Method of geophysical prospecting by electric means

FIELD: geophysics.

SUBSTANCE: method comprises generating low-frequency electromagnetic field with the use of ungrounded loop at the day earth surface, measuring Cartesian components of magnetic induction from parallel profiles at given levels, measuring real and imaginary parts of the Cartesian components of the magnetic induction with respect to the phase of the vertical component of the magnetic induction in the epicenter of the ground loop, determining deviations of measured components from the normal values for a homogeneous medium, and determining sections of increased electric conductivity from the sign and magnitude of these values.

EFFECT: enhanced accuracy and efficiency of measuring.

5 dwg

 

The present invention relates to Geoelectromagnetic alternating current induced in the earth by an inductive method, and can be used for prospecting and exploration of conductive objects in a non-conductive and conductive medium, such as seawater. Field of application: the search for ore deposits, occurring at depths up to 500 m and more.

The known method [1] radiokanalane and direction finding (radiorip), which measures the spatial components of the electromagnetic fields of remote stations sverhdlitelnogo (SDR) range. However, this method has significant drawbacks, namely that it has a small depth research and low measurement accuracy due to the use of relatively high frequencies (>10 KHz) and variations (from short-term to long-period) of the signal in time.

Also known geoelectroprospecting [2], in which low-frequency electromagnetic field to excite a vertical cable, properly grounded at both ends well, and measure the Cartesian components of the magnetic induction at a given elevation on parallel profiles, which allows sorting of magnetic field anomalies caused by deep and near-surface objects.

However, the way of inherent existence is significant disadvantages: 1) requires vertical uncased wells: 2) when measuring the real and imaginary components of the magnetic field required landing near the power cable to compensate for the initial phase shifts in the measuring apparatus complex.

The closest technical solution is geoelectroprospecting [3], taken as the prototype method. In the method prototype electromagnetic field created by a vertical cable, properly grounded at both ends well, and measure the Cartesian components of the magnetic induction parallel profiles at specified altitudes. The main advantage of this method is that by placing the deep electrode above and below the depth of the anomalous object, according to the measurement results, more clearly observed anomalous object lying at a depth of up to 2.8 km

However, the prototype method, like the method [2], also have significant drawbacks: 1) for measurements require the presence of the vertical or malonaselennai uncased borehole certain depth; 2) the need landing near vertical cable (power line AB) to compensate for the initial phase shifts in the measuring instrument in the study of the real and imaginary component of the Cartesian components of the magnetic induction, at the same time as the real time axis of the coordinate system is taken to be the phase of the current in the ground wire.

The purpose of the proposed technical solutions - improving the accuracy of measurements and about what socialnet in areal studies.

This objective is achieved in that in the geoelectroprospecting, which excite low-frequency electromagnetic field using an ungrounded loop on the surface of the Earth, measured Cartesian components of the magnetic induction parallel profiles at specified altitudes, measure the real and imaginary components of the orthogonal components of the magnetic induction relative to the phase of the vertical component of the magnetic induction in the midst of an ungrounded loop, determine the deviations of the measured component from the normal homogeneous media values and their size and sign of highlight areas of increased conductivity.

Figure 1 shows a block diagram of a device which implements the proposed method, figure 2 are graphs of the real and imaginary component of the vertical component of the magnetic induction Inzand phase angles on the profile passing through the center of the square loop 400*400 m at a frequency of 78 Hz at an altitude of 100 m when the conductivity of the medium (Cm/m): curve 1 to 10-3, 2-2*10-3, 3-10-2(position loop marked with a solid line 4). Figure 3, 4, 5 are curves of the imaginary components of the vertical component of Bzoutside ungrounded loop in three profiles: 1 - measured, 2 - calculated for a homogeneous half-space, 3 - abnormal value equal to the difference between the computed and measured imaginary component.

The device comprises a receiving antenna 1, side panel 2, including the receiver reference phase 3, three-channel out phase-sensitive analog-to-digital Converter (ADC) 4, a memory 5 and the device 6, the sensor unit 7, the generating device 8, the transmitter 9, the shunt 10, ungrounded loop 11 and the transmitting antenna 12.

The proposed method is implemented as follows. On the surface of the Earth are laid ungrounded loop 11 (1) square shape with a side α . This loop is passed the current rectangular shape without permanent component of frequency ω . The source is generating device 8. In series with the loop 11 is switched shunt 10 resistance RW. The shunt 10 and the loop 11 current flows J=JmSign Cos ω t, where Jmthe amplitude of the rectangular current, the Sign of Cos ω t - the sign function argument Cos ω t. The voltage from the shunt 10

UW=RW*JmSign Cos ω t

fed to the input of the modulation of the transmitter 9, the load of which is the transmitting antenna 12. From the receiver reference phase 3 of the rectangular signal, proportional to UWused as a reference for forming the quadrature voltages, one of which, without regard to phase shifts in the channel, the common mode current in the ungrounded loop 11, and the other is shifted na° .

The current flowing in the loop, excites the electromagnetic field, the parameters (amplitude and phase) Cartesian components on the surface and in the air depend on the electrical conductivity of rocks. Measurements in air at height h is performed in parallel profiles with velocity V(figure 1).

