RU2244322C1 - Method and device for radiolocation sounding of underlaying surface - Google Patents

Abstract

FIELD: examination of soil subsurface structure.

SUBSTANCE: sounding pulses are formed by means of gas discharger and irradiated. Reflected waves are registered and registered signal is subject to preliminary processing. Wave form of signal is achieved by means of comparing with threshold value preset by quantization scale. Data is put onto liquid crystal indicator and recorded to memory. During preliminary processing the signal amplitude quantization quazi-logarithmic scale is formed. Logarithmic full-wave shape of registered signal is represented in form of sequent row of signal wave shapes in three-dimensional form, i.e. "amplitude-delay time-length of profile" with color coding of signal amplitude. Dielectric constant and signal attenuation values in underlaying layers are determined from which values the presence of subsurface objects is judged. Simultaneously with full-wave signal shape the binary frame is put onto screen of liquid crystal indicator. Binary frame is composed of sequent row of full-wave shapes selected at preset threshold value. Device for radiolocation sounding has transmitter, gas discharge-based sounding pulse former, transmitting aerial and receiving unit which has receiving aerial connected in series with aerial amplifier and amplifier-limiter which both are connected in series and compose separate unit of aerial amplifier. Amplifier-limiter is connected with first output of synchronization unit. Basic amplifier is connected with second output amplifier-limiter. Device also has control board, memory unit, liquid crystal indicator, processing unit. First input of processing unit is connected with output of basic amplifier, the second input is connected with output of 7-digit digital-to-analog converter and the third input is connected with output of controller. Output of processing unit is connected with input of controller which is in turn connected with synchronization unit, memory unit and liquid crystal indicator. Controller is connected with attenuator control unit through7-digit digital-to analog converter. Attenuator control unit is connected with controlled attenuator of aerial amplifier. Transmitter is brought to operation by means of breaking of photon-coupled pair which is connected with control board of basic unit and transmitter voltage converter. Photon-coupled pair is made of IR diode and photoreceiver.

EFFECT: prompt data receiving.

2 cl, 6 dwg

Description

The invention relates to geophysics and is intended to study the subsurface structure of the soil and detect objects to depths of several tens and hundreds of meters and is applicable for solving scientific and engineering problems in various fields, such as geophysics, geology, construction, archeology.

A known method of radar sensing of the underlying surface, including the formation of sounding pulses using a gas spark gap, their radiation by the transmitting antenna, registration of reflected waves by the receiving antenna, preliminary processing of the registered signal in the receiving unit using an attenuator and an amplifier-limiter, obtaining a waveform of the signal by comparing with a value threshold, set on a quantization scale, the output of information on the screen of a liquid crystal display (LCD) and recording e in the memory (RU 2080622 C1, 27/05/1997).

The disadvantage of this method is that adopted for the main binary mode does not allow in difficult situations to produce the correct interpretation of the data.

The objective of the invention is the creation of a new registration mode - “full wave form logarithmic”.

A device is known for radar sensing of the underlying surface, comprising an autonomous transmitter including a serially connected timer and a voltage converter connected to a power source, and a probe pulse generator on a gas spark gap, and a transmit antenna connected via a connector, a receiving unit including a receiving antenna connected in series and structurally series-connected attenuator and amplifier boundaries combined in a separate unit of the antenna amplifier rer coupled to the first output of the synchronization unit, coupled to the second output of the limiter amplifier main amplifier, and a device comprises a control panel, LCD and memory unit (RU 2080622 C1, 05/27/1997).

The disadvantages of the device are insufficient dynamic range, which leads to a limitation of the amplitude of the signal upon receipt of the wave form, as well as to a complete loss of information about the amplitude of the signal in binary mode.

The objective of the invention is to provide a device for implementing a new registration mode - “full wave form logarithmic”.

The technical result is the rapid receipt of information about subsurface structures and objects in the form of two-dimensional pictures in real time, the detection of subsurface objects with higher spatial resolution compared to the binary mode.

