Because of the different detection objects, the survey area, survey network, working device, instruments and equipment, field construction and so on are different. The following mainly expounds several main problems in working technology.

1. Several technical problems in field work of near-field magnetic source transient electromagnetic method

(1) Selection of working device

The selection of working devices should comprehensively consider the exploration purpose, construction conditions and the characteristics of various devices. If the depth of the detection target is within 100m, high resolution is required, and the surrounding rock has good electrical properties (easy to generate current collection effect), the same point device is preferred. If deep exploration is required, or the survey area is rugged or there are other obstacles such as river valleys, and it is difficult to lay the mobile source loop, the large loop source determination device should be selected.

(2) Selection of loop size

Increasing the side length of transmitting loop and receiving loop will enhance the signal strength, prolong the duration of effective signal and increase the detection depth. However, the increase of the two makes the field work more difficult, and at the same time expands the influence range of the measurement results, thus reducing the lateral resolution. In addition, increasing the side length of the receiving loop will not only increase the effective signal strength, but also increase the interference signal strength. Therefore, under the condition of ensuring the predetermined exploration depth, the length of the ring edge should generally be as small as possible. The simulation results and field examples show that the same-point device can reliably detect good conductors with linear size equal to the side length of the loop and buried depth twice as long as the side length of the loop. Therefore, when using the same point device, the side length of the line should be equal to or slightly greater than 0.5 times the detection depth. When a frame-ring device is used, the side length of the ring sent by a large fixed source can be equal to or slightly larger than the depth to be detected.

(3) Several technical problems in field data collection.

Circular layout

The power supply loop should use wires with low resistance and good insulation performance, and the resistance per kilometer is generally required to be less than 6 Ω, so as to output enough current under the limited power supply voltage. Traverses shall be laid according to the points laid by geodesy. If there are residual wires on the wire rack, they should be laid in zigzag on the ground to avoid strong induction signals caused by wire winding. All metal objects near the loop will produce strong interference signals, and the strong interference signals of high-voltage power lines will even damage the measuring circuit. So the loop layout should avoid all metal objects and stay away from high-voltage power lines.

Selection of observation time range and superposition times

When writing the working design of a survey area, it is often necessary to determine the recording time range according to the required detection depth and the resistivity change range of the survey area. Formula of current diffusion depth of "smoke ring"

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It can be inferred that the detection depth of TEM is similar to that of tem. If the detection depth is assumed to be half of the current smoke ring depth, it can be used.

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Determine the observation time range. In the above two formulas, tmin and tmax are the minimum delay and the maximum delay, respectively; Hmin and hmax are the required minimum and maximum detection depths, respectively; ρmin and ρmax are the lowest and highest resistivity of strata in the survey area.

Generally speaking, in practical work, it is desirable to record useful signals in as wide a time range as possible. However, because there is a certain transition process in the measurement loop itself, the minimum delay time of observation should not be too early. The existence of interference electromagnetic field and instrument noise level in the measurement area limits the maximum delay time of observation. The recording time range is too wide, in fact, the observation quality of the later traces can not be guaranteed. It is better to do some experimental work before investigating the regional work. If the last few readings are noise levels, it means that all useful signals have been recorded; If the last few readings exceed the noise level, the observation time range should be increased. Of course, after selecting the observation time range, if you encounter an anomaly with slow attenuation in actual observation, you should immediately extend the observation time range and repeat the observation, so as to record the useful signals completely.

In order to suppress the interference electromagnetic signals in the survey area and improve the signal-to-noise ratio of the observed data, most modern transient electromagnetic instruments use the technology of "accumulation and average" for data inversion. Increasing the stacking times can reduce the interference noise level in the recorded data, but increasing the stacking times will increase the observation time and slow down the observation speed. The selection of stacking time should consider both data quality and observation speed. The minimum stacking time should be selected so that the useful signal above the instrument noise level can be recorded with sufficient signal-to-noise ratio.

