration measurement can be performed with high resolution to a required exploration depth. A control method is provided.

[0005]

In order to achieve this object, a method of controlling a sweep vibration source in ground exploration according to the present invention is to pre-vibrate an electromagnetic vibrator based on a desired sweep reference signal and transmit the electromagnetic vibrator. Obtaining a function, obtaining an inverse filter of the transfer function,
A step of obtaining a drive signal by integrating the reference signal and the inverse filter; and applying an oscillation wave equivalent to the sweep reference signal from the electromagnetic vibrator to the ground to be measured by driving the electromagnetic vibrator with the drive signal. And a step of performing the operation.

[0006]

As shown in FIG. 2A, the electromagnetic vibrator used in the present invention has a reaction mass (mass M) oscillated by a signal (reference signal) used for oscillation.
_r , displacement X _r ) and a base plate (mass M _b , displacement X _b ) that vibrates relative thereto and transmits vibration to the ground. The force by which the vibrator presses the ground is the sum of the reaction mass and the acceleration of the base plate weighted by each mass.

Thinking is widely recognized that it is [number 1] ^{_{(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 (excitation force) as a control signal
The vibration of the vibrator is controlled so that the phases match. _Ft is a static force for pressing the base plate to the ground. In order to completely execute this control for all waveforms, the sweep source uses the concept of a transfer function. Assuming that 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) becomes

H (ω) = F (ω) / S (ω) (1) Therefore, the spectrum P (ω) equal to the reference signal
To obtain a control signal with

A (ω) = P (ω) H (ω) ^-1 (2) A signal (drive signal) having a spectrum A (ω) obtained by:
Drives the vibrator. Here, H (ω) ^-1 is an inverse filter, which is given as a time-domain coefficient sequence.

That is, a vibrator is excited by a reference signal to obtain a transfer function, and an inverse filter is calculated. By summing this with the reference signal and calculating the drive signal, and by vibrating the vibrator, a ground reaction force causing deformation vibration to the desired sweep reference signal to be applied to the ground, The control signal equal to the reference signal can be obtained by eliminating all obstacles in the system such as the air bag.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail. FIG. 1 is a connection system diagram of an apparatus for carrying out the method of the present invention. Here, reference numeral 1 denotes an electromagnetic vibrator, which is excited by a drive signal amplified by a power amplifier 2.
a and a base plate 1b that vibrates relative to the reaction mass by the vibration of the vibrator 1a pressed against the ground and transmits the vibration to the ground. Reference numeral 3 denotes 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 generates 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. This is an extended IO box for performing Reference numeral 5 denotes a control device that includes a personal computer and a transputer and performs high-speed calculation processing and control required for the present invention.

According to the present invention, a high resolution is obtained by using an electromagnetic vibrator capable of generating a high frequency for reflection search, and a few k from DC.
It is possible to generate vibration up to Hz. Usually, a commercially available electromagnetic vibrator is composed of an electromagnet in the center and an electromagnet constituting an outer shell, and by applying a signal current to the electromagnet, a desired vibration is generated on the same principle as the 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 adopt a method in which the vibrating inner core mainly transmits the vibration to the ground. In the present invention, an electromagnetic vibrator configured as shown in FIG. 2B can be used so that the vibrating core vibrator directly transmits vibration to the ground. At this time, a mechanism for movably holding the outer shell at a predetermined range position with respect to the inner core is required, and the airbag 1c is disposed between the inner core oscillator and the outer shell. When the upward acceleration exceeds 1 G, the device comes into contact with the ground and floats up, and the signal waveform transmitted to the ground becomes a deformed waveform in which a part of a negative (pulling) peak is missing. This may be driven vibration acceleration less than 1G in order to prevent it, it may be attached a hold-down mass F _t as a separate pre-load to the baseplate 1b to vibrate with a larger vibration energy.