The technical result is achieved by the fact that the method of radar sounding of the underlying surface includes the generation of probe pulses using a gas spark gap, their radiation by the transmitting antenna, registration of reflected waves by the receiving antenna, preliminary processing of the registered signal in the receiving unit using an attenuator and an amplifier-limiter, obtaining a waveform of the signal by comparing with a threshold value set on a quantization scale, displaying information on a liquid crystal screen indicator (LCD) and its recording in memory. When pre-processing the registered signal to increase the dynamic range of registration using a multi-bit DAC and an attenuator control unit, a quasi-logarithmic scale of quantization of the signal amplitude is formed. Using the logarithmic full-waveform of the registered signal, presented in the form of a series of waveforms of the waveform in three-dimensional form - “amplitude - delay time — profile length” with color coding of the signal amplitude, the dielectric constant and signal attenuation in the underlying layers are determined, the value of which determines the presence of subsurface objects, and for operational control, a binary frame composed of a sequence is displayed on the LCD screen simultaneously with the full-waveform frame a series of full-wave forms isolated at a given threshold value.

In addition, the technical result is achieved by the fact that the device for radar sensing of the underlying surface comprises a transmitter including a serially connected timer and a voltage converter connected to a power source, and a probe pulse generator on a gas spark gap, and a transmit antenna connected via a connector, a receiving unit including serially connected receiving antenna and structurally combined into a separate unit of the antenna amplifier are serially connected controllable attenuator and a limiting amplifier, coupled to the first output of the synchronization unit, coupled to the second output of the limiter amplifier main amplifier, and a device comprises a control panel, a memory and LCD. The main unit of the device, connected to the receiving unit via a cable, additionally contains a processing unit, the first input of which is connected to the output of the main amplifier, and the second to the output of the 7-bit DAC, and the third input to the output of the controller, the output of the processing unit is connected to the input of the controller , also connected to the synchronization unit, memory unit, control panel and LCD, the controller is connected via an 7-bit DAC to an attenuator control unit, which is connected via cable to an antenna amplifier controlled attenuator of Tell, wherein the transmitter is carried out by launching gap optoelectronic pair associated with the base unit control panel and voltage inverter and a transmitter constructed as an infrared LED and photodetector.

Figure 1 presents a block diagram of a device.

Figure 2 presents a typical logarithmic full-waveform of the signal reflected from subsurface structures and objects, the decimal logarithm of the signal amplitude is plotted along the horizontal axis, and the delay times of the reflected signal in nsec (billionth of a second) are plotted along the vertical axis.

Figure 3 presents for comparison a frame of a linear waveform with a limitation of the amplitude of the signal at the top of the frame. The horizontal axis represents the amplitude value on a linear scale.

Figure 4 presents a color (printed on a black and white printer) frame from logarithmic full-wave forms, illustrating a higher spatial resolution compared to a binary frame.

Figure 5 presents for comparison with figure 4 a binary frame of the same section.

Figure 6 presents a frame of a logarithmic full-wave form with 4 sections, where the envelopes of the signal amplitudes are approximated by straight lines.

The device comprises a transmitter and receiving blocks, the transmitter 1 including a battery 15, a timer 16, a voltage converter 17, a probe pulse generator 18, a transmit antenna 2 detachable from the block; the photodetector of the optoelectronic pair 5, and the receiving units include a receiving antenna 3, an antenna amplifier 4 connected to it through a connector, comprising a controlled attenuator 19, an amplifier-limiter 20 and an LED of the optoelectronic pair 5, and a structurally separate main unit connected by a cable to the antenna amplifier synchronization 6, main amplifier 7, processing unit 8, controller 9, LCD 10, internal memory 11, control panel 12, 7-bit DAC 13 and attenuator control unit 14.

The device has a mode of “binary forms", i.e. full-waveform waveforms allocated at a given threshold value. It is possible to register at the choice of one or three frames, with a threshold of 0 and two additional thresholds within + 16 ÷ -16, which somewhat enriches the ability to interpret data in difficult situations. The mode of “binary forms” compares favorably with the mode of “full-wave” forms in the small amount of internal memory required, but its use is limited by relatively simple tasks, such as detecting pipes and communications.

