3.7. 1 Contents, methods and steps of geological interpretation of gravity and magnetic data
3.7. 1. 1 Pre-analysis of gravity and magnetic data
In order to ensure the integrity, reliability and convenience of interpretation, the following conditions and factors should be analyzed before interpretation.
1) Analyze and check whether the gravity and magnetic anomalies, the basic data for interpretation, meet the requirements of measuring the anomalies caused by the studied geological factors in the required degree of detail within the allowable error range, that is, analyze the accuracy of gravity and magnetic survey, whether the shape and density of the survey network are appropriate and whether the anomalies are reliable, which is the premise of achieving good geological results.
2) It is necessary to study and analyze whether there are differences between gravity and magnetic anomalies caused by different research objects and between gravity and magnetic anomalies caused by research objects and non-research objects (or interference factors) in a work area. If there is such a difference, it is necessary to choose the corresponding data processing method purposefully, distinguish the gravity and magnetic anomalies produced by different research objects, and at the same time eliminate or suppress the anomalies produced by interferers, so as to obtain effective (target) anomalies caused by a single geological factor, which is conducive to making correct geological judgments.
3) The explanation of anomalies generally starts with "looking at the picture" or identifying anomalies, grasping the overall situation first, and then going deep into the local area. That is, firstly, the anomalies are divided or classified, and the possible internal relationship between the abnormal characteristics of each district (class) and the regional geological environment is analyzed. On this basis, the geological factors that form local anomalies in each district are further explained reasonably.
4) Exception interpretation should follow the principle from known to unknown. The anomalies caused by similar geological conditions have similar characteristics, especially in the interpretation of local anomalies. After using a drilling data or a seismic profile data as a contrast interpretation, the successful experience obtained can be extended to anomalies in areas with similar surrounding conditions, which can get twice the result with half the effort.
5) Fully collect, analyze and utilize geophysical (geochemical) data such as geology, drilling and physical properties in the work area, increase known conditions or constraints as much as possible, and limit the multiplicity of inverse problems.
3.7. 1.2 processing and conversion of gravity and magnetic anomalies
The processing and transformation of gravity and magnetic anomalies is an important part of gravity and magnetic interpretation theory.
In the discussion of the positive and negative problems of gravity and magnetic anomalies, for the sake of simplicity, some assumptions are made about the problems discussed, such as the regular shape, uniform density or magnetization, monomer and horizontal observation surface of geological bodies. The relationship between geological model and the characteristics of gravity and magnetic anomalies is established, and a set of interpretation theories is established. However, the actual situation is often quite different from these theoretical assumptions. If the above methods are directly used to explain the control anomalies, it will be difficult or lead to incorrect conclusions.
The gravity and magnetic anomaly we obtained contains all the effects of density inhomogeneity and magnetic inhomogeneity from the deep to the surface, which is a superimposed anomaly. The anomalies caused by different geological factors are different in amplitude, distribution range and change speed, so they contain a lot of information. However, the superposition of anomalies caused by different factors has brought great difficulties to people's identification, identification and research. Therefore, the decomposition of superimposed anomalies into isolated anomalies, or highlighting some anomalies and suppressing others, has become the main component of gravity and magnetic anomaly processing and conversion.
Gravity and magnetic exploration can only get δ G, Za and δ T generally. Sometimes, in order to make the actual anomalies meet the needs of interpretation, it is necessary to carry out component transformation, such as calculating Hax by Za and Vxz by δ G, so as to provide various anomaly information and meet the requirements of some interpretation methods.
Practice has proved that the processing and transformation of magnetic anomalies is very important to improve the interpretation and inference effect. With the continuous improvement of gravity and magnetic measurement accuracy, the amount of reliable information contained in the measured anomalies is also increasing. How to extract and use this information effectively has become an important topic in the theoretical research of gravity and magnetic anomaly interpretation.
There are many methods to deal with gravity and magnetic anomalies, each of which has its own characteristics and functions, and also has its own specific application conditions, so it cannot be used blindly. We should carefully analyze the characteristics of gravity and magnetic anomalies, the physical properties, geological conditions and geological problems to be solved in the survey area, and reasonably choose the processing methods. The processing and transformation of gravity and magnetic anomalies is only a mathematical processing, which can make some information in the data more prominent and obvious, but it can't obtain information that is not in the observed data. Therefore, we must pay attention to the accuracy of the actual data and the accuracy of the processing method itself when applying various methods.
The processing and conversion of gravity and magnetic anomaly data can be carried out in spatial domain and frequency domain. The purpose and significance of various methods are briefly introduced below, and the theoretical basis and implementation steps of the methods can be found in relevant books.
