NL2030048B1 - A CSAMT near-field prospecting method and device - Google Patents
A CSAMT near-field prospecting method and device Download PDFInfo
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- G01V3/08—Electric or magnetic prospecting or detecting; Measuring magnetic field characteristics of the earth, e.g. declination, deviation operating with magnetic or electric fields produced or modified by objects or geological structures or by detecting devices
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Abstract
The invention provides a CSAMT near-f1eld prospecting method and device, which exciting audio frequency signal down to the underground through the grounded wire, receiving the vertical signal from the underground target within a certain range near the grounded wire source (the separation range between transmitter and receiver position is 20m to 60m for 300m depth target survey), measures the vertical magnetic field component, measures the response information of the steeply inclined ore body within the depth of 300m, and obtains the plane location information of the geological target body of the deep steeply inclined ore body. The invention can be applied to CSAMT near-field source distance, so as to obtain more precise information of geological target body. Compared with the traditional method (the separation between transmitter and receiver position is 2000m for 300m depth target survey), it is convenient to construct in mining area and mountainous area. 1
Description
A CSAMT near-field prospecting method and device
The invention belongs to the technical field of near-field prospecting, in particular to a CSAMT near-field prospecting method and device.
Background technology
The geophysical method can realize the perception of underground structure by passive and active observation of geophysical characteristics. Among the geophysical methods, electromagnetic method is very sensitive to the electrical difference of underground medium, so it becomes the main geophysical method for mineral resources exploration. Based on the natural source magnetotelluric method, large-scale and large-depth geotectonic exploration is realized. Based on the magnetotelluric method, the artificial source is introduced to enhance the excitation field signal in the audio frequency segment, and the observation is carried out in the far-field area to ensure that the excitation field at the measuring point meets the plane wave hypothesis, so as to inherit the advantages of the natural source method, at the same time, it 1s more conducive to carry out deeper mineral resources exploration .
The traditional method of artificial source frequency sounding generally observes a group of orthogonal horizontal electric and magnetic field components, and requires the observation distance to meet the requirements of the far-field, that is, greater than 6 times skin depth, so as to ensure that the electromagnetic wave is similar to the plane wave with vertical incidence. Because the near-field electromagnetic field has no plane wave property, the traditional method can not be used.
To solve the above technical problems, the invention provides a CSAMT near-field prospecting method and device. In order to have a basic understanding of some aspects of the disclosed embodiments, a brief summary is given below. This summary is not a general review, nor is it intended to identify key / important 1 components or to describe the scope of protection of these embodiments. Its sole purpose is to present some concepts in a simple form as a preface to the detailed description later.
The invention adopts the following technical scheme:
In some optional embodiments, a CSAMT near-field prospecting method is provided, including:
In the case of near-field, the audio signal is transmitted to the underground, and the vertical signal from the underground target is received to obtain the magnetic field response data;
The vertical magnetic field component is measured, and the response information of steeply inclined ore body within 300 m underground depth is measured to obtain the plane position information of geological target body of deep steeply inclined ore body.
In some optional embodiments, the audio signal is transmitted to the underground through the grounded wire, and the vertical signal from the underground target is received within a certain range near the grounded wire source.
In some optional embodiments, the measurement range is one third of the emission line in the direction parallel to the measuring line, and the vertical magnetic field component is measured in the range of 20-60 meters in the direction perpendicular to the transmission line.
In some optional embodiments, a CSAMT near-field prospecting device is provided, including:
The observation system, which is used to transmit audio signals to the underground and receive vertical signals from underground targets to obtain magnetic field response data;
The analysis system, which is used to measure the vertical magnetic field component, measure the response information of steeply inclined ore body within 300m depth, and obtain the plane position information of geological target body of deep steeply inclined ore body. 2
In some optional embodiments, the observation system includes a transmitter, a 60 receiver and a magnetic sensor.
In some optional embodiments, the transmitter transmits audio signal to the underground through a grounded wire; the receiver receives a vertical signal from the underground target within a certain range near the grounded wire source.
The invention has the following beneficial effects: 65 1. The invention can be applied to CSAMT near-field distance, so as to obtain more precise information of geological target body, and is convenient for construction in mining area and mountain area compared with traditional method; 2.Compared with the traditional method, the observation area is moved from the far-field area to the near-field area, so as to obtain the response signal with higher 70 signal-to-noise ratio, wider bandwidth and higher spectral resolution, and realize the mineral resources exploration with higher resolution. It often depends on the conductivity, magnetic conductivity and electrochemical action of the geological target. The observation of electric potential difference between two points on the ground in general electrical method is changed into point by point observation of 75 magnetic field. Due to the observation of magnetic field parameters, it is less affected by topography. The magnetic anomaly observed in electromagnetic method mainly depends on the conductivity, magnetic conductivity and electrochemical action of geological target; 3. The method is not limited to low-resistivity ore bodies. If the geophysical 80 premise and instrument sensitivity and signal-to-noise ratio are met, the high-resistivity and other ore bodies can also be detected; it can not only measure the magnetic field, but also the electric field, which are both caused by the change of current density.
