SU911413A1 - Device for registering elastic oscillations at acoustic probing of rocks - Google Patents

Description

(54) DEVICE FOR FIXING ELASTIC VIBRATIONS WITH ACOUSTIC SENSING OF MOUNTAIN BREEDS

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The invention relates to the field of investigation of the kinematic and dynamic characteristics of elastic waves in rocks intersecting a well.

A device is known whose operation is based on fixing elastic oscillations converted into an electrical signal by means of a threshold device which is triggered at the first positive half-period of the signal when the latter reaches the threshold. To prevent the threshold device from being triggered from acoustic and electrical interference arising from the downhole tool movement, the threshold is set above the noise level. In this case, the signal is locked at the gentle front of the fixed half-period and is delayed relative to the moment of arrival of the signal, measured at the beginning of the half-period. Time from start

of the fixed half-period of the signal and, prior to the moment of fixation, determines a temporary error when fixing the signal 11

A disadvantage of the known device is that this error at a high threshold is quite significant and depends on the amplitude of the signal. The signal amplitude, in turn, depends on the dynamic characteristics of the rock, quality, centering and p

10 other hard-to-consider factors, i.e. can be considered a random variable. Therefore, a commit error is a random variable and cannot be taken into account.

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Thus, the moment of arrival of the signal is fixed with an error, which distorts the reliability of the estimate of the kinematic and dynamic characteristics of elastic waves in the investigated rocks.

It is also known a device in which two sweats are used to record the moment of arrival of the yn-i oscillations.

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Horn devices: coarse with a trigger threshold above the noise level and accurate with a trigger threshold at the noise level. Received from the receiver and the amplified information signal simultaneously arrives at the coarse threshold device and at the delay line, which delays the information signal for a time equal to half the oscillation period of the information signal. The output of the delay line is connected to an operating amplifier on the operational amplifier to a sensitive threshold device, which in its initial state is locked with the help of a voltage supplied to its non-inverting input. When the first positive half-period of the information signal reaches the threshold of the coarse threshold device, the latter generates a command pulse that, through the key, acts on the non-inverting input and reduces the reference voltage to 10 mV. Thus, in each cycle, after the coarse threshold device is triggered, the trigger threshold of the exact threshold device is lowered to a low level of 10 mV. It is triggered by the delayed information signal arriving at the inverting input at the very beginning of its entry into its steepest part, where the moment of fixation is practically independent of the amplitude of the information signal, i.e. instead of the hardly recognizable random error associated with the amplitude of the signal and the height of the threshold, a systematic and predetermined error was introduced, equal to the delay in the delay line. This error is systematic and easily compensated by the processing of the information received. The effect of interference on the device is insignificant, since the time interval between the direct and delayed information signal is a small part in the O71 interval of the signal and the probability of interference in this interval is small, 21.

The drawbacks of this device are due to the relatively large amount of delay, which should be provided by a delay line, since acoustic sounding of rocks is often used in an information signal in a wide range of frequencies, starting from

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4-5 kHz. In this case, to ensure the normal operation of the device, a delay by a line must be at least 50 ISS. A curve using a line with such a delay is cumbersome, and a line with a long delay at v wide spectrum of the information signal introduces phase distortions, which leads to deformation of the first inputs of the information signal and, accordingly, causes an additional error of fixation due to phase distortions.

The aim of the invention is to improve the accuracy of the device.

This goal is achieved by the fact that in a device for fixing elastic vibrations during acoustic sounding of rocks, containing

0 two threshold devices sensitively connected to semipernetisms of oscillations of one polarity; coarse with a trigger threshold above the noise level and accurate with a trigger threshold at the noise level, a square pulse shaper with a duration of not less than a quarter of the period of recorded oscillations and a sector were additionally introduced, with the square pulse pulse generator input connected to the output of the exact threshold device, the selector inputs are connected respectively to the output of the shaper of rectangular impulses and the output of the coarse threshold device.