Determination of transmitting and measuring signal waveform and power supply current intensity

Waveforms of transmitting and measuring signals-The transmitting current of the instruments currently used is a positive and negative rectangular wave with a duty ratio of 1: 1, as shown in Figure 3-45 (transmitting current and primary field waveform). The waveform in the middle of Figure 3-45 (induced electromotive force) is the induced electromotive force waveform generated by the transmitting magnetic field in the receiving loop. This is not the signal to be observed, but the secondary field generated by underground induced eddy current needs to be observed.

Figure 3-45 Schematic Diagram of Transmitting and Receiving Waveforms of Transient Electromagnetic Method

Determination of power supply current intensity-the power supply current intensity can be calculated according to the equipment used and the maximum delayed observation signal reaching the lowest resolvable signal level. For example, for overlapping loop devices, there are

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Where: I is the power supply current intensity; ρmax is the estimated maximum apparent resistivity of the measurement area; Tmax is the maximum delay required corresponding to the maximum detection depth; Vmin is the lowest resolvable voltage; L is the length of the ring.

Measurement of noise level

The noise level in the working area has a decisive influence on the lowest resolvable level η. Although the instrument itself has a strong ability to suppress external electromagnetic interference such as industrial electricity, and the signal-to-noise ratio can be improved by high-order superposition method, the noise level output by the instrument itself varies greatly in various regions. Therefore, it is necessary to know the interference noise level of instrument input and output in each section of the work area. For example, the average noise level of EM-37 system is lower than 0.5nV/m2 in low interference area, generally 0. 15 ~ 0.24 NV/m2, and it can reach n× 10nV/m2 in strong interference area.

When working in industrial and mining areas, it is generally required to measure the output interference level of instruments at each measuring point or at intervals. This measurement is made by feeding current into a matching load. For periodic low-frequency signals (such as 50Hz power frequency interference), the noise level input to the instrument can be detected by using transistor millivoltmeter directly connected in parallel at both ends of the receiving loop. For aperiodic random interference (such as antenna power), special instruments are needed to continuously sample and record waveforms; You can also install a simple audio amplifier and use speakers for monitoring and detection.

2. Selection of transient electromagnetic sounding instrument system

Generally speaking, the transient electromagnetic method requires high instrument sensitivity, strong anti-interference ability and large dynamic range, and requires that the time range and emission power are suitable for detection purposes. According to the division of detection targets, instruments can be roughly divided into four categories.

1) is a portable instrument with low power consumption for shallow sounding. The detection depth is n× 10m ~ n× 100m, and the required time window is n×104μ s. The transmitter usually uses the portable rechargeable battery of 10 A h as the power supply, and the voltage is 12 ~ 60 V.

2) Instrument for detecting the target layer of100m ~ n×100m. For some instruments specially used for mineral survey, the time window range is n×10-1~ n×10 ms, and the power supply current does not exceed 10A. For example, Sirotem-Ⅱ from Australia and WDC-2 from China. These instruments started late; Due to the low transmission power, the data after about 30ms is lower than or close to the noise level. Therefore, it can not be applied to the detailed investigation of overlying strata, and it can only be improved in the low resistance coverage area.

3) Instruments used to detect medium power, with a depth of about 100 m to 1000 m ... Some instruments used to explore deep minerals and coalfields, such as EM-37, digital PEM, GDP-32, etc. The time window is from n× 10-2 to n× 102 ms, and the maximum power supply current reaches 20A, which basically meets the requirements of structural exploration in metal ore fields and coalfields.

4) High-power instruments applied to oil and gas fields or deep structural exploration. The time window of this kind of instrument ranges from n× 10- 1 ms to several seconds, the power supply current is about 100A, and the detection depth is from n×102 to n×103m, such as EM-42 in Canada and ц in the Soviet Union. LOTEM method is a transient electromagnetic sounding method developed in western countries, and its representative instrument is DEMS Ⅳ Ⅳ system in Germany.

3. Data collation and interpretation

(1) Contents of data collation

Data collation includes the following aspects:

1) to transmit and print field observation data;

2) Check and accept the records of original recorded data, site measuring point status and instrument working status;

3) sorting, numbering and summarizing the original recorded data, and compiling indexes and descriptions;

4) filter data as needed;

5) Transform various derived parameters (such as τs, τ s, τ h, τ τ) as required.