Description of drawings 85 Fig. 1 1s the measurement diagram of the invention;
Fig. 2 is the simulation result of virtual component of vertical magnetic field of the invention; 3
Fig. 3 is the simulation result of real component of the vertical magnetic field of the invention; 90 Fig. 4 is the magnetic field amplitude curve of the invention;
Fig. 5 shows the field observation results of the invention.
Specific implementation mode
The following description and the accompanying drawings fully demonstrate specific embodiments of the invention to enable those skilled in the art to practice 95 them. Other implementations may include structural, logical, electrical, process, and other changes. The embodiments represent only possible changes. Individual components and functions are optional and the order of operation can be varied unless explicitly required. Partial features of some embodiments may be included in or substituted for partial features of other embodiments. 100 As shown in Fig. 1, in some illustrative embodiments, the invention provides a
CSAMT near-field prospecting method, including the following steps:
In the case of near-field, the audio signal is transmitted to the underground, and the vertical signal from the underground target is received to obtain the magnetic field response data; 105 The vertical magnetic field component is measured, and the response information of steeply inclined ore body within 300m underground depth is measured to obtain the plane position information of geological target body of deep steeply inclined ore body.
Furthermore, the audio signal is transmitted to the underground through the 110 grounded wire, and the vertical signal from the underground target is received within a certain range near the grounded wire source.
Furthermore, in the direction parallel to the measuring line, the measurement range is one third of the emission line, and in the direction perpendicular to the transmission line, the vertical magnetic field component is measured within the range 115 of 20-60 meters.
The ultrashort-offset frequency domain electromagnetic method 1s used to detect 4 the steeply inclined gold deposits in central China. In the near-field condition, it is found that the electromagnetic responses of high-resistivity bodies to different frequencies are basically the same, which belongs to near-field detection, while the 120 electromagnetic responses of low-resistivity bodies to different frequencies are different, which belongs to mixed field detection to a certain extent. According to this characteristic, the distance between receiver and transmitter is equal to 30 meters, and the anomaly of steeply inclined ore body is found, with the depth of 100 meters to 300 meters. The detection results show that the current-magnetic field method has good 125 results in searching for steeply inclined vein metal deposits in thick coverage area.
The invention also provides a CSAMT near-field prospecting device, which comprises: an observation system for transmitting audio signals to the underground, receiving vertical signals from underground targets, and obtaining magnetic field response data; and an analysis system for measuring vertical magnetic field 130 components, measuring the response information of steeply inclined ore bodies within the depth of 300 meters underground, and obtaining plane position information of deep steeply inclined ore bodies.
The observation system comprises a transmitter, a receiver and a magnetic
Sensor. 135 Further, the transmitter transmits audio signal to the underground through the grounded wire; the receiver receives the vertical signal from the underground target within a certain range near the grounded wire source.
Magnetic field measurement in near-field low-frequency case belongs to conductive electrical method. Through transmitting electrodes A and B, a certain 140 frequency electromagnetic field is emitted to the underground to form a pseudoflow -field. Through the receiver, the increasing abnormal data of current density and electric field intensity amplitude of pseudo-flow field can be collected. Based on the similarity between the electric field or magnetic field distribution with current field of underground ore body, the location and scale of the steeply inclined orebody are 145 analyzed and judged through the observation data and curves. This method is fast.
The electric field in the near region is basically horizontal, and the horizontal and vertical components of the magnetic field have the same order of magnitude.
After entering the near region, the time-varying field approaches to the constant current field, and the coupling relationship between the electric field and the magnetic 150 field is very weak and negligible. Therefore, the electric field is only related to the resistivity, and the magnetic field is only related to the permeability.
Analysis of electromagnetic near-field detection mechanism: the electromagnetic near-field detection mechanism is analyzed from the angle of the size of constant magnetic field and constant electric field generated from constant current. 155 Steady current can produce steady electric field and steady magnetic field, but the steady electric field and steady magnetic field can exist independently, which is different from the coupling relationship between electric field and magnetic field produced by time-varying current. However, since the current density J is related to the electric field E, the formula is as follows: 160 J=cE,
The current strength I is also related to the current density J as follows: 1=[[J-dS=[[cE-dS
Where S is the cross section through which the current density vector passes.
Therefore, the magnitude of the constant magnetic field is related to the constant 165 current intensity which produces the magnetic field, the current intensity is related to the current density, and the current density is related to the resistivity of the medium.
