RU2061248C1 - Device for seismic prospecting in water areas - Google Patents

Abstract

FIELD: acoustic instruments for seismic prospecting. SUBSTANCE: device has channels 1 and 2 for detection of sound pressure and oscillation rate. Said channels are connected to input of multiplier 3. Single half-period direct-polarity peak detector 4 is connected to output of multiplier 3. Detector 4 is connected to recording unit 5. Device provides possibility to suppress secondary signal caused by reflection from water surface due to opposite polarity of these signals. Level hydrodynamic noise is decreased due to relative- correlation processing of signals from channels 1 and 2 in multiplier 3. In addition single half-period inverse-polarity peak detector 6 is introduced in order to use secondary signal caused by reflection from water surface. Detector 6 detects secondary signal and sends it to input of adder 9 through inverter 7. Line 8 provides time delay that is equal to ratio of double depth of tugged antenna to speed of sound in water. Output of delay line 8 is connected to input of adder 9. This results in in-phase addition of direct and secondary signals. This results in possibility to nearly double signal-to-noise ratio without change in level of noise. In addition signals from channels 1 and 2 for detection of sound pressure and oscillation rate are sent to identical units of band-pass filters 10. This results in possibility of spectral analysis of wide-band probing signals. Secondary signals caused by reflection from water surface are clipped by direct-polarity peak detectors. EFFECT: increased signal-to-noise ratio, increased precision. 3 cl, 5 dwg

Description

 The invention relates to acoustic instrumentation and is intended for seismic exploration in the water.

Known and widely used devices for seismic-acoustic reconnaissance in water areas containing towed antennas, in which non-directional sound pressure receivers are used as acoustic sensors [1]
The disadvantage of this device is the low noise immunity from the secondary signal reflected by the surface of the water and the acoustic interference of the towing vessel.

Closest to the proposed device is the acoustic sensors of the towed antenna of which contain sound pressure channels, vibrational velocity and the adder. The addition of the signals of the channels of sound pressure and vibrational velocity in the adder allows you to generate a cardioid directional characteristic, oriented at least toward the surface of the water, thereby achieving the suppression of the signal reflected by the surface of the water-air [2]
The disadvantage of the prototype is low noise immunity.

 To eliminate this drawback, a device for seismic-acoustic reconnaissance in water areas is proposed, the technical result of which is expressed in increased noise immunity in the lower part of the sound and infrasound frequency ranges.

 The technical result is achieved by the fact that in a device for seismic-acoustic reconnaissance in water areas, including a towed antenna containing sound pressure and vibrational velocity channels, the outputs of sound pressure and vibrational velocity channels are connected to the inputs of the multiplication unit, the output of which is connected to the input of the recording device through a half-wave peak detector .

 In addition, a second half-wave peak detector is connected to the output of the multiplication unit, having a polarity opposite to the first, the output of which is connected through an inverter to one of the inputs of the adder, the second input of which is connected via a delay line to the output of the first half-wave peak detector, and the output of the adder is connected to the recording device, and the delay time of the delay line is equal to the ratio of the doubled depth of the towed antenna from the surface of the water to the speed of sound in water.

 In addition, the outputs of the channels of sound pressure and vibrational velocity are connected to the same blocks of bandpass filters, the corresponding outputs of which are connected in pairs with the inputs of the multiplication units, the outputs of which are connected through single-half peak detectors to the individual inputs of a multi-channel recording device.

 The proposed device allows you to simultaneously suppress the secondary signal reflected by the surface of the water, and the acoustic noise of the towing vessel, as well as hydrodynamic noise, which as a result leads to a significant increase in noise immunity and, accordingly, to improve the quality of the obtained geophysical information.

 The fact that the multiplicative processing of signals from sound pressure and vibrational velocity channels increases noise immunity due to the simultaneous suppression of a signal reflected by the surface of the water, towing acoustic noise and flow noise does not follow the prior art explicitly from a specialist.

 Thus, the technical solution meets the criterion of "Inventive step".

 The invention is illustrated in figures 1-5.

