Method for positioning and predicting concealed orebody based on parallel double-tunnel transient electromagnetic exploration

Abstract

Provided is a method for positioning and predicting concealed orebody based on parallel double-tunnel transient electromagnetic exploration, and belongs to the applied geophysics exploration technology. The method can locate and predict concealed orebodies with low resistivity around the tunnel in a full spatial domain based on a parallel double-tunnel transient electromagnetic method. The implementation of the method mainly includes measuring an electrical parameter, observing a tunnel transient electromagnet, calculating an apparent resistivity, determining an upper limit of an apparent resistivity abnormity, delineating the apparent resistivity abnormity and positioning and predicting concealed orebodies. The method can effectively solve the double-solution problem of positioning concealed orebodies in the full spatial domain of the tunnel.

Claims

1. A method for positioning and predicting concealed orebody based on parallel double-tunnel transient electromagnetic exploration, comprising following steps: step (1), measuring electrical parameters of ores and ore-hosted rocks in a mining area, calculating a geometric mean of a resistivity, and determining that an orebody is a low-resistance element with respect to a surrounding rock, step (2), defining a forward direction of a tunnel as a positive direction of an X-axis, an upward direction normal to the X-axis as a positive direction of a Z-axis, and a direction normal to the X-axis and normal to the Z-axis as a positive direction of a Y-axis according to a right-hand rule; when two tunnels are parallel upper tunnel and lower tunnel, assuming that a distance between the two tunnels is c, and specifying a first measuring point of the lower tunnel as the origin O with an coordinate of (0,0,0); specifying a coordinate of a first measuring point of the upper tunnel as (0,0,c), wherein c is between 0 and 500 m; arranging tunnel transient electromagnetic measuring points along the tunnel for measurement, wherein a distance between the measuring points is 20 m; carrying out measurement by using a tape measure and a plan of a middle section of the tunnel for positioning the tunnel transient electromagnetic measuring points; carrying out measurement by using a coplanar equivalent counter-flux device to obtain data about electric potential observation value V(t), current I and time t, wherein a number of turns of a transmitting coil is 6, a radius of the transmitting coil is 0.42 m, power supply current is 100 A, sampling time is 0.17-10 ms, a number of superposition times is 30, and detection distance is 500 m; calculating an apparent resistivity as follows: $\begin{array}{cc}{}_{}=0.00632L^{\frac{4}{3}}{q}^{\frac{2}{3}}{\left[V\left(t\right)/I\right]}^{-\frac{2}{3}}{t}^{-\frac{5}{3}}& \left(1\right)\end{array}$ wherein L is a side length of the transmitting coil in unit of m; q is an effective area of a receiving coil in unit of m.sup.2, and the effective area of the receiving coil is equal to an area of a single-turn coil multiplied by a number of turns; V(t) is the electric potential observation value in unit of V; V(t)/I is a normalized electric potential observation value in unit of v/A; t is an observation time in unit of ms; .sub. is the apparent resistivity in unit of .Math.m; carrying out an inversion calculation on a visual depth as follows: $\begin{array}{cc}{h}_{}=4/\left({}_0{S}_{}\right)\left(V\left(t\right)/\frac{\mathrm{dV}}{\mathrm{dt}}-t/4\right)& \left(2\right)\end{array}$ $\begin{array}{cc}{S}_{}=\frac{16{}^{1/3}}{{\left(3\mathrm{Mq}\right)}^{1/3}{}_0^{4/3}}{\left(V\left(t\right)\right)}^{5/3}/{\left(\frac{\mathrm{dV}}{\mathrm{dt}}\right)}^{4/3}& \left(3\right)\end{array}$ wherein .sub.0=410.sup.7H/m; S.sub. is a longitudinal conductance in unit of 1/; V(t) is the electric potential observation value in unit of V; dV/dt is a derivative of an observation electric potential in unit of V/s; t is an observation time in unit of ms; M is a transmitting magnetic moment in unit of m.sup.2A; q is an effective area of the receiving coil in unit of m.sup.2, and the effective area of the receiving coil is equal to the area of the single-turn coil multiplied by the number of turns; h.sub. is the visual depth in unit of m; obtaining the apparent resistivity according to a calculation result of Formula (1), then calculating and obtaining the visual depth according to Formula (2) and Formula (3), and drawing a cross-sectional view of resistivity-visual depth distribution according to the apparent resistivity and the visual depth; step (3), determining an upper limit of an apparent resistivity abnormity; through statistics of normal distribution of the apparent resistivity in tunnel transient electromagnetic measurement, obtaining a median and a standard deviation of the apparent resistivity, wherein the upper limit of the apparent resistivity abnormity is equal to the median of the apparent resistivity minus 1 to 3 times the standard deviation, or the upper limit of the apparent resistivity abnormality is equal to an average of the apparent resistivity minus 1 to 3 times the standard deviation; step (4), delineating the apparent resistivity abnormity; according to the upper limit of the apparent resistivity abnormity determined in step (3), delineating apparent resistivity abnormities of the two tunnels; wherein there are two areas with the apparent resistivity abnormity, which are axially symmetrically distributed with the tunnel transient electromagnetic measuring points; step (5), positioning and predicting concealed orebody; for the apparent resistivity abnormities delineated in step (4), forming a distribution map of four apparent resistivity abnormities of the two tunnels in X-O-Z plane according to a parallel correspondence relationship between vertical planes of the two tunnels, defining a part above the upper tunnel as an upper part, a part below the lower tunnel as a lower part, and a part between the two tunnels as a middle part; comparing coincidences of the four apparent resistivity abnormities of the two tunnels in the plane, and in a case that two apparent resistivity abnormities of the two tunnels overlap in a certain direction, determining that a prediction area is within an overlapping area; wherein in the X-O-Z plane, when the apparent resistivity abnormities of the two tunnels overlap in the upper part, the prediction area is located in the upper part; when the apparent resistivity abnormities of the two tunnels overlap in the lower part, the prediction area is located in the lower part; and when the apparent resistivity abnormities of the two tunnels overlap in the middle part, the prediction area is located in the middle part.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic diagram of determining the prediction area using a method of positioning and predicting concealed orebody based on parallel double-tunnel transient electromagnetic exploration, where (1) is a schematic diagram of a vertical and parallel double-tunnel coordinate system, (2) is a schematic diagram of the situation that the prediction area is located in the middle part, (3) is a schematic diagram of the situation that the prediction area is located in the upper part, and (4) is a schematic diagram of the situation that the prediction area is located in the lower part.

