Research on the application of high-precision magnetic method in iron ore detection

1Instruments and equipment

The measurement equipment used this time is the MZC-4 proton magnetometer produced by Chongqing Dingfeng Geological Exploration Instrument Co., Ltd. MZC-4 is a high-precision proton magnetometer developed by our company. It has complete measurement functions, high measurement accuracy, and good data repeatability. It is easy to carry and operate, and can perform base station mode, walking mode and gradient mode measurements. MZC-4 adopts dual operation mode, which can operate the host through the panel and control the measurement host through Bluetooth . The instrument data storage is complete, which can provide more complete data information for later analysis and processing.

n  allowed gradient: ≤8,000nT/m

n  GPS positioning accuracy: < 2.5m CEP

n  Base station measurement interval: 2 ~ 60 seconds, configurable

nMeasurement  range: 20,000nT ~ 120,000nT

n  Number of measurement channels: 1 channel (standard configuration , can measure gradient in time)

   2 channels (optional, can measure gradient at the same time)

nStorage  data: 8GB storage capacity, with power-off protection function

nSensitivity  : ±0.05nT (fine mode); ±0.1nT (normal mode)

nBluetooth  interface: store data on your smartphone via Bluetooth connection

nMeasurement  accuracy: ±0.2nT (fine mode); ±0.5nT (normal mode)

nMeasurement  speed:  ≤3 seconds / reading (fine mode); ≤2 seconds / reading (normal mode)

nHost  weight: 2.8kg

nProbe  weight: 1.5kg

nCommunication  distance: 0 ~ 15 meters

nPower  supply: built-in rechargeable lithium battery

nHost  volume: 219mm × 85mm × 271mm

nProbe  volume: 174mm long × 75mm diameter

nOperating  interface: panel /android system operating interface

n  Communication interface: It has a Bluetooth interface to connect the host and Android system devices.

Figure 1.1 MZC-4 measurement system

2 Principles of high-precision magnetic method, consistency and data correction

2.1 Principle of magnetic method

Magnetic exploration is a geophysical exploration method that uses magnetic anomalies caused by magnetic differences among various rocks ( ores ) in the earth's crust to find useful minerals or identify underground geological structures.

2.2 Consistency experiment

2.2.1 Instrument consistency experiment

In order to ensure the accuracy of measurement data from different instruments and reduce errors caused by the performance of the instruments themselves, we conducted consistency experiments on all proton magnetometers participating in this project before formally collecting data. Then the steps to carry out the magnetic instrument consistency experiment are:

Select an area with a stable magnetic field, arrange a 30 -point measuring line with a point distance of 5m , and set an interference source in the middle of the measuring line that is strong enough to have a strong and stable impact on the magnetometer measurement data. Use rope measurement instead of handheld GPS to reduce errors caused by satellite positioning. Use Instrument No. 6 , Instrument No. 7 , Instrument No. 8 , Instrument No. 9 and Instrument No. 10 in turn to perform round-trip measurements with a point distance of 5m .

Figure 2.1 Instrument consistency curve

2.2.2 Instrument noise experiment

In order to determine the signal-to-noise ratio of the measurement area and understand how to distinguish or eliminate interference in the measurement area, a noise experiment was conducted on the instruments and equipment before formal collection. The noise experiment time was selected before 7 a.m., when it is less affected by daily changes. It best reflects the signal-to-noise ratio of the measurement area.

Calculated based on the instrument consistency data, the No. 6 instrument with the most stable performance is used as the daily variable base station instrument. Choose a site with a stable T value and close to the basic field. Collect daily data every 5 seconds with instruments No. 6 , 7 , 8 , 9 and 10 at a distance of 20m , and organize and calculate the collection results.