(2) the result of the graph.

Generally speaking, the results of transient electromagnetic method are as follows:

1) multichannel or configuration file;

2) ρ ρ pseudo-section;

3) ρ ρ curve type diagram;

4)Sτ-hτ curve type diagram;

5) Plane contour map of ρ ρ or some trajectories.

When the main purpose of the work is to detect local conductors, the above 2 ~ 4 diagrams can be omitted. When the purpose of the work focuses on the stratification of the earth, the above 2 ~ 4 kinds of maps are important basic maps. These profiles are often summarized and drawn into comprehensive profiles for comprehensive interpretation.

(3) data interpretation of transient electromagnetic method

Transient electromagnetic data interpretation is to analyze the temporal and spatial characteristics of transient electromagnetic response and determine the spatial distribution characteristics of geological structures according to the geological and geophysical characteristics of the work area. For example, the change of overburden thickness, vertical stratification of rock and lateral change of rock stratum; Location, shape, occurrence, scale, buried depth, etc. Fault zone and other local geological structure targets of interest. Like other geophysical methods, qualitative analysis and interpretation of data is the most important and basic part of data interpretation, and quantitative interpretation is generally based on qualitative interpretation. Some simple and practical quantitative calculation methods are derived from simple electrical conditions, so the calculation results can only be regarded as semi-quantitative, and its limitations should be paid attention to when applying.

Because the transient electromagnetic method has both profile method and sounding method, in most cases, it is necessary to explain the whole work area or profile with the focus on profile method, and also use sounding data to explain the time characteristics of transient electromagnetic response of some measuring points.

(2) Application examples

1. Profile measurement of Zhangjiagou pyrite mine in Liaoning by pulse transient method

Figure 3-46 is a typical curve of profile measurement of Zhangjiagou pyrite mine in Liaoning Province by pulse transient method. The ore body is located in pre-Sinian metamorphic rocks, and its surrounding rocks are dolomite marble, muscovite granite and high-resistivity rocks. The ore body is pyrrhotite with a resistivity of 0.05 Ω m. As can be seen from the figure, there are obvious anomalies above the ore body. Using attenuation curve to find apparent time constant by ratio method

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It is found that TS = 7.7 ms, that is, the apparent time constant of the conductor is large, and the equivalent current center of the anomaly is roughly estimated by vector interpretation method, which is near the top of the ore body [Figure 3-46(c)]. Figure (a) shows the observation results of * * circle method of 40m×40m.

Figure 3-46 Observation Results of Pulse Transient Method in Zhangjiagou Pyrite Mine, Liaoning Province

(a)*** circle mode 40m×40m；; ; (b) Loop mode 100m× 100m: solid line-vertical component, dashed line-horizontal component; (c) Geoelectric profile:1-Quaternary, 2-dolomite marble, 3-muscovite granite and 4-pyrite; Attenuation curve; TS = 7.7ms ms.

Supplement ※

The induced electromotive force in the receiving coil is

ε=ke-mαt

Where: k is a constant independent of time; M is the coefficient related to the shape of ore body; α is the comprehensive parameter of ore body, and the unit is s- 1. In single logarithmic coordinates:

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The calculation shows that m =10 for a sphere; Cylinder, m≈5.8. In the wild, I don't know the shape of the ore body, so I can't know.

M, order, can be obtained by m =,)

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2. Application in Shuikoushan Pb-Zn-Au ore field, Hunan Province.

Shuikoushan Pb-Zn-Au ore field in Hunan Province is a famous old mine, and Kangjiawan Pb-Zn-Au ore field in Shuikoushan is a large stratabound deposit. The ore body occurs in the contact fracture zone (QBf) between Jurassic bottom conglomerate, Qixia limestone, Hutian limestone and Dangchong siliceous rock, which is layered and gently inclined nearly horizontally, with a buried depth of 200 ~ 500m and a total thickness of 1 ~ 25m. The red beds of Cretaceous Tokyo Formation cover Jurassic and Permian strata, showing unconformity contact. The measurement results of electrical parameters of rocks and minerals show that the average resistivity of lead-zinc-gold deposits is 0.1~1ω m, which is more than three orders of magnitude lower than that of surrounding rocks (the resistivity is greater than1000 ω m). The resistivity of overlying red layer (K 1d3) is 50 ~ 50 ~100Ω m, which is a typical low-resistance coating.