We believe that the measured components of the first harmonic of the magnetic field BxByBzare expressions

Inx=umCos(ω t-ϕx),

By=BmindCos(ω t+ϕy),

Bz=BzmCos(ω t+ϕz),

where BumBmindBzmrespectively the amplitudes of the components of BxByBz; ϕxthat ϕythat ϕzrespectively the phase shifts measured components of BxByBzrelative to the first harmonic current in the ungrounded loop 11 (figure 1). The output voltages from the sensors x, y, z of the sensor unit 7 (Fig 1), is proportional ToxByand Inzarrive at the three-out phase-sensitive analog-to-digital Converter 4 (figure 1), in which the determination of the real and imaginary components of these voltages for a time interval that is a multiple of the period of the useful signal T=2π /ω . Output codes of the Converter 4 is determined by the following expression:

where n is an integer ϕx0that ϕV0that ϕz0respectively the phase shifts in the channels x, y and z, ϕp- phase shift introduced when transmitting the reference signal in the radio channel, i - integer (current value measurement points). We denote the Kx=kx*nT/π , Ky=ky*nT/π , Kz=kz*nT/π modules conversion factors measuring channels x, y, z; then

Rex,i=Kx*InumCos(ϕxx0p);

Rey,i=Kx*InymCos(ϕyy0p);

Jmy,i=Ky*InmindSin(ϕxx0p);

Rez,i=Kz*BzmCos(ϕzz0p);

Jmz,i=Kz*BzmSin(ϕzz0p);

Above the center of the loop at a height h output codes of the real and imaginary component of the vertical component of the magnetic induction is determined by the following expression:

Rez,c=Kz*BzmCos(ϕz,cz0p)=

Kz*BzmCosΔ ϕz0;

Jmz,c=Kz*BzmSin(ϕz,cz0p)=

Kz*BzmSinΔ ϕz0;

where ϕz,c- phase shift of the vertical component of B above the center of the loop. Given that | ϕz,c|is small and does not exceed 2-2 .5° (figure 2), this phase shift is taken for conventional zero, and the phase angle Δ ϕz0is determined from the expression

Δ ϕz0=arctan(Jmz,c/Rez,c).

Given, due to the approximate identity of measuring channels approximate equality of phase shifts ϕxϕyand ϕz,ctrue real and imaginary components of the spatial components are defined by the expression

ReBx,i=Rex,i*CosΔ ϕz0+Jmx,i*SinΔ ϕz0;

JmBx,i=-Rex,i*SinΔ ϕz0+Jmx,i*CosΔ ϕz0;

ReWoo,i=Rey,i*CosΔ ϕz0+Jmy,i*SinΔ ϕz0;

JmWoo,i=-Rey,i*SinΔ ϕz0+Jmy,i*CosΔ ϕz0;

ReBz,i=Rez,i*CosΔ ϕz0+Jmz,i*SinΔ ϕz0;

JmBz,i=-Rez,i*SinΔ ϕz0+Jmz,i*CosΔ ϕz0.

Of course, when determining real and imaginary components of the horizontal components x and y contains error due to relative identity characteristics of the channels x and y relative to the measuring channel z. However, this error is of second order of smallness in comparison with the phase shifts ϕx0pand &x003D5; y0p.

The digital output codes of the inverter 4 are fed into the memory 5, which is also synchronous with the incoming code is provided with the information from the navigation device 6, for example, the type of GPS unit position sensor 7 in the space (height, latitude and Meridian coordinates).

Figure 3, 4, 5 as an example, the results of measurements of the imaginary components of the vertical component of JmBz,i, (curves 1) and calculated Jmz0coordinates of observation points for a homogeneous half-space (curves 2). The difference between the measured and calculated values of JmBza=JmBz,i-Jmz0) (curves 3) due to the presence of rocks with different conductivity. Place of transition through zero of JmBza(figure 3, 4, 5 are marked in bold below 4) correspond to the conducting object in the Ground.

The proposed method was tested in field conditions in the study of promising areas of the Middle Urals. In total, there were studied 14 sites with a total area of more than 400 km2. Field experiments have shown high efficiency and performance of geophysical survey. In the proposed method does not require the landing of the helicopter near the wire ungrounded loop to determine and compensate for phase shifts in the measuring channel and the transmission path of the reference signal over the air.

Offers the proposed method allows for the search of major ore deposits in large, undeveloped, inaccessible areas (wetlands, forested), where there is no deep search uncased wells and impossible landing helicopter near the loop with current.

Thus the proposed method has significant advantages in comparison with known methods.

Sources of information

1. Electromagnetics. Handbook of Geophysics in two books. Edited by V.K. Khmelevsky and V.M. Bondarenko. Book 2. - M., Nedra, 1989, p.46-52.

2. Astafiev PF, Pigianos SHE, Alpatov B.A. report on the performed experimental-methodical works on the development of methods euroresidue works in search of copper-pyrite ores within the upper Ural ore district. - Sverdlovsk, 1987, S. 7-60, № state registration 40-35-30/19a.

3. Patent RU No. 2076344 C1 (Russia). Geoelectroprospecting, G 01 V 3/30, 27.03.97 (Prototype).

Geoelectroprospecting, which excite low-frequency electromagnetic field using an ungrounded loop on the surface of the Earth, measured Cartesian components of the magnetic induction, characterized in that it is parallel profiles at specified altitudes measured real and imaginary components of the Cartesian components of the magnetic induction relative to the phase of the vertical component of the magnetic induction in the midst of an ungrounded loop, determine the deviations of the measured component is t normal for a uniform medium values and their size and sign of highlight areas of increased conductivity.



 

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