In order to more effectively highlight the target body information and suppress the non-target body information, the measured single parameter is converted into multiple parameters needed for interpretation.
In the positive problem of the previous section, the curves or equations obtained are derived from many assumptions. The actual situation is quite different from these theoretical assumptions. For example, some approximately equiaxed magnetic bodies can be treated as a magnetic sphere when the magnetic measuring section is far above the magnetic body. When the profile is very close to the magnetic body, this assumption may bring greater error. At this time, if the section can be mathematically processed to convert the abnormalities on the higher plane, the interpretation results may be improved.
The purpose of magnetic anomaly handling and conversion
Make the actual anomaly meet or approach the assumptions required by the interpretation theory. For example, the measured anomalies distributed on the curved surface are converted into anomalies distributed on the same plane; the superimposed anomalies are divided into isolated anomalies. That is, complex exceptions are processed into simple exceptions for easy explanation.
Make the actual exception meet the requirements of the interpretation method. For example, the measurement result of a single component of the magnetic field is converted into the value of other components; the oblique magnetization is converted into the perpendicular magnetization; or the magnetic field value is converted into a spectral value, etc., which can provide various abnormal information to meet the requirements of some interpretation methods.
Highlight one aspect of the magnetic anomaly. For example, methods such as upward continuation are used to suppress the anomalies of shallow magnetic bodies, and relatively highlight the anomalies of deep magnetic bodies; through directional filtering or conversion of directional derivatives to relatively highlight the characteristics of magnetic anomalies in a certain direction.
At present, the processing and conversion of magnetic anomalies mainly include roundness and division anomalies (such as the separation of regional and local fields, the separation of deep and shallow source fields, etc.); the spatial conversion of magnetic anomalies (measured anomalies are converted to other passive Spatial magnetic field); component conversion (compute the components between dT, Za, Ha and Ta from the measured anomaly); derivative conversion (calculate the vertical and horizontal direction derivative from the measured anomaly); between different magnetization directions Conversion (such as magnetic poles, etc.) and magnetic anomaly conversion on curved surfaces, etc.
When handling and converting magnetic anomalies, two issues must be clarified. One is that the methods of processing and conversion should be selected reasonably. The user must master the principles and practices of various nursing and conversion methods, and have the ability to correctly interpret the results; second, the treatment and conversion of magnetic anomalies only change the abnormal signal-to-noise ratio, and cannot provide new information. Don't forcefully propose or pursue requirements that cannot be met by mathematical processing alone.
Smoothness, interpolation and data networking
1. The least squares of magnetic anomaly is smooth
The measured anomalies include accidental errors and interference caused by non-uniform magnetic bodies near the surface, which make the measured magnetic field show irregular fluctuations. First, smooth, eliminate interference, and highlight the subject's abnormality.
2. Interpolation of magnetic anomalies
When the local anomaly and the regional anomaly are superimposed, the local field and the regional field need to be divided. The interpolation function is constructed through the measurement point interpolation which is not affected by the local abnormal field, and the regional field value is generated. The difference between the measured value and the calculated regional field value is the local field value.
3. Data gridding
Use irregularly distributed interpolation node values to calculate the value on regular grid nodes.
Treatment and conversion of magnetic anomalies in space domain
1. Basic theory of magnetic anomaly space conversion
According to the measured magnetic anomalies of a certain observation surface, the magnetic anomalies in other spatial locations other than the field source are converted. Function U has continuous first-order and second-order derivatives at any point in D in the domain, and satisfies the Laplace equation, then U is harmonic in the D domain. The magnetic potential and magnetic field in the outer space of the magnetic body are such harmonic functions.
2. Analysis and extension of magnetic anomalies
In the magnetic survey work, it is carried out on the surface or in the space near the surface. The harmonic domain is the space above the observation surface, and the magnetic field attenuates as the distance from the bottom surface increases, and tends to zero at infinity.
However, we generally measure the total strength of the magnetic field, and there is no normal guide number. However, in the special case of the plane observation surface, the normal guide number can be eliminated and the magnetic anomaly can be extended.
Known conditions: (1) The value of the harmonic function at each point on the plane, that is, the aforementioned gridded magnetic anomaly---Dirichlet's problem
(2) The vertical direction value of the harmonic function at each point on the plane (the vertical direction of the magnetic anomaly) --- Neumann's problem
Result: the harmonic function of any point above the plane
Conversion of the second-degree magnetic anomaly of the horizontal survey line
1, improvement extension
The longer the profile and the more points, the higher the calculation accuracy, and the sum of the extension coefficients is 1.
Upward continuation is a common processing method, and its main purpose is to weaken local interference anomalies and reflect deep anomalies. A small and shallow magnetic field decays much faster with distance than a large and deep magnetic field.
An example of a general survey of ultrabasic rocks in Inner Mongolia using magnetic surveys. There is a thin layer of basalt in the shallow caprock, which makes the magnetic field appear as a strong beating. In order to suppress the interference of basalt, the magnetic field was extended upward by 500m.
