The EM-4H system is a low-frequency electrical survey in an aero variant that implements the dipole induction profiling (DIP-A) method. Among the aerogeophysical methods that are actively used in practice, electrical exploration in its various modifications occupies a very important place. The history of the development of airborne electrical exploration systems goes back more than half a century [1] . Despite such a solid age, this branch of practical geophysics continues to thrive. Moreover, in recent years all over the world, aero electrical prospecting has experienced a rise quite comparable with the development dynamics of relatively young aero gravimetry. Almost every year new systems appear, or new modifications of already well-established complexes are proposed [2] . Practically all the existing variants of electrical aerial survey were somehow tested on the territory of the Russian Federation. Today we can say with confidence that in our country the most widely used electrical exploration system that implements the method of low-frequency DIP-A. The equipment developed for this method allows shooting and obtaining maps of effective conductivities of 1: 5000 scale [3] . At the same time, the average productive speed of shooting is about 100 km / h. This is an EM-4H system.
Content
EM-4H Technical Description
The EM-4H equipment performs simultaneous measurements at four frequencies of 130, 520, 2080 and 8320 Hz. The horizontal multi-turn frame (vertical magnetic dipole) fixed on the fuselage of the aircraft is used as the source of the field. In the implemented systems, the carriers were the An-2 , An-3 and Mi-8 helicopters. In aircraft versions, the transmitter loop is tensioned between the biplane racks and the rear mooring unit. As a result, it has the shape of a triangle with an area of about 40 m2. When mounted on a Mi-8 helicopter, a special frame is constructed for mounting the transmitter, to which a loop is attached (Fig. 1). The coil area is already about 60 m2. The current form is the sum of the harmonic signals of the corresponding frequencies. Dipole moments differ for different types of carriers, their approximate values for the four frequencies in ascending order are: 20,000, 10,000, 6,000, 3,000 A • m2. The measurement of the parameters of the alternating magnetic field is carried out by the receiving frames located in the gondola, towed on a cable 70 meters long. The receiver has measurement channels along three orthogonal axes. Its sensitivity at operating frequencies is hundredths of μA / m (100 ppm ). With such geometric parameters of the system, there is a well-known problem - compensation of the influence of currents induced on board the aircraft. For its solution, two compensating loops are installed in the EM-4H system. In each loop of the compensator pumped the current of its frequency, different from the workers. The signal at these frequencies is measured by the receiver along with the rest. The law of propagation of the magnetic field is the same for all frequencies. Therefore, at altitudes of more than 500 meters, where the response from the earth is negligible, you can choose a linear combination of the vectors of the major semiaxes of polarization ellipses at compensating frequencies equal to the projection of the imaginary vector of the operating frequency on their plane. Thus, by subtracting this linear combination, a linearly polarized signal is obtained. This operation is done with each of the operating frequencies. Next, the detection phase is chosen so that the excitation vectors are valid. Since the coefficients are selected automatically by the on-board computer, these procedures take quite a bit of time: 2-3 minutes. The stability of the system is ensured by the introduction of a special reference signal, as a result of which it is sufficient to compensate twice for the departure - at the beginning and at the end.
Measured Parameters
The result of the EM-4H system [4] is the in-phase and quadrature components or the amplitudes and phases of the components of the alternating magnetic field vector at each of the operating frequencies of 130, 520, 2080 and 8320 Hz. In addition, the traditional for the DIP-A method the ratios of the semi-axes, the squares of the major semi-axes and the angles in the axes of the receiver of the large semi-axes of polarization ellipses are calculated. Additionally, the position of the gondola relative to the aircraft is determined [5] . The calculation of the effective conductivities is performed as a solution of the inverse problem for a conducting homogeneous half-space at each frequency.
Fig. 2. Graphs of the in-phase (ReHz) and quadrature (ImHz) component of the response at the specified frequency of the longitudinal conductivity for the S-plane model.
Traditionally, when processing data of similar systems, interpretation is performed using ellipticity, which is informatively comparable to the quadrature component of the response, and height. In the region of maximum, the plot of the quadrature component of the response from the resistance (Fig. 2) requires additional processing techniques in order to distinguish the left branch from the right one [3] . In particular, the method of mapping with respect to signals of quadrature components at two adjacent frequencies is used [6] . However, all modern methods of interpreting the data of frequency aeroelectromagnetic exploration use both quadrature and in-phase response components to calculate apparent resistivity and depths [7] . To determine the in-phase response component in the EM-4H system, a special technique has been developed [8] .
Fig. 3. Graphs of the total in-phase field component (ReHz), the quadrature field component (ImHz), the in-phase response component (ReHzS), and the height. The highlighted zone is the salt lake Tus, Khakassia. Fig. 4 Pseudo section of apparent resistance. Salt Lake Tus, Khakassia.
The technique was tested on a number of conducting objects with a specific resistance of a fraction of Om • m, for example, a salt lake, and gave an impressive result. It allowed us to reach a qualitatively new level in the processing of data obtained in low-resistance regions. The data presented in Figures 3, 4 were obtained over the salt lake Tus in Khakassia. The resistivity of the composing rocks is about 100 Ohm * m, the resistance of salt water is about 0.1 Ohm * m.
Notes
The EM-4H low-frequency inductive aero-electro-prospecting system is a convenient, effective means of studying the conductivity properties of rocks. From year to year, the volume of shooting with its use is growing. The equipment performed well in the works in the Urals, in the Norilsk region, on Taimyr, in Yakutia, on the Kola Peninsula, near the Arkhangelsk, in the central regions of Russia, in Transbaikalia, Kazakhstan, etc.
Literature.
- ↑ [Fountain D. Airborne electromagnetic systems - 50 years of development // Exploration Geophysics. - 1998. - No. 29. - P. 1-11.]
- ↑ [Exploration Trends & Developments // ed .: Werniuk G. 2007, 2008, 2009, 2011, 2012, 2013]
- ↑ 1 2 [Petrov S.I., Novak V.D., Tikhomirov O.A. Aeroelectro-prospecting using the DIP-A method // Exploration and protection of mineral resources. - 2006. - № 5. - p. 38-42.]
- ↑ [Volkovitsky A. K., Karshakov E. V., Popovich V. V. Low-frequency inductive aero-electrical prospecting system EM-4H // Proceedings of the XXXV session of the International Seminar. D. G. Uspensky “Questions of the theory and practice of the geological interpretation of gravitational, magnetic and electric fields” .- Ukhta, 2008.- P.51-54.]
- ↑ [Pavlov B. V., Volkovitsky A. K., Karshakov E. V. Low-frequency electromagnetic system of relative navigation and orientation // Proceedings of the XVI St. Petersburg International Conference on Integrated Navigation Systems, - St. Petersburg, 2008 - P.236 -243.]
- ↑ [Haoping Huang and Douglas C. Fraser The use of quad-quad resistivity in helicopter electromagnetic mapping // Geophysics.- 2002.- Vol.67, No.2.- Pp.459-467]
- ↑ [Klaus-Peter Sengpiel and Bernhard Siemon Advanced Inversion Methods for Airborne electromagnetic Exploration // Geophysics.- 2000.- Vol.65, No.6.- Pp.1983-1992.]
- ↑ [Volkovitsky AK, Karshakov Ye. V., Moilanen Ye. V. Determination of the full response vector in frequency aero-electrical survey systems with a non-rigid base // Proceedings of the IV All-Russian Workshop on Electromagnetic Sounding of the Earth, - Moscow, 2009, - C .210.]