JP7051050B2 - Geophysical exploration method and sound source - Google Patents

Description

The present invention relates to a geophysical exploration method and a sound source, and more particularly to a geophysical exploration method and a sound source for exploring topography, resources, etc. by oscillating pseudo-random waves in water or the ground.

Abundant resources (eg oil, natural gas, methane hydrate, multi-metal baby booms, manganese nodules, submarine hydrothermal deposits, etc.) exist on the continental shelf and deep sea floor, and the ocean has risen due to the recent rise in resource prices. The need for resource development is increasing. In addition, natural resources on land are unevenly distributed in certain areas, and resources with low domestic production have to rely on imports from foreign countries, and there are many geopolitical risks. In Japan, which is surrounded by the sea on all sides, the marine region is attracting attention as a new frontier for resource development in order to ensure a stable supply of resources.

By the way, a pseudo-random wave (a binary pseudo-random sequence expressed by a wave phase difference) is used in the field of underwater acoustic communication because it has a high correlation between a vibration signal and a vibration received signal. For the same reason, the applicability of pseudo-random waves is being investigated and studied in the field of geophysical exploration (see, for example, Patent Document 1).

In Patent Document 1, a plurality of pseudo-random waves are simultaneously oscillated and received from a plurality of oscillators using pseudo-random code signals having different code arrangements, and analysis is performed to obtain only the signals of the individual oscillators. A non-destructive measurement method by multiple vibrations using a pseudo-random wave characterized by separation is disclosed.

Japanese Unexamined Patent Publication No. 2004-163322

By the way, when the sound wave used during physical exploration is a sweep wave (sound wave having a frequency that changes at a constant speed), a positive phase (upward convex signal) and a negative phase (downward convex signal). And are repeated alternately, so that the vibration of the vibrating plate (sound source) that oscillates the sound wave always repeats the reciprocating motion.

On the other hand, when the sound wave used during physical exploration is a pseudo-random wave, the positive phase (upwardly convex signal) and the negative phase (downwardly convex signal) are randomly arranged. , The positive phase (upwardly convex signal) may be continuously repeated, or the negative phase (downwardly convex signal) may be continuously repeated. When the same phase is continuously repeated in this way, the displacement amount (amplitude) of the diaphragm exceeds the drive limit of the device, and the sound wave cannot be temporarily generated. ..

Therefore, if the displacement (amplitude) of the diaphragm exceeds the drive limit of the device, the sound pressure level of the sound wave will drop, and the waveform of the oscillated sound wave will be different from the planned pseudo-random wave. It causes problems such as cross-correlation becoming impossible.

The present invention has been devised in view of such a problem, and it is possible to prevent the displacement amount (amplitude) of the diaphragm from exceeding the drive limit of the device even when a pseudo random wave is used. It is an object of the present invention to provide a physical exploration method and a sound source.

According to the present invention, it is a physical exploration method in which a pseudo-random wave is oscillated to perform physical exploration, in which a signal generation step for generating a predetermined modified acceleration signal and a modified pseudo-random wave based on the modified acceleration signal are generated. The signal generation step includes an oscillating step, which includes an acceleration signal generation step for generating an acceleration signal capable of oscillating a pseudo-random wave, and an amplitude signal for calculating an amplitude signal of the pseudo-random wave from the acceleration signal. A calculation step, a modified amplitude signal generation step of cutting off a predetermined low frequency component from the amplitude signal to generate a modified amplitude signal, and a modified acceleration signal generation step of generating the modified acceleration signal from the modified amplitude signal. A physical exploration method characterized by inclusion is provided.

Further, the signal generation step includes a limit confirmation step for confirming whether or not the modified amplitude signal exceeds the drive limit of the sound source, and if the modified amplitude signal does not exceed the drive limit of the sound source, the modified acceleration signal. It may be set to proceed to the generation step and return to the acceleration signal generation step or the correction amplitude signal generation step when the modified amplitude signal exceeds the drive limit of the sound source.

Further, the predetermined low frequency component may be a component having a frequency of 20 Hz or less.

Further, the predetermined low frequency component may include a region in which the blocking amount changes from 0 to 100%.