2. Downward extension
Downward continuation is the continuation of the measured magnetic field in the direction of the magnetic source. There is a small change in dT, which causes a big solution, and the curve jumps sharply and needs to be smooth. At present, the approximate method is used for downward continuation of the spatial domain. Such as polynomial interpolation, series regularization and harmonic analysis.
Continuation downwards can deal with lateral superimposed anomalies, such as two adjacent deep-buried plate-shaped bodies, when they are close to the surface, the superimposed display is a wide and flat anomaly. It can be extended downward to separate the magnetic body.
Frequency and magnetic anomaly conversion
The calculation of the three-dimensional space domain is complicated, and frequency domain conversion is generally used.
The basic process of magnetic anomaly conversion in the frequency domain is to first use the forward Fourier transform to obtain the original magnetic field spectrum, and then perform various conversions in the frequency domain, and perform the inverse Fourier transform of the converted spectrum to obtain the converted magnetic field. abnormal.
There are two types:
Simple type: carried out on the horizontal observation surface, in the mid-high latitudes with a small north-south span
Complex type: in the high mountainous area, low latitude area or large north-south survey area in the undulating observation surface
Magnetic anomaly inverse problem
Know the spatial distribution characteristics of the magnetic field to determine the characteristics of the field source body corresponding to the underground, such as determining the spatial location, geometric parameters, shape, occurrence and magnetization of the magnetic parameters, etc.
1. Several simple inversion methods:
- Feature point method
- (2) The tangent method, which uses the relationship between the intersections of the tangents of some characteristic points on the abnormal curve (such as extreme points, inflection points) to calculate the occurrence of magnetic elements. The Airborne Geophysical Remote Sensing Center for Land and Resources has conducted a systematic study on the tangent method, and has accumulated a lot of experience in its application for reference.
- Magnetic anomaly gradient integration method
- Hilbert transform method of magnetic anomaly
- Vector interpretation
2. Fast and automatic inversion of magnetic anomalies
Interpretation of large-area aeromagnetic data requires a large number of inversion depth parameters to determine the location of magnetic bases, magnetic rocks, and structures. Automatically invert the depth of the magnetic anomaly section and area data, and draw the geological section combined with geological and other geophysical data; this depth data can also be used as an initial value to provide an initial model for optimal inversion.
(1) Werner method
(2) Magnetic anomaly total gradient modulus method
(3) Euler's method, which automatically estimates the location of the field source, uses the potential field anomaly, its spatial derivative, and the specific "structural index" of various geological bodies to determine the location of the anomaly field source, especially for large-area heavy Interpretation of magnetic measurement data.
(4) Optimization method
(5) Three-dimensional physical property method
(6) Human-computer interaction method
(7) Inversion method of complex field intensity and magnetic field spherical harmonic series expansion
(8) Magnetic field spectrum inversion method
The nature of the rock
Generally depends on the amount of silica.
Acid rock: The content of silicon dioxide is 65%~75%. The amount of silicon and aluminum minerals greatly exceeds that of iron-magnesium minerals. The feldspar is mainly alkaline feldspar, and the content of quartz is about 1/4~1/ of the rock. The magmatic rock in 3 is acid rock. The plutonic facies are represented by granite, and the expelled facies are represented by rhyolite. The color is generally shallow, plutonic facies are widely distributed, and most of them present large-scale rock foundations. There are many minerals related to acid rocks. Such as gold, silver, copper, iron, tin, lead, zinc, molybdenum, tungsten, antimony, mercury, beryllium, niobium, tantalum and rare earth elements.
Basic rock: The magmatic rock with a silica content of 45%~52% and a high content of iron and magnesium is called basic rock. Common basic rocks include gabbro in plutonic rocks, gabbro diabase in diagenetic rocks, diabase and basalt in extruded rocks. In plutonic rocks, titanium magnetite and nickel ore can be found, and some vanadium-bearing formations form vanadium-titanium magnetite deposits.
Ultrabasic rock: The magmatic rock with a content of silicon dioxide less than 45%, rich in magnesium and iron, deep color, and high specific gravity is called ultrabasic rock. Its main ingredients are olivine and pyroxene. Representative rocks are peridotite, pure peridotite, and Kimberlite. Minerals related to ultrabasic rocks are chromium, nickel, platinum, diamond and asbestos.
Neutral rock: The magmatic rock with a silica content of 52%-65%, which contains less iron and magnesium, and more potassium, sodium, and aluminum than basic rocks, is called neutral rock. The mineral composition is mainly neutral feldspar and hornblende, with few quartz, mostly in gray or light green gray. Common neutral rocks include diorite in intrusive rocks, andesite in eruptive rocks. Both are related to iron and copper deposits
For more information, please refer to xiaok marine surveying and mapping network or the public account of the same name