Fig, 1 shows a diagram of the device; in fig. 2 - the scheme of his work.

Device for fixing elastic

Vibration 0 contains a coarse threshold device I, an exact threshold device 2, a square pulse former 3 and a selector 4. A driver input is connected to the output of a precise threshold device, two selector inputs are connected one to the output of the square pulse generator, and the other to the output of a rough threshold device.

Information signal 5 in the form of a package of electrical oscillations; received as a result of the conversion of the elastic oscillations received at the receiving sensor, together with the noise, arrives at the inputs of the coarse with the threshold 6 and exactly with the threshold of 7 threshold devices made on the basis of operational amplifiers. The mode and switching circuit of the operational amplifiers ensures the fixation of positive half-periods of the information signal, with the coarse threshold set higher than the interference and the height of the exact threshold set to the order of a coarse threshold and commensurate with the level of the most intense interference. The operational amplifier of the coarse threshold device is set to the comparator mode and when the positive signal reaches 8 signal, the response threshold (corresponding to the reference voltage I.) generates voltage drops P. Interference does not reach the amplitude of the coarse threshold device in amplitude and therefore the threshold device has no effect. The logic circuit at the output of the operational amplifier in the circuit of the coarse threshold device generates voltage drops 2 at the output, the front 13 of which corresponds to the first trigger of the coarse threshold device in the interval of waiting. The operational amplifier of the exact threshold device is also set to the comparator mode, but with a lower reference voltage of about 0.1 Id. Therefore, it is triggered at a lower level, forming voltage drops 14 at the output from positive half-periods of the signal, as well as from interference with their occurrence and sufficient amplitude. The voltage drops from the output of the exact threshold device arrive at the square pulse shaper and trigger it with the fronts of the differences corresponding to the trigger points of the threshold device. In response to each launch, the driver generates rectangular pulses 15, each having a duration equal to a quarter of the oscillation period of the signal (T / 4). The output voltage of the coarse threshold device and the square pulse shaper are fed to separate inputs of the selector, which is complete according to the logic trigger circuit. When the front hits the interval of one of the pulses at the output of the selector, a rectangular pulse 16 is outputted, the leading edge of which corresponds to the instant of operation of the coarse threshold device, and the rear edge corresponds to the instant of triggering of the exact threshold device at the beginning of the first positive period, shifted by a time equal to a constant duration puller forerunner. From the output of the device, pulse 16 enters the instrumentation measurement units, where the leading edge of pulse 6 is used in a circuit. Measuring the interval time for the passage of elastic waves to estimate porosity and in a damping circuit to estimate the fracture of the rock sections, t, e, to evaluate their kinematic and dynamic characteristics. The role of a coarse threshold device is reduced to detecting (recognizing) a half-period of a signal, but fixing it, with complete safety (due to a high threshold) in terms of interference. The role of the exact threshold device is reduced to the fixation of signals (in t., H and interference) with high accuracy (due to a very low threshold). The coincidence of two events - the coarse tangency of the threshold device and its falling into a limited interval (about a quarter of a signal's period), tied to the moment of signal fixation by the exact threshold device, confirms that the working half-period of the signal is fixed, not the disturbance. The existing error, determined by the duration of the impulses of the forward-impulse former, is systematic and easily taken into account when processing the received information or automatically compensated (in two-channel equipment). Failure of interference due to the low threshold of the exact threshold device is low, since a very small interval at the front of the recorded signal input, which is always less than a quarter of the signal period, is unprotected from interference. A comparative analysis of the proposed and known device showed that when the other characteristics are identical, the dimensions of the proposed device are many times smaller, and the measurement error associated with the phase distortions of the information signal is absent and fundamentally impossible.

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Claims (2)

1. Collector Geophysical equipment .M. , Nedra, 1972, K 43, pp.62-65. 2.Patent UK No. 1444337, cl. H 2 H, 1976 15 (prototype).