Further, according to the present invention, the control device includes at least one vibrating plate, a driving means for driving the vibrating plate, and a control device for transmitting a drive signal to the driving means, and the control device has a predetermined modification. The control device is configured to transmit an acceleration signal as the drive signal to the drive means, and the control device includes a signal generator that generates an acceleration signal capable of generating a pseudo-random wave, and the pseudo-random wave from the acceleration signal. The arithmetic unit includes an arithmetic unit for calculating the amplitude signal of the above and a high-pass filter for generating a modified amplitude signal by blocking a predetermined low frequency component from the amplitude signal, and the arithmetic unit includes the modified acceleration signal from the modified amplitude signal. A sound source is provided characterized in that it is configured to generate .

Further, the control device may generate a new acceleration signal by the signal generator or change the conditions of the high-pass filter when the modified amplitude signal exceeds the drive limit of the diaphragm. It may be configured.

Further, the predetermined low frequency component may be a component having a frequency of 20 Hz or less.

Further, the predetermined low frequency component may include a region in which the blocking amount changes from 0 to 100%.

According to the above-mentioned physical exploration method and sound wave according to the present invention, a low-frequency component having a large displacement (amplitude) of the diaphragm is generated by generating a modified acceleration signal in which a predetermined low-frequency component is cut off from a pseudo-random wave. The sound wave can be removed, and the displacement amount (amplitude) of the diaphragm can be prevented from exceeding the drive limit of the device.

It is a flow chart which shows the geophysical exploration method which concerns on one Embodiment of this invention. It is a figure which shows the characteristic of a pseudo-random wave, (a) shows the amplitude of a sweep wave, and (b) shows the amplitude of a pseudo-random wave. It is a figure which shows an example of the acceleration signal generated in the acceleration signal generation step. It is a figure which shows the amplitude signal calculated in the amplitude signal calculation step, and the correction amplitude signal generated in the correction amplitude signal generation step. It is a conceptual diagram which shows the blocking method of a low frequency component, (a) shows the first example, (b) shows the second example. It is a figure which shows the sound pressure level before and after the cutoff of a low frequency component, (a) before cutoff (amplitude signal), (b) after cut off (corrected amplitude signal). It is a figure which shows the correction acceleration signal generated in the correction acceleration signal generation step. It is a figure which shows the sound source which concerns on one Embodiment of this invention, (a) shows the first example, (b) shows the second example. It is a figure which shows the sound source which concerns on one Embodiment of this invention, (a) shows the 3rd example, (b) shows the 4th example.

Hereinafter, embodiments of the present invention will be described with reference to FIGS. 1 to 9 (b). Here, FIG. 1 is a flow chart showing a geophysical exploration method according to an embodiment of the present invention. 2A and 2B are diagrams showing the characteristics of a pseudo-random wave, in which FIG. 2A shows the amplitude of the sweep wave and FIG. 2B shows the amplitude of the pseudo-random wave.

As shown in FIG. 1, the physical exploration method according to the embodiment of the present invention is a physical exploration method in which a pseudo-random wave is oscillated to perform physical exploration, and is a modified acceleration signal that cuts off a predetermined low frequency component. It includes a signal generation step Step 1 for generating Ac, and a vibration step Step 2 for oscillating a modified pseudo-random wave Wc based on the modified acceleration signal Ac.

The signal generation step Step 1 includes, for example, an acceleration signal generation step Step 11 that generates an acceleration signal Ar capable of generating a pseudo random wave Wr, and an amplitude signal calculation step Step 12 that calculates an amplitude signal Sr of the pseudo random wave Wr from the acceleration signal Ar. Step 13 for generating a modified amplitude signal that generates a modified amplitude signal Sc by blocking a predetermined low frequency component from the amplitude signal Sr, and step 14 for confirming whether the modified amplitude signal Sc exceeds the drive limit of the sound source. And the modified acceleration signal generation step Step 15 for generating the modified acceleration signal Ac from the modified amplitude signal Sc.

