RU81812U1 - DEVICE FOR RADAR SENSING OF SUBSURFACE SPACE - Google Patents

Description

The proposed utility model relates to techniques for receiving and transmitting radio signals, in particular, to sensing by radio waves of a subsurface medium and can be used in geophysics, geology, construction industry, road construction, oil and gas industry, etc.

There are many devices for radar sensing of the subsurface space, in which the processing uses the conversion of the signal spectrum to a lower frequency region either by the gating method (United States Patent 3806795, Ap.23, 1974) or by comparing (RU Pat. 2080622 C1, 05/27/1997 )

The main disadvantage of these processing methods is the lengthening of the time period of signal processing by a thousand times with the stroboscopic method and hundreds of times with the method of comparing in connection with the conversion of the signal to a lower frequency region.

Closest to the proposed utility model is a device for radar sensing of the underlying surface (RU Pat. 2244322 C1, 01/10/2005, prototype), which uses an autonomously working transmitter that generates a probe pulse with a high-voltage gas spark gap, and the receiver is synchronized with the transmitter impulse, after passing through the air the gap between the transmitting and receiving antennas, and to expand the dynamic range of the signal display, a quasi-logarithmic I scale representation of amplitude.

The disadvantage of the synchronization method in the prototype device is the overload of the input circuits of the receiver by a powerful probing pulse that has come through the air and the appearance of a nonlinear signal amplification mode for this reason. The disadvantage should also include the use of a quasi-logarithmic amplitude scale, in which a linear amplitude scale is used instead of the logarithmic scale in the region of small signals.

The proposed utility model solves the technical problem of creating a device with direct signal processing that has the highest possible speed and a wide range of sensing operating frequencies, neutralizing the effects of a powerful probe pulse on the input circuits, which leads to the appearance of a nonlinear operation mode of the amplifier, and realizing a purely logarithmic amplitude recording scale giving the correct idea of the process of signal attenuation in deep layers.

The stated technical problem is solved in that in a sensing device containing a transmitting antenna with a transmitter of the probe pulse, a receiving antenna with an amplifier of pulses reflected from subsurface objects and an analog-to-digital converter of the input signal connected to it, the transmitter is launched synchronously with the receiver turned on and the transmitting and receiving antennas are shielded in the frequency range from 5 MHz to 1 GHz in the form of several layers of a radar absorbing pile mat With an ohmic resistance of the layer closest to the antenna equal to the wave impedance of the free space of 377 ohms, and with a gradual decrease in the resistance exponentially from the inner layer to the outer, the analog-to-digital converter is capable of directly processing the input signal in the frequency range from 5 MHz to 1 GHz with display of the output signal on a logarithmic amplitude scale.

Recent advances in computer technology make it possible to implement a device for radar sensing of subsurface space with direct signal processing. Due to the specifics of operation (omnidirectional antennas, scattering on a variety of inhomogeneities, reflection from underground and airborne objects, man-made interference, the need to ensure a large dynamic range of at least 100 dB, etc.), the scan along the time axis of the received signal (full-wave signal) has complex view and carries a lot of information. Therefore, for processing the signal, a high-speed 8-bit ADC (analog-to-digital converter) with a sampling rate of 1 GHz was selected, which makes it possible to operate the georadar in the frequency band from 5 MHz to 300 MHz (the lower limit is not limited, but the lower frequencies are not

apply if good spatial resolution is required). A probe pulse with an amplitude of 5 kV in the transmitter is formed by a precision gas spark gap. A wide operating frequency band and a powerful impulse provide a sounding depth from ~ 0.5 m to ~ 200 m in medium soils (sandy loam, light loam, permafrost, etc.). The dynamic range of signal registration ~ 120dB is achieved by three consecutive measurements using 3 pulses: without attenuation; with attenuation of the signal by 40db; with attenuation of 80db. To further increase the sensitivity, i.e. dynamic range expansion, averaging of measurement data is used. The device’s flash memory stores data in its raw form. Then, using a special interactive program, the obviously faulty measurements are rejected, the optimal number of measurements for averaging is selected, and the measurements are processed using the moving average method. Note that, in existing devices, already averaged measurements are stored in memory, which significantly reduces the averaging efficiency. Compared with other analogs, this device has hundreds or thousands of times higher data acquisition speed.

As noted earlier, the disadvantage of using a powerful sounding pulse that has come through the air to synchronize is overloading and entering the non-linear mode of the amplifier input circuits. In the proposed device, the launch of the transmitter and further synchronization of the receiver is performed by the command "start" from the control unit. Therefore, in the proposed device, the air pulse is suppressed by effective shielding (40 dB or more) of the transmitting and receiving antennas from interference in the upper (air) hemisphere. Also, when synchronizing from the control unit, it becomes possible to take measurements even if there is an obstacle (for example, a wall or a small structure) between the receiver and the transmitter.

Effective shielding of antennas significantly improves the quality of the recorded information and makes it possible to work in places with a strong level of interference of a different nature. Especially screening has become relevant in recent years due to a sharp increase in man-made interference, even away from settlements (television, the Internet, mobile communications, etc.). Need to

also note that the elimination of interference of various nature greatly simplifies the complex problem of interpretation of measurement data.