Cross-sectional survey adopts 200 m×200 m overlapping ring device. The instrument used is SIROTEM electromagnetic system made in Australia. The time delay is selected within 0.4 ~ 22.2 ms (i.e. sampling channel 1 ~ 18), and the observation parameter is v (t)/i.

In order to improve the signal-to-noise ratio, the emission current is required to be greater than 5A, and the observation is carried out by using a two-turn receiving loop. The selection of stacking times depends on the interference level of each observation point, which is 5 12 in mountainous areas far from the power grid and 2048 or 4096 in areas near industrial facilities. The observed value of each sampling channel is calculated according to the following formula:

ψτ= 6.32× 10-3 l8/3[V(t)/I]-2/3t-5/3

Converted into apparent resistivity ρ (t) data. The unit of each parameter in the formula is: ρ ρ is the apparent resistivity (ω m); L is the ring edge length (m); V(t)/I is the normalized induced voltage value (μ v/a) observed on the receiving loop; T is the corresponding delay (ms) of each channel. Usually, the observed value of V(t)/I is used to draw multi-channel profile curves [Figure 3-47(a)] and pseudo profile of ρ ρ (t) [Figure 3-47(b)] to analyze the variation law of geoelectric profile in horizontal and vertical directions.

As shown in Figure 3-47(a), the first eight curves of multi-channel V(t)/I profile mainly reflect the lateral changes of shallow geological bodies, and the curves are stepped. The high value area in the east reflects the distribution of low resistivity red beds in the thick layer of the upper member of Cretaceous Tokyo Formation (K 1d3). With the increase of sounding trajectory, the turning point of the step moves eastward, reflecting the feature that the thickness of red bed increases eastward. The low-value response in the middle of the curve reflects that Jurassic and Permian are strata with relatively high resistivity. The response of joints is mainly reflected behind the 10 track. It can be seen from the curves of measuring points 24 ~ 32 on line I and 57 ~ 63 on line II that although it is unusually slow, it is still clearly distinguishable from the background, and the anomaly is more obvious with the increase of the track. Because the buried depth of the I-line ore body (300m) is greater than that of the II-line ore body (180 m), it is relatively late to start showing anomalies. The comprehensive parameters (attenuation index) α of the anomaly are 13s- 1 and 14s- 1 respectively, indicating that it is caused by a good conductor of a certain scale.

Figure 3-47(b) is a pseudo-section of apparent resistivity ρ ρ, which clearly shows the horizontal and vertical changes of geoelectric section. The ρ-ρ isoline intuitively explains the undulating form of low resistivity red bed (K 1d3) and the uplift of deep resistivity layer (P 1q, P 1D3). The pseudo-profile does not clearly reflect the coal seam, but only shows it on the closed circle of 60 Ω m and 40 Ω m isolines.

3. Experimental application effect of transient electromagnetic sounding.

Take Lianshao coalfield in Hunan as an example to illustrate.

Stratigraphic and electrical characteristics of (1) area

The exposed strata in the survey area are Quaternary (Q), Lower Triassic Daye Formation (T 1D), Upper Permian Dalong Formation (P2d), Longtan Formation (P2l), Lower Permian Dangchong Formation (P 1d) and Qixia Formation (P 1q). The Quaternary system is composed of clay, sandy clay and gravel, with a thickness of 0 ~ 15 m and a resistivity range of n×10 ~ n×100Ω m, which is a low-resistance overburden. Daye Formation is distributed in the center of the survey area, with a total thickness of more than 500m, and is mainly composed of marl, argillaceous limestone and limestone. Dalong Formation is composed of siliceous limestone, argillaceous limestone, thick gravel limestone and thin siliceous rock, with thin calcareous mudstone at the bottom, and the thickness of the whole formation is generally 70 ~ 80 m. The resistivity of Daye Formation and Dalong Formation is generally above100Ω m, which becomes the overlying high-resistivity layer of coal-bearing strata. Longtan Formation is a coal-bearing stratum in this area, which is divided into upper member and lower member according to lithology and coal-bearing property. Upper member (P2l2) is a coal-bearing section, consisting of black mudstone, sandstone mudstone and light gray sandstone, with a thickness of about 100m and four coal-bearing layers. The lower member (P2l 1) contains no coal, and is composed of mudstone, sandy mudstone and sandstone, with a thickness of about 300 m. The whole coal measure stratum is a low resistivity stratum, and the resistivity is generally n×10Ω m m. Dangchong Formation and Qixia Formation are siliceous limestone, limestone and mudstone. , which is the marker layer of high-resistivity basement in the survey area, with resistivity greater than 300 ~ 500 Ω m.