 In FIG. 1-3 show sound pressure channel 1, vibrational velocity channel 2, multiplication block 3, half-wave peak detectors 4 and 6, “+” designate the polarity of the half-wave peak detector, recording device 5, inverter 7, delay line 8, adder 9, block 10 bandpass filters, multi-channel recording device 11.

In figure 4.5, the following notation:
With ₁ direct signal;
C ₂ signal reflected by the surface of the water;
PA interference is acoustic;
GHG hydrodynamic interference;
A radiation pattern of analog sensors;
P radiation pattern of the prototype sensors;
And the radiation pattern of the sensors of the invention;
"+", "-" radiation pattern polarity signs;
U electrical voltage at the output of the multiplication unit;
τ is the delay time between signals C1 and C2.

 The proposed device contains (Fig. 1) a sound pressure channel 1 and a vibrational velocity channel 2, each of which contains respective sound pressure and vibrational velocity sensors (pressure gradient) and amplifiers. Depending on the type of sensor, an integrator can be included in channel 2 of the vibrational velocity, by analogy with the prototype. The outputs of channels 1 and 2 of sound pressure and vibrational velocity are connected to the input of the multiplication unit 3, the output of which is connected to the input of a half-wave peak detector 4 having direct polarity. The output of the detector 4 is connected to the input of the recording device 5.

 The device (figure 2) contains channels 1 and 2 of sound pressure and vibrational velocity, connected to the inputs of the unit 3 multiplication. Two single-half-peak peak detectors 4 and 6 are connected to the output of the multiplication unit 3. The first half-wave peak detector 4 is in direct polarity and the second half-wave peak detector 6 is inverse. The output of the detector 4 is connected via a delay line 8 to one of the outputs of the adder 9. The output of the block 6 is connected through an inverter 7 to the other input of the adder 9. The output of the adder 9 is connected to the recording device 5.

 The device (figure 3) contains channels 1 and 2 of sound pressure and vibrational velocity, each of which is connected to the same blocks of 10 bandpass filters. The corresponding (identical in frequency) outputs of the bandpass filters are connected in pairs to the multiplication blocks 3, the number of which is equal to the number of frequency bands allocated during filtering. A separate half-wave peak detector 4 is connected to the output of each multiplication block 3. The outputs of all half-wave peak detectors 4 are each connected to a separate input of a multi-channel recording device 11.

 The proposed device for seismic exploration in the water operates as follows.

The vessel towing (figure 4) tows the emitter and a flexible towed antenna. An acoustic signal is emitted by the emitter, reflected from the bottom structures and supplied (C1) to the acoustic sensors (A, P, I) of the towed antenna. Then the useful signal is again reflected by the water surface and also enters (C2) to the acoustic sensors (A, P, I) of the towed antenna, distorting the C1 signal. At the same time, the acoustic sensors (A, P, I) of the towed antenna are exposed to acoustic noise (PA), the source of which is the towbar. In addition, hydrodynamic noise (GH) structural vibrations and turbulent pressure pulsations act on acoustic sensors (A, P, I). Thus, at the output of channel 1 of sound pressure (Fig. 1) of the proposed device, a signal of the form
P _C1 + P _C2 e ^{i ω τ} + P _PG + P _PA , (1) where P _{C1 is the} sound pressure of signal C1;
P _C2 sound pressure signal C2;
P _GH sound pressure hydrodynamic interference;
P _PA sound pressure of acoustic noise,
τ = 2kr, where k is the wave number;
r deepening the antenna from the surface of the water.

At the output of channel 2 of the vibrational velocity, a signal of the form
V _C1 -V _C2 e ^{i ω τ} + V _PG + V _PA , (2) where C1 is the vibrational velocity in the sound wave of signal C1;
C2 vibrational speed in the sound wave of the signal C2;
V _GH vibrational velocity of hydrodynamic interference;
V _PA vibrational velocity in the sound wave of acoustic noise.