(2) FIG. 2 is a plan of a measuring point in a middle section of 670 m of a lead-zinc deposit in northeast Yunnan.

(3) FIG. 3 is a plan of a measuring point in a middle section of 610 m of a lead-zinc mineral deposit in northeast Yunnan.

(4) FIG. 4 is a histogram of the normal distribution of an apparent resistivity at middle sections of 670 m and 610 m of a lead-zinc mineral deposit in northeast Yunnan, in which the middle section of 670 m is on the left, and the middle section of 610 m is on the right.

(5) FIG. 5 is a schematic diagram of positioning low-resistivity orebody in (Transient electromagnetic method) TEM abnormal areas of a double-tunnel section at middle sections of 670 m and 610 m of a lead-zinc mineral deposit in northeast Yunnan, in which two graphs in the left column are the cross-sectional views of the tunnel in the middle section of 670 m, two graphs in the middle column are the cross-sectional views of the tunnel in the middle section of 610 m, and two graphs in the right column are overlapping schematic diagrams of the cross-sectional views of the two tunnel.

(6) FIG. 6 is a cross-sectional view of a concealed orebody predicted by using a parallel double-tunnel transient electromagnetic method in a lead-zinc mineral deposit in northeast Yunnan.

DETAILED DESCRIPTION OF THE EMBODIMENTS

(7) The present disclosure will be further described in detail with reference to examples hereinafter, but the scope of protection of the present disclosure is not limited to the above contents, and the methods in the embodiments are all conventional methods unless otherwise specified.

(8) Embodiment 1: The method for positioning and predicting concealed orebody based on parallel double-tunnel transient electromagnetic exploration is applied in a lead-zinc mineral deposit tunnel in northeast Yunnan, which has good ore-prospecting results, and includes steps (1)-(6).

(9) In step (1), measurement of electrical parameters is carried out as follows.

(10) 163 specimens were collected, and the identification results showed that: there are 85 lead-zinc ores, 25 limestones and 53 dolomites. The statistical results of resistivity measurement showed that the geometric average resistivity of lead-zinc ores was 10 .Math.m, the geometric average resistivity of the ore-hosting limestone was 30218 .Math.m, and the geometric average resistivity of the ore-hosting dolomite was 17403 .Math.m. Compared with surrounding rocks, lead-zinc orebody are low-resistance elements, which have the electrical premise to carry out measurement using a tunnel transient electromagnetic method.

(11) In step (2), observation of a tunnel transient electromagnet is carried out as follows.

(12) The method is applied to two tunnels corresponding to each other up and down in the middle sections of 670 m and 610 m of a lead-zinc deposit in northeast Yunnan (FIGS. 2-3). The transient electromagnetic measuring points of the tunnel are arranged along the tunnel. Measurement is carried out by using a tape measure in conjunction with a plan of a middle section of the tunnel for positioning the measuring point, in which a distance between points is 20 m. Measurement is carried out by using a coplanar equivalent counter-flux device to obtain the data about voltage V(t), current I and time t, where the number of turns of a transmitting coil is 6, the radius of the transmitting coil is 0.42 m, and the side length of the equivalent square transmitting coil is 0.753 m; the power supply current is 100 A, the equivalent single-turn power supply current is 600 A, the sampling time is 0.17-10 ms, and the number of superposition times is 30. In order to overcome the iron interference, the receiving coil is placed 100 cm above the two tracks and 200 cm away from the iron pipe for data acquisition.