Figure 2.2 Instrument noise test curve

2.2.3 Probe height experiment

In order to study the influence of uneven surface magnetic bodies, the optimal probe height was studied to obtain more stable and accurate data. Select the relatively most stable No. 6 instrument as the experimental instrument, select a site with a stable T value and close to the basic field, conduct diurnal variation observations with probe heights of 0.5m , 1m , 1.5m , and 2m respectively, and select stable measurement data at each height For 50 points, the mean square error is statistically calculated. When the probe height is 0.5m , the mean square error is 0.05 ; when the probe height is 1m , the mean square error is 0.03 ; when the probe height is 1.5m , the mean square error is 0.02 ; when the probe height is 2m , the mean square error is 0.02. The variance is 0.01 .

2.3 Data correction

The collected data needs to be organized into a data format that can be recognized by professional software (a data format with three vertical columns of X , Y , and Z. The value T) ; then perform diurnal variation correction, height correction, and normal field correction on the compiled data to obtain the ΔT value. Then select appropriate gridding parameters for gridding processing, and use professional software to process the plane data in the frequency domain. Then based on the human-computer interaction results, exceptions are extracted for further analysis and processing.

2.3.1 Daily changes and corrections

In high-precision magnetic surveys, diurnal variation stations must be selected for diurnal variation observations. Among them, the selection of diurnal variation base stations requires choosing a place in the work area with less interference and relatively stable data. A magnetometer is used to conduct 24- hour diurnal variation observations to observe In the results, the data of about 3 hours with relatively stable data is used as the benchmark point of the important work area. Since the area of the work area is not large, there is no need to design sub-base stations for measurement work. All measurement data should be measured and zeroed based on this benchmark point. The diurnal variation station selects a group of machines with the best consistency for diurnal variation observation, sets the MZC-4 proton magnetometer to automatic measurement mode, and the measurement time interval is 20 seconds for automatic observation and recording.

Diurnal correction is to normalize the measurement data of each measurement team for one day in time, that is, each measurement point on each measurement line is equivalent to measurement in the same time period, eliminating the daily variation of the geomagnetic field with time. The error is about 20nT . Moreover, the changing characteristics of the geomagnetic field can also be seen based on the diurnal variation observation curve. If there are phenomena such as magnetic storms, and the diurnal variation values change violently and irregularly, it means that all the data on that day are unavailable, avoiding erroneous data results in the measurement area. Measure the impact of your work.

2.3.2 Height correction

Since the geomagnetic field also changes to a certain extent in the vertical height, all data should be normalized based on the base point elevation, that is, all measurement data should be equal to the measurement points that are lower or higher than the base point elevation after correction. on the same level as the base point.

2.3.4 Horizontal gradient correction

Since the geomagnetic field is not static in the horizontal north-south direction, the measured values should be latitude corrected according to the value of the base point, that is, based on the horizontal field value of the base point, the IGRF model using the international geomagnetic reference field is used for normalization processing to eliminate horizontal Error in the direction of the geomagnetic field.

Taking high-precision ground magnetic survey data as an example, the collected raw data, elevation data, diurnal variation data and horizontal gradient information are corrected for diurnal variation, height and gradient. Use Jinwei GeoIPAS software to perform calculations, as shown in the figure (Figure 2.3 ).

Figure 2.3 GeoIPAS software magnetic method correction interface

After being modified by GeoIPAS , the ΔT data obtained by diurnal correction, latitude correction and altitude correction are shown in Figure ( 2.4 ).

Figure 2.4 Schematic diagram of the magnetic method correction results of GeoIPAS software

3Data processing and interpretation

After the data after diurnal variation correction, altitude correction, and horizontal gradient correction are completed, mathematical and physical methods need to be used for further processing and processing of the data.

First of all, the purpose of gridding the completed data is to convert the points and lines of the survey line data into plane data, and select reasonable gridding parameters based on the measurement network degree and the shape of the actual magnetic anomaly. After gridding the data Interpolation and filtering need to be carried out according to the result requirements of the plane display (for example, when there are many rounding points, the plane data needs to be processed, and the measurement plane is theoretically made more complete according to the interpolation algorithm; when the abnormal jump is large, smooth filtering processing and filtering are required Remove some useless interference anomalies and retain useful magnetic anomalies).

