RU2502092C2 - Method and apparatus for induction frequency probing - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: apparatus has a flat radiating loop, an alternating current source, a nonconducting bar lying at an angle to the plane of the radiating loop, a set of receiving coils mounted on the nonconducting bar. Vertical magnetic flux of the direct field through the receiving coils which is close to zero is achieved by selecting a tilt angle of the nonconducting bar close to 35 degrees. The set of receiving coils on the bar matches the number of selected operating frequencies.

EFFECT: high reliability of obtained data, simple design of the device, higher sensitivity and effective probing depth.

8 cl, 1 dwg

Description

The invention relates to the field of electromagnetic induction non-destructive soundings, in particular to devices for measuring the electrical conductivity of soil and soil.

Methods of electromagnetic sounding and profiling are widely used in geophysics, as well as in solving problems such as the detection of underground objects and anomalies in archeology, soil science, geophysical engineering surveys.

Hand-held instruments used for direct (in-situ) measurements of apparent electrical resistivity (resistivity) in the near-surface layer of soil (soils and soils) are usually divided into soil conductivity meters (TCM) and multi-frequency electromagnetic probes (multi-frequency electromagnetic) sounders - MEMS).

The corresponding class of devices for non-contact multi-frequency electromagnetic sounding (EMC) used to study rocks and soils at shallow depths is able to measure variations in the electromagnetic (induction) coupling between the device and the ground (and objects in the ground) as a function of the frequency of the emitted signal. An example is airborne electromagnetic sensors (AEM) produced by various companies (Barringer Research, Dughem, Goetech, Geophex, Aerodat - USA). Terrestrial systems such as Apex Double-Dipole ™, Geophex-300 ™ (Geophysical Survey Systems), which operate in the frequency range from 330 to 20,000 Hz, are also presented.

Known single-frequency and multi-frequency electromagnetic devices for non-contact electromagnetic sounding of near-surface objects. Among them are EM-34, EM-38, manufactured by Geonics Ltd. (Canada), geophysical exploration devices GEM-2 and GEM-3 from Aeroquest (USA) and CMD-Explorer (GF Instruments, Czech Republic).

A device for electromagnetic sounding of the soil is described in US Pat. No. 6,534,985 to "Modular Electromagnetic Sensing Apparatus Having Improved Calibration" (Geosensors, Inc., USA). The sensing apparatus includes an electromagnetic emitter, one or more detectors, and one calibration device, as well as a switch and a processor. The calibration device is positioned so that the distance of the transmitter-receiver is greater than the distance of the transmitter-calibration receiver. The processor has one input for the useful signal and one input for the calibration signal, which allows you to track distortion in the transmitted signal.

In patent application US 20111037462 (Geonics Ltd.) discloses a device multi-coil electromagnetic detector. The distance between the coils is changed and adjusted using the control unit. The device is designed to detect metallic and non-metallic objects (explosive devices) underground and is not intended to measure the soil conductivity profile.

In the patent of the Russian Federation 2136021 a device for electrical exploration is described, having one emitting circuit and n pairs of pairwise symmetrical receiving circuits. In this case, the receiving circuits are arranged so that the moment of mutual induction between each receiving circuit and the radiating circuit in non-conductive space (in air) is zero. The use of n pairs of receiving circuits in this device provides performance and accuracy sufficient for shallow research. At the same time, the dimensions of such an installation are quite large and the measurement circuit implements only a single-part operating mode.

Also known is a device for induction frequency sensing, described in RF patent 2152058 (Institute of Geophysics SB RAS, Russia). In the patent of the Russian Federation 2152058 describes the generation of an alternating magnetic field sequentially at many frequencies. The direct magnetic field signal is compensated by the use of two opposite-connected receiving sensors, and the field compensation is adjusted when the device is located in a non-conductive medium (in air). This configuration of the device allows for a fixed position of the generator loop and receiving sensors (rigid rod) to perform frequency sounding in the surface space. At each frequency, the real and imaginary components of the secondary magnetic field are measured.

In most geophysical devices for electromagnetic sounding, local heterogeneities of the apparent conductivity of the rock (soil) are detected when an electromagnetic field is generated by a generating coil and the signal from currents induced in the earth is measured. Measurement schemes are two- or three-coil with direct field compensation (field from the generator coil).