As shown in the left figure of FIG. 2A, when the sound wave used during physical exploration is a sweep wave (a sound wave having a frequency that changes at a constant velocity), the acceleration signal has a positive phase (upper). A convex signal) and a negative phase (a downwardly convex signal) are alternately repeated. Here, if the phase of the sound wave is displayed in binary, the positive phase can be displayed as "0" and the negative phase can be displayed as "1".

By integrating this acceleration signal in the second order, the amplitude signal of the diaphragm (sound source) can be calculated as shown in the right figure of FIG. 2 (a). Since the sweep wave alternately repeats the positive phase (0) and the negative phase (1), the amplitude of the diaphragm (sound source) that vibrates the sound wave also repeats the reciprocating motion. Therefore, the displacement amount (amplitude) of the diaphragm does not exceed the drive limit of the device (the line shown by the dotted line in the figure).

On the other hand, when the sound wave used during physical exploration is a pseudo-random wave, the positive phase (0) and the negative phase (1) are random as shown in the left figure of FIG. 2 (b). Since it is arranged in, the positive phase (0) or the negative phase (1) may be continuously repeated. The figure shows a case where the positive phase (0) is repeated 5 times continuously.

When the same phase is continuously repeated in this way, the displacement amount (amplitude) of the diaphragm is shown by the drive limit of the device (dotted line in the figure) as shown in the right figure of FIG. 2 (b). It will exceed the line) and fall into a state where sound waves cannot be oscillated. As a result, the sound pressure level of the sound wave may decrease, or the waveform of the oscillated sound wave may be different from the planned pseudo-random wave, and cross-correlation may not be obtained.

Therefore, the inventor of the present application pays attention to the fact that the low-frequency component of the sound wave has a large amplitude, and if a predetermined low-frequency component is removed from the pseudo-random wave, the pseudo-random wave is oscillated while maintaining the phase of the pseudo-random wave. I came up with the idea that the amplitude of the vibrating plate could be reduced. Hereinafter, the details of the flow chart shown in FIG. 1 will be described with reference to FIGS. 3 to 7.

Here, FIG. 3 is a diagram showing an example of an acceleration signal generated in the acceleration signal generation step. FIG. 4 is a diagram showing an amplitude signal calculated in the amplitude signal calculation step and a modified amplitude signal generated in the modified amplitude signal generation step. 5A and 5B are conceptual diagrams showing a method of blocking a low frequency component, in which FIG. 5A shows a first example and FIG. 5B shows a second example. FIG. 6 is a diagram showing the sound pressure level before and after the cutoff of the low frequency component, showing (a) before the cutoff (amplitude signal) and (b) after the cutoff (corrected amplitude signal). FIG. 7 is a diagram showing a modified acceleration signal generated in the modified acceleration signal generation step.

The acceleration signal generation step Step 11 is a step of generating an acceleration signal Ar capable of generating a pseudo-random wave Wr, as shown in FIG. The acceleration signal Ar is generated by a random number sequence of 0 (positive phase) and 1 (negative phase). In the case of pseudo-random wave Wr, a pseudo-random number sequence obtained by a certain calculation is used instead of a true random number sequence. The pseudo-random sequence can be generated, for example, by a pseudo-random sequence generator that includes an algorithm that generates a pseudo-random sequence.

The amplitude signal calculation step Step 12 is a step of calculating the amplitude signal Sr by second-order integrating the acceleration signal Ar. In FIG. 4, the amplitude signal Sr shown by the thick dotted line is calculated based on the acceleration signal Ar shown in FIG. Here, if the drive limit of the diaphragm is ± γ, the drive limit can be drawn as shown by the thin dotted line in FIG. When the amplitude signal Sr is calculated from the acceleration signal Ar of the normal pseudo-random wave Wr, the drive limit γ is exceeded as shown in FIG.

The modified amplitude signal generation step Step 13 is a step of generating the modified amplitude signal Sc shown by a thick solid line from the amplitude signal Sr, as shown in FIG. Specifically, a high-pass filter is used to block a predetermined low-frequency component from the amplitude signal Sr to generate a corrected amplitude signal Sc.