This is the mechanism that the near-field magnetic field, which apparently has nothing to do with resistivity, can detect well-conducting ore bodies. When the geophysical premise exists and the differences caused by different emission frequencies can be 170 observed, it is possible to measure them.
Similarly, it is the same reason to detect the well-conducting ore bodies with the constant electric field. That is to say, the current density that produces the constant electric field changes, so does the electric field strength. It shows that there is an 6 inhomogeneous ore body which changes the current density at a certain depth or in 175 the horizontal direction, so as to find ore in the near-field.
According to Ampere rule and Biot-Savart law, the magnetic field intensity of around the ore body is positively related to the power supply current. If there is a low-resistivity overburden on the upper part of the ore body, the underground current distribution has the skin effect, and the current entering the ore body is also related to 180 the power supply frequency. Low-resistivity overburden can shield current, while low-frequency current has certain ability to penetrate low-resistivity overburden. The lower the frequency, the more current will enter the ore body through the overburden, and the stronger the magnetic field. If the power is supplied along the direction parallel to the ore body on the ground, the current emitted by current sources A and B 185 gathers and flows in the rock and ore with low resistivity. When the position of the mineralized zone to be predicted is met, and the resistivity of the mineralized zone and ore body is low to a certain extent, the current will gather on the ore body in large amount, which can predict the location of the ore body.
When the offset distance is fixed, the smaller the resistivity, the larger the 190 conductivity value and the higher the frequency, the more difficult it is to meet the near-field conditions. On the contrary, the higher the resistivity, the smaller the conductivity value and the lower the frequency, the easier to meet the near-field conditions. When the conductivity of the ore body is much greater than that of the surrounding rock, and the response of the surrounding rock is near-field component, 195 the response of the ore body may partly have the component of the transition field. In this way, the metal ore body may be detected.
When the offset is fixed, the lower the resistivity, the higher the conductivity and the higher the frequency, the more difficult it is to meet the near-field conditions.
On the contrary, the higher the resistivity, the lower the conductivity and the lower the 200 frequency, the easier to meet the near-field conditions. When the conductivity of the ore body is much greater than that of the surrounding rock, and the response of the surrounding rock is near-field component, the response of the ore body may partly 7 have the component of the transition field. In this way, the metal ore body may be detected. 205 For ore body and surrounding rock, due to their different conductive properties, and different magnetic current densities, the conditions to meet the near-field are not the same. Therefore, when the excitation frequency is different, the amplitude of magnetic field component is different, so the position of underground ore body can be distinguished. 210 Carry out simulation calculation and analysis, the transmitting dipole distance
AB = 1500 m, the transmitting and receiving distance R = 30 m, the buried depth of the top interface H = 100 m, the plate body extending 100-400 m, the plate resistivity is 1 £m the surrounding rock resistivity is 1000 £7 and the emission frequencies are 29Hz, 2-'Hz, 2°Hz, 27 Hz, 2*Hz, 2%Hz, 2°Hz, respectively. The real part and 215 imaginary part of the vertical magnetic field component are calculated respectively.
It can be seen from figures 2 and 3 that the amplitude of virtual component of vertical magnetic field is relatively small at high frequency and large at low frequency; while the field value of real component of vertical magnetic field is exactly the same at high frequency and low frequency, and the amplitude does not change with 220 frequency. However, both the virtual component and the real component of the vertical magnetic field have changes in the horizontal direction, so the position of the mineralized body can be inferred according to the amplitude change of the vertical magnetic field in the horizontal direction.
The invention uses a high-power field source emission system to emit current 225 with different frequencies to the underground, and uses the receiving instrument to measure the magnetic field intensity above the ore body at different frequencies near the source, and judges whether the metal ore body exists or not and its position according to the magnetic field change. This method avoids the volume effect of electrical prospecting in the case of large transmitting and receiving distance. The 230 anomaly is easy to explain and the positioning effect is good.
The observation system is composed of transmitter, receiver and magnetic 8 sensor. The V8 multi-functional electrical exploration system of Phoenix company of
Canada and relevant magnetic probes are used. It has high transmitting power, high receiving sensitivity and strong anti-interference ability. The frequency range of 235 TXU3O0 transmitter is 0.0039 ~ 10000Hz; the frequency range of V8-RXU receiver is 10000 ~ 0.00005Hz; the frequency range of MTC-50H receiving magnetic probe is 400 ~ 1 / 50000Hz; the power of Yuchai KW-30gf generator is 30kW.