The design of the screen (О Ен Ден. A lightweight, compact tent screen for GPR antennas. Special equipment, 2006, No. 5, p.32-35.) Is made in the form of a frameless tent of several layers of radar absorbing pile material covering a dipole resistively loaded antenna, mounted on a pallet of thin radiolucent material. To reduce the dimensions of the screen, the layer closest to the antenna is as close as possible (3-5 cm) to the antenna, and the distance between the layers is chosen equal to 2-3 cm. For coordination, the first layer closest to the antenna is made of pile material with a resistance close to the wave resistance of the free 377 ohms, and this choice minimizes the effect of the screen on the radiative characteristics of the antenna. The resistance of each subsequent layer is selected by half the resistance of the previous layer (more precisely, the basis of the exponential function is determined experimentally for each type of antenna).

Further development of the ideas laid down in the screen design was the expansion of the range (200-300 MHz and higher) of the screen to the low-frequency region (up to 25 MHz and lower). Recall that for frequencies above 200 MHz there are no problems with shielding the antennas. A screen and a pallet at a working frequency of 200 MHz are made of three sections (the central part and two wings), and these sections in working condition are connected together with the antenna placed inside (75 cm long). To switch from a frequency of 200 MHz to a lower frequency of 100 MHz, a short antenna of 0.75 m is replaced by a long antenna of 1.5 m, and sections of the screen are moved apart and intermediate sections-inserts 37.5 cm long are inserted between them.For a frequency of 50 MHz ( antenna 3 m) in addition to having sections of the screen 75 cm long are inserted, and in this way it is possible to lower the working frequency of the screen to 25 MHz (antenna 6 m) and lower. If you use 7 layers of pile in the screen, then the last layer has a resistance of ~ 6 ohms, while the coefficient of suppression of air interference exceeds 40 dB at a frequency of 50 MHz.

The advantage of such a screen design is its versatility (for various frequencies) and low weight (2-3 kg per linear meter of antenna) and relatively low cost. Note for comparison that the best

foreign georadars for a frequency of 100 MHz are equipped with shielded (in magnetic component) antennas with an interference suppression factor of ~ 20 dB with a single screen weight exceeding 80 kg.

The proposed device measures two parameters in the underlying layers - the delay time (nsec) of the signal reflected from the object and the attenuation of signal G in the medium during propagation. It is known that when a signal propagates in a layer with ohmic conductivity, the signal amplitude decreases exponentially. Then, on a logarithmic scale, the envelope of the signal will be straight, and the degree of slope of the envelope can be used to determine the attenuation coefficient G (dB / m). Strictly speaking, the coefficient found in this way takes into account, in addition to ohmic attenuation, the signal attenuation due to the divergence and scattering of the waves, but they are practically small compared to ohmic attenuation. In the prototype device, to determine the attenuation G by the waveform of the signal, a quasilogarithmic amplitude scale is used - a linear scale for small signals that make up a third of the dynamic range and logarithmic in the region of large signals, and this method leads to a significant distortion of the signal attenuation pattern in deep, most interesting for research layers.

The proposed device provides for recalculation of the signal amplitude from a linear scale to a purely logarithmic with the subsequent color coding of the amplitude, which allows us to correctly estimate the parameter G. Typically, the signal envelope experiences a kink or a break at the transition boundary between the layers, and this boundary can be identified if other additional features, such as a change in the frequency composition of the signal, confirm this conclusion.

Figure 1 shows a diagram of the proposed device. The device consists of a control unit and information storage 1, a receiver unit 2, consisting of an input amplifier 3, an ADC 4 and an integrated power supply 5, a shielded receiving antenna 6, a shielded transmitting antenna 7, and a probe pulse transmitter 8 with an integrated power source 9.

The measurement process begins with the “start” command from the control unit 1, transmitted via the Wi-Fi interface to the receiver unit 2. By this command, the receiver unit enters the registration mode and simultaneously transmits via

fiber optic cable to the transmitter unit 8 permission to emit one or a series of probing pulses. The transmitter unit 8 sends a signal via the fiber optic cable to the receiver unit 2 about the moment of the start of the pulse, by which the delay of the reflected pulses is timed. Using the transmitter antenna 7, the probe pulse is sent to the subsurface space, reflected from various objects and gets through the receiver antenna 6 to the receiver unit 2, where after amplification and direct processing on the ADC 4 it is transferred to flash memory for storage, from where it is sent Wi-Fi in the control unit 1.

Thus, the wide operating frequency range, high speed of information recording and reliable noise immunity gives the proposed device significant advantages over other analogues.

Claims (1)

A device for sensing a subsurface space containing a transmitting antenna with a transmitter of the probe pulse, a receiving antenna with an amplifier of pulses reflected from subsurface objects and an analog-to-digital converter of the input signal associated with it, characterized in that the transmitter is started up synchronously with the receiver turned on, and the transmitting and receiving antennas are shielded in the frequency range from 5 MHz to 1 GHz in the form of several layers of radar absorbing pile material with the ohmic resistance of the layer closest to the antenna equal to the wave impedance of the free space 377 Ohm, and with a gradual decrease in resistance exponentially from the inner layer to the outer, the analog-to-digital converter is capable of directly processing the input signal in the frequency range from 5 MHz to 1 GHz with display of the output signal on a logarithmic amplitude scale.