Here, a method of blocking low frequency components will be described. When the amplitude signal Sr is redisplayed at the sound pressure level, it can be illustrated as shown in FIG. 5 (a). Here, the horizontal axis is the frequency (Hz) and the vertical axis is the sound pressure level (dB). Then, by blocking all the frequency bands below the frequency α (the shaded portion in the figure), the low frequency component of the pseudo-random wave Wr can be blocked. In general, it is known that 20 to 60 Hz sound waves are predominant in air guns (compressed air sounding method) that generate sound waves in water (for example, Port and Airport Research Institute Document No. 456, June 1983, sound wave exploration). Examination of the current state of the law and its examples, published by the Port and Airport Research Institute, Ministry of Transport).

Therefore, by setting α = 20 Hz as the low frequency component to be cut off, it is possible to cover a frequency band at least as high as that of an air gun. Further, when it is desired to make the exploration depth deeper than that of the air gun, α may be set to a value smaller than 20 Hz (for example, 15 Hz, 10 Hz, 5 Hz, etc.).

Further, as shown in FIG. 5B, it may include a region in which the blocking amount of the low frequency component changes from 0 to 100%. For example, the low frequency component up to the point A is blocked 100%, the blocking amount is gradually reduced from the point A, and the blocking amount is set to 0% at the point B. At this time, the blocking amount may be reduced at a constant rate, the blocking amount near point A may be increased to gradually decrease the blocking amount near point B, or the blocking amount near point A may be reduced. It may be reduced and the blocking amount near the point B may be gradually increased.

When the amount of cutoff is changed between points A and B in this way, the frequency of point B may be set to 20 Hz or less than 20 Hz, but the frequency of point A is the frequency of point B. It should be set lower than the frequency. Further, the intermediate point between the points A and B may be set to be 20 Hz or less than 20 Hz. At this time, the frequency at point B may exceed 20 Hz.

Here, FIG. 6A shows the amplitude signal Sr shown in FIG. 4 in terms of sound pressure level, where the horizontal axis represents frequency (Hz) and the vertical axis represents sound pressure level (dB). .. Then, the sound pressure level in which a predetermined low frequency component is cut off from the graph shown in FIG. 6 (a) is shown in FIG. 6 (b). Here, the cutoff amount up to 5 Hz is set to 100%, the cutoff amount of 10 Hz is set to 0%, the cutoff amount near 5 Hz is increased, and the cutoff amount near 10 Hz is gradually reduced.

Then, when the sound pressure level shown in FIG. 6B is displayed again on the amplitude signal, the corrected amplitude signal Sc shown in FIG. 4 can be obtained.

The limit confirmation step Step 14 is a step of confirming whether or not the modified amplitude signal Sc exceeds the drive limit γ. When the modified amplitude signal Sc does not exceed the drive limit γ of the sound source (No), the process proceeds to the next step (corrected acceleration signal generation step Step 15) as shown in FIG.

On the other hand, when the corrected amplitude signal Sc exceeds the drive limit γ of the sound source (Yes), as shown by the solid line in FIG. 1, the process returns to the acceleration signal generation step Step 11 and the acceleration signal Ar of the pseudo-random wave Wr You may try to start over from the generation. Further, as shown by the dotted line in FIG. 1, the modification amplitude signal generation step Step 13 may be returned to change the cutoff condition of the low frequency component.

The limit confirmation step Step 14 can be omitted if necessary. For example, if the cutoff condition of the low frequency component can be set so as not to exceed the drive limit γ of the sound source no matter what pseudo-random wave Wr is generated, the limit confirmation step Step 14 is omitted. be able to.

Further, the modified amplitude signal Sc that does not exceed the drive limit γ of the sound source is generated in advance without performing the signal generation step Step 1 at the time of geophysical exploration, and the modified amplitude signal Sc is stored in the sound source and used for geophysical exploration. In that case, the limit confirmation step Step 14 can be omitted.

The modified acceleration signal generation step Step 15 is a step of generating a modified acceleration signal Ac by second-order differentializing the modified amplitude signal Sc. The modified acceleration signal Ac generated from the modified amplitude signal Sc shown in FIG. 4 is shown in FIG. 7.