Figure 3-47 Comprehensive Profile of Transient Electromagnetic Method on Lines Ⅰ and Ⅱ

(a) multichannel V(t)/I profile curve; (b) ρ ρ pseudo cross section; (c) Schematic diagram of geological section:

K 1d 3—— Upper member of Cretaceous Tokyo Formation (red bed); J 1g—— Jurassic Gaojiatian Formation; P2d 1- Permian Douling Formation; P1d-Permian Dangchong Formation; P2q-Permian Qixia Formation; C2+3- Carboniferous Hutian Group; QBf—— Silicified fracture zone

To sum up, the electrical properties of each layer in the survey area are obviously different, and the electrical exploration method has a good physical premise for coal exploration.

(2) Experimental application effect

The central loop device is used in the work, the loop length is L = 250 m and 400m, and the transmission current is I = 17A. The average electromagnetic interference level in the survey area is 0.24nV/m2, which belongs to moderate interference area. Coupled source devices are also used in a few areas, ab = 1000m, r = 750 ~1250 m. Total * * * completed the workload of 45 sounding points in three sections. After processing the field observation data, the ρ ρ curve diagram, ρ ρ pseudo-section diagram and S ρ (H ρ) curve diagram are drawn. According to these maps and computer inversion results, the top and bottom interfaces of coal measures strata are inferred and determined.

Fig. 3-48 is a comprehensive profile of 13 line transient electromagnetic sounding. As can be seen from the figure, ρ ρ curves are mostly H-shaped, and their minimum values are all in the range of 20 ~ 30 Ω m; The distribution of low-value isolines in ρ ρ pseudo-section map reflects the syncline structural outline.

Figure 3-48 13 line transient electromagnetic sounding comprehensive profile

Central loop l = 250 m; I = 17A； The time window is 0.8 ~ 71.9mst1d-Lower Triassic Daye Group; P 1d—— Dangchong Formation of Lower Permian; P2l 2—— Upper member of Longtan Formation of Upper Permian (including coal seam); P2l 1- Lower member of Longtan Formation of Upper Permian; P2d-Upper Permian Dalong Formation; F- failure; ○○○○○○○○○○○○○○○○○○○○○○○○○○○○○967

The top and bottom boundaries of coal measures strata are determined by the turning point of the corrected Sτ(hτ) curve. Table 3-5 gives the comparison data between the inferred results and drilling data, with an average relative error of 6.4%. Therefore, it can be considered that the inferred top-bottom interface of coal measures strata can basically outline its distribution.

Table 3-5 Comparison between Inference and Drilling Results

On the basis of man-machine joint fitting interpretation, the interpreter carried out automatic fitting inversion calculation for six sounding points on the profile. The total average relative error of six-point fitting is 5.9%, and the depth of the upper interface of coal measures is close to that inferred from Sτ(hτ) curve, with an average relative error of 12.3%.

The test results show that the medium-power transient electromagnetic sounding system can determine the top-bottom interface of coal measures strata with buried depth of 1 ~ 1.5 km in Lianshao coalfield or similar geological conditions. In the result diagram, the distribution outline of coal measures strata can be roughly delineated from ρ ρ (t) curve diagram and ρ ρ (t) pseudo-section diagram. It is an effective method to determine the top-bottom interface of coal measures strata by using the corrected Sτ(hτ) curve.