(1) data gridding
In practice, due to some objective reasons, some measuring points can not be measured, resulting in uneven distribution of measuring points. Therefore, the data on regular grid nodes must be converted from the actual data on irregular grid, which is the process of data gridding.
The essence of data gridding is to interpolate irregular data points, which is usually calculated by binary Lagrangian interpolation polynomial.
(2) Abnormal smoothness
Due to measurement error, correction error and random interference near the surface, abnormal curves often show irregular sawtooth shape. Smoothing must be done before interpretation, and the commonly used method is least square smoothing.
(3) Analytical continuation
The anomaly outside the field source is calculated from the anomaly observed on the horizontal plane (or horizontal line), which is called analytical continuation of anomaly. The calculation of anomalies in the upper half space (or upper half plane) is called upward continuation, and vice versa. The upward continuation is solved by using the solutions of the first boundary value problem in the potential field half space, namely (1. 1-58) and (1. 1-59). Taking the δ T anomaly of two-dimensional magnetic body as an example, its continuation formula is:
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For downward continuation, the downward continuation formula is derived by interpolation polynomial.
The main function of upward continuation is to highlight deep geological body anomalies and suppress shallow and small geological body anomalies. Downward extension can highlight shallow geological body anomalies and distinguish horizontal superimposed anomalies.
(4) Component conversion
Generally, gravity exploration can only measure δ g, while magnetic exploration can generally measure δ z and δ T.. Abnormal interpretation sometimes requires other quantities, and then it needs to be converted into components, such as calculating Hax from Δ g; Sometimes, in order to make the explanation of anomalies easier, it is necessary to change the magnetization direction, such as changing Δ z into, or changing Z a into bedding magnetization z "a, h" ax.
(5) Derivative calculation of anomalies
The derivatives of gravity and magnetic anomalies are widely used to explain. At the same time, in order to highlight shallow anomalies and distinguish horizontal superimposed anomalies, it is often necessary to calculate the derivatives of anomalies, such as calculating Vxz and Vzz with Za.
(6) Division of regional field and local field
Regional field and local field are relative concepts. Usually, the regional field is caused by deep geological factors, and the local field is caused by shallow geological factors. In order to study the regional field or local field separately, they must be separated from the measured superposition field. The commonly used methods are graphic method, mean field method and trend analysis method.
3.7. Qualitative explanation of1.3 anomaly
Qualitative interpretation includes two aspects: one is to preliminarily judge the geological causes of anomalies, and the other is to roughly judge the shape, occurrence and scope of geological bodies. In geophysical exploration, the object directly sought is called the target object, and the object finally sought is called the target object. For example, using gravity exploration combined with magnetic exploration to find magnetite target area is the target; In oil and gas fields, the target is not the target, but the geological factors (including igneous rocks, strata and structures, etc. ) is related to the occurrence position of minerals. Therefore, the qualitative explanation of anomaly is to determine whether the target exists, infer its occurrence state and judge the possibility of the target existence.
Because of the complexity of underground geological conditions, it is often difficult, sometimes even impossible, to judge the geological causes of its anomalies with the data obtained by geophysical methods. Considering that the research object often has various physical properties, comprehensive analysis of the data obtained by various geophysical methods can determine the geological causes of anomalies more accurately. Therefore, as long as the method is used properly, better geological effects can be achieved.
3.7. 1.4 Quantitative interpretation of anomalies
Quantitative interpretation is usually carried out on the basis of qualitative interpretation, and the results can often supplement the results of interpretation. There is no strict boundary between the two, and they complement each other. Quantitative interpretation is to further judge the geological causes of anomalies according to the spatial position, geometric parameters and physical parameters of geological bodies obtained by inversion; Provide the changes of structure, dip angle and thickness of rocks (strata) or basement on the plane or section, so as to infer the underground geological structure; Provide the projection position, depth and dip angle of geological body on the plane. , so as to arrange the drilling project reasonably.
3.7. 1.5 Geological conclusion and schematic diagram
Geological conclusion is the result of abnormal interpretation and the final result of gravity and magnetic work. It is a brief summary or generalization of the geological conditions reflected by gravity and magnetic data, and also a geological inference made by combining qualitative interpretation, quantitative interpretation and geological laws. It should be noted that this geological conclusion is not necessarily the same as that of geologists.
Geological map is a concentrated expression and image description of gravity and magnetic work results. The results of gravity and magnetic work should be expressed as far as possible in the form of inferred results map, such as geological profile map, geological sketch map, inferred structural outline map, mineral prediction map, etc.