The device uses the new mode of “logarithmic full-wave forms” as the main mode. The attenuator is switched programmatically using a 7-bit DAC so that on the LCD screen the signal amplitude is recorded in a logarithmic scale. The frame on the LCD screen has 128 horizontal pixels, of which the left 63 pixels are for displaying positive signals, the right 64 pixels are for negative signals, and the 64th pixel corresponds to a zero signal (zero threshold). At small amplitudes of the measured signal, the attenuator does not turn on. Up to plus and minus 8 pixels (8 pixels to the left and right of the 64th pixel zero) the quantization step (increment of the signal amplitude when increasing by 1 threshold module) is the same, the next 4 pixels the quantization step of the threshold doubles (increment by two threshold levels by one pixel), or, in other words, the steepness of a straight line segment is halved, the next 4 pixels are quadrupled, the next 4 pixels are eight times. After this (plus or minus the twentieth pixel), the controlled attenuator is activated: in each rectilinear segment, consisting of 3 subsequent pixels, the last 3 threshold values are set again, but with the attenuator connected with a sequentially increasing attenuation in 4 dB steps (4; 8 etc. up to 56 dB). Therefore, the steepness of the rectilinear sections (within 3 pixels) sequentially decreases, and as a result of increasing the pixel number from the middle (64th pixel), the signal amplitude on the LCD screen increases according to the quasi-logarithmic law (piecewise-linear approximation), and thus increase the dynamic range of the recorded signal to ~ 100 dB (42 dB gives a 7-bit DAC and 56 dB a controlled attenuator) without using a multi-bit high-frequency ADC. The advantage of the proposed registration scheme for full-wave forms should also include the fact that the sensitivity of the receiving path is higher at low thresholds (50-75 μV at zero threshold), since at high signal amplitudes the receiver’s noise is not significant.

The device consists of three structurally separate blocks (figure 1). The transmitter 1 is powered by a built-in power source 15 and consists of a timer 16 that sets the pulse repetition rate (~ 1 kHz), a voltage converter 17 that increases the voltage from 10-15 V to 5 kB, and a probe pulse generator 18 based on a precision gas discharge . When a voltage is applied to the main unit, the operation of the voltage converter 17 is blocked using an optoelectronic pair 5, and only when the “start” button is pressed in the registration mode, the voltage converter 17 is released, the storage capacitor in the probe pulse generator 18 is charged. The voltage on the spark gap gradually increases, breakdown of the spark gap, the capacitor closes to the transmitting antenna 2, forming a powerful probing ultra-wideband video pulse. The probe pulse first reaches the receiving antenna 3 through the air gap between the antennas, and a clock pulse is generated on the steep leading edge of this pulse in the synchronization block 6, which serves as a time reference for the entire signal processing process. The delayed signals reflected from subsurface objects, depending on the distance and the depth of their location, are sequentially transmitted to the receiving antenna 3. The received signal is pre-processed in the antenna amplifier 4 by a controlled attenuator 19 and a limiting amplifier 20. The signal amplitude is limited in order to protect the receiving path from overload. The signal from the output of the antenna amplifier 4 is transmitted via cable to the main unit, where it is amplified in the main amplifier 7 and gets into the processing unit 8, in which the signal is compared with the threshold set by the 7-bit DAC 13 and the attenuator control unit 14 on the quasi-logarithmic quantization scale. Exceeding the threshold signal is registered in binary code as “1”, the absence of exceeding is registered as “0”. To bind the time delays of the moments when the threshold signal is exceeded, a clock pulse is used, generated in the synchronization block 6. From the output of the processing unit, information through the controller 9 is displayed on the LCD 10 and recorded in the memory unit 11. The controller operating mode is set via the menu and submenu on the control panel 12. Information is transferred from the memory block 11 to a personal computer via the serial port using a null modem cable. The amount of internal memory 11 allows you to save 500 frames of “binary” form or 3000 frames of “full wave” form in packed form. Information is transferred under the control of a computer via a communication protocol, either by individual frames or by groups of frames combined into so-called “trace-lines”. An external 12 V battery with a capacity of 7 Ah is used as a power source for the antenna amplifier and the main unit.