The vibration step Step 2 is a step of controlling the displacement amount (amplitude) of the diaphragm (sound source) based on the corrected acceleration signal Ac to generate a modified pseudo random wave Wc. By vibrating this modified pseudo-random wave Wc, the displacement amount (amplitude) of the diaphragm does not exceed the drive limit γ of the device, and it is possible to prevent the sound wave from falling into a state in which it cannot be vibrated. Therefore, it is possible to suppress a decrease in the sound pressure level of the sound wave. In addition, the waveform of the oscillated sound wave does not differ from the planned one, and the cross-correlation between the oscillating signal and the vibration receiving signal can be easily obtained.

The above-mentioned signal generation step Step 1 includes, but is not limited to, the acceleration signal generation step Step 11 to the modified acceleration signal generation step Step 15.

Next, the sound source used in the geophysical exploration method described above will be described with reference to FIGS. 8 (a) to 9 (b). Here, FIG. 8 is a diagram showing a sound source according to an embodiment of the present invention, (a) is a first example, and (b) is a second example. 9A and 9B are views showing a sound source according to an embodiment of the present invention, where FIG. 9A shows a third example and FIG. 9B shows a fourth example.

The sound source 1 of the first example shown in FIG. 8A includes two diaphragms 11, a drive means 12 for driving each diaphragm 11, and a control device 13 for transmitting a drive signal to each drive means 12. , And the control device 13 is configured to transmit a modified acceleration signal Ac that cuts off a predetermined low frequency component to the driving means 12 as a driving signal.

The diaphragm 11 has, for example, a disk shape, and is connected to the surface of the casing 14 accommodating the driving means 12 and the control device 13 via an elastic body 15. The two diaphragms 11 are arranged on opposite surfaces of the casing 14. Since the elastic body 15 is arranged on the outer periphery of the diaphragm 11, the diaphragm 11 is configured to be able to reciprocate in a direction perpendicular to the arrangement surface.

The drive means 12 is, for example, a hydraulic cylinder provided with a reciprocating piston rod 12a, and is configured to be operable by operating the servo valve 12b. The tip of the piston rod 12a is connected to the back surface of the diaphragm 11. Therefore, the piston rod 12a and the diaphragm 11 can be reciprocated by adjusting the flow rate of the working fluid supplied to the hydraulic cylinder by the servo valve 12b.

The control device 13 is a computer that transmits a drive signal for driving the piston rod 12a to the servo valve 12b. The control device 13 is predetermined from a signal generator 13a that generates an acceleration signal Ar capable of generating a pseudo random wave Wr, an arithmetic unit 13b that calculates an amplitude signal Sr of the pseudo random wave Wr from the acceleration signal Ar, and an amplitude signal Sr. It may include a high-pass filter 13c which cuts off the low frequency component of the above and generates a corrected amplitude signal Sc. Further, the arithmetic unit 13b may be configured to generate a corrected acceleration signal Ac from the corrected amplitude signal Sc. With such a configuration, the geophysical exploration method described above can be carried out.

The control device 13 may include a storage device that stores the acceleration signal Ar in advance. In this case, the signal generator 13a can be omitted. Further, a storage device in which the corrected acceleration signal Ac is stored in advance may be provided. In this case, the signal generator 13a, the arithmetic unit 13b, and the high-pass filter 13c can be omitted.

The sound source 1 of the second example shown in FIG. 8B includes a single diaphragm 11, a driving means 12 for driving the diaphragm 11, and a control device 13 for transmitting a drive signal to the driving means 12. The control device 13 is configured to transmit a modified acceleration signal Ac that cuts off a predetermined low frequency component to the drive means 12 as a drive signal. The sound source 1 of the first example includes two drive units including the diaphragm 11 and the drive means 12, whereas the sound source 1 of the second example includes one drive unit. Since each configuration of the sound source 1 of the second example is the same as that of the sound source 1 of the first example described above, detailed description thereof will be omitted.

The sound source 1 of the first example is suitable for, for example, a geophysical exploration method in which a pseudo-random wave is intermittently emitted in water while moving the oscillation position. Further, the sound source 1 of the second example is suitable for, for example, a geophysical exploration method in which a pseudo-random wave is emitted toward the ground or the seabed.

