1. Inspection and checking calculation
Check whether all items in the record are complete and clear, and check the average reading.
2. Calculate the reading difference
Subtract the early base point from the average reading Ti of each measuring point.
Calculate the reading difference Δ ti from the average reading T0 at (0), that is, Δ δTi = Ti-T0. 3. Daily change correction
Find out the daily variation influence value δ' 1 from the daily variation curve.
The specific method is: set the reading time t0 of the early base point on the time axis (horizontal axis), draw a straight line parallel to the vertical axis at the point intersection t0, and draw a straight line parallel to the horizontal axis at the intersection of this straight line and the diurnal variation curve as shown in Figure 2-4- 1, which is the zero line of the diurnal variation curve of this instrument, so as to calculate the diurnal variation influence value δ1i' of each measuring point. For MP-4 proton precession magnetometer, the diurnal variation observer can be directly connected with the measuring instrument to automatically correct the diurnal variation, or the above method can be used to correct it. The accuracy of diurnal variation correction is measured by instrument noise level.
Fig. 2-4- 1 daily variation curve
4. Normal field correction and height correction
For high-precision magnetic survey, when the measurement area is large, it is necessary to correct the normal gradient. The Gaussian coefficients provided by the geomagnetic field model in the corresponding year of the International Geomagnetic Reference Field (IGRF) can be used for normal field correction or as a normal field. The spherical harmonic expression of IGRF is
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Where: gmn and hmn are Gaussian coefficients (which can be obtained by looking up the table); Pmn(cosθ) is an N-order M-order combination and legendre polynomials; λ is the longitude value of each node; θ is the complementary latitude value of each node.
The geomagnetic component values of each point calculated by the above formula are derived from the following formula.
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According to the normal gradient formula of geomagnetic field, namely
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The elevation correction value Δ t can be calculated, where radius of the earth r=637 1200m and Δ h is the elevation difference between the measuring point and the total base point. When the height difference is 30m and the geomagnetic field T0=5000nT, the vertical variation is -0.72nT, which needs height correction. The height correction is calculated according to the elevation of the total base point. Generally, when T0=5000nT, the height difference of 1nT is corrected every 42m, and this correction can be almost ignored if it is high.
To sum up, the final calculation steps for finding δ T anomalies are as follows:
(1) Substitute the geodetic latitude and longitude coordinates λ and θ measured by GPS into equation (2-4-9) to get X, Y and Z. ..
(2) Using x, y and z, the T0 value of the measuring point is obtained by the formula (2-4- 10).
(3) Subtract the T0 value from the corrected total magnetic field strength t of each measuring point to obtain δ t. ..
5. Calculation method of comprehensive impact correction value
Proton precession magnetometer is not affected by temperature change and has no zero drift. However, after the influence of daily variation δ 1 is eliminated twice, there is still a difference, and this difference δ 2 is called comprehensive influence. that is
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This δ 2' should be linearly distributed to each measuring point in time, and its correction value is
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Where: Δ t is the time difference between the readings of two base points; Ti and t0 are the reading times of the measuring point and the early base respectively.
But this internship has requirements for the total δ 2:
If the mean square deviation of quality inspection is multiplied, the measurement is invalid;
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When the mean square error of double-quality test is double-quality test, make linear distribution on the measuring points according to the above method; Practical guidance of applied geophysics teaching
When checking the mean square deviation of double quality, it can be ignored, and it will not be recorded without correction. 6. Total base point correction
The magnetic field difference of each base point relative to the total base point obtained during the joint measurement of base points is the correction value of the total base point. If the base point A is 50nT higher than the total base point, the correction value of the total base point is δ 3 = 50 nt; If the base point A is 30nT lower than the total base point, the total base point correction value δ 3 =-30 nt.
7. Normal gradient correction
Because the survey area in this practice is small (only 0.5km in the north-south direction), the normal gradient correction of geomagnetic field is not carried out.
To sum up, the formula of abnormal value of each measuring point should be
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The calculation of all data must be 100% checked, and the calculator and checker must sign to show their responsibilities.
Records and calculations require complete contents, true records, clear handwriting, uniform format and no tearing of pages. The original record cannot be modified or corrected with an eraser. If there is a mistake in the record, you can only cross it out with a thin line and write it beside it.
Finally, it needs to be pointed out that the total mean square error of measuring the absolute value of the total geomagnetic field intensity in teaching practice is designed to be 5 nt. If the quality inspection fails, we should find out the reasons and do some or all of the work.
(2) Determination of magnetic parameters of rock (ore) samples
Magnetic parameters refer to magnetic susceptibility κ and residual magnetization Mr, κ is scalar and Mr is vector. Therefore, in order to measure the size and direction of specimen magnetism, directional specimens must be collected, that is, the magnetic north direction and vertical downward direction must be marked on the outcrop before collection, as shown in Figure 2-4-2. The method of measuring magnetic parameters with MP-4 proton precession magnetometer (Gaussian first position) is as follows.
1. Ready
(1) Place and level the special instrument rack for magnetic measurement (Figure 2-4-3), put on the compass, set the direction, rotate the sample rack to the static direction of the magnetic needle, adjust the tilt direction of the sample rack to be consistent with the local magnetic field direction, and then lock it. At this time, the MP-4 probe should be in the magnetic east-west direction.
(2) Establish a spatial rectangular coordinate system in the center of the specimen, as shown in Figure 2-4-2.
Figure 2-4-2 Schematic Diagram of Directional Specimen Collection
Figure 2-4-3 Special Instrument Frame for Measuring Magnetism
Step 2: Determine the steps
(1) Read T0 (reading of samples not placed);
(2) Place the sample on the sample tray, set the reading of T 1 when the Z axis is inclined downward, and set the reading of T2 when the Z axis is inclined upward. T3, T4, T5 and T6 can also be read for the X and Y axes; The center of the probe is located at the first Gaussian position of the x, y and z components of the sample magnetic distance;
(3) reading T0' (taking down the sample);
(4) measuring the distance r (in m) from the center of the sample to the center of the probe;
(5) Measure the volume of the sample (m3).
3. Calculation formula of magnetic parameters
The formula for calculating the apparent magnetic susceptibility of the sample is
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Where: r is in m; V unit is cubic meters; T0, T 1, T2, T3, T4, T5 and T6 are in nT units.
The formula for calculating the residual magnetization is
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The calculation formulas of deflection angle φ and inclination angle θ of residual magnetization are respectively
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4. Technical and accuracy requirements
(1) should be greater than or equal to t0 respectively;
(2) The distance r between the sample center and the probe center should be less than or equal to 0.45m;
(3)| T0-T ' 0 |≤2nT；
(4) Repeat measurement according to a certain proportion, and calculate the relative error between apparent magnetic susceptibility and residual magnetization.