An unpleasant problem with shock excitation of an antenna is the occurrence of periodic oscillations at its own frequency, the so-called “ringing” of the antenna. In GPR, the monitored objects are in the near zone, and the first reflected pulses from the objects come with a minimum delay after the probing pulse. Ringing, superimposed on these signals, completely disguises them, therefore, to reduce the blind zone, it is necessary to take measures to eliminate them. The device uses resistively-loaded dipole antennas with a suppression ratio of 0.35. With this damping, only one spurious ringing pulse is noticeable. Potential losses due to antenna damping are compensated by introducing averaging of the signal over 16 measurements. With this in mind, it is possible to easily calculate the theoretical GPR potential of ~ 150-160 dB from the ratio of the transmitter voltage to the sensitivity of the receiving path of the GPR. The real potential of the GPR can be very different from the theoretically calculated one, therefore a lower potential of 100-130 dB is declared for the device compared to the theoretical one. The experimental determination of the GPR potential is a very difficult problem due to the uncertainty of many factors, such as the properties of antennas and the propagation conditions of the signal.

To improve the operational control of the material obtained in the device when using a color LCD, a composite frame of full-wave forms with color coding of the signal amplitude in three-dimensional form is displayed on one half of the indicator - “amplitude - delay time - profile length” and simultaneously on the other half of the indicator - “binary form” ”, I.e. full-wave form allocated at a given threshold value.

Frames of “binary forms” are easily decrypted even in the field, and if necessary, the decision is made on binary frames to make adjustments to the measurement process. The measurement process consists in the following: the studied area is divided into a number of points with a certain step, and at each point a frame of a full-wave signal is recorded, a characteristic view of which is shown in Fig.2. For comparison, figure 3 shows the waveform with a limited amplitude. In the event of a malfunction or interference, the last full-wave frame is deleted and the frame is re-registered.

A composite three-dimensional frame of full-waveforms with color-coded amplitude allows several times to improve spatial resolution in the field of small time scans due to amplitude discrimination of signals from objects (in Fig. 3 such an area is surrounded by a rectangular frame), as well as to select objects that are invisible on “binary” forms ”due to the superposition of a direct probe pulse on the signal from the objects.

A sequential series of full-wave forms contains all the information on dielectric permeability and attenuation of a signal in a subsurface medium that is accessible to georadars. Representation of the amplitude of the recorded signal on a logarithmic scale is also convenient because by the “picture” on the indicator it is possible to quickly evaluate the signal attenuation in subsurface layers, since the envelope of the signal amplitude is a straight line with a constant signal attenuation in the layer, which is clearly seen in Fig. 6 . In this figure, 4 layers with a rectilinear envelope can be confidently identified up to a signal delay time of approximately 140 nsec. From the slope of the direct envelopes, the ohmic attenuation of the signal in the layer can be estimated, and the sections of kink or rupture of the envelope correspond to the boundaries of the transition between the layers. In theoretical considerations, a plane-layered model of the medium parallel to the earth’s surface is usually used, and in such a medium each layer is assumed to be uniform with constant dielectric constant and ohmic attenuation due to conductivity. In such a model, the probe signal is reflected only at the boundaries of the transition between the layers and partly on objects that have electrodynamic parameters different from the surrounding medium. But the practice of georadar studies, in particular observations on the device, shows that such a model is far from reality in most cases. The subsurface medium, in contrast to the air, is substantially heterogeneous. In each layer there are many reflective objects, such as stones, inclusions of another rock, anthropogenic objects, etc., located at different distances from the georadar antennas, and the straight lines in Fig. 6 are the envelopes (on average) of signals from many reflective objects in the layer.

Another parameter of the subsurface medium, determined by GPR, is the dielectric constant, the size of which is equal to the square of the refractive index of the medium. The georadar image of intense signal reflectors, which, in particular, cables and pipes, are in the form of hyperbolic curves, the steepness of the wings of which depends on the speed of signal propagation and, accordingly, on the refractive index of the medium. But when calculating the dielectric constant, the reflecting object is considered as a mathematical point, which significantly increases the error. The refractive index is determined by hyperbolas, and the final dimensions of reflecting objects are taken into account. For this, in the geometric optics approximation, an equation of the 6th degree in variable parameters is solved. With a pipe diameter of 0.5 m and a depth of 1-2 m and a refractive index of 3, taking into account the diameter of the pipe reduces the depth by 20% (or increases the refractive index by 20%).