The sound source 1 of the third example shown in FIG. 9A is a modification of the configuration of the driving means 12 in the sound source 1 of the first example. The drive means 12 includes, for example, a reciprocating drive rod 12c and an electric coil 12d that drives the drive rod 12c by an electromagnetic force. Even with this configuration, by passing a current through the electric coil 12d, an electromagnetic force in the direction of the arrow in the figure can be generated to reciprocate the drive rod 12c.

When changing the driving direction of the driving rod 12c, the direction of the current flowing through the electric coil 12d may be reversed. Further, conditions such as the direction, magnitude, and time of the current flowing through the electric coil 12d are controlled by the control device 13.

The sound source 1 of the fourth example shown in FIG. 9B is a modification of the configuration of the driving means 12 in the sound source 1 of the second example. Since the configuration of the driving means 12 is the same as that of the sound source 1 of the third example described above, detailed description thereof will be omitted.

The present invention is not limited to the above-described embodiment, and can be used for various purposes such as not only research on marine resources but also research on mining resources, civil engineering / environmental research, and the like. Of course, various changes can be made without deviation.

1 Sound source 11 Vibration plate 12 Drive means 12a Piston rod 12b Servo valve 12c Drive rod 12d Electric coil 13 Control device 13a Signal generator 13b Arithmetic unit 13c High pass filter 14 Casing 15 Elastic body Ac Corrected acceleration signal Ar Acceleration signal Sc Corrected amplitude signal Sr Amphitheater signal Wc Modified pseudo-random wave Wr Pseudo-random wave Step1 Signal generation step Step11 Acceleration signal generation step Step12 Vibration signal calculation step Step13 Modified amplitude signal generation step Step14 Limit confirmation step Step15 Modified acceleration signal generation step Step2 Vibration step

Claims (8)

It is a geophysical exploration method that performs geophysical exploration by oscillating pseudo-random waves.
A signal generation step that generates a predetermined modified acceleration signal, and
Including a focal step that oscillates a modified pseudo-random wave based on the modified acceleration signal.
The signal generation step includes an acceleration signal generation step for generating an acceleration signal capable of generating a pseudo-random wave, an amplitude signal calculation step for calculating an amplitude signal of the pseudo-random wave from the acceleration signal, and a predetermined amplitude signal. Includes a modified amplitude signal generation step that cuts off low frequency components to generate a modified amplitude signal, and a modified acceleration signal generation step that generates the modified acceleration signal from the modified amplitude signal.
A geophysical exploration method characterized by this. The signal generation step includes a limit confirmation step for confirming whether or not the modified amplitude signal exceeds the drive limit of the sound source, and if the modified amplitude signal does not exceed the drive limit of the sound source, the modified acceleration signal generation step. The physical exploration method according to claim 1 , wherein the process is set to return to the acceleration signal generation step or the modified amplitude signal generation step when the modified amplitude signal exceeds the drive limit of the sound source. The geophysical exploration method according to claim 1, wherein the predetermined low frequency component is a component having a frequency of 20 Hz or less. The geophysical exploration method according to claim 1, wherein the predetermined low frequency component includes a region in which the blocking amount changes from 0 to 100%. It is provided with at least one diaphragm, a driving means for driving the diaphragm, and a control device for transmitting a driving signal to the driving means.
The control device is configured to transmit a predetermined corrected acceleration signal as the drive signal to the drive means .
The control device includes a signal generator that generates an acceleration signal capable of generating a pseudo-random wave, an arithmetic unit that calculates an amplitude signal of the pseudo-random wave from the acceleration signal, and a predetermined low frequency component from the amplitude signal. The calculator is configured to generate the modified acceleration signal from the modified amplitude signal, including a high pass filter that shuts off and produces a modified amplitude signal.
A sound source characterized by that. The control device is configured to generate a new acceleration signal by the signal generator or change the conditions of the high-pass filter when the modified amplitude signal exceeds the drive limit of the diaphragm. The sound source according to claim 5 . The sound source according to claim 5 , wherein the predetermined low frequency component is a component having a frequency of 20 Hz or less. The sound source according to claim 5 , wherein the predetermined low frequency component includes a region in which the blocking amount changes from 0 to 100%.

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