The source signal transmission mode similar to CSAMT method is adopted. The power supply wire is arranged in rectangle with side length of 1500m x 750m. The 240 power supply electrode AB is located at both ends of a long side, and the transmitter and generator are located in the middle of the other long side. When measuring, the emission current is fixed at 20A, the magnetic probe is placed vertically, and the vertical component of magnetic field amplitude of 2°Hz. 2-1Hz, 22Hz. 2*Hz. **Hz. 2- Hz. 2%Hz are measured at the same measuring point. 245 The experiment area is located in Anhui Province in Central China. The bedrock is the xiyudui formation of the upper Archean-lower Proterozoic Wuhe group. The main lithology is amphibole plagioclase gneiss, amphibolite and striated migmatite, interspersed with diorite porphyrite and granite porphyry. The overburden is the
Cenozoic quaternary system with a thickness of 98m, mainly composed of loam, clay 250 and sand, containing calcareous sand and iron manganese nodules.
In the exploration area, the resistivity of amphibolite and migmatite is the highest, about 4000-6000Q:m; the second 1s amphibolite, plagioclase gneiss, granulite, diorite, etc., about 2000 Q-m; the resistivity of polymetallic sulfide ore is the lowest, about nx10% - 10? Q'm; the resistivity of structural altered rock is higher than that of 255 polymetallic sulfide ore, much lower than that of intact surrounding rock, and the
Quaternary resistivity 1s 20 -100Q-m. The electrical characteristics of main rocks and ores are shown in the following table: 9
Electrical parameters
Numb Number of | (geometric mean value)
Name of rock and ore er specimens | Resistivit
Polarizability y gold bearing polymetallic 2 10 42.7 68.57 sulfide gold bearing polymetallic 3 sulfide quartz vein, quartz | 12 119.3 18.79 vein diorite porphyrite, diorite 6 1322.0 quartz syenit, syenite 7 1508.8 1.26 porphyry biotite plagioclase gneiss, 36 1809.8 1.39 plagioclase gneiss
Amphibolite, plagioclase 2400(cro | 1.60(crowd 66 (biotite) amphibolite wd value) | value 4200(cro | 2.0(crowd 13 migmatite 66 wd value) | value) biotite plagioclase 6400(cro | 3.2(crowd 14 129 amphibole gneiss wd value) | value)
It can be seen from the above table that the resistivity of ore body and 260 mineralized alteration is low, and the resistivity of plagioclase amphibolite and 10 migmatite is high. The orebody and mineralized alteration zone are good conductors with low resistivity relative to surrounding rock, and have geophysical conditions for conducting current magnetic field method.
As shown in Figures 4 and 5, the high-frequency part of the magnetic field 265 amplitude curve is basically straight, like the underground horizontal bamboo root, without obvious abnormal display; in the high-frequency part, with the decrease of frequency, the magnetic field amplitude curve gradually bulges, and the abnormal intensity gradually increases.
The amplitude curves of 2°Hz and 2'Hz current magnetic field are basically 270 straight without obvious anomalies. They mainly reflect the electrical uniformity of
Quaternary overburden, indicating that the current does not enter into bedrock.
When the power supply frequency is 2?Hz, the amplitude curve of current magnetic field above the ore body, corresponding to 1500-1400 position, is slightly downward curved, showing a weak negative anomaly, which is slightly lower than the 275 current magnetic field strength of the surrounding rock of the ore body, which is speculated to be caused by the increase of silicification alteration resistivity around the orebody.
The amplitude curve of 27Hz current magnetic field is straight, which indicates that the current entering bedrock increases, the current magnetic field generated in the 280 mineralization zone is enhanced, and the amplitude intensity is equal to the weak negative anomaly of silicification.
The amplitude curves of 2*Hz, 2“Hz and 2°Hz current and magnetic field are protruding upward above the ore body, and the lower the frequency, the greater the intensity of anomaly, indicating that the more current enters the bedrock. 285 The drilling results show that the buried depth of the ore body is 98m-390m, and the upper end is in contact with the quaternary system, and the dip angle 1s 65 © and tends to the southeast. The thickness of mineralization alteration zone is 10-15m, in which the thickness of ore body is 1.25-3.35m. The natural types of ore are pyrite-polymetallic sulfide-quartz vein type, pyrite-quartz vein type and altered rock 11
290 type. The main metal minerals are natural gold, silver gold, galena, chalcopyrite and pyrite. The gangue minerals are mainly quartz, mica feldspar and calcite, with gold content of 5.17-31.8g/t and lead content of 0.79-2.27 %.
It should also be understood by those skilled in the art that various illustrative logic boxes, modules, circuits, and algorithm steps described in connection with the 295 embodiments herein can be implemented as electronic hardware, computer software or a combination thereof. In order to clearly illustrate the interchangeability between hardware and software, various illustrative components, frames, modules, circuits and steps are described in general around their functions. Whether this function is realized in hardware or software depends on the specific application and the design constraints 300 imposed on the whole system. Skilled technicians may implement the described functions in a flexible manner for each specific application, but such implementation decisions should not be interpreted as departing from the scope of protection of the present disclosure. 12
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