In those cases when the approximation of parallelism of the transition boundaries of the Earth's surface layers is applicable, to determine the refractive index, a hodograph is constructed - a hyperbolic curve of the reflected signal from the boundaries of the layers, obtained with an increasing distance between the transmitting and receiving antennas, and the refractive index in the layer is determined from the hodograph and, accordingly, , the dielectric constant.

The found values of permittivity and signal attenuation are important for the reliable identification of the composition of subsurface soil (for example, sand-sandy loam-loam-clay). After finding the refractive indices of the subsurface layers, the signal delay times are transformed into depths, and for each layer, its own conversion coefficient “delay time - depth” is set.

The full-waveform is a two-dimensional frame (amplitude - delay time), and a composite frame of a consecutive set of full-waveforms is three-dimensional (amplitude - delay time - profile length), which is very inconvenient when displaying and displaying information. To process full-waveforms, instead of the third (amplitude) coordinate, the color gradation of the signal amplitude is used. The number of colors (usually 32 colors) and the color palette are selected and entered into the frame at the choice of the operator. The lines of maximum (positive) and minimum (negative) amplitudes are highlighted on color frames, and the so-called common-mode lines of the signal, which correspond to subsurface objects and layers, are determined by these lines and by lines of signal polarity change (binary frame with threshold 0). In addition to a color frame, you can get any binary frame on any of 128 thresholds in the form of a black-and-white or monochrome frame. Both color and black and white frames are processed according to the standard method of binary forms.

The above representation of full-wave forms compares favorably with the widespread method of “wiggle trace - image of amplitudes by deviations”, when wave forms are compressed into a narrow ribbon and a frame is drawn from a series of such ribbons. (In the literature, it is customary to call full-wave forms “traces” - a translation of the English word “trace”.)

Thus, successful technical solutions in the device made it possible to realize optimal operational characteristics.

Claims (2)

1. A method of radar sounding of the underlying surface, including the formation of sounding pulses using a gas spark gap, their emission by a transmitting antenna, registration of reflected waves by a receiving antenna, preliminary processing of the registered signal in the receiving unit using an attenuator and an amplifier-limiter, obtaining a waveform of the signal by comparing with the threshold value set on the quantization scale, the output of information on the screen of a liquid crystal indicator (LCD) and recording it in the memory characterized in that when pre-processing the registered signal to increase the dynamic range of registration using a multi-bit DAC and an attenuator control unit, a quasi-logarithmic scale for quantizing the amplitude of the signal is formed using the logarithmic full-waveform of the recorded signal, presented in the form of a sequential series of waveforms of the signal in three-dimensional form " amplitude - delay time - profile length "with color coding of signal amplitude, dielectric values are determined eskoy constant and attenuation in the underlying layers, the magnitude of which is judged on the presence of subsurface objects, and for the operative control of the LCD screen frame simultaneously with the full-wave waveform outputted binary frame consisting of a succession of full-wave forms allocated for a given threshold value. 2. A device for radar sensing of the underlying surface, comprising a transmitter including serially connected timers and a voltage converter connected to a power source, and a probe pulse generator on a gas spark gap, and a transmit antenna connected via a connector, a receiving unit including a receiving antenna connected in series and structurally combined in a separate unit of the antenna amplifier in series connected controlled attenuator and amplifier-limiter connected to the first output of the synchronization unit, connected to the second output of the limiter amplifier, the main amplifier, as well as the device includes a control panel, a memory unit and an LCD, characterized in that the main unit of the device connected to the receiving unit via a cable further comprises a processing unit, whose first input is connected to the output of the main amplifier, and the second to the output of the 7-bit DAC, and the third input to the output of the controller, the output of the processing unit is connected to the input of the controller, also connected to the unit m synchronization, memory unit, control panel and LCD, the controller via a 7-bit DAC is connected to the control unit of the attenuator, which is connected via a cable to the controlled attenuator of the antenna amplifier, while the transmitter is launched by breaking the optoelectronic pair associated with the control panel of the main unit and transmitter voltage transmitter and made in the form of an infrared LED and photodetector.

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