(13) In step (3), an apparent resistivity is calculated as follows:

(14) $\begin{array}{cc}{}_{}=0.00632L^{\frac{4}{3}}{q}^{\frac{2}{3}}{\left[V\left(t\right)/I\right]}^{-\frac{2}{3}}{t}^{-\frac{5}{3}};& \left(1\right)\end{array}$

(15) the visual depth is calculated by using following formulas:

(16) $\begin{array}{cc}{h}_{}=4/\left({}_0{S}_{}\right)\left(V\left(t\right)/\frac{\mathrm{dV}}{\mathrm{dt}}-t/4\right)& \left(2\right)\end{array}$$\begin{array}{cc}{S}_{}=\frac{16{}^{1/3}}{{\left(3\mathrm{Mq}\right)}^{1/3}{}_0^{4/3}}{\left(V\left(t\right)\right)}^{5/3}/{\left(\frac{\mathrm{dV}}{\mathrm{dt}}\right)}^{4/3}& \left(3\right)\end{array}$

(17) An apparent resistivity is obtained according to the Formula (1), then a visual depth is calculated and obtained according to Formula (2) and Formula (3), and a profile view of resistivity-visual depth distribution is drawn according to the apparent resistivity and the visual depth.

(18) In step (4), determination of an upper limit of an abnormity is carried out as follows.

(19) Referring to the histogram of the normal distribution of an apparent resistivity according to the transient electromagnetic method of the tunnel in the middle section of 670 m in FIG. 4, it can be seen that the apparent resistivity obeys a logarithmic normal distribution. Therefore, the upper limit of an abnormity is determined by subtracting 1 to 3 times the standard deviation from the median of the apparent resistivity. The median minus 1 time the standard deviation is 307 .Math.m, the median minus 2 times the standard deviation is 153 .Math.m, and the median minus 3 times the standard deviation is 76 .Math.m. The upper limit of the abnormity is determined to be 200 .Math.m by combining FIG. 4 with the observed electric potential abnormity and the known orebody abnormity.

(20) Referring to the histogram of the normal distribution of an apparent resistivity according to the transient electromagnetic method of the tunnel in the middle section of 610 m in FIG. 4, it can be seen that the apparent resistivity obeys the logarithmic normal distribution. Therefore, the upper limit of an abnormity is determined by subtracting 1 to 3 times the standard deviation from the median of the apparent resistivity. The median minus 1 time the standard deviation is 327 .Math.m, the median minus 2 times the standard deviation is 175 .Math.m, and the median minus 3 times the standard deviation is 93 .Math.m. The upper limit of the abnormity is determined to be 200 .Math.m by combining FIG. 4 with the observed electric potential abnormity and the known orebody abnormity.

(21) In step (5), delineation of the apparent resistivity abnormity is carried out as follows.

(22) According to the upper limit of the apparent resistivity abnormity determined in step (4), the apparent resistivity abnormities of the cross section and the plane of the tunnel in the middle section of 670 m and 610 m are delineated (FIG. 5).

(23) In step (6), positioning and predicting concealed orebody are carried out as follows.

(24) For the apparent resistivity in a single tunnel, there are two longitudinally symmetrical apparent resistivity abnormities. Two apparent resistivity abnormities, which are symmetrical to each other up and down, of the tunnel in the middle section of 670 m are TEM-670-Q and TEM-670-S, and two apparent resistivity abnormities, which are symmetrical to each other up and down, of the tunnel in the middle section of 610 m are TEM-610-Q and TEM-610-S. According to the up-and-down correspondence relationship between the two tunnels, the apparent resistivity abnormities of the two tunnels are overlapped on the section, and the overlapped range of the apparent resistivity abnormities of the low-resistivity orebody under the tunnel in the middle section of 610 m is determined as the prediction area, namely TEM-670-S and TEM-610-S (FIG. 5). The prediction area is located in the ore-hosted formation of Carboniferous Weining Formation (C.sub.2w) and Baizuo Formation (C.sub.1b), and the center position is located in the ore-hosting formation of C.sub.2w.sup.1-3 with 370 m200 m, the dip direction in the northwest (NW), and the plunge in the northeast (NE) (FIG. 6).

(25) The position of the concealed orebody predicted by this method is generally consistent with the verification results of drilling engineering, which shows that the method is feasible.