WO2004111685A1

WO2004111685A1 - Method for radar sounding an underlying surface and device for carrying out said method

Info

Publication number: WO2004111685A1
Application number: PCT/RU2003/000150
Authority: WO
Inventors: Alexandr Evgenievich Reznikov; Den Oe
Original assignee: Zakritoe Aktsionernoe Obschestvo 'timer'
Priority date: 2003-04-02
Filing date: 2003-04-10
Publication date: 2004-12-23
Also published as: RU2244322C1; AU2003271240A1

Abstract

The invention is used for studying a subsurface soil structure and detecting objects at a depth of several tens and hundreds of meters. The inventive method consists in forming sounding pulses with the aid of a gas-discharge arrester, sending said impulses by means of a transmitting antenna, recording reflection waves by a receiving antenna and in processing a recorded signal. A quasi-logarithmic signal amplitude quantizing scale is formed during processing. The logarithmic full-waveform of the recorded signal is represented in the form of a sequence of the signal waveforms in a three-dimensional form, i.e. amplitude-delay time-profile length associated with a coloured encoding of the signal amplitude. The values of a dielectric constant and decaying signal in underlying layers are defined, thereby making it possible to detect subsurface objects by using the size of said values. A binary exposure consisting of the sequence of the full-waveforms selected at a predefined threshold value is displayed on a LCD screen simultaneously with the exposure of the full-waveform of the signal. The inventive device consists of a transmitter provided with a pulse driver based on the gas-discharge arrester, a receiving unit provided with a receiving antenna and an antenna amplifier unit which comprises a controlled attenuator and a clipping amplifier, a main unit comprising a processing unit whose inputs are connected to the outputs of an amplifier, a seven-bit DAC and a controller, the outputs of the processing unit being connected to the controller input which is connected to a synchronisation and memory units, a control panel and the LCD. Said controller is connected to the attenuator control unit through the seven-bit DAC.

Description

Method for radar sounding of underlying surface and device for its implementation

Technical field

The invention relates to geophysics and is intended to study under the surface 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.

State of the art

A known method of radar sensing of the underlying surface, including the formation of sounding pulses using a gas spark gap, their radiation, 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 its memory (RU 2080622 Cl, 05/27/1997).

The disadvantages of the method are that adopted for the main binary mode does not allow in difficult situations to produce the correct interpretation of the data. A device for radar sensing of the underlying surface, comprising an autonomous transmitter comprising 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 in series and series-connected attenuator and amplifier-limbs structurally combined into a separate antenna amplifier unit the amplifier connected to the first output of the synchronization unit, connected to the second output of the limiter amplifier, the main amplifier, and the device also contains a control panel, a memory unit and an LCD (RU 2080622 Cl, 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.

SUMMARY OF THE INVENTION

The objective of the invention is the creation of a method and device for implementing a new registration mode - "full-wave form l ographic". 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 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 threshold value set on a quantization scale, displaying information on a liquid crystal screen Skogen display (LCD) and its entry into 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 on the LCD screen simultaneously with the frame of the full-waveform waveform output a binary frame composed of consecutive series of full-wave forms isolated at a given threshold value.

In addition, the technical result is achieved in that the device for radar sensing of the underlying surface comprises a transmitter including a series-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 combined into a separate antenna amplifier unit, connected in series e controlled attenuator and limiter amplifier connected to the first output of the synchronization unit, connected to the second output of the limiter amplifier, the main amplifier, and the device also contains a control panel, a memory unit and an 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. Brief Description of the Drawings

Figure 1 presents a block diagram of a device.

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

Fig. 3 shows for comparison a linear waveform frame with a limitation of the signal amplitude 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 best embodiment of the invention

The device contains a transmitter and receiving blocks, and the transmitter 1 includes a battery 15, a timer 16, a converter voltage 17, probing pulse shaper 18 transmitting antenna 2 detachable from the unit; 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 “binary form” mode, 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 “binary form” mode compares favorably with the “full wave” mode 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 “logarithmic full-waveform” mode 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 slope of the line segment is halved, the next 4 pixels are four times, the next 4 pixels are eight times. After this (plus or minus the twentieth pixel), a controlled attenuator is activated: in each rectilinear segment consisting of 3 subsequent pixels, the last 3 threshold values are set repeatedly, but with the attenuator connected with a sequentially increasing attenuation in increments of “4 dB (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 (42dB gives a 7-bit DAC and 56dB a controlled attenuator) without using a multi-bit high-frequency ADC. To the advantage of the proposed registration scheme for full-wave forms It should also be noted that the sensitivity of the receiving path is higher at small thresholds (50-75 μV at a zero threshold), since at large signal amplitudes the receiver’s own noise is not significant.