Secondly, the processed data is processed to convert the ΔT in the plane domain into the vertically magnetized ΔZ ( Figure 3.1 ), and then based on the grading processing, upward analytical continuation processing is performed (upward extension eliminates the ground front Internal anomalies, highlighting anomalies with a certain scale in deep parts) 25 , 50 , 75 , 100 , 150 , 200m processing (Fig. 3.2 ), and the three-dimensional visualization slice display (Fig. 3.3 ) based on the upward processing can be seen more intuitively , there is only one main anomaly in the work area that extends to a certain depth and scale. The rest of the positive anomalies on the ground are mostly caused by the influence of shallow magnetism. The scale and impact are not large. They basically disappear when they extend up to 25m . Therefore, this work The main study on the relationship between the main anomaly and the state of magnet occurrence is numbered as anomaly A. The anomaly is generally northwest-oriented, with a length of about 500m and a width of about 200m . The boundary delineation value is about 800nT , and the anomaly peak is 6149.9nT . It can be seen from the upward extension results The maximum extension of the anomaly is about 200m , the center is offset to the northeast, and the inferred tendency is also northeast.

Figure 3.1 Contour diagram of magnetic data, positive and negative anomaly monochrome diagram, cross-sectional plan diagram, polarization diagram

Figure 3.2 Contour diagram of magnetic data contours extending 25 , 50 , 75 , 100 , 150 and 200m

Figure 3.3 Three-dimensional slice diagram extending on the isoline of magnetic data

According to the abnormal shape delineated on the plane, the vertical A abnormal direction is selected to arrange the precise measurement section (section length 330m , point spacing 5m ) ( Figure 3.4) , using the MCH-1 digital magnetic susceptibility meter produced by Chongqing Dingfeng Geological Exploration Instrument Co., Ltd. , for The magnetic susceptibility of the ZK1 drill core ore body and surrounding rock was measured here , and finally 2.5D human-computer interactive profile inversion was performed. According to the fitting inversion results, it can be found that the ZK1 drill hole was constructed at the outcrop of the ore body. In places where the ore body extends, ZK2 should be designed to better control the burial depth and occurrence of the ore body. The final drilling verification results are consistent with the theoretical inference, indicating that high-precision magnetic method work can better guide the prospecting work. , by optimizing data processing and analysis in the plane domain, and auxiliary verification of precise measurement profiles combined with known geological information, not only can direct anomaly verification guidance be provided, but also the deficiencies of existing data can be analyzed through known parameter information to optimize subsequent measurements. The construction plan is of great significance to the prospecting work.

Figure 3.4   2.5D plate-shaped body model section fitting inversion diagram

4 Conclusion

1. The workload and quality accuracy completed by this high-precision ground magnetic survey have met the design and specification requirements. The original data and inferred interpretation results obtained are authentic and reliable, and have achieved good geological effects, providing information for the next development of the mining area. provided the basis for geophysical prospecting.

2. In order to ensure the true reliability of the results, when inferring and judging the properties of magnetic anomalies, not only the results of precise profile inversion must be based, but also calculations such as △ T contours, polarized magnetic pole processing, and analytical continuation. The data are supplemented by the measured regional geological data to make inferences.

3. Using 2.5D magnetic profile fitting and inversion, the geophysical model of the magnetic parameters was determined. Through model comparison and drilling, the true accuracy of the results was verified, indicating that the model of the magnetic parameters is important for this mining area. It has guiding significance for prospecting.

4. This high-precision ground magnetic survey has achieved good results. According to the results of magnetic inversion, it provides a geophysical basis for the next step of work. However, due to the limitations of working methods in the region and the problems encountered in actual work, there are other insights that need to be verified.