-

e^2iωτ+

(3) где "-" означает осреднение во времени по длительности прямого сигнала С1, вносимое блоком 3 перемножения.The minus sign in front of the second term, taken from the side of the water surface, takes place due to the out-of-phase of the petals of the dipole radiation pattern of the acoustic sensor And (figure 4). In addition, due to the presence of a minimum radiation pattern of the acoustic sensor AND in the direction of the towing vessel, the acoustic noise (PA, Fig. 4) is substantially suppressed, i.e. in expression (2)
v _PA _ → 0
Given the uncorrelated signals and interference at the output of the block 3 multiplication (figure 1) is observed in this case, a signal of the form

-

e ^2iωτ +

(3) where “-” means time averaging over the duration of the direct signal C1 introduced by the multiplication unit 3.

0
На выходе однополупериодного детектора 4, имеющего прямую полярность, остается только импульс положительной полярности.As shown by V.S. Petrovsky (Unsteady problems of hydroacoustics, L. Sudostroenie, 1988, p. 10-11), the pressure and vibrational velocity of pseudo sound (hydrodynamic interference) are not correlated, therefore, in expression (3)

0
At the output of a half-wave detector 4 having a direct polarity, only a pulse of positive polarity remains.

=

(4)
Для сравнения сигналы для аналога и прототипа имеют соответственно следующий вид

+

(5)

+

(6)
Следовательно, в предлагаемом устройстве (фиг.1) сигнал С1 очищается от искажений и помех, что эквивалентно росту помехозащищенности.Thus, the signal on the recording device 5 has the form

=

(4)
For comparison, the signals for analogue and prototype are respectively as follows

+

(5)

+

(6)
Therefore, in the proposed device (figure 1), the signal C1 is cleared of distortion and interference, which is equivalent to an increase in noise immunity.

 The signal C2 (figure 5) in the proposed device (figure 2) can be used to increase the level of the useful signal C1. Indeed, after the multiplication block 3 (see relation (3)), the signals C1 and C2 can be easily separated by polarity (Fig. 5). Obviously, with small depths of the towed antenna, usually used in practice (≈10 m), the signal C2 differs from the signal C1 mainly in the phase shift ω τ 2kr, where ω is the cyclic frequency; τ rise time of signal C2; k wave number; r deepening the antenna from the surface of the water.

 Thus, if the signal C1 is delayed for a time τ, then the signals C1 and C2 are added in phase, thereby increasing the level of the useful signal by almost half, which is equivalent to an increase in noise immunity. In the device (FIG. 2), the signal C1 from the half-wave peak detector 4 of direct polarity arrives at delay line 8. At the same time, the signal C2 from the half-wave peak detector 6 of the reverse polarity is supplied to the inverter 7. In the adder 9, the in-phase addition of signals C1 and C2 occurs, the sum is registered by the recording device 5.

When working with broadband probing signals, an additional increase in noise immunity and information content is achieved through spectral analysis. In the proposed device, spectral analysis is performed as shown in FIG. 3. Signals from channels 1 and 2 of sound pressure and vibrational velocity are supplied to identical blocks of 10 bandpass filters. The outputs of the bandpass filter blocks 10 that are identical in frequency bands are connected in pairs with the multiplication blocks 3, where the component P _C1 V _C1 is calculated for each frequency band. Component P _C2 V _C2 is cut off by half-wave peak detectors 4 of direct polarity. The signal C1 for each of the frequency bands is recorded by a multi-channel recording device 11.

Claims (3)

 1. A device for seismic-acoustic exploration in water areas, including a towed antenna containing sound pressure and vibrational velocity channels, characterized in that the outputs of the sound pressure and vibrational velocity channels are connected to the inputs of the multiplication unit, the output of which is connected to the input of the recording device through a half-wave peak detector.  2. The device according to claim 1, characterized in that a second half-wave peak detector is connected to the output of the multiplication unit, having a polarity inverse to the first, the output of which is connected through an inverter to one of the inputs of the adder, to the second input of which the output of the first half-wave is connected via a delay line peak detector, and the adder output is connected to a recording device, and the delay time of the delay line is equal to the ratio of the doubled depth of the towed antenna from the surface of the water and the speed of sound in water.  3. The device according to claim 1, characterized in that the outputs of the sound pressure and vibrational velocity channels are connected to the same blocks of bandpass filters, the corresponding outputs of which are connected in pairs with the inputs of the multiplication units, the outputs of which are connected through single-semiconductor peak detectors to separate inputs of a multi-channel recording device.

Images