In methods of induction sounding of shallow depths, it should be taken into account that in the region of receiving coils the magnetic field has several sources. Earth's constant magnetic field does not induce an emf in a fixed coil and may not be taken into account in geological exploration devices. The generating coil of the induction device induces currents in the ground or conductive objects in the ground. In the method of induction sensing, an alternating magnetic field from an induced (induced) current is called a secondary field and is a useful signal, indicating the conductivity of underground structures.

Existing devices for induction sensing of subsurface space have a number of disadvantages. If the direct field compensation (double-coil probe) is not performed, this reduces the information content of the signal, since the signal from the primary field (generator loop) acts as an interference.

Primary field compensation circuits work better, but their setup and calibration require the use of complex electrical circuits and structural solutions.

In addition, in the case of two- or three-coil devices, one underground metal object (an anomaly in electrical conductivity) located near the surface can create two or three images when visualizing data.

The objective of the invention is to increase the reliability of the data and simplify the design of the device with electromagnetic multi-frequency sounding. This problem is solved using the following technical solution.

Figure 1 presents the structural diagram of a contactless device for induction multi-frequency sounding of underground space.

A device for electrical exploration by induction frequency sensing (see Figure 1) contains a flat radiating circuit 1 connected to a power amplifier 2. To the bottom of the device carrying the radiating circuit 1 is connected a support rod 3 made of non-conductive and non-magnetic material. The angle of the rod 3 to the plane of the radiating circuit 1 is selected so as to provide a zero vertical component of the magnetic field strength H _z on most of the carrier rod. On an inclined rod receiving coils 4 are installed in which the emf is induced from alternating magnetic current. The magnetic moments of the generator and all receiving coils located on the rod are parallel to each other. The signals from the receiving circuits are sequentially received, processed and stored in the microprocessor module 5.

In one embodiment of an electromagnetic induction sensing device, an emitting circuit in the device body emits sequentially electromagnetic sounding pulses at a selected frequency sequence. To do this, a sequence of alternating current pulses with different frequencies is supplied from the power source to the emitting circuit. For sensing soil and terrestrial rocks, these sounding frequencies are selected in the range from 2.5 kHz to 250 kHz.

Each receiver coil is pre-tuned to a specific frequency from a selected frequency sequence. Lower frequencies correspond to a greater depth of induction sounding of soil (rock) [1].

In the main version of the device, the receiving coils are located on an inclined non-conductive rod with an equal interval between the receiving coils. The axes of the receiving coils are parallel to the axis of the generating coil (radiating circuit).

The inclined rod shown in FIG. 1 and used to fasten the set of receiving coils should be made of non-conductive material so as not to interfere with the conductivity currents above the ground. As the material for the rod, dielectric materials with an electrical conductivity of less than 10 ^-7 S / m can be selected.

An additional requirement for the rod is the lightness of the rod, since the device is usually used as a portable sensing device. Typically, the support rod is made in the form of a hollow pipe made of materials with low conductivity such as dry wood, polymer, plastic, glass composite material or other composite material with a low specific gravity based on a dielectric matrix.

When arranging the receiving coils on an inclined rod, the effect of the existence of a conical surface with a vertex in the center of the generator loop is used, on the generatrix of which the vertical component of the intensity of the alternating primary magnetic field in a homogeneous non-conducting space H _z changes sign. For a guide in the form of a circle, the generatrix of this surface can be approximated by a straight line with an angle of inclination to the horizontal plane of 35 degrees.

Receiving coils are located along one generatrix line on a direct non-conductive rod. In other words, the tilt angle of the non-conductive rod on which the induction receiving sensors are located is approximately 35 degrees with the plane of the generator loop. The generator loop in the process of sensing is located substantially parallel to the earth's surface.

. Для выбранного диапазона рабочих частот от 2,5 до 250 кГц глубинность зондирования для полупространства с УЭС 50 Ом·м (сухой грунт) составляет от 8 до 10 м [2]. Этой глубины достаточно для проведения приповерхностного зондирования грунта или породы.The values of operating frequencies are determined by calculating the equality of the increase in sounding depth in magnitude

. For the selected operating frequency range from 2.5 to 250 kHz, the sounding depth for half-space with a resistivity of 50 Ohm · m (dry soil) is from 8 to 10 m [2]. This depth is sufficient to conduct near-surface sounding of soil or rock.