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

In a vibrator by electromagnetic vibration, the phase difference between the reference signal used for vibration generation and the vibration of each part of the vibrator is large, and it is difficult to make the phase of the reference signal uniform with simple feedback. FIG. 5 is a correlation record between a signal received in a nearby well and a reference signal obtained by vertically exciting the ground with an electromagnetic vibrator having a base plate attached to a small electric shaker that does not perform any control. . Although it is recognized that the P wave and the S wave of the direct wave proceed, the waveform shows remarkable ringing, and due to 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 performed as follows. A vibrator is driven by the reference signal, a control signal waveform in this case is taken out, noise is removed by a filter, a transfer function is obtained using Fourier transform, and an inverse filter is calculated. A signal for driving the vibrator is calculated by integrating an inverse filter with the reference signal. When the vibrator is driven again using this drive signal, the control signal obtained at this time matches the reference signal.

According to this operation, all waveforms are controlled, so that the amplitude of the reference signal is also controlled to be constant. That is, this method is a waveform control in which harmonic suppression, amplitude control, and phase control are performed simultaneously. Thus, the reflected wave is different from the case of FIG. 5 and is as shown in FIG.
Based on 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
It is necessary to excite twice. Also, the control operation using this transfer function requires three FFT calculations, but the sweep time needs to be extended to about 10 seconds. Is impossible.

Therefore, a transputer (parallel processing board) which has recently become commercially available as shown in FIG. 1 was attached to a personal computer, and a control device was prototyped by achieving a calculation speed close to that of a workstation. In the prototype device, the upper limit frequency of the reference sweep signal was about 200 Hz,
Using a sweep time of 15 seconds, an observation time of about 1 second, and a sampling rate of 1 ms, 16,384 points of fast Fourier transform (FFT) were used. With this device,
A processing time of 0 seconds was achieved. At such a processing speed, one shot is completed in about one minute by adding the time required for two excitations, so that the resource exploration is performed.
If it is not a multi-source, multi-channel operation such as a three-dimensional exploration, it is possible to work with sufficient efficiency.

In the prototype control device shown in FIG. 1, similarly to the large hydraulic vibrator, the acceleration of the base plate, the acceleration of the reaction mass and the vibrating force are used as control signals. 6, 7, and 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 therefrom. FIG. 7 is a diagram comparing an amplitude ratio and a phase difference of a control signal with respect to a reference signal when vibration is generated. In the case of (a) in which the control device is not used, a remarkable change in amplitude and phase due to resonance is observed. However, when control is performed using the respective transfer functions, the amplitude spectrum becomes as shown in FIG. It becomes flat and the phase difference becomes zero in the entire frequency range, indicating that the control by this transfer function is extremely effective. Note that the amplitude ratio slightly decreases at both ends of the frequency range, which is due to the influence of the band-pass filter for removing harmonics.

[0016]

As described above in detail, 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.
Ground measurement can be performed stably with a high resolution to the required depth of search with a small measurement error, and it is extremely large that contributes to the improvement of the technology of ground search.

[Brief description of the 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) showing an electromagnetic vibrator used in the present invention, and a sectional structural diagram (b).

FIG. 3 is a waveform chart 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 measured waveform according to 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]

 DESCRIPTION OF SYMBOLS 1 Electromagnetic vibrator 1a Vibrator (reaction mass) 1b Base plate 2 Power amplifier 3 Preamplifier 4 Expansion IO box 5 Control device

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-51-71201 (JP, A) JP-A-56-33578 (JP, A) JP-A-4-188091 (JP, A) 24268 (JP, U) (58) Field surveyed (Int. Cl. ⁷ , DB name) G01V 1/02

Claims (2)

(57) [Claims] A step of obtaining a transfer function of the electromagnetic vibrator by performing a preliminary vibration of the electromagnetic vibrator based on a desired sweep reference signal; a step of obtaining an inverse filter of the transfer function; Sweep in ground exploration including a step of obtaining a drive signal by integration and a step of driving the electromagnetic vibrator with the drive signal to apply a vibration wave equivalent to the sweep reference signal from the electromagnetic vibrator to the ground to be measured. Source control method. 2. The electromagnetic vibrator includes an outer shell, an inner core that is arranged in the outer shell via an airbag, and mainly vibrates, and one end of the inner core at a position outside the outer shell. The method according to claim 1, further comprising a base plate for applying vibration to the ground to be measured.