The device consists of three structurally separate blocks (Fig.l). The transmitter 1 is powered by a built-in power source 15 and consists of a timer 16 that sets the repetition rate (~ lKHz) probe pulses, a voltage converter 17, increasing the voltage from 10-15V to 5 KB, and a probe pulse shaper 18 based on a precision gas spark gap. 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 “pyc” 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 occurs 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 their depth, 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 an amplifier-limiter 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 recorded in binary code as "1", the absence of exceeding is recorded as "0". To bind the time delays of the moments when the threshold signal is exceeded, a sync pulse is used, generated in the synchronization unit 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 the “binar” form or 3000 frames of the “full-wave” form in packed form. Information is transferred under the control of a computer using a communication protocol, either by individual frames or by groups of frames combined into so-called “trunk 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 the antenna is the occurrence of periodic oscillations at the natural frequency, the so-called “ring” 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, overlapping these signals, completely mask them, therefore, to reduce the blind zone, measures must be taken 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 the introduction of signal averaging over 16 measurements. With this in mind, you can easily calculate the theoretical GPR potential ~ 150 -lB ODB from the ratio of the transmitter voltage to the sensitivity of the receiving path of the GPR. The actual ground penetrating radar potential can be very different from the theoretically calculated one, therefore a lower potential of 100-lЗОдБ is declared for the device. 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 the 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 at the same time on the other half of the indicator - “binar form ", i.e. full-wave form allocated at a given threshold value.

The “binary form” frames 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 section is divided into a number of points with a certain step, and a frame of a full-wave signal is recorded at each point, a characteristic view of which is shown in FIG. 2. For comparison, FIG. 3 shows a waveform with amplitude limitation. In the event of a malfunction or interference, the last full-wave frame is erased and the frame is re-registered.

Composite three-dimensional frame of full-wave forms 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. 5, such an area is surrounded by a rectangular frame), as well as to distinguish objects that are invisible to “binary forms” due to the direct sounding pulse being applied to the signal from 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 straight envelope can be reliably identified up to a signal delay time of approximately 140Hsec. 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, unlike 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 georadarograms, 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 are cables and pipes, have the form of hyperbolic curves, the steepness of the wings of which depends on the speed of propagation of the signal 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 determination of the refractive index is carried out by hyperbole, while taking into account the final dimensions of the reflecting objects. 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 decreases the depth by 20% (or increases the refractive index by 20%).

In those cases when the approximation of parallelism of the boundaries of the transitions 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 delay times of the signal 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 in 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 lines of change of signal polarity (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-waveforms compares favorably with the widespread method of “wiggle trace - image of amplitudes by deviations)), when the waveforms (wave form) are squeezed into a narrow tape and a frame is drawn from a consecutive row of such tapes. (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.

Industrial applicability

The invention is used in geophysics and is intended to study the subsurface structure of the soil and detect objects to a depth of several tens of meters and several hundred meters. It is also applicable for solving scientific and engineering problems in various fields, such as geology, construction, archeology, etc.

Claims

Claim

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 of quantization of the signal amplitude is formed using the logarithmic full-waveform of the recorded signal, presented in the form of a series of three-dimensional waveforms of the signal - "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. Device for radar sensing of the underlying surface, comprising 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 a receiving antenna connected in series and constructively connected in a separate unit of the antenna amplifier, a series-connected controlled attenuator and an 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 synchronization, memory unit, control panel and LCD, the controller via a 7-bit DAC is connected to an attenuator control unit, which is connected via a cable to a controlled attenuator of the antenna amplifier, while the transmitter is launched by breaking the optoelectronic pair connected to the control panel of the main unit and the converter transmitter voltage and made in the form of an infrared LED and photodetector.