, где l - расстояние от центра генераторной петли до приемного датчика, f - рабочая частота из набора частот. Это дополнительное условие на геометрию расположения датчиков является вторым отличием данного устройства.In the proposed device and method, unlike others, in addition to frequency sounding, geometric sounding is performed, where an isoparameter equal to

where l is the distance from the center of the generator loop to the receiving sensor, f is the operating frequency from the set of frequencies. This additional condition on the geometry of the location of the sensors is the second difference of this device.

A device for surface electrical prospecting by induction frequency sensing works as follows. The power amplifier 2 creates a harmonic current in the radiating coil 1, generated sequentially at a number of selected frequencies. The primary electromagnetic field induces induction currents in the earth, which create a secondary electromagnetic field. This secondary electromagnetic field creates an emf. in the aggregate of the receiving coils 4. At a low frequency, a signal from the far coil is recorded and, accordingly, at a high frequency, a signal from the near (to the generator) coil is recorded, and this process is monitored using a microprocessor unit 5. EMF values at each frequency are directly related to the electrical conductivity of a certain underground volume, which is the basis of the method of frequency sounding.

Procedure for setting and calibrating the emf zero in each reel next. The center of each coil is located on the rod of non-conductive material described above, approximately on the line of equality to zero of the vertical component of the field strength H _z . After that, such an (almost vertical) position of each receiving coil is selected and fixed that provides the minimum emf amplitude After setting the final position of all receiving coils, the described device for electromagnetic sounding rises on the ropes into the air to a height of 10 m (usually more than 3-4 meters), and the place for testing and tuning is selected away from all conductive objects (iron structures, power lines etc). At such a distance from the earth's surface, the environment can be considered uniform in magnetic permeability and completely non-conductive (air). EMF values all receiving coils recorded for suspended in the air devices for sensing the electrical conductivity of the soil are taken as the zeros of the device and are subtracted from all subsequent field measurements of the emf

The described device has the following advantages. Compensation of the direct field of the generator is implemented using special placement of single receiving coils. Vertical spacing of the generator and receiver coils leads to the predominance of a single image (maximum) in the signal from a local near-surface heterogeneity in the soil conductivity and, in addition to frequency sounding, simultaneously performs geometric sounding. This allows you to get rid of false signals. Several receiver coils located at different distances from the generator coil are tuned to the corresponding (selected) frequencies in order to increase the sensitivity and effective depth of induction sounding by the described non-contact electromagnetic sounding device.

Literature

1. Balkov E.V., Manstein A.K. Comparison of the characteristics of two- and three-coil implementation of induction probes for shallow frequency sounding // Geophysical Bulletin. - 2006. - No. 1. - S.12-17.

2. Balkov E.V., Epov M.I., Manstein A.K. Estimation of the depth of ground electromagnetic induction frequency sounding // Geophysics. - 2006. - No. 3. - S. 41-45.

Claims (8)

1. A non-contact device for electromagnetic induction sensing, containing a flat radiating circuit, an alternating current source, a non-conductive rod, located at an angle to the plane of the radiating circuit, a set of receiving coils mounted on a non-conductive rod, characterized in that the vertical magnetic flux of the direct field is close to zero through receiving coils is achieved using the angle of the rod. 2. The device according to claim 1, characterized in that the angle of inclination of the non-conductive rod for mounting the receiving coils has an angle close to 35 °. 3. The device according to claim 2, characterized in that a sequence of alternating current pulses with different frequencies is supplied to the radiating circuit. 4. The device according to claim 3, characterized in that each receiving coil is tuned to an electromagnetic field signal of a certain frequency from the generated frequency set. 5. The device according to any one of claims 2 to 4, characterized in that the frequencies of the probe radiation generated in the generator loop are selected from the interval from 2.5 to 250 kHz. 6. The device according to claim 1, characterized in that the inclined rod is made of non-conductive glass composite, plastic, composite material with a low specific gravity. 7. The device according to claim 1, characterized in that the receiving coils are spaced at equal intervals between them. 8. The method of using a non-contact device for electromagnetic induction sensing according to claim 1, characterized in that the signal (amplitude and phase) from the receiving coils tuned to high sounding frequencies corresponds to sounding of shallow depths, and the signal from the receiving coils with lower frequencies